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Numerical simulations using the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) are performed to elucidate the effects of the interplanetary magnetic field (IMF) on the middle thermosphere composition during a "geomagnetically quiet" period from the day of year (DOY) 110–111 in 2019 (when the Auroral electrojet (AE) index never exceeded 300 nT and the Kp never exceeded 2). In particular, this paper aims to examine how the Global‐scale Observations of the Limb and Disk (GOLD) mission observed daytime thermospheric O and N2 column density ratio (∑O/N2) depletion at mid‐latitudes originated under such a "geomagnetically quiet" condition. A comparison of electric potential, Joule heating rate per unit mass, ion velocity, neutral temperature and winds in the middle thermosphere (∼160 km) between real IMF and without the IMF east‐west component (By) indicates that a By dominant condition can enhance their strengths under this "geomagnetically quiet" condition. Consequently, ∑O/N2 depletion with a stronger magnitude (30% compared with ∼8% without By) and larger disturbed area was introduced in the post‐midnight sector at high‐latitudes due to strong and localized upwelling associated with the enhanced Joule heating rate per unit mass. The ∑O/N2 depletion was transported equatorward and corotated from local post‐midnight to early morning, and was observed by GOLD at middle latitudes during daytime. Plain Language Summary: This study utilizes numerical simulations with a climatological tide in the lower boundary to investigate how thermospheric composition disturbance is produced and transported to mid‐latitudes during a geomagnetically quiet period (DOY110‐112, 2019, maximum Kp = 1.7, AE < 300 nT). Through comparing parameters from simulations under real IMF and By = 0, it is found that the IMF By, which was dominant over this period, can significantly enhance the ionospheric potential, Joule heating rate per unit mass, ion drift and neutral winds compared with Bz only condition. As a result, the geomagnetic activity with AE < 300 nT can trigger stronger depletion (30%) of the column density ratio of thermospheric O and N2 (∑O/N2) in the polar region. Then the accelerated meridional wind transports the ∑O/N2 depletion equatorward to mid‐latitudes. The depletion is also transported from local post‐midnight to local early morning. This study shows the crucial role of dominant By during "quiet time". Additionally, it suggests that the response of ∑O/N2 to geomagnetic activity during the "quiet period" is similar to the one during geomagnetic storms. Key Points: IMF with dominant By enhances ion convection pattern, Joule heating rate and neutral windsStrong thermospheric composition disturbance can be generated and propagate into mid‐latitudes with dominant By during "quiet" timeResponse of thermospheric composition to geomagnetic activity with AE < 300 nT is similar to its response to geomagnetic storms [ABSTRACT FROM AUTHOR]

We conduct a global survey of magnetosonic waves and compute the associated bounce‐ and drift‐averaged diffusion coefficients, taking into account colocated measurements of fpe/fce, to assess the role of magnetosonic waves in radiation belt dynamics. The average magnetosonic wave intensities increase with increasing geomagnetic activity and decreasing relative frequency with the majority of the wave power in the range fcp < f < 0.3fLHR during active conditions. In the region 4.0 ≤ L* ≤ 5.0, the bounce‐ and drift‐averaged energy diffusion rates due to magnetosonic waves never exceed those due to whistler mode chorus, suggesting that whistler mode chorus is the dominant mode for electron energization to relativistic energies in this region. Further in, in the region 2.0 ≤ L* ≤ 3.5, the bounce‐ and drift‐averaged pitch angle diffusion rates due to magnetosonic waves can exceed those due to plasmaspheric hiss and very low frequency (VLF) transmitters over energy‐dependent ranges of intermediate pitch angles. We compute electron lifetimes by solving the 1D pitch angle diffusion equation including the effects of plasmaspheric hiss, VLF transmitters, and magnetosonic waves. We find that magnetosonic waves can have a significant effect on electron loss timescales in the slot region reducing the loss timescales during active times from 5.6 to 1.5 days for 500 keV electrons at L* = 2.5 and from 140.4 to 35.7 days for 1 MeV electrons at L* = 2.0. Plain Language Summary: The modeling and forecasting of the Earth's radiation belts, composed of charged particles trapped by the Earth's magnetic field, is important for mitigating damage to satellites during extreme solar events. Interactions with plasma waves in the magnetosphere can energize and scatter these charged particles, depending on the intensity of the plasma wave and local plasma conditions. One such plasma wave is the magnetosonic wave, which can contribute to both electron energization and loss to the atmosphere. In this paper, we study the role of magnetosonic waves using simultaneous measurements of their intensity and local plasma conditions and find a reduction in the loss timescales of high energy electrons in the near‐Earth region when magnetosonic waves act in combination with other plasma waves. Key Points: Pitch angle diffusion rates for magnetosonic waves exceed those of hiss and VLF transmitters over energy‐dependent ranges of pitch anglesMagnetosonic waves have a significant effect on electron loss timescales in the region 2.0 ≤ L* ≤ 3.5Magnetosonic waves reduce the loss timescales during active times from 140.4 to 35.7 days for 1 MeV electrons at L* = 2.0 [ABSTRACT FROM AUTHOR]

This paper reports a rare interaction between two fronts of nighttime medium scale traveling ionospheric disturbances (MSTIDs) and associated phenomena observed over geomagnetic low‐mid latitude transition region on a geomagnetically quiet night (Ap = 5) of 21 May 2020. The event was observed for nearly 8 hours in O(1D) 630.0 nm airglow images captured by an all‐sky imager installed at Hanle (32.7°N, 78.9°E; Mlat. ∼24.1°N), Leh Ladakh, India and supplemented by a digisonde and a global positioning system receiver located within the imager's field of view (FOV). The images revealed a slow‐moving MSTID front being followed by a fast‐moving MSTID front, both traveling southwestward. The third front of the MSTID caught up with the second one, initiating the interaction. As a consequence, a plasma channel was created during the interaction and the MSTIDs' fronts began decaying. Subsequently, they became thin strips and later dissipated within the FOV of the imager. The polarization electric field of the merged region of the two interacting MSTIDs' fronts played a key role in creating the plasma channel. In the later hours of the night, another MSTID with a rotating front was observed, which is a rarely reported phenomenon. The horizontal gradient in the westward wind is a plausible reason for the observed rotation. This study elucidates the complex nature of geomagnetic low‐mid latitude transition region. Plain Language Summary: Medium scale traveling ionospheric disturbances (MSTIDs) are plasma depleted and rich bands propagating in the mid latitude ionospheric F region. Nighttime MSTIDs in northern hemisphere are usually aligned in the northwest to southeast direction and propagate southwestward. Despite the extensive study of MSTIDs, a few rarely reported phenomena, such as their interaction and rotation, are not well understood. A better understanding of these phenomena is crucial because of their effects on radio communication and navigation. On the night of 21 May 2020, we observed southwestward propagating multiple plasma depleted and rich MSTIDs' fronts in 630.0 nm all‐sky airglow images. The characteristics (horizontal velocity, orientation with respect to geomagnetic N‐S, and wavelength) of the observed fronts of the MSTIDs were different. As the MSTIDs' fronts propagated in the imager's field of view, the faster front caught up with the slower one, resulting in the interaction. Later on, MSTIDs' fronts decayed into thin strips and eventually disappeared. In about 1 hour after the disappearance of thin strips, another MSTID started propagating southwestward. However, its orientation and horizontal velocity decreased gradually over time. Using multi‐instrument data sets, we have explored possible mechanisms behind these phenomena. Key Points: The interaction of Medium scale traveling ionospheric disturbances (MSTIDs)' fronts resulted in creating a plasma channel followed by the decay of the MSTIDsPolarization electric field played a key role in creating the plasma channelA significant change in the orientation of one of the MSTIDs' fronts was observed [ABSTRACT FROM AUTHOR]

Using ground‐based magnetic field measurements and numerical simulations from the Thermosphere‐Ionosphere Electrodynamic General Circulation Model (TIEGCM), a first paper (Zhang, Yamazaki, et al., 2021, doi: https://doi.org/10.1029/2021JA029681; under review) introduced the potential roles of disturbance dynamo electric field due to subauroral polarization streams (SAPS) on the equatorial electrojet (EEJ) during a moderate geomagnetic storm on 1 June 2013. Our second study investigated the temporal responses of equatorial electrojet to SAPS. At noon, the residual EEJ (ΔEEJ) owing to SAPS flows westward, that is, counter equatorial electrojet (CEJ). The temporal variation of CEJ excited by the dynamo electric field was basically consistent with that by SAPS, and the effects of zonal wind were larger than those of meridional wind. The relative time delay of CEJ and SAPS was related to the propagation time of disturbance wind from mid‐latitudes to low‐latitudes. It took 2–3 hr for SAPS‐related disturbance wind to propagate to the equatorial region and change the polarity of EEJ. The influence of meridional winds on the temporal variations of ΔEEJ is related to the generation of eastward currents at mid‐latitudes, which can accumulate the positive charges at dusk terminator and then generate a westward electric field at lower latitudes. Plain Language Summary: The equatorial electrojet (EEJ) represents a ribbon of intense electric current flowing in the ionospheric E region (approximately 110 km) on the dayside along the dip equator. Its behaviors are controlled by the daytime ionospheric electric field and ionospheric conductivity. However, the temporal variations of EEJ in response to the subauroral polarization streams (SAPS), an interesting and important physical phenomenon at subauroral latitudes, are seldom explored and poorly understood. The understanding of EEJ behaviors and their associated physical drivers during SAPS periods can contribute to the modeling and forecasting of the equatorial space environment, and the understanding of the coupling between ionosphere‐thermosphere systems at high‐latitudes and dip equator. Key Points: ubauroral polarization streams‐induced counter equatorial electrojet (CEJ) shows a 2–3 hr delay with respect to SAPSThe time delay of CEJ attributes to the traveling atmospheric disturbances in zonal windsThe effects of meridional winds on CEJ depend greatly on the geomagnetic declination [ABSTRACT FROM AUTHOR]

The formation of the evening time F3 layer remains elusive. One of the hypotheses for the formation of the evening time F3 layer relies on the height gradient in the vertical plasma drift. In this paper, we test the validity of the hypothesis using Jicamarca incoherent scatter radar observations of height profiles of electron density and vertical plasma drift. Results show that the evening F3 layer could occur when the plasma drift is moderate and the height gradient in the vertical plasma drift is small or insignificant. Given that the production in the evening hours is negligible, these results call for interpretation for understanding the formation of the evening F3 layer. We propose that in addition to the known role of vertical plasma drift, the height variation in the recombination loss of electrons plays an important role in the formation of evening F3 layer over the geomagnetic equator. Key Points: First simultaneous observations of height profiles of electron density and vertical drift linked with evening time F3 layer are presentedResults show that the evening time F3 layer could occur when the height gradient in vertical drift is small or insignificantThe roles of height varying vertical drift and recombination loss are discussed [ABSTRACT FROM AUTHOR]

