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We review important advances in the understanding of auroral kilometric radiation (AKR) resulting from observations by the Wideband Data instruments on the four Cluster spacecraft. AKR is an intense radio emission originating in the Earth's auroral regions with frequencies typically in the range 50–700 kHz, usually observed from space. It is now widely accepted that AKR is generated by the cyclotron maser instability (CMI) in density cavities in the auroral acceleration region. Multi‐point observations by the Cluster spacecraft with a time delay of arrival technique have allowed the source locations of many individual AKR bursts to be determined. The position uncertainty is around 500 km at the source region or about 200 km when mapped on to the auroral zone. AKR is emitted in a narrow beam close to the tangent to the magnetic field vector in the source region. This has important implications for the possible generation mechanisms, being incompatible with filled or hollow cone beaming models. It also implies that an observer at a given location can only see AKR from a fraction of possibly active source regions. The complex frequency time structure of AKR sometimes shows regular striations or pulsations. Cluster observations of these phenomena have been interpreted as modulation of the CMI by disturbances propagating through the generation region. Exceptionally, AKR can sometimes be observed from low altitude spacecraft or even on the ground. Recent work has involved simultaneous observations of AKR on the Cluster spacecraft and on the ground at the South Pole. Plain Language Summary: A review of Cluster Wideband Data observations of auroral kilometric radiation (AKR). Multi‐point observations have allowed the source locations of many individual AKR bursts to be determined. The angular beaming pattern was found to be close to the tangent to the magnetic field vector in the source region, which has important implications for the possible generation mechanisms. The complex frequency time structure of AKR sometimes shows regular striations or pulsations, which have been interpreted as modulation by disturbances propagating though the generation region. The most recent work has involved simultaneous observations of AKR on the Cluster spacecraft and on the ground at the South Pole. Key Points: The Cluster spacecraft Wideband Data instruments have allowed important advances in the understanding of auroral kilometric radiation (AKR)Studies include source locations, angular beaming, and fine structure including pulsations and striationsSimilar AKR fine structures are sometimes observed simultaneously on the spacecraft and at ground level [ABSTRACT FROM AUTHOR]

Geomagnetically induced currents (GICs) can be driven in terrestrial electrical power grids as a result of the induced electric fields arising from geomagnetic disturbances (GMD) resulting from the dynamics of the coupled magnetosphere‐ionosphere‐ground system. However, a key issue is to assess an optimum spacing for the magnetometer stations in order to provide appropriate monitoring of the GIC‐related GMD. Here we assess the vector correlation lengths of GMD and related amplitude occurrence distribution of the variations of horizontal magnetic field dBH/dt. Specifically, we study the GMD response to two storm‐time substorms using data from two magnetometer arrays, the Baltic Electromagnetic Array Research Project in Scandinavia and the Canadian Array for Realtime Investigations of Magnetic Activity array in North America, so as to determine the appropriate magnetometer spacing in latitude and longitude, for monitoring and assessing GIC risk. We find that although magnetic disturbances are well‐correlated up to distances of several hundred kilometers at mid‐latitudes, the vector correlation length rapidly drops off for station separations of less than 100 km within the auroral oval. In general geomagnetic fluctuations are stronger and more localized in the auroral zone. Since the auroral oval is pushed equatorward during intense magnetic storms, we highlight that networks using a station separation of ∼200 km should provide an excellent basis for monitoring both small and large scale geomagnetic disturbances. A monitoring network with this station spacing is recommended as being appropriate for assessing the role of GMD in driving GICs in the electric power grid. Plain Language Summary: One of the most dangerous effects of space weather is the induction of currents on wires and pipelines on Earth, which can destroy electrical transformers in the power grid and damage infrastructure. Part of our effort to deal with that threat involves monitoring the variability of the Earth's magnetic field at the Earth's surface, since that can give us an indication of how and when such currents may be induced. A key issue is the required spatial separations of observing magnetic monitoring stations. Using two existing arrays of ground stations we statistically analyzed their observations during two days of strong space‐related geomagnetic activity. From that analysis we have found that regions in the auroral zone have intense and more localized geomagnetic variations, compared to regions equatorward of them. Since the auroral zone can be pushed equatorward at times of intense geomagnetic storms, which is when the induced currents will likely pose the greatest risk, we need to account for separations which can additionally monitor small‐scale fluctuations. We conclude that arrays of ground magnetometers with a ∼200 $\sim 200$ km separation are appropriate for the required magnetic monitoring. Key Points: Occurrence distributions of dBH/dt during two similar storms at two different ground magnetometer arrays are found to be log‐normalTwo‐point dBH/dt vector correlations between distant stations are lower within the auroral oval than at mid‐latitudesMagnetometers placed 200 km apart should provide required coverage of localized large dBH/dt events for geomagnetically induced current applications [ABSTRACT FROM AUTHOR]