The effect of radial interplanetary magnetic field (IMF BX ${\mathrm{B}}_{\mathrm{X}}$) has been ignored in the solar wind‐magnetosphere‐ionosphere (S‐M‐I) coupling. We present a statistical study of IMF Bx effects on the Heppner‐Maynard Boundary (HMB) midnight latitude calculated from SuperDARN measurements between January 2002 and December 2017. The HMB represents the equatorward extent of the ionospheric convection pattern and can be used as a proxy for the equatorward boundary of the auroral oval. That is, the HMB expands toward lower latitude when the level of S‐M‐I coupling is enhanced. It is found that the averaged HMB midnight latitudes during both sunward and antisunward radial IMFs are around 64–65° magnetic latitude (MLAT). HMB midnight latitude is negatively correlated with the magnitude of IMF Bx as well as solar wind speed, while no significant trend is found between HMB midnight latitude and IMF Bz or By during radial IMF. For antisunward radial IMF, the relationship between upstream parameters and HMB is more dependent than in sunward radial direction. As the first long‐term statistical study focused on HMB during radial IMF conditions, this work provides observational evidence that the IMF Bx plays an important role in the coupling process and the traditional coupling function could not be applicable in the case of radial IMF. Plain Language Summary: The magnetic reconnection between solar wind and magnetosphere can drive a convection structure in the high‐latitude ionosphere. Heppner and Maynard developed a method to calculate the equatorward boundary of the ionospheric convection, which is called Heppner‐Maynard Boundary (HMB). HMB could be used as the monitor of the interaction process between the solar wind and geomagnetic field. In this paper, we report the effects of the radial (sunward or antisunward) interplanetary magnetic field (IMF) on HMB in the northern hemisphere. For normal cases, the Bz component o IMF is the most important. But our statistical results indicate that there is a decreasing trend between the strength of IMF Bx and HMB latitudes at midnight. No significant trend is found between HMB midnight latitude and IMF Bz or By during radial IMF. Moreover, the results provide observational evidence the Bx term should be considered in the coupling function during radial IMF. Key Points: The HMB midnight latitude has a negative trend on the IMF Bx, especially for negative IMF BxThe averaged values of HMB during radial IMFs are larger than those for substorm periods and a long‐term period of 17 yearsThe traditional coupling function may not be applicable in the case of radial IMF [ABSTRACT FROM AUTHOR]

Newly built Jiamusi (JME) radar and Hokkaido East (HOK) radar data reveal statistical features of the mid-latitude ionospheric irregularities during quiet times. In this paper, the distribution features and seasonal variations of the scattering occurrence rate and convection velocity of the ionospheric, F-region irregularities between 45°N and 60°N magnetic latitude (MLAT) during geomagnetic quiet times are studied by using one-year observation data from the JME radar and HOK radar. The results show that the scattering occurrence rate is strongest in summer and weakest in winter in this region, and the highest value always appears on the dusk side, especially in summer and the equinox. Line-of-sight (LOS) velocity also shows significant seasonal variation. It is stronger in winter and the equinox than in summer, which is in good agreement with previously reported observations from six midlatitude Super Dual Auroral Radar Network radars in the North American sector. An interesting new finding is that on the night side, LOS velocity decreases first and then increases as the MLAT increases from 45°N to 56°N, particularly in summer and the equinox. This implies that there is a velocity reversal region during the geomagnetic quiet time in regard to night side, mid-latitude, ionospheric convection. Plain Language Summary The distribution features and seasonal variations of the scattering occurrence rate and convection velocity of the F-region irregularities between 45° to 56° magnetic latitude during the geomagnetically quiet time are studied by using one-year observation data from the Jimausi radar and the Hokkaido East radar. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive study of geomagnetic storm time currents using magnetic field recorded by multispacecraft polar‐orbiting mission, Swarm. During geomagnetic storm period, the magnetic field variations obtained after removing the internal geomagnetic field and quiet time contributions can be considered as a proxy for storm time currents and are found to follow the temporal profile of Dst index very closely. These variations at the equatorial crossings recorded by multiple‐spacecraft are used to estimate the Dst values and are found to have a good match with the ground‐based Dst index. The average deviation between these two is around 4–13%. We have estimated the asymmetry by taking the difference between the magnetic field variations at two local time sectors separated by 12 hr. The estimated asymmetry shows a good match with the AsyH‐index, especially when satellite traverses in the dawn‐dusk sector. In general, the magnetic field variations are stronger in the night‐to‐dusk sector than day‐to‐dawn sector, which could be due to the larger pressure‐gradients near night‐to‐dusk caused by ion movements. The important advantage of Swarm mission is that it provides an opportunity to investigate the longitudinal gradients in the storm time magnetic fields. It is observed that in general, the gradients are stronger during the main phase of the storm, centered near the equator with a latitudinal width of ~20–30° in both the hemispheres, and are supportive to the scenario of particle‐injection from the magnetotail. The stronger gradients are observed at higher latitudes (~40°) during the episodes of substorms and might be associated with the ionospheric/field‐aligned currents. Plain Language Summary: The space weather events such as geomagnetic storms can have a huge impact on the terrestrial magnetic and plasma environment, producing large currents, which in turn, impact spaceborne communication, navigation systems, aviation and satellite operations, and electric power grids, etc. The paper presents a new approach to estimate geomagnetic indices equivalent to Dst and AsyH during geomagnetic storms, making use of the increased observations from space‐based instruments in recent times, and demonstrates their utility in quantifying the geomagnetic environment. The important advantage of multispacecraft Swarm mission is that it provides an opportunity to investigate the spatial gradients in the storm time magnetic fields. The paper reports that strong azimuthal gradients in the magnetic field, near the equatorial sector, are associated with particle injection from the magnetotail. The paper demonstrates the utility of space‐based measurements in understanding the geomagnetic environment. Key Points: Geomagnetic storm time magnetic field variations are stronger in the dusk‐night than the dawn‐day sectorLongitudinal gradients in the storm time magnetic field are stronger near the equator and support the idea of particle injection from tailDuring episodes of substorm, stronger longitudinal gradients are observed at higher latitudes (~40°), which might be associated with FACs [ABSTRACT FROM AUTHOR]

A two‐dimensional particle‐in‐cell (PIC) simulation is performed to study the formation of the pitch angle distribution of energetic electrons at dipolarization fronts (DFs) driven by magnetic reconnection. The energetic electrons at DFs are originated from the lobe region, and experience a two‐step acceleration process at the reconnection x‐line and the DFs, respectively. Three kinds of pitch angle distributions commonly observed in the magnetotail, Pancake, Rolling pin, and Cigar distributions, are formed in sequence during the propagation of the DFs. In the early stage, Pancake distributions are formed through betatron acceleration. During this stage, the flux of energetic electrons with pitch angles around 0° and 180° is low because these electrons have no time to be reflected many times at the DFs to obtain sufficient Fermi acceleration. However, the electrons with pitch angles around 90° are difficult to be trapped around the DFs for a long time, and their flux saturates quickly; while the electrons with pitch angles around 0° and 180° can be trapped inside the closed field lines and they get continuous Fermi acceleration during the propagation of the DFs. Therefore, in the later stage, the flux of energetic electrons with pitch angles around 0° and 180° gradually increases and at last exceeds that of energetic electrons with pitch angles around 90°, forming Rolling pin and Cigar distributions in sequence. Plain Language Summary: Energetic electrons in the magnetotail usually show anisotropic pitch angle distributions, such as Pancake, Cigar, and Rolling‐pin distributions. All of the three kinds of pitch angle distributions are frequently observed to be related to dipolarization fronts. Pancake distribution is characterized by the enhancement of energetic electron flux with pitch angles around 90°; Cigar distribution is characterized by the enhancement of energetic electron flux with pitch angles around 0° and 180°; Rolling‐pin distribution is characterized by the enhancement of energetic electron flux with pitch angles around 0°, 90°, and 180°. It has been long accepted that electrons with pitch angles around 90° are generated by betatron acceleration, and electrons with pitch angles around 0° and 180° are generated by Fermi acceleration. However, the differences in the conditions to form the three kinds of distributions are far less understood. In this paper, we study when and where these distributions can be formed, and the detailed acceleration processes that lead the energetic electrons to form these distributions. Key Points: The energetic electrons at the dipolarization fronts (DFs) experience a two‐step acceleration process at the reconnection x‐line and DFsPancake, Rolling pin, and Cigar distributions are formed in sequence during the propagation of the DFsThese distributions are resulted from the different time scales of betatron and Fermi mechanisms at the DFs [ABSTRACT FROM AUTHOR]

A quasi‐semidiurnal type pattern was observed earlier in the diurnal UT variation of the geomagnetic storms studied using mainly Kyoto Dst (disturbance storm‐time) index. However, the pattern has been argued as apparent due to uneven longitude distribution of the four Dst observatories. Unlike earlier studies, this paper investigates the diurnal UT variation of the storms automatically identified in six available indices including Kyoto Dst, USGS (United States Geological Survey) Dst, SymH (symmetric‐H), RC (ring current), Dcx (corrected extended Dst), and AER (Atmospheric and Environmental Research) in 50, 50, 36, 21, 5, and 7 years, respectively. The indices are derived using 4, 4, 12, 14, and 15 ground observatories (with maximum longitude separations of ∼120°, 120°, 70°, 110°, and 50°) and four DMSP (Defense Meteorology Satellite Program) satellites, respectively. The UT distribution of the storm intensity (minimum value of an index during the storm main phase) in all indices shows a striking quasi‐semidiurnal type variation with maxima around 06–08 UT and 21–23 UT and minima around 03–05 UT and 13–15 UT. Similar quasi‐semidiurnal variation is also observed in the computed values of the main energy input in the ring current. The variation correlates well with the variations of the dipole tilt angles μ and θ involved in the equinoctial hypothesis and Russell‐McPherron (RM) effect, respectively. These observations indicate that the quasi‐semidiurnal variation is real. Plain Language Summary: Large disturbances in the geomagnetic field lasting form several hours to several days are known as geomagnetic storms. The variations of the occurrence and intensity of the storms with solar activity and season have been understood thanks to the works of a large number of scientists. The variation of the storms with the time‐of‐day studied using mainly the low latitude geomagnetic activity index Dst has shown a quasi‐semidiurnal pattern. The pattern, however, has been argued as apparent due to the uneven longitude distribution of the four magnetic observatories used for deriving Dst. The present study investigates the diurnal UT variation of the storms using six available indices. The results show similar striking quasi‐semidiurnal patterns in the UT distribution of the storm intensity in all indices and computed value of the main energy input in the ring current. The quasi‐semidiurnal pattern also correlates well with the angles μ and θ involved in the mechanisms of equinoctial hypothesis and RM effect. These observations indicate that the quasi‐semidiurnal variation is real. Key Points: A striking quasi‐semidiurnal pattern is observed in the UT distribution of the geomagnetic storm intensity in six low latitude indicesSimilar pattern exists in the UT variation of the computed value of the main energy input in the ring currentThe quasi‐semidiurnal pattern correlates well with the angles μ and θ involved in the mechanisms of equinoctial hypothesis and RM effect [ABSTRACT FROM AUTHOR]

This paper reports the characteristics of the topside ionospheric O+ diffusive flux (ΦO+) during both geomagnetically quiet (0 ≤ Kp ≤ 2) and moderate (2 < Kp ≤ 4) times using incoherent scatter radar observations at Millstone Hill (42.6°N, 288.5°E) for solar minimum from 1970 to 2018. ΦO+ partially characterizes plasma mass exchange between the upper and lower part of the topside ionosphere through diffusion and sometimes serves as upper boundary conditions for ionosphere‐thermosphere models. The altitude where the flux sign changes (mainly during daytime) is termed the transition height and the time when the flux sign changes (mainly at dawn and dusk) is termed the transition time. At quiet times, the daytime transition height is ∼100 km above the F2 peak height (hmF2) in summer, and it is about 50 km above hmF2 in other seasons; the transition time is before 18 solar local time (SLT) in spring and winter, but after 18 SLT in summer and autumn. The daytime average upward ΦO+ above the transition height shows a significant seasonal variation with a minimum of 2.2×108cm−2s−1 in summer and a maximum of 3.7×108cm−2s−1 in autumn. Under geomagnetically moderate conditions, the transition height increases by ∼20 km in spring, winter, and autumn, but moves up by about 20–50 km in summer. The transition time occurs later by ∼1 hr in summer but ∼1 hr earlier in other seasons. The mean upward ΦO+ peaks in summer and minimizes in spring. Plain Language Summary: The topside ionospheric O+ diffusive flux (ΦO+) is a part of the total plasma flux that is a critical parameter to characterize the plasma exchange between the upper and lower part of the topside ionosphere, but has been poorly known due to lack of direct measurements. Using long‐term incoherent scatter radar (ISR) data observed at Millstone Hill for nearly five decades, this study investigates ΦO+ variations as a function of the altitude above the F2 peak height (hmF2), local time, and season in both geomagnetically quiet and moderate times, and further explores the physical mechanism causing these variations. During quiet times, the summer daytime transition height where ΦO+ changes from downward to upward along magnetic field lines occurs at ∼hmF2+100 km. However, it is usually near hmF2+50 km in other seasons. In addition, the transition time when ΦO+ changes from upward to downward at a fixed height is earlier than 18 SLT in spring and winter, but later than 18 SLT in summer and autumn. We also found that an increase in geomagnetic activity decreases the vertical gradient of plasma density during the daytime in all seasons, resulting in a ∼20 km increase in the transition height and an earlier transition time near dusk in spring, autumn, and winter, but a larger transition height change of about 20–50 km and a later transition time in summer. The change in the seasonal variations of the upward ΦO+ results from the competition between the variations of plasma diffusive velocity and density when the geomagnetic activity increases, as well as the changes in plasma vertical density profiles. Key Points: Topside ionospheric O+ diffusive flux (ΦO+) climatology at solar minimum is derived from incoherent scatter radar data at Millstone HillO+ diffusive flux has seasonal, local time, and altitude dependenceGeomagnetic activity changes the spatiotemporal variations of Φ(O+) by modulating the vertical gradient of the plasma density [ABSTRACT FROM AUTHOR]

Within the inner magnetosphere, the occurrence of plasma waves and/or magnetic field structures plays an important role in the dynamics of the particle populations, accelerating some particles to higher energies while scattering others. The 2013 Cluster Inner Magnetosphere Campaign was designed to investigate this process based on observations made at small separation scales. In addition, it provides an ideal opportunity for the study of small‐scale structures. This paper presents observations of previously unreported small‐scale magnetic field structures and investigates their characteristics and spatial properties. These structures, seen in the STAFF‐SC data set, are characterized by a rotation in the field direction and are observed to have spatial sizes of a few tens of kilometers, with a central core region around 10 km and are probably filamentary current structures. Using a variance analysis, it was determined that the maximum variance direction was close to being perpendicular to the external magnetic field. Based on the size of the central core region the current density is of the order of 10–40 nAm−2. While they may affect the profile of these structures, data calibration effects were ruled out as their origin. Key Points: Cluster STAFF‐SC magnetometer observations of previously unreported nonlinear structures in the inner magnetosphere are presentedThe small separation of two of the Cluster satellites enables their spatial and temporal properties to be investigatedWe argue that these structures can be generated by thin current filaments whose size is a few tens of kilometers [ABSTRACT FROM AUTHOR]

A geomagnetic storm is a phenomenon of weakening of Earth's magnetic field from a few hours to several days, which directly or indirectly affects navigation, transportation, communication, power grids, satellite electronics, etc. In general, the duration of the recovery phase is larger. Thus, its cumulative effect is greater than the main phase. Therefore, it is important to understand the physical processes involved during the recovery phase. In this paper, we have studied the recovery phase of 31 extreme geomagnetic storms that occurred from 1990 to 2020. Each storm demonstrates two distinct features of the recovery phase that is, initial fast and later slow recovery phase. During the fast recovery phase, the rate of recovery either linearly or non‐linearly depends on SYM−H, which has been characterized by an exponential or hyperbolic decay function in various reported studies. In our study, we have noted that the hyperbolic function explains the complete recovery phase of only 11 extreme events. Furthermore, both decay functions fail to explain the late recovery phase of storms. Interestingly, we have found that the rate of recovery during the slow phase shows steady variation (independent to SYM−H), that is, d(SYM−H)dt=constant. Our analysis suggests that the magnitude of the recovery rate during the late phase is proportional to the magnitude of the storm. Key Points: The exponential or hyperbolic decay function can only explain fast recovery phase of a stormsWe observe a constant recovery rate during the late (slow) recovery phase of extreme storms that is, d(SYM−H)dt=constantWe also find that the recovery rate during the late phase is directly proportional to the amplitude of the storm [ABSTRACT FROM AUTHOR]

The Ionospheric Data Assimilation Four‐Dimensional (IDA4D) technique has been coupled to Sami3, which is another model of the ionosphere (SAMI3). In this application, ground‐based and space‐based GPS total electron content (TEC) data have been assimilated into SAMI3, while in‐situ electron densities, autoscaled ionosonde NmF2, and reference GPS stations have been used for validation. IDA4D/SAMI3 shows that night‐time ionospheric localized enhancements (NILE) are formed following geomagnetic storms in November 2003 and August 2018. The NILE phenomenon appears as a moderate, longitudinally extended enhancement of NmF2 at 30°–40°N MLAT, occurring in the late evening (20–24 LT) following much larger enhancements of the equatorial anomaly crests in the main phase of the storms. The NILE appears to be caused by upward and northward plasma transport around the dusk terminator, which is consistent with eastward polarization electric fields. Independent validation confirms the presence of the NILE, and indicates that IDA4D is effective in correcting random errors and systematic biases in SAMI3. In all cases, biases and root‐mean‐square errors are reduced by the data assimilation, typically by a factor of 2 or more. During the most severe part of the November 2003 storm, the uncorrected ionospheric error on a GPS 3D position at 1LSU (Louisiana) is estimated to exceed 34 m. The IDA4D/SAMI3 specification is effective in correcting this down to 10 m. Plain Language Summary: Geomagnetic storms have important effects on signals of different frequencies, including GPS and HF, because they modify the ionosphere. This paper presents a new data assimilation modeling technique for specifying the ionospheric response to geomagnetic storms, and highlights poorly understood aspects of these events. One such feature is a localized night‐time enhancement of the ionosphere that occurs at midlatitudes. The new technique provides major improvements in terms of correcting GPS position estimates and maximum usable frequencies in the HF band. Key Points: The IDA4D data assimilation scheme has been coupled to the SAMI3 ionospheric modelIDA4D/SAMI3 shows night‐time ionospheric localized enhancements (NILE) at midlatitudes after stormsFormation of the NILE appears to be caused by upward/northward plasma transport near the dusk terminator [ABSTRACT FROM AUTHOR]

Accurate representation of ensemble members of coupled Ionosphere‐Thermosphere (I‐T) is crucial to the ionosphere forecast in the ensemble Kalman filter (EnKF) data assimilation system. In this paper, besides the model drivers used in previous studies, including the solar 10.7 cm radio flux (F10.7), auroral hemispheric power (HP) and cross‐tail potential drop (CP), the eddy diffusion coefficient (ED), and nighttime vertical O+ flux at the top boundary are also perturbed to generate ensemble members. Based on our developed EnKF data assimilation system, the impact of perturbing different model external forcing parameters on ensemble generation and forecast capability of ionosphere and thermosphere has been investigated in detail through a series of sensitivity tests and data assimilation experiments. This system uses the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) as a background model. The findings are summarized as follows: (a) Associated with perturbing two additional model forcing parameters, better ensemble members of ionosphere and thermosphere states of the background model can be generated during both daytime and nighttime. (b) The improvement of the forecast capability of the ionosphere and thermosphere variables can be further enhanced. This study can provide a reference for ensemble generating strategy for the coupled I‐T EnKF data assimilation in the future. Plain Language Summary: The ensemble member of EnKF data assimilation considering as much as possible variabilities existed in the coupled I‐T system should represent model uncertainties better. In the previous researches on EnKF ionosphere forecast, ensemble members are mainly generated by perturbing solar and geomagnetic activity index common model drivers (e.g., F10.7, KP). However, solely perturbing those indices is not sufficient to distribute the ionosphere and thermosphere states at all local times. In this study, in addition to perturbing those indices, the key model forcing parameters of the lower and upper boundary of the TIEGCM are also perturbed to generate the ensemble member of the coupled I‐T system. We found that an improved ensemble spread can be obtained at all local times in this way, which can further contribute to the forecast capability of the ionosphere and thermosphere states. Key Points: The eddy diffusion and upper boundary are additionally perturbed to generate ensembles in EnKF I‐T assimilation systemEnsemble members can be better obtained at no matter daytime or nighttimeThe forecasting capability of I‐T states can be further improved [ABSTRACT FROM AUTHOR]

Three new methods for estimating a ratio of the ultralow frequency (ULF; 1–100 mHz) wave equatorial electric field amplitude in the Earth's magnetosphere to ground magnetic field amplitudes for field line resonances (FLR) are described. These methods use ratios of the time series extrema, ratios of the envelope waveform and the ratio of the spectral amplitude at the FLR frequency. These methods were applied to four ULF resonance intervals; three detected by the Van Allen Probe A spacecraft and one detected by the POLAR spacecraft. The intervals were conjoined with the CARISMA and IMAGE ground magnetometer arrays. The spectral ratio results for the Van Allen Probe intervals were approximately twice to three times the ratios estimated from the two time series based methods. The POLAR interval showed similar values across all three methods. The differences are attributed to broadband frequency signals that modify the time series amplitudes, while the spectral method avoids these off‐resonant frequencies. Based on the results of this study, a spectral based method for calculating the ratio at the FLR frequency is best. Plain Language Summary: A reliable method to remote sense the amplitude of electric fields of natural frequencies in space using ground magnetic fields would remove difficulties in obtaining sufficient spatial information of fields that are thought to energize electrons to high energies. One recently proposed method maps ground magnetic field variations, b, to electric field variations, e, in space via the ratio, e/b. However, there are different ways to calculate e/b and the uncertainties of this metric have not been reported. This paper presents a study of three methods used to calculate e/b and the uncertainty limits; a ratio of the peaks/troughs of the fields, ratios of each point of the envelope waveform of the fields, and a ratio of the spectral amplitude of the frequency where resonance occurs. These methods were applied to electric field data obtained from two satellites with corresponding magnetic field data taken from ground magnetic field detectors at the footprint of the resonant magnetic field line. It was found that the frequency‐based method gave two to three times larger values and smaller uncertainty values compared with the other methods. Based on the results of this study, the frequency‐based method is preferred for estimating e/b. Key Points: Three methods for estimating the ratio of ultralow frequency electric fields in space to magnetic fields on the ground, e/b, are presentedRatio values for 106 field line resonant events were calculate from spacecraft and ground data over September 25, 2012 to February 25, 2014The uncertainties in e/b for using the spectral method were consistently smaller than the other methods [ABSTRACT FROM AUTHOR]