An explicit proxy of solar activity on the Earth is the auroral displays. The auroral oval can extend to lower latitudes during geomagnetic storms triggered by explosive solar activities. The lower the latitude of auroral oval is, the stronger the solar activity is. Systematic auroral records in Europe (mainly central Europe with geomagnetic latitude lower than 55°) can be dated back to 1000 AD and can be used to manifest solar activity in the past millennium. However, the temporal distribution of the 6,262 auroral records during 1000 and 1900 AD is seriously uneven, with 85.6% of the records appeared after 1700 AD. Here we use the extreme value theory (EVT) to evaluate the effectiveness of characterizing the solar activity with the auroral records before 1700 AD. Due to the inhomogeneity of the auroral records, the EVT has been conducted separately for the data before 1700 and after 1700, finding that the 100‐year auroral frequency is 25.66 [19.67, 33.67] based on the auroral data in 1000–1699, and 283.71 [183.14, 417.58] based on the auroral data in 1700–1900. The predictions of both the 50‐year and 100‐year auroral frequencies before 1700 AD agree well with the solar activity index. Key Points: Extreme value theory was applied to the ancient auroral records to calculate the N‐year frequencyHundred‐year space weather event can be connected with the observed auroraeResults of the aurorae calculated by extreme value theory can provide a basis for reconstructing the solar activity [ABSTRACT FROM AUTHOR]

Meso‐scale plasma convection and particle precipitation could be significant momentum and energy sources for the ionosphere‐thermosphere (I‐T) system. Following our previous work on the I‐T response to a typical midnight flow burst, flow bursts with different characteristics (lifetime, size, and speed) have been examined systematically with Global Ionosphere‐Thermosphere Model (GITM) simulations in this study. Differences between simulations with and without additional flow bursts are used to illustrate the impact of flow bursts on the I‐T system. The neutral density perturbation due to a flow burst increases with the lifetime, size, and flow speed of the flow burst. It was found that the neutral density perturbation is most sensitive to the size of a flow burst, increasing from ∼0.3% to ∼1.3% when the size changes from 80 to 200 km. A westward‐eastward asymmetry has been identified in neutral density, wind, and temperature perturbations, which may be due to the changing of the forcing location in geographic coordinates and the asymmetrical background state of the I‐T system. In addition to midnight flow bursts, simulations with flow bursts centered at noon, dawn, and dusk have also been carried out. A flow burst centered at noon (12.0 Local Time [LT], 73°N) produces the weakest perturbation, and a flow burst centered at dusk (18.0 LT, 71°N) produces the strongest. Single‐cell and two‐cell flow bursts induce very similar neutral density perturbation patterns. Key Points: Neutral density perturbations due to a midnight flow burst are most sensitive to scale sizeNeutral density perturbation magnitudes due to flow bursts at different local times depend on the background stateSingle‐cell and two‐cell flow bursts induce similar perturbation patterns [ABSTRACT FROM AUTHOR]