In our recent paper, Yi, Reid, Xue, Younger, Spargo, et al. (2017, https://doi.org/10.1002/2017JA024446, hereinafter Y17A), we presented a first analysis of 9‐ and 6.75‐day periodic oscillations observed in the Earth's neutral mesospheric density during 2005 and 2006. These observations reveal a direct coupling between the Sun's upper atmosphere and the Earth's mesosphere. Tsurutani et al. (2019, https://doi.org/10.1002/2016JA022499, hereinafter T19) have commented on T19 by way of suggesting possible physical mechanisms for Y17A's observations (Y17A do not address possible mechanisms in their paper). Key Points: Reply to comments by Bruce Tsurutani on our paperNeutral mesospheric density over Antarctica decreases as geomagnetic activity enhances [ABSTRACT FROM AUTHOR]

The interaction of the solar wind with the Earth's magnetic field produces geomagnetic activity, which is critically dependent on the orientation of the interplanetary magnetic field (IMF). Most solar wind coupling functions quantify this dependence on the IMF orientation with the so‐called IMF clock angle in a way, which is symmetric with respect to the sign of the By component. However, recent studies have suggested that the sign of By is an additional, independent driver of high‐latitude geomagnetic activity, leading to higher (weaker) geomagnetic activity in Northern Hemisphere (NH) winter for By > 0 (By<0). In this paper we quantify the size of this explicit By effect with respect to the solar wind coupling function, both for northern and southern high‐latitude geomagnetic activity. We show that high‐latitude geomagnetic activity is significantly (by about 40%–50%) suppressed for By < 0 in NH winter and for By > 0 in Southern Hemisphere winter. When averaged over all months, high‐latitude geomagnetic activity in NH is about 12% weaker for By < 0 than for By > 0. The By effect affects the westward electrojet strongly, but hardly at all the eastward electrojet. We also show that the suppression of the westward electrojet in NH during By < 0 maximizes when the Earth's dipole axis points toward the night sector, that is, when the auroral region is maximally in darkness. Key Points: IMF By is an explicit driver of high‐latitude geomagnetic activityHigh‐latitude geomagnetic activity is suppressed in local winter for By < 0 in Northern Hemisphere and for By > 0 in Southern HemisphereExplicit By effect maximizes when the Earth's dipole axis points toward night [ABSTRACT FROM AUTHOR]

Metallic and silicate ions carry essential information about the evolution of the Earth and near-Earth small bodies. Despite this, there has so far been very little focus on ions with atomic masses higher than oxygen in the terrestrial magnetosphere. In this paper, we report on abundances and properties of energetic ions with masses corresponding to that of silicon (Si) and iron (Fe) in Earth's geospace. The results are based on a newly derived data product from the Research with Adaptive Particle Imaging Detectors on Cluster. We find traces of both Si and Fe in all of the regions covered by the spacecraft, with the highest occurrence rates and highest intensities in the inner magnetosphere. We also find that the Fe and Si abundances are modulated by solar activity. During solar maximum, the probability of observing Fe and Si in geospace increases significantly. On the other hand, we find little or no direct correlation between geomagnetic activity and Si and Fe abundance in the magnetosphere. Both Si and Fe in the Earth's magnetosphere are inferred to be primarily of solar wind origin. [ABSTRACT FROM AUTHOR]

Over the last few decades, the role of ionospheric outflow for the loss of atmospheric constituents, as a plasma supplier to the magnetosphere and hence for the evolution of the Earth's atmosphere has been recognized. A substantial amount of the outflow is thought to be caused by the presence of an ambipolar electric field aligned with the open magnetic field lines of the polar region. To better understand how the changes in outflow are influenced by the solar and geomagnetic activity, it is critical to get a better understanding of the impact of this electric field, and to be able to measure it under various conditions. However, such measurements are not possible with present techniques. In this paper, we propose a new technique to measure the tiny electric field. This technique builds on existing instrument technology but extends the capability to measure the very small electric fields. We present the underlying design concept and demonstrate that this concept is viable and able to measure the very small ambipolar electric fields thought to play a key role in the polar wind. [ABSTRACT FROM AUTHOR]

This paper presents and discusses electron density depletion regions observed with the incoherent scatter EISCAT Svalbard Radar (ESR) located at 75.43°N geomagnetic latitude. The data include several decades of measurements, which make them suitable for studying statistical features and characteristics of the ionospheric parameters. Here we focus on the electron density depletions and their dependence on diurnal and seasonal variations and solar activity. An electron density depletion region is identified in the ESR data in the early morning sector. This depletion region seems to be clearest during equinox and winter and moderate/high solar activity. An enhancement in the ion temperature is often colocated with the electron density depletion region. The ion temperature enhancement could indicate that ion frictional heating is related to the electron density depletion region. However, during summer when the solar activity is low, the electron density depletion is not observed although the ion temperature is enhanced, suggesting that formation of the electron density depletion regions due to ion frictional heating may depend on the background effective temperature and O/N2 ratio. In addition, seasonal changes in the solar zenith angle could also contribute to the formation of the depletion region. [ABSTRACT FROM AUTHOR]

With the interplanetary magnetic field (IMF) that turns from northward to southward, the global simulation successively reproduces the growth phase, the onset, and the expansion phase of the substorm. The calculated ionospheric convection for the growth phase reproduces the development of the Harang reversal (HR), the upward field-aligned current (FAC), and the upcoming onset point as observed. Magnetic field lines traced from the center of the nightside upward FAC are open, while the magnetic field line traced from the onset point is closed. These open magnetic field lines map to flow shear just outside the O/C boundary. Seen from the magnetic configuration, the growth phase proceeds as the replacement process of nulls. Two new nulls appear on the dayside under the southward IMF, while two old nulls under the northward IMF retreat tailward forming two bifurcation regions on the dawn and dusk flanks. Flow shear around the O/C boundary forms by magnetospheric convection that returns to the dayside via bifurcation regions. The expansion phase proceeds through a topological change by the nearearth neutral line (NENL). The NENL occurs inside the thinned structure of the northward IMF remnant, on the low-latitude side of the flow shear, and projects down to the low-latitude edge of the upward FAC. Associated with the NENL, the convection return path changes to the center of the plasma sheet and reveals in the ionosphere as the release of the HR. By the shrinkage of projecting magnetic field line, the O/C boundary migrates poleward in the ionosphere. [ABSTRACT FROM AUTHOR]

Assessing the statistical significance of electromagnetic anomalies in the ultralow frequency (ULF) range observed prior to earthquakes is a necessary step toward determining whether these perturbations constitute actual earthquake precursors. A statistical epoch analysis (SEA) was recently performed by Han et al. (2014, https://doi.org/10.1002/2014JA019789) to analyze earthquakes happening between 2001 and 2010 near the geomagnetic observatory of Kakioka, Japan; the authors found a significant number of anomalies 6 to 15 days prior to the earthquake day within 100 km from Kakioka, while no significant pre‐earthquake activity was observed for the farther region 100 to 216 km from the observatory. In this current paper, we describe the application of our independent software implementation of their method. Despite using a different outlier rejection scheme, we manage to approximate their results. Upon validation of our program, we conduct multiple sensitivity studies. First, we explore how different outlier rejection schemes impact the results. We then restrict the analysis to only mantle earthquakes, highlighting a marginally significant number of anomalies prior to the earthquake day. Next, we test a higher band‐pass filter than the one initially used but find no anomalous pre‐earthquake activity in this higher‐frequency band. We then use a different catalog to establish the list of qualifying "earthquake days" which also leads the anomalous pre‐earthquake episode to vanish, thus raising concerns about the robustness of the results. Finally, we apply the SEA to another time window, ranging from 2013 to 2018: No significant pre‐earthquake episode can be observed for this interval. We conclude our study by providing guidelines for upcoming work. Key Points: We reproduce the results of Han et al. (2014), introducing an algorithm aiming to assess the significance of prequake magnetic signalsWe show the sensitivity of this method to the choice of outlier rejection scheme, hypocentral depth, frequency band, and earthquake catalogThis method does not highlight precursory activity when applied to an interval (2013–2018) newer than the one initially used (2001–2010) [ABSTRACT FROM AUTHOR]

In this paper, we have evaluated the role of interplanetary magnetic field (IMF) By on the asymmetry of the ring current during the main phase of geomagnetic storms. The mean H variations have been calculated using 31 ground magnetic stations over magnetic latitudes of 09–45° following the methodology of Li et al. (2011, https://doi.org/10.1029/2011JA016886). Further, the magnetic local time (MLT) variations in the H component at these stations w.r.t. the mean H were investigated for three cases of geomagnetic storms with varying southward IMF Bz and IMF By conditions. Significant ring current asymmetries were observed during the main phase of geomagnetic storms. The primary role of IMF Bz on the asymmetry of the ring current is observed from these cases. More importantly, the investigation brings out for the first time, the additional role of IMF By in influencing the MLT distribution of ring current observed at ground magnetic stations. Under southward IMF Bz conditions, it is shown based on SuperDARN and AMPERE data that IMF By can alter the MLT distribution of ring current under suitable conditions. The timescales of IMF By also play very important role in determining the asymmetry in the ring current. Under steady convection state, IMF By can rotate the convection cells based on its polarity, which in turn can change the MLT distribution of ring current observed by low‐latitude ground stations. This investigation, thus, brings out the important role of IMF By on the asymmetric MLT distribution of ring current under southward IMF Bz. Key Points: IMF By plays important role in asymmetry of the ring current observed at ground stations in addition to IMF BzSuperDARN convection cells and AMPERE‐derived FACs show the association of IMF By with MLT distribution of the ring currentUnder suitable conditions, IMF By can alter the MLT distribution of the ring current [ABSTRACT FROM AUTHOR]

The solar wind plasma is a major plasma source for the Earth's magnetosphere, which has a strong influence on the magnetotail plasma and field properties. The relative importance of different plasma entry mechanisms and pathways is largely determined by the solar wind conditions. Therefore, the spatial and temporal dependence of magnetotail plasma and field properties under different kinds of solar wind conditions is critically important for understanding the Earth's magnetosphere. This study presents a statistical study of fundamental magnetotail plasma properties in a normalized reference frame by utilizing 12+ years of data from NASA's Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. These statistical maps are mostly in agreement with the magnetosheath of MHD runs from the CCMC BATS‐R‐US model, but some features in the maps can be explained by kinetic particle physics, not present in the MHD. The results are also used to investigate the presence of any magnetotail plasma parameter asymmetries and their possible causes. Plain Language Summary: Earth's intrinsic magnetic field, generated by the currents in the Earth's interior, protects our planet from solar radiation and plasma called the solar wind. However, physical processes such as magnetic reconnection occurring at the boundary of this magnetic barrier, the magnetosphere, can break this shield, enabling access of solar wind plasma into the Earth's magnetosphere. Earth's ionosphere provides another source for magnetospheric plasma. This plasma can be further accelerated to huge energies, which provides a threat for astronauts and satellites. The effectiveness of physical mechanisms that control the entry and acceleration of this plasma strongly depends on the local magnetic field geometry and plasma properties, which in turn are affected by the solar wind. However, the solar wind properties are not constant but vary as it can originate from different regions of the sun. While the magnetic field of the Sun, the interplanetary magnetic field (IMF), on average forms an "Archimedean Spiral," flapping of the heliospheric current sheet and fluctuations along field lines will make the IMF "hit" the Earth at different orientations, thus impacting the shock geometry in front of the planet, which in turn affects the downstream plasma and field properties in the turbulent boundary layer called the magnetosheath. In this paper, we have characterized the dependence of the large‐scale plasma properties in the Earth's magnetosphere and magnetosheath on the solar wind and IMF conditions by using over 12 years (thus covering over one solar cycle) of data from NASA's Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. Key Points: A statistical study using 12+ years of magnetospheric, magnetosheath, and solar wind data is performedDawn‐dusk asymmetries in the magnetotail are consistent with kinetic drift physics, source plasma properties and entry mechanismsGeneral large‐scale features agree quite well with MHD simulations, but this study motivates the consideration of new type of code initialization methods [ABSTRACT FROM AUTHOR]

We present observations from five Mars Express (MEX) orbits in September 2016 while the spacecraft passed through the Martian induced magnetotail at altitudes up to 3,500 km. On these orbits, the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument was operated in Active Ionospheric Sounding (AIS) mode at much higher altitude than normal, acting as a local sounder and detecting cold plasma structures in this region. In this paper we combine MARSIS tail measurements with solar wind data from the Solar Wind Ion Analyzer (SWIA) instrument and the Magnetometer (MAG) from Mars Atmosphere and Volatile EvolutioN (MAVEN) in order to investigate possible factors affecting plasma transport from the dayside and through the terminator. MARSIS observed structured cold ionospheric plasma along its trajectory, at all altitudes and solar zenith angles (SZAs). Isolated regions of cold plasma were also observed on each orbit as the spacecraft crossed the terminator, even at high altitudes. We conclude that the variability of plasma seen in the tail results from a multifactorial transport process, the development of which cannot be attributed to a sole parameter influencing it, despite the availability of simultaneous high quality solar wind measurements. Key Point: Special MARSIS high‐altitude operations in the Martian‐induced magnetotail showed high variability of the cold ionospheric plasma structures [ABSTRACT FROM AUTHOR]

This paper focuses on unique aspects of the ionospheric response at conjugate locations over Europe and South Africa during the 7–8 September 2017 geomagnetic storm including the role of the bottomside and topside ionosphere and plasmasphere in influencing electron density changes. Analysis of total electron content (TEC) on 7 September 2017 shows that for a pair of geomagnetically conjugate locations, positive storm effect was observed reaching about 65% when benchmarked on the monthly median TEC variability in the Northern Hemisphere, while the Southern Hemisphere remained within the quiet time variability threshold of ±40%. Over the investigated locations, the Southern Hemisphere midlatitudes showed positive TEC deviations that were in most cases twice the comparative response level in the Northern Hemisphere on the 8 September 2017. During the storm main phase on 8 September 2017, we have obtained an interesting result of ionosonde maximum electron density of the F2 layer and TEC derived from Global Navigation Satellite System (GNSS) observations showing different ionospheric responses over the same midlatitude location in the Northern Hemisphere. In situ electron density measurements from SWARM satellite aided by bottomside ionosonde‐derived TEC up to the maximum height of the F2 layer (hmF2) revealed that the bottomside and topside ionosphere as well as plasmasphere electron content contributions to overall GNSS‐derived TEC were different in both hemispheres especially for 8 September 2017 during the storm main phase. The differences in hemispheric response at conjugate locations and on a regional scale have been explained in terms of seasonal influence on the background electron density coupled with the presence of large‐scale traveling ionospheric disturbances and low‐latitude‐associated processes. The major highlight of this study is the simultaneous confirmation of most of the previously observed features and their underlying physical mechanisms during geomagnetic storms through a multi–data set examination of hemispheric differences. Key Points: The storm led to TEC increase which was at least twice in Southern Hemisphere than Northern Hemisphere midlatitudes on 8 September 2017Processes of low‐latitude origin, thermospheric composition changes, and TIDs contributed to the observed positive ionospheric storm effectsIonospheric bottomside and topside/plasmasphere contributions to TEC were different in both hemispheres during the storm main phase [ABSTRACT FROM AUTHOR]

A dense network of ground global navigation satellite system receivers detected ionospheric total electron content (TEC) changes starting ~40 min before the 2011 Tohoku‐oki (Mw9.0) earthquake around the ruptured fault, together with the long‐lasting postseismic TEC drop. In this paper, we robustly estimate three‐dimensional (3‐D) distribution of both preseismic and postseismic ionospheric anomalies of the 2011 Tohoku‐oki earthquake by tomographic inversions of electron density anomalies. We set up >6,000 blocks, as large as 1.0° (east‐west) × 0.9° (north‐south) × 60 km (vertical), over the Japanese Islands, the Sea of Japan, and the Korean Peninsula, up to 870 km altitude. By using TEC anomalies of pairs exceeding ~1,300 stations and eight satellites obtained using reference curves, we estimated electron density anomalies within individual blocks. The results showed that the preseismic and postseismic anomalies do not overlap in space. The preseismic anomalies are composed of low (~300 km height) positive and high (~600 km height) negative anomalies. They occurred above the land of NE Japan without extending offshore, suggesting its origin related to surface electric charges. On the other hand, the postseismic electron depletion occurred offshore above the region where large coseismic uplift took place. These results demonstrate that the preseismic and postseismic ionospheric anomalies are independent not only temporarily but also spatially and certainly in underlying physical mechanisms. We propose a simple model to explain how surface charges redistribute ionospheric electrons to make the observed preseismic electron density anomalies. Plain Language Summary: A dense network of GNSS/GPS receivers found that redistribution of ionospheric electrons started ~40 min before the 2011 Tohoku‐oki earthquake around the fault. It was also found that a long‐lasting electron depletion occurred after the earthquake. We studied the three‐dimensional structures of the electron density anomalies immediately before and after the 2011 earthquake. We found that the preseismic anomaly occurred above land, but the postseismic anomaly occurred offshore. The preseismic change is characterized by the simultaneous growth of positive and negative electron density anomalies, while the postseismic change is dominated by an electron decrease. These differences reflect the different physical origins of the preseismic and postseismic anomalies. Key Points: We robustly analyzed the 3‐D structure of ionospheric anomalies immediately before and after the 2011 Tohoku‐oki (Mw9.0) earthquakeThe preseismic anomalies consist of positive (~300 km height) and negative (~600 km height) anomalies over the land of NE JapanThe postseismic electron depletion occurred offshore above the ruptured fault [ABSTRACT FROM AUTHOR]

In this paper, ionograms from São Luís (SL, 2°S; 44°W, I: −3.8°) and Cachoeira Paulista (CP, 22.42°S; 45°W, I: −34.4°) are analyzed to examine the characteristics of the intermediate descending layers (ILs) over the Brazilian equatorial and low‐latitude regions under different solar flux conditions. The solar flux effects on the ILs are investigated in terms of the rate of occurrence of the IL; the seasonal behavior of some parameters such as the height, frequency, and velocity; and the duration and the number of ILs events observed per day. One of the main results of this work is that unlike over CP, the ILs over SL presented some peculiarities, such as a lower rate of occurrence during a period of solar maximum activity (2003) when compared with a period of solar minimum activity (2009). This apparent variation was likely caused by the magnetic equator moving away from SL during this period. The duration of the ILs was also investigated, and it was found that in 2009, the ILs presented higher life time than in 2003. The descending velocity of the ILs is compatible with the semidiurnal and quarter‐diurnal tides. Over SL, the larger descending rate in some cases (>10 km/hr) may reveal the additional influence of the gravity waves in the IL's dynamics. Key Points: The ILs occurrence over low latitudes seems to be independent of the solar fluxOver SL, the lower IL occurrence in 2003 than in 2009 can be related to the dip equator displacement from this region during that periodThe IL's duration over SL and CP was higher during the solar minimum; further, the duration was shorter at the equator [ABSTRACT FROM AUTHOR]

Recent advances in reconstructing Earth's magnetic field and associated currents by utilizing data mining of in situ magnetometer observations in the magnetosphere based on geomagnetic indices and solar wind parameters have proven remarkably accurate at reproducing observed ion diffusion regions. We investigate the effect of placing regions of localized resistivity in global simulations of the magnetosphere at specific locations inspired by the data mining results for the substorm occurring on 6 July 2017. When explicit resistivity is included, the simulation forms an x‐line at the same time and location as the Magnetospheric Multiscale Mission observation of an ion diffusion region at 15:35 UT on that day. Without this explicit resistivity, reconnection forms later in the substorm and far too close to Earth (≳−15RE), a common problem with global simulations of Earth's magnetosphere. A consequence of reconnection taking place farther down the tail due to localized resistivity is that the reconnection outflows transport magnetic flux Earthward and thus prevent the current sheet from thinning enough for reconnection to take place nearer Earth. As these flows rebound tailward from the inner magnetosphere, they can temporarily and locally (in the dawn‐dusk direction) stretch the magnetic field allowing for small scale x‐lines to form in the near Earth region. Due to the narrow cross‐tail extent of these x‐lines (≲5RE) and their short lifespan (≲5 min), they would be difficult to observe with in situ measurements. Future work will explore time‐dependent resistivity using 5 min cadence data mining reconstructions. Plain Language Summary: Recently, data mining of spacecraft magnetometers have created highly accurate reconstructions of Earth's magnetic field. These reconstructions capture the location and timing of so‐called x‐lines on the night side of Earth. Magnetic field lines reconnect at x‐lines and convert magnetic energy to plasma energy via heating and acceleration of flows. Nightside reconnection is a crucial part of geomagnetic storms and substorms that pose a danger to spacecraft, astronauts, and the power grid on Earth. By identifying x‐lines in reconstructions we can influence simulations of specific substorms to increase their accuracy and make them more physically realistic. We demonstrate this ability by matching simulation results with observations and find that reconnection further from Earth suppresses extended x‐line formation near Earth. Additionally, the flows from reconnection can bounce off the strong magnetic field close to Earth (a well‐known effect) and locally stretch the magnetic field creating small scale x‐lines near Earth. Key Points: Localized Resistivity in global magnetohydrodynamic simulations can "encourage" magnetic reconnection in specific locations to match data mining resultsActive x‐lines at XGSM ≲ −20RE can suppress the formation of extended x‐lines at XGSM ≳ −15REReconnection outflows rebound from the near‐Earth region causing transient and narrow (in YGSM) secondary reconnection [ABSTRACT FROM AUTHOR]

This paper examined the secular displacement of the dip equator and the geomagnetic poles as well as the variations of the global magnetic inclination and declination angles using magnetometer measurements onboard different low‐Earth orbit (LEO) satellite. The secular variation of the dip equator and geomagnetic poles has different impacts on different applications—from affecting the long‐term characterization of the low‐latitude ionosphere to degrading the precision of geomagnetic navigation. The strong displacement of the dip equator can result in a systematic error in the determination of the long‐term equatorial electric field variations and hence in the characterization of ionospheric density structure, especially in the region, where the displacement of dip equator is large enough (more than 20 km or 0.2°/year) within the time scale of a solar cycle or less. Similarly, the slowly moving locations of magnetic poles, estimated from magnetometer observations onboard LEO satellite, exemplify noticeable discrepancy with that of world magnetic model (WMM) and International Geomagnetic Reference Field (IGRF) values, indicating inevitable possible impact on the precise geomagnetic navigation for commercial and military applications. Thus, accurately estimated locations of the dip equator and magnetic poles, as well as declination angles, are critically important. Key Points: The migration of dip equator has significant impact on several applicationsThe observed Erratic Motion of Magnetic Poles has strong discrepancy with the modelSystematic error in characterizing the long‐term equatorial electric field and ionospheric density occurs due to the erratic motion dip equator [ABSTRACT FROM AUTHOR]