In this study, the effect of disturbance dynamo electric field (DDEF) induced by subauroral polarization streams (SAPS) on the variations of the equatorial electrojet (EEJ) and its counter electrojet (CEJ) during the geomagnetic storm on June 1, 2013 is analyzed in detail for the first time. Observations from ground‐based magnetometers showed that the SAPS‐induced EEJ flows westward and eastward in the daytime and dawn/dusk sectors, respectively. The effects of SAPS on EEJ are mainly associated with the changes of zonal ionospheric electric field, while the changes in the ionospheric conductivity play a secondary role. By using Thermosphere Ionosphere Electrodynamic General Circulation Model simulations, the zonal electric field induced by SAPS associated with the DDEF is examined. The results of the simulations show that the DDEF has a significant impact on the EEJ variability. The daytime westward EEJ at the dip equator is mainly driven by disturbance zonal wind, with secondary contributions from disturbance meridional wind. A similar mechanism can be observed in the dawn/dusk sector when the eastward EEJ is produced; however, it has a much weaker intensity than that during the daytime. Plain Language Summary: The equatorial electrojet (EEJ) is an intense electric current flowing along the dip equator in the ionospheric E region (∼110 km) during the daytime. This intense current is an important physical phenomenon in space weather, because it can significantly deflect the Earth's magnetic field. Subauroral polarization streams (SAPS) are an interesting phenomenon that occurs at high latitudes. The effects of SAPS on the thermospheric wind, temperature, and ionospheric density have been studied by previous literatures since its discovery. However, no dedicated study has focused on the behavior of EEJ during the SAPS period. The full decomposition of SAPS effects on EEJ is thus critical to understand the coupling processes of the ionosphere‐thermosphere system from high‐latitudes to the dip equator, and for the improvement of near‐Earth space weather modeling and forecast. Key Points: The daytime counter equatorial electrojet is generated by the subauroral polarization streams (SAPS)The Cowling conductivity induced by SAPS plays a negligible role in the changes of equatorial electrojetThe counter equatorial electrojet is associated with the disturbance dynamo electric field due to disturbance zonal wind [ABSTRACT FROM AUTHOR]

South Pole Station, Antarctica (SPA, magnetic latitude = −74.5°, magnetic local time (MLT) = UT–3.5 h), is a unique observatory which can capture daytime auroral forms throughout austral winter season. We have studied the properties and origin of ultralow‐frequency (ULF) range modulation of daytime diffuse aurora, using data acquired on June 23, 2017 by multi‐instrument measurements at SPA and in situ measurements in the dayside outer magnetosphere. At 1500–1600 UT, monochromatic Pc5‐range pulsations (period ∼10 min) emerged in the midday diffuse auroral region. The sequential 2‐D images reveal that the auroral pulsations result from the repetitive formation of faint, diffuse auroral patches, propagating poleward at a speed of ∼1.5 km s−1. Interestingly, no obviously similar magnetic pulsations were found at SPA. The results differ fundamentally from the ground optical and magnetic signatures expected for a standing field line resonance. On the other hand, the co‐located riometer and VLF receiver observed clearly synchronized pulsations, suggesting that tens‐of‐keV electrons interact with modulated chorus waves and then are scattered down to the auroral pulsation region. During the same interval, the THEMIS‐D spacecraft detected corresponding Pc5 oscillations in the dayside outer magnetosphere (9–10 RE and ∼15 MLT). The compressional component of the magnetospheric Pc5 waves, presumably driven by an external source, exhibited a good correspondence to the daytime Pc5 auroral pulsations. The simultaneous SPA–THEMIS observations highlight the role of compressional Pc5 pulsations in the dayside outer magnetosphere in determining the periodicity of daytime high‐latitude diffuse auroral pulsations. Key Points: Daytime Pc5 auroral pulsations occurred concurrently with Pc5 CNA and VLF pulsations at South Pole, Antarctica2‐D signature of the auroral pulsations represented poleward‐moving periodically reforming diffuse auroral patches, not discrete arcsCompressional Pc5 pulsations in the dayside outer magnetosphere act as an oscillatory source of the daytime Pc5 diffuse auroral pulsations [ABSTRACT FROM AUTHOR]