Recent work has demonstrated that global Pc5 pulsations observed by ground‐based magnetometers may be accompanied by periodic oscillations in the total electron content (TEC) of the ionosphere measured by GPS receivers. These TEC observations may provide new insights into magnetosphere‐ionosphere coupling mechanisms, especially when combined with other ground‐based observational techniques. Presented in this paper is a large‐scale morning sector Pc5 event which was observed simultaneously by ground‐based magnetometers, two high‐frequency Super Dual Auroral Radar Network (SuperDARN) radars, and several Global Positioning System (GPS)/TEC receivers. The transient 2.6 mHz pulsations observed by the ground magnetometers and radars are accompanied by periodic fluctuations in the time rate‐of‐change of TEC (ROT) at the same frequency. To investigate possible mechanisms for the TEC modulation by ultralow frequency (ULF) waves, we determine the ratios between the spectral amplitudes of the magnetic, ionospheric Doppler velocity and ROT oscillations. The relationship between the simultaneous magnetic field and ionospheric Doppler velocity oscillations can be reasonably well interpreted using the theory of Alfven wave interaction with the thin ionospheric layer. Though the observed ratio between ROT and Doppler velocity amplitudes can be explained by the occurrence of local steep gradient of the topside ionosphere plasma at auroral latitudes, the responsible modulation mechanism cannot be considered as firmly established. Key Points: Large‐scale 2.6 mHz Pc5 waves are detected simultaneously by ground magnetometers, SuperDARN radars, and several GPS/TEC receiversRelationship between geomagnetic and ionospheric velocity oscillations is interpreted using theory of MHD wave interaction with ionosphereObserved ratio between rate‐of‐change of TEC and Doppler velocity amplitudes can be explained by occurrence of local steep gradient [ABSTRACT FROM AUTHOR]

The ionosphere is very active and complex due to photo‐ionization from the solar activity, while traditional empirical models can only give a rough description of its actual variations. Nowadays, global ionosphere maps (GIMs) derived from denser Global Navigation Satellite Systems (GNSS) world‐tracking data provide an excellent total electron content (TEC) data set for global ionospheric research and modeling. In this paper, long‐tern variations of 16‐year (2003–2018) TEC time series from GIMs are investigated by using the principal mode analysis (PCA) technique. We analyze the resulting modes in the time‐spectral domain and parameterize the main contributions in terms of solar and magnetospheric forcing, local solar time (LST), and annual variations. The results show that the TEC variability is strongly dependent on the geographical location of the Earth's magnetic field, and the Earth's diurnal rotation modulates its spatial patterns of variability. The latitudinal asymmetry in the global distribution of TEC variations is due to the effects caused by the irregular shape of the Earth's magnetic field along with its diurnal rotation. The analyses of residuals show that periodicities are correlated to the solar wind speed and magnetospheric forcing, especially those located near the southern dip pole at the night side. Furthermore, we found a TEC anomaly at about 15° from the South magnetic dip at the night side, more prominent around 52°S 155°E. Key Points: The solar and the magnetospheric forcing are the main drivers of nonperiodic ionospheric TEC variationsMain periodic contributions to TEC variations are related to the frequencies of the solar rotation, annual, and subharmonicsTEC anomaly has been found at about 15° from the South magnetic dip at the night side, more prominent around 52°S 155°E [ABSTRACT FROM AUTHOR]

Magnetosonic waves, with the frequency between proton cyclotron frequency and lower hybrid frequency, mainly occur near the Earth's magnetic equatorial plane and play an important role in the magnetospheric dynamics. In this paper, we report unusual magnetosonic waves with nonlinear harmonics observed by Magnetospheric Multiscale mission in the magnetotail. These magnetosonic waves have multiband enhanced electromagnetic power spectral densities (Bw/B0 up to 1/45) with the frequency from below to above the lower hybrid frequency and are quasi‐perpendicular propagating and linearly polarized. The frequency of the fundamental band is much higher than the proton cyclotron frequency fci (i.e., ~35 fci). We identified these emissions as high‐frequency magnetosonic waves with nonlinear harmonics. These are the first observations of such high‐frequency magnetosonic waves with the frequencies of harmonics higher than the lower hybrid frequency in the Earth's magnetosphere to the best of our knowledge. Given the absence of ring distributions for the protons and the easy coupling between compressed mode and its electromagnetic term, we propose that these nonlinear harmonic structures of the observed magnetosonic waves are likely generated by the nonlinear wave‐wave coupling among electromagnetic terms of the fundamental and higher harmonic waves. Key Points: First observations of high‐frequency multiharmonic magnetosonic waves with the frequencies of harmonics higher than the lower hybrid frequency are observed in the Earth's magnetosphereSuch magnetosonic waves have nonlinear harmonicsNonlinear harmonics are likely to be generated by nonlinear wave‐wave coupling interaction [ABSTRACT FROM AUTHOR]

It is generally accepted that extreme space weather events tend to be related to strong flares and fast halo coronal mass ejections (CMEs). In the present paper, we carefully identify the chain of events from the Sun to the Earth induced by all 12 X‐class flares that occurred in 2002. In this small sample, we find an unusual high rate (58%) of solar sources with a longitude larger than 74°. Yet all 12 X‐class flares are associated with at least one CME. The fast halo CMEs (50%) are related to interplanetary CMEs (ICMEs) at L1 and weak Dst minimum values (more than −51 nT), while five (41%) of the 12 X‐class flares are related to solar proton events (SPEs). We conclude that (i) all 12 analyzed solar events, even those associated with fast halo CMEs originating from the central disk region, and those ICMEs and SPEs were not very geo‐effective. This unexpected result demonstrates that the suggested events in the chain (fast halo CME, X‐class flares, central disk region, ICME, and SPE) are not infallible proxies for geo‐effectiveness. (ii) The low value of integrated and normalized southward component of the interplanetary magnetic field (Bz*) may explain the low geo‐effectiveness for this small sample. In fact, Bz* is well correlated to the weak Dst and low auroral electrojet activity. Hence, the only space weather impact at Earth in 2002 we can explain is based on Bz* at L1. Key Points: We discussed geo‐effectiveness of the 12 X‐class flare‐CMEs in 2002 based on the available observationsThey failed to evoke strong disturbances since weak interplanetary magnetic field southward componentsThe usual solar proxies (complex active region, X‐class flare, and fast halo CME) are not sufficient to forecast geo‐effectivity in 2002 [ABSTRACT FROM AUTHOR]

We simulated the impact of long‐term changes in the geomagnetic field on the spatial pattern of the Weddell Sea Anomaly (WSA). The Weddell Sea Anomaly, belonging to the region near the tip of the Antarctic Peninsula, Falklands, and surrounding seas, is the best‐known example of ionospheric reversed diurnal cycle. This paper investigates whether and to what extent long‐term changes in the geomagnetic field influence spatial morphology of the WSA. The major concept centers on a normalized density difference index, serving as a measure of difference between dayside and nightside ionospheric density. Through combined analysis of in situ ionospheric electron density data from past mission CHAMP, ongoing ESA mission Swarm, and simulations based on the NCAR TIEGCM model, we examine changes in the spatial pattern of the WSA‐like anomalies, which could be linked to long‐term changes in the geomagnetic field. A series of simulations provides theoretical analysis of changes in the morphology of the WSA on time scales longer than several decades. Numerical analysis shows that from the time when the WSA was discovered (around 1960) till present, maximum of the WSA has migrated by approximately 7° in longitude towards the Pacific Ocean, showing clear westward drift, consistent with temporal evolution of the geomagnetic dipole component. Key Points: Numerical reconstruction of the WSA is derived with the TIEGCM modelTwo LEO satellite missions are used for determining and tracking maximum of the WSAThe WSA exhibits westward migration towards the Pacific Ocean [ABSTRACT FROM AUTHOR]

The sources for the variabilities of the ionospheric tidal oscillations are complex and not totally understood yet. In this paper, both the seasonal and short‐term variations of the ionospheric diurnal and semidiurnal oscillations over a meteor radar chain (Wuhan [114.4°E, 30.6°N], Beijing [116.5°E, 39.9°N], and Mohe [121.1°E, 50.1°N]) are investigated with critical frequency of the F2 layer (foF2) observations during 2014. The collocated mesospheric horizontal winds, the daily 10.7‐cm solar flux, and the daily Ap index are also examined to determine the sources of the ionospheric tidal variabilities from the lower atmosphere, solar and geomagnetic activities, respectively. We found that the seasonal variations of the ionospheric tidal oscillations at the three sites are consistent, which are not in‐phase with the seasonal variations of mesospheric tides at the same location. This indicates that mesospheric tides are unlikely the main cause of the seasonal variations of the ionospheric tides. Besides, the ionospheric tidal oscillations show clear periodical signals, which could correspond well with those in the local mesospheric wind or Ap index. Nevertheless, the periodical solar flux oscillation does not induce similar short‐term ionospheric tidal variabilities. We also found that the influence of geomagnetic activities on the ionospheric tidal variabilities is with either enhancement or depression patterns during geomagnetic storms. Key Points: The ionospheric tidal oscillations and their sources are investigated with ionosonde foF2 and meteor radar neutral wind during 2014The ionospheric tidal variations induced by lower atmospheric forcing, geomagnetic and solar activities are all exhibitedThe periodical solar flux exhibits less impact to the ionospheric tidal variabilities than the planetary wave and geomagnetic activities [ABSTRACT FROM AUTHOR]

We present an empirical model of thermospheric winds (High‐latitude Thermospheric Wind Model [HL‐TWiM]) that specifies F region high‐latitude horizontal neutral winds as a function of day of year, latitude, longitude, local time, and geomagnetic activity. HL‐TWiM represents the large‐scale neutral wind circulation, in geomagnetic coordinates, for the given input conditions. The model synthesizes the most extensive collection to date of historical high‐latitude wind measurements; it is based on statistical analyses of several decades of F region thermospheric wind measurements from 21 ground‐based stations (Fabry‐Perot Interferometers and Scanning Doppler Imaging Fabry‐Perot Interferometers) located at various northern and southern high latitudes and two space‐based instruments (UARS WINDII and GOCE). The geomagnetic latitude and local time dependences in HL‐TWiM are represented using vector spherical harmonics, day of year and longitude variations are represented using simple harmonic functions, and the geomagnetic activity dependence is represented using quadratic B splines. In this paper, we describe the HL‐TWiM formulation and fitting procedures, and we verify the model against the neutral wind databases used in its formulation. HL‐TWiM provides a necessary benchmark for validating new wind observations and tuning our physical understanding of complex wind behaviors. Results show stronger Universal Time variation in winds at southern than northern high latitudes. Model‐data intra‐annual comparisons in this study show semiannual oscillation‐like behavior of GOCE winds, rarely observed before in wind data. Key Points: We developed a comprehensive empirical model of high‐latitude F region thermospheric winds (HL‐TWiM)Universal Time variations in high‐latitude winds are stronger in the Southern than Northern HemisphereHL‐TWiM provides a necessary benchmark for validating new high‐latitude wind observations and tuning first principal models [ABSTRACT FROM AUTHOR]

The atmospheric effects of precipitating electrons are not fully understood, and uncertainties are large for electrons with energies greater than ~30 keV. These electrons are underrepresented in modeling studies today, primarily because valid measurements of their precipitating spectral energy fluxes are lacking. This paper compares simulations from the Whole Atmosphere Community Climate Model (WACCM) that incorporated two different estimates of precipitating electron fluxes for electrons with energies greater than 30 keV. The estimates are both based on data from the Polar Orbiting Environmental Satellite Medium Energy Proton and Electron Detector (MEPED) instruments but differ in several significant ways. Most importantly, only one of the estimates includes both the 0° and 90° telescopes from the MEPED instrument. Comparisons are presented between the WACCM results and satellite observations poleward of 30°S during the austral winter of 2003, a period of significant energetic electron precipitation. Both of the model simulations forced with precipitating electrons with energies >30 keV match the observed descent of reactive odd nitrogen better than a baseline simulation that included auroral electrons, but no higher energy electrons. However, the simulation that included both telescopes shows substantially better agreement with observations, particularly at midlatitudes. The results indicate that including energies >30 keV and the full range of pitch angles to calculate precipitating electron fluxes is necessary for improving simulations of the atmospheric effects of energetic electron precipitation. Plain Language Summary: The study presented here investigates the effects from energetic electron precipitation (EEP) in the southern hemisphere winter of 2003. Electron precipitation is common during periods of enhanced geomagnetic activity and can create reactive nitrogen oxides and hydrogen oxides that can destroy ozone. Most global climate models currently do not include precipitating electrons with energies greater than 30 keV. To test whether this deficiency is important, this investigation compares observations with model simulations that included electrons with energies greater than 30 keV, as observed by the Medium Energy Proton and Electron Detector (MEPED) satellite instruments. In addition, one of the EEP data sets used in the simulations included data from just one of the telescopes on the MEPED instruments, whereas the other included data from both of the telescopes. We found that including both of the telescopes is important for capturing chemistry changes at polar and subpolar latitudes. The model simulation that only included only one of the telescopes showed significant improvement compared to a simulation with only low energy electrons. However, it did not perform as well as the model simulation that included both MEPED telescopes. This work is important because it shows that including energies >30 keV and the full range of precipitating electron pitch angles is necessary to show the impact electrons have on the atmosphere and provides an EEP data set for use in future model simulations. Key Points: Effects of energetic electron precipitation in southern hemisphere 2003 simulated with Whole Atmosphere Community Climate ModelSimulations of middle atmosphere chemistry improve significantly by including >30‐keV precipitating electronsIncluding the full range of pitch angles is necessary to capture precipitating electron impacts at subpolar latitudes [ABSTRACT FROM AUTHOR]

Comprehensive pattern of ionospheric troughs' location in winter for all local times, longitudes, high (HSA), and low (LSA) solar activity, in both hemispheres, is at first investigated. Statistical analysis based on a large dataset of Interkosmos‐19, CHAMP, and Kosmos‐900 satellites was performed for quiet geomagnetic conditions Kp = 1–3. Three troughs were considered: high‐latitude trough (HLT) located inside the auroral oval, main ionospheric trough (MIT) located equatorward from the auroral oval, that is, at subauroral latitudes, and mid‐latitude ring ionospheric trough (RIT). The main purpose was to study the formation of the troughs' diurnal pattern in different conditions. The main problem was to distinguish MIT from RIT and MIT from HLT. For this purpose, early morning hours (04–06 LT), late morning hours (07–10 LT), day, evening, and night conditions were examined. In the early morning sector, RIT was separated from MIT and eliminated from the dataset. In the late morning sector, MIT and HLT were first clearly divided, although only for HSA. During the day, in the Northern Hemisphere under all conditions, HLT is mainly observed, in the Southern Hemisphere at poorly lit longitudes only the daytime MIT, and at well‐lit longitudes only the HLT is observed. The division of MIT and HLT was carried out according to the (statistical) position of the equatorial boundary of the auroral oval precipitation. At night, longitudinal variations in the MIT position determine the asymmetry of the hemispheres. Thus, the occurrence and position of MIT and HLT depend on the hemisphere, longitude, and solar activity. Plain Language Summary: Dynamics of the ionization troughs determines the variation in a structure of the winter and equinoctial subauroral and auroral ionosphere. Unfortunately, many aspects of this dynamics are still poorly known, which does not allow creating an adequate model of the high‐latitude ionosphere. Such a model is necessary to describe the conditions for propagation of short radio waves. In the international reference model of the ionosphere IRI, the trough as a structure is absent; therefore, IRI poorly describes high latitudes. In this paper, for the first time, a comprehensive pattern of diurnal variations in the different troughs position in winter high‐latitude ionosphere is constructed. They are found to be highly dependent on hemisphere, longitude, and solar activity. The results of this work will be used to build an adequate model of the high‐latitude ionosphere, which is being now created in the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation of the Russian Academy of Sciences (IZMIRAN). Key Points: Comprehensive pattern of diurnal variations in winter ionospheric troughs' position in Northern and Southern hemispheres was obtainedStatistical analysis based on a large dataset of Interkosmos‐19, CHAMP, and Kosmos‐900 satellites was performedDiurnal variations in trough position depend strongly on solar activity, hemisphere, and longitude [ABSTRACT FROM AUTHOR]

The existence of an additional stratification in the daytime equatorial ionospheric F region (the F3 layer) was known since the 1940s. However, its characteristics and the underlying physical mechanism have been uncovered only recently. In this paper, we present and discuss the F3 layer characteristics observed by six ionosondes distributed over equatorial and low latitudes (−20° to +25° dip latitudes) in the Brazilian longitude sector during the strongest geomagnetic storm (DstMin = −223 nT) of solar cycle 24, the St. Patrick's Day storm of 17 March 2015. Two eastward prompt penetration electric field (PPEF) events, as seen in equatorial electrojet, occurred during the main phase of the storm on 17 March 2015, a strong one (~100 nT) at around ~1200 UT and a weak one (~50 nT) at around ~1725 UT. Local time variations in the F3 layer occurrence and ionospheric base height (h′F), peak height (hmF), and peak electron density (Nmax) are investigated. Notably, the F3 layer occurred at all six locations, more distinctly during the stronger PPEF event. The large latitudinal extend in the occurrence of the F3 layer in opposite hemispheres (−20° to +25° dip latitudes) covering the equatorial ionization anomaly crests observed for the first time is interpreted in terms of the combined effect of the super plasma fountain generated by the eastward PPEF and storm time equatorward neutral wind. Key Points: F3 layer occurred over a large latitudinal extent from 20°S to 25°N dip latitudes during 17 March 2015 storm‐induced PPEF phasesThe PPEF‐generated superfountain led to the F3 layer occurrence earlier at the equator and later at higher latitudes in both hemispheresF3 layer was more distinct and stronger and lasted longer in the Northern Hemisphere due to the storm time asymmetric equatorward neutral winds [ABSTRACT FROM AUTHOR]

We study the contribution of the solar wind Poynting flux S→sw to the total power input into the magnetosphere. The dominant power delivered by the solar wind is the kinetic energy flux of the particles, which is larger than Ssw by a factor of order MA2, where MA is the Alfvén Mach number. The currents J→ flowing in the bow shock and magnetosheath and the electric field E→ of the solar wind give regions where J→.E→<0, which are sources of Poynting flux, generated from the kinetic energy flux. For southward interplanetary magnetic field, E→ is duskward and the currents in the high‐latitude tail magnetopause are also sources of Poynting flux. We show transfer of kinetic energy into the magnetosphere is less efficient than direct entry of S→sw by a factor MA. Because MA is typically of order 10, this means that although the power density in the solar wind due to S→sw is typically only 1%, it is responsible for of order 10% of the energy input to the magnetosphere. To investigate the effect of this, we add a term to the solar wind‐magnetosphere energy coupling function that allows for S→sw and that increases the correlation with the geomagnetic am index for 1995–2017 (inclusive) from 0.908 to 0.924 for 1‐day averages and from 0.978 to 0.979 for annual means. The increase for means on daily or smaller timescales is a small improvement but is significant (at over the 3σ level), whereas the improvement for annual or Carrington rotation means is not significant. Plain Language Summary: Space weather is caused by energy extracted from the solar wind by the magnetosphere, the volume of space surrounding the Earth that is dominated by Earth's magnetic field. That energy arrives in two main forms in the solar wind: the kinetic energy of the particle flow and an electromagnetic energy flux. The most successful predictors of space disturbances have considered only the kinetic energy flux. The paper shows typically 10% of the power input comes from the electromagnetic energy flux in the solar wind and allowing for this can make a small, but significant, improvement to our ability to predict terrestrial space weather disturbances. Key Points: Solar wind Poynting flux is about 1% of the total solar wind energy flux but provides typically 10% of the power entering the magnetosphereEquations for adding solar wind Poynting flux to kinetic energy flux are presentedAllowing for solar wind Poynting flux gives correlations with geomagnetic activity that are slightly but significantly raised [ABSTRACT FROM AUTHOR]

In this paper, we study electron radial diffusion coefficients derived from Pc4‐Pc5 ultralow frequency (ULF) wave power during the intense geomagnetic storm on 17–18 March 2015. During this storm the population of highly relativistic electrons was depleted within 2 hr of the storm commencement. This radial diffusion, depending upon the availability of source populations, can cause outward radial diffusion of particles and their loss to the magnetosheath, or inward transport and acceleration. Analysis of electromagnetic field measurements from Geostationary Operational Environment Satellite (GOES), Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite, and ground‐based magnetometers shows that the main phase storm‐specific radial diffusion coefficients do not correspond to statistical estimates. Specifically, during the main phase, the electric diffusion (DLLE) is reduced, and the magnetic diffusion (DLLB) is increased, compared to empirical models based on Kp. Contrary to prior results, the main phase magnetic radial diffusion cannot be neglected. The largest discrepancies, and periods of dominance of DLLB over DLLE, occur during intervals of strongly southward IMF. However, during storm recovery, both magnetic and electric diffusion rates are consistent with empirical estimates. We further verify observationally, for the first time, an energy coherence for both DLLB and DLLE where diffusion coefficients do not depend on energy. We show that, at least for this storm, properly characterizing main phase radial diffusion, potentially associated with enhanced ULF wave magnetopause shadowing losses, cannot be done with standard empirical models. Modifications, associated especially with southward IMF, which enhance the effects of DLLB and introduce larger main phase outward transport losses, are needed. Key Points: Event‐specific electric and magnetic diffusion both show coherent energy dependence during the intense March 2015 magnetic stormMain phase event‐specific diffusion coefficients are poorly represented by empirical Kp‐dependent solar cycle averaged statistical modelsIntervals of southward IMF during the main phase drive strong compressional perturbations significantly enhancing magnetic diffusion [ABSTRACT FROM AUTHOR]

The Mars Advanced Radar for Subsurface and Ionosphere Sounding on board the Mars Express spacecraft measures the frequency of local plasma oscillations, which can be used to determine electron densities local to the spacecraft. This paper provides an overview of electron densities in the upper Martian ionosphere, obtained by investigating over 400,000 ionograms, during the course of about 11 years, corresponding to a full solar cycle. The data cover wide latitude and longitude ranges, 180° of solar zenith angle (SZA), and altitudes from about 250 to 1,550 km. The electron density profiles show large fluctuations within each orbit and also for any given altitude and SZA range. However, the median electron density is almost constant on the dayside at a fixed altitude range, with the exception of a dip at around 30° SZA, at altitudes between 300 and 600 km. A sudden drop in density is observed as the terminator is approached from the dayside. For a fixed SZA range, the median electron density decreases exponentially with increasing altitude. The high‐altitude scale height is composed of two exponential functions of SZA joined near the ionospheric terminator. The e‐folding height changes between 45 and 214 km from the subsolar point up to 120°, corresponding to effective temperatures between about 165 and 780 K. Solar activity has a clear effect on the median electron densities above 500 km and on e‐folding height. The median electron density is higher during northern winters, as well as above regions of strong crustal fields on the dayside. Key Points: Electron density increases with decreasing altitude with an e‐folding height changing between 45 and 120 km from 0 to 120 degrees of SZAData for a full solar cycle show that the average electron density values are directly related to the solar activityElectron densities are higher during northern winters and above strong crustal magnetic fields on the dayside [ABSTRACT FROM AUTHOR]