Enhanced precipitation of magnetospheric energetic electrons during substorms increases ionospheric electron density and conductance. Such enhancements, which have timescales of a few hours, are not reproduced by the existing ionospheric models. We use the linear prediction filter (LPF) method to reconstruct the substorm‐related response of electron densities and integral conductances from long‐term ionospheric observations made by the European Incoherent SCATer radar located at Tromsø. To characterize the intensity of substorm dipolarization at a 5 min time step, we use the midlatitude positive bay index. We build response functions (LP filters) as a function of substorm time between T0−1 h and T0 + 4 h (T0 is a substorm onset time) in different magnetic local time (MLT) sectors to estimate the magnitude and delays of the ionospheric density response at different altitudes. Systematic and large relative changes are mostly observed in the D‐ and E regions. The duration of the response is about 3 h. It starts and reaches maximum magnitude near midnight, propagating from there toward the east and decaying after passing into the noon‐evening sector. The reliability of LPF results is confirmed by the consistency of D‐region response with independently derived response of the auroral absorption. Whereas strong ionization increases are seen in both E‐ and D‐regions on the nightside, the D‐region response is stronger in the morning‐dayside sector. Such MLT variation corresponds to the drift motion and precipitation of the high‐energy electrons injected in the nightside magnetosphere during substorm dipolarization. The inferred ionization changes result in strong enhancements of integral Hall (and Pedersen) conductance in the nightside auroral zone, where intense auroral currents are known to occur during substorms Key Points: Substorm‐related relative increase of electron density is small in F‐ and large in D and E regions, it mainly increases Hall conductanceThe electron density and conductance response starts near midnight at substorm onset, propagates eastward and decays after passing noonStrongest D‐region response in the pre‐noon MLT sector suggesting hard electron precipitation is observed [ABSTRACT FROM AUTHOR]

Throat aurora is believed to be associated with magnetopause indentations and has direct implications on magnetopause reconnection. In this study, for the first time, we use Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager observations over ∼14 years to characterize the throat aurora occurrence, latitudinal extent, seasonal, and its solar cycle dependence. We identified 386 throat aurora cases during the different passes of the satellite over the northern hemisphere. The latitudinal extent of these throat aurorae are estimated and are divided into small, medium, and large categories. The small and medium latitudinal extent throat aurorae account for about 91% of the total cases. The throat aurorae are found to occur most frequently in the post‐noon hours. The throat aurorae were also observed more frequently during winter, likely due to summer‐winter asymmetry in the ionospheric conductance. The occurrence of the throat aurora is also found to be anti‐correlated with the solar cycle, likely because low solar activity gives good chances for sporadic reconnection that favors the occurrence of throat aurora. The dependence on interplanetary magnetic field reveals positive Bx favoring the occurrence of throat aurora. It also shows positive Bx (negative By) supports pre‐noon (post‐noon) occurrence. The difference in the trend and average values of background interplanetary magnetic field conditions with the throat aurora confirms the favorable conditions for the occurrence of throat aurora. Key Points: Statistical analysis of ∼14 years long Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager observation shows the occurrence of throat aurora is dominant in post‐noonThe occurrence of throat aurora show anti‐correlation with sunspot number/solar cycleThe changes in the interplanetary magnetic field in comparison with background facilitates throat aurora occurrence [ABSTRACT FROM AUTHOR]

• New polar caps emerge at lower latitudes with axial non-dipolar magnetic fields. • Axial non-dipolar magnetic fields have more polar caps and auroral zones but may cover smaller areas. • Total area of auroral zones is larger than the area of polar caps with axial degrees larger than 3. • Latitudes of auroral zones and polar caps adjacent to the geographical poles increase with degree. The geomagnetic field, modified by the solar wind, determines the shape, area and location of polar caps and auroral zones, among other magnetosphere and upper atmosphere characteristics. Since the field varies greatly with time it is of interest to analyze polar caps and auroral zones variations linked to magnetic field variations of intensity and pattern. Polar caps and auroral zones locations and areas for various single harmonic axial field configurations are obtained analytically assuming a simple magnetopause model. As the axial degree n increases, the polar caps and auroral zones total number, given by n + 1 and 2n respectively, also increase. However, their total areas decrease from a larger value in the case of an axial dipole to a minimum for an axial octupole (n = 3), and then increase for increasing degrees. The increasing rate is much higher in the auroral zones case to the point that it exceeds the dipolar value at n = 5 while in the polar caps case this occurs at n = 8. The absolute latitudes of the auroral zones and polar caps that reside around the geographical poles increase with axial degree. Our results represent an end-member case of the evolution of auroral zones and polar caps during polarity reversals if the transition involves axial dipole energy cascade to higher axial degrees. Evidence for such an energy transfer is found in the historical record of the geomagnetic secular variation. [ABSTRACT FROM AUTHOR]