Usual depletions in the Northern Hemisphere form a tree fork junction feature near the south (i.e., toward the equator) in all‐sky airglow images, and its branches surge toward the north (i.e., poleward). We report in this paper unusual airglow depletions in OI 630.0‐nm imaging over India that surged equatorward and formed an inverted tree fork junction feature on 3 January 2011. Airglow images showed faint signatures of medium‐scale traveling ionospheric disturbances in the beginning that were east‐west aligned and propagated toward equator. Within 06 to 18 min of its passing, turbulent structures were noted in the field of view, and two depleted patches appeared out of low airglow background over the off‐equatorial edge. The apex height of the associated geomagnetic flux tubes varied from ~ 1,400 to 1,600 km. Later, these dark regions intensified and surged equatorward while drifting slowly toward west. While remaining almost steady, one of them further intensified and continued to surge equatorward. The second dark patch got linked up with another isolated depletion to form an elongated depleted feature. Next, the southern end of this attached feature surged equatorward. When two depletions were well formed, an inverted tree fork junction was noticeable. During this time, the equatorward motion of the equatorial ionization anomaly structure is also seen. To the best of our knowledge, this is the first imaging observation of an inverted tree fork junction feature and brings out the unknown facets of ionospheric irregularities. Plain Language Summary: Usual depletions form a tree fork junction feature near the equatorial edge in all‐sky airglow images; its branches surge poleward. On 3 January 2011, unusual depletions were noted over India that surged equatorward and formed an inverted tree fork junction. We present an independent usual depletion event. Such an inverted feature has been not reported earlier to the best of our knowledge and brings out the unknown facets of ionospheric irregularities. Key Points: This is the first report of off‐equatorial edge initiating and equatorward surging airglow depletionsUnlike usual depletions in the Northern Hemisphere, observed depletions appeared like an inverted tree fork junction (somewhat similar to those reported from the Southern Hemisphere) [ABSTRACT FROM AUTHOR]

Under northward interplanetary magnetic field conditions, magnetic reconnection at the Earth's magnetopause is usually thought to operate through the merging of magnetosheath magnetic field lines and open magnetic field lines from the magnetospheric lobe. However, reconnection also occurs between magnetosheath field lines and closed magnetic field lines in the magnetosphere. Under certain conditions, this nonlobe field line reconnection has distinct plasma and magnetic field signatures that distinguish it from reconnection of lobe field lines. A survey of these conditions suggests that nonlobe reconnection at the Earth's magnetopause may be common even for relatively strong northward IMF. Plain Language Summary: Magnetic reconnection interconnects magnetic field lines across a current sheet like that at the Earth's magnetopause or like that in the solar corona. This paper describes a type of interconnection of magnetic field lines at the magnetopause that was first reported for the solar corona. It appears that this type of reconnection is fairly common at the magnetopause. The different topology of this reconnection suggests a different type of interaction between the magnetic field lines in the solar wind and the field lines of the Earth. Key Points: Different magnetic field topologies at the magnetopause can be distinguished with plasma and magnetic field dataLobe and nonlobe reconnection occur at the magnetopause for northward IMFNonlobe reconnection appears to be common at the Earth's magnetopause for northward IMF [ABSTRACT FROM AUTHOR]

In this paper, we present our investigation of geomagnetic crochets observed following the X‐class solar flare events on 9 August 2011 (X6.9) and 24 September 2011 (X1.4). Both events occurred during geomagnetically quiet times (Kp ≤ 3). The analysis of the geomagnetic crochets near the magnetic equator revealed very different responses in the ionospheric currents between these two events. During the 9 August 2011 event, a reduction of the eastward equatorial electrojet was observed in all daytime stations studied, while the daytime eastward equatorial electrojet was observed to enhance during the 24 September 2011 event. A spherical harmonic analysis was applied to the observations from nearly 160 ground stations at middle and low latitudes to derive the equivalent ionospheric current system for the geomagnetic crochets as well as background solar‐quiet (Sq) variations for both events. The equivalent current system of the geomagnetic crochets for the 24 September 2011 event showed a similar pattern with the background Sq current system and thus can be attributed to the enhancement of Sq currents. However, the crochet current system for the 9 August 2011 event revealed a different pattern from the background Sq current system in both Northern and Southern Hemispheres. It is suggested that unusual geomagnetic crochets during the 9 August 2011 event may be dominated by the ionospheric currents at D‐region altitudes. Key Points: Following the X6.9 solar flare event of 9 August 2011, negative crochets were observed at dayside equatorial stationsGlobal ionospheric current system due to the solar flare that cannot be explained by an enhancement of the background Sq current systemContribution from the D‐region currents is suspected for the changes of the global ionospheric system [ABSTRACT FROM AUTHOR]

Medium‐scale ionospheric ionization structures are a persistent global feature of the Earth's ionosphere. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation measurements are well suited to address the incomplete global observational picture of plasma density irregularities, including the global climatology in both bottomside and topside F region layers, and their structure in the vertical dimension. A climatological database of F region ionospheric irregularities and their characteristics has been developed through detection of total electron content perturbations by Global Positioning System receivers onboard COSMIC satellites. This paper presents global occurrence rates and detailed characteristics of equatorial to midlatitude medium‐scale irregularities under quiet geomagnetic conditions. The study covers 4 years, two during solar minimum (2008–2009) and two during the ascending phase of solar cycle 24 (2012–2013). Irregularities were found to occur frequently at high latitudes and during nighttime in equatorial to midlatitude regions in both bottom and topside F region layers. Longitudinal‐seasonal occurrence trends at equatorial and midlatitudes are consistent with previous irregularity climatology, which reaffirms that localized enhancements in plasma instability growth rates contribute to irregularity occurrence. Seasonal occurrence patterns also indicate a high occurrence of irregularities in regions corresponding to the solar terminator, confined primarily to altitudes below ~300 km. The local time‐altitude distributions of equatorial and midlatitude irregularity occurrence, amplitude, and scale size provide further insight into irregularity generation mechanisms, and include features consistent with "spread F" irregularities and traveling ionospheric disturbances. Key Points: COSMIC radio occultation TEC measurements are used to examine the global climatology of medium‐scale ionospheric plasma irregularitiesObservation of vertical irregularity structure and distribution provides new insight into irregularity occurrence and characteristicsIrregularities are persistently observed in nighttime equatorial to midlatitude regions, near the solar terminator, and at high latitudes [ABSTRACT FROM AUTHOR]

Auroral finger‐like structures appear equatorward of the auroral oval in the diffuse auroral region and contribute to the auroral fragmentation into patches. A previous report of the first conjugate observation of auroral finger‐like structures using a Time History of Events and Macroscale Interactions during Substorms (THEMIS) Ground‐Based Observatories camera and the THEMIS‐E satellite at a radial distance of ∼8 RE showed antiphase oscillations of magnetic and plasma pressures in the dawnside plasma sheet. In the present study, we report another simultaneous observation of auroral finger‐like structures at Gillam, Canada, at ∼0900 UT (0230 magnetic local time) on 14 November 2014 with the Radiation Belt Storm Probes satellites at 5.8 RE in the inner magnetosphere. From this simultaneous observation event, we obtained the following observations. (1) Auroral finger‐like structures developed poleward in the equatorward moving auroral arc at the equatorward edge of the auroral oval. (2) Both the electron and ion OMNI fluxes measured by HOPE increased at ∼0900 UT as the satellite footprint entered the auroral region, indicating that the satellite was crossing the observed auroral finger‐like structures. (3) The absolute value of magnetic pressure was several times that of the plasma pressure, and no systematic phase relationship was identified between the magnetic and plasma pressures, unlike that in the THEMIS case. Based on these observations, we discuss two possible causes of the observed finger‐like structures, namely, pressure‐driven instability in the magnetosphere and gradient drift instability in the ionosphere. In this paper, the latter possibility is newly suggested to develop in the equatorward moving aurora associated with the westward electric field in the equatorward ionospheric density gradient. Plain Language Summary: We report the conjugate observation of auroral finger‐like structures using ground‐based all‐sky imager and the RBSP‐A satellite. Such a report of conjugate observation of auroral finger‐like structures with the RBSP‐A satellite in the inner magnetosphere is for the first time. The auroral finger‐like structures can be a cause of the auroral fragmentation into patches. From the present observation, we suggest that the gradient drift instability in the ionospheric plasma can be a cause of the auroral finger‐like structures. Key Points: Conjugate space‐based measurements are combined with ground‐based observations of auroral finger‐like structuresForce balance between pressure gradient and tension forces in the magnetosphere are quantified and compared with prior studiesWe suggest a gradient drift instability in the ionosphere as the possible cause of auroral structure in this event [ABSTRACT FROM AUTHOR]

We present a new technique to experimentally measure the spatial spectrum of ionospheric disturbances in the spatial scale regime of 40–200 km. This technique produces a two‐dimensional (2‐D) spectrum for each time snapshot over two dense Global Positioning System (GPS) receiver networks (GPS Earth Observation Network in Japan and Plate Boundary Observatory in the Western United States). Because this technique created the spectrum from an instantaneous time snapshot, no assumptions are needed about the speed of ionospheric irregularities. We examine spectra from 3 days: one with an intense geomagnetic storm, one with significant lightning activity, and one quiet day. Radial slices along the 2‐D spectra provide one‐dimensional spectra that can be fit to a power law to quantify the steepness of the falloff in the spatial scale sizes. Continuous data of this type in a stationary location allow monitoring the variability in the 2‐D spectrum over the course of a day and comparing between days, as shown here, or even over a year or many years. We find that the spectra are highly variable over the course of a day and between the two selected regions of Japan and the Western United States. When strong traveling ionospheric disturbances (TIDs) are present, the 2‐D spectra provide information about the direction of propagation of the TIDs. We compare the TID propagation direction with horizontal wind directions from the Horizontal Wind Model. TID direction is correlated with the horizontal wind direction on all days, strongly indicating that the primary source of the TIDs measured by this technique is tropospheric. Plain Language Summary: The ionosphere is a part of the upper atmosphere that is ionized (made into a plasma) by incoming solar radiation. Turbulence and waves can be created in this plasma by various processes from above (solar activity) and below (e.g., thunderstorms and earthquakes). The spatial spectrum of these perturbations ranges from thousands of kilometers to meters. Characteristics of the spectrum impact the distortion of radio signals passing through the ionosphere, between ground and satellite. This paper shows a new way of measuring the spatial spectrum by using hundreds of closely spaced ground‐based GPS receivers in Japan and in California. We create a map of ionospheric variation and waves at each time and then measure variations with scale sizes of 50–200 km with a new mathematical technique. We look at the difference in the spatial spectrum between a calm day, a day with significant solar activity, and a day with significant thunderstorm activity. We find that the spectrum changes a lot over the course of a day and from day to day. We also can see that waves in the ionosphere often move opposite to the direction of the ionospheric wind, indicating that they are originating from below the ionosphere. Key Points: We demonstrate a new technique that uses densely spaced GPS receiver arrays to measure 2‐D spatial spectrum of ionospheric irregularitiesVariations in the ionospheric spatial spectrum due to geomagnetic activity and tropospheric thunderstorms are examinedTraveling ionospheric disturbance direction of propagation is found to correlate with Horizontal Wind Models in nearly all cases studied [ABSTRACT FROM AUTHOR]