Altitudinal source regions and generation mechanisms of ionospheric scintillation are investigated by employing the incoherent scatter radar (ISR) at Poker Flat, Alaska (PFISR) and nearby global positioning system (GPS) receivers. Analysis of PFISR and GPS observations during the September 2017 geomagnetic disturbances reveals that phase scintillation is seen during auroral disturbances, and can occur at any storm phase. Statistical comparisons of the rate of change of the total electron content (TEC) index (ROTI) with the amplitude (S4) and phase (σϕ) scintillation in July‐December 2017 show a significant increase with geomagnetic activity and considerable correlations between ROTI and σϕ, whereas S4 showed no noticeable increase or correlations. The PFISR‐GPS comparisons show that scintillation occurs when the electron density is enhanced in the auroral E‐region and is stronger for larger E‐region peak density and/or lower E‐region peak altitude. This implies that scintillation is stronger for more intense and/or deeper auroral particle precipitation. A detailed analysis of an event with a well‐defined and propagating E‐region density enhancement imaged by PFISR reveals that scintillation is stronger on the trailing edge of the density enhancement. This feature is discussed in the context of the gradient‐drift instability as a potential mechanism for generation of plasma density waves that scatter GPS radio signals and lead to scintillation. The results of this study add to the growing body of evidence that ROTI can be used as a proxy for phase scintillation and that the ionospheric E‐region is an important source region for ionospheric scintillation at auroral latitudes. Key Points: Phase scintillation is stronger for deeper and/or more intense auroral particle precipitation into the ionospheric E‐regionThe ionospheric E‐region is a key source region for phase scintillation at auroral latitudesScintillation occurs on the trailing edge of an E‐region density enhancement, suggesting that E‐region plasma instability is a cause [ABSTRACT FROM AUTHOR]

This publication addresses the collisional superthermal electron dynamics below the auroral acceleration region (AAR). This region is the portion of an auroral field line with a field‐aligned electric field that leads to the formation of precipitating monoenergetic keV electron fluxes that produce the discrete auroral displays observable from the ground. It is assumed that these precipitating electron fluxes are monoenergetic and accelerated through a potential drop, V, such that these electrons are peaked at an energy E0 = eV, where e is the electron charge. Monoenergetic electrons precipitating into the upper atmosphere degrade to lower energies via many different collisional processes and produce the secondary electron population with energies of 10–100s eV which escapes back to magnetospheric altitudes and becomes geomagnetically trapped between the AAR and the upper ionosphere. The secondary electrons in this geomagnetic trap transfer energy via elastic Coulomb collisions to the thermal electrons. That energy is then returned to the topside ionosphere as heat flux carried by the electron thermal conduction which is essential to maintaining the topside electron temperature. Key Points: Superthermal electron (SE) dynamics in the geomagnetic trap below the AAR is consideredHow SE distribution function evolve in response to the existence of geomagnetic trap is discussedThe SE energy interplay in the geomagnetic trap and the formation downward energy fluxes are considered [ABSTRACT FROM AUTHOR]

The Feature Tracking of Aurora (FTA) model was constructed using 1.5 years of Polar Ultraviolet Imager data and is based on tracking a cumulative energy grid in 96 magnetic local time (MLT) sectors. The equatorward boundary, poleward boundary, and 19 cumulative energy bins are tracked with the energy flux and the latitudinal position. With AE increasing, the equatorward boundary moves to lower latitudes everywhere, while the poleward boundary moves poleward in the 2300-0300 MLT region and equatorward in other MLT sectors. This results in the aurora getting wider on the nightside and becoming narrower on the dayside. The peak intensity of the aurora in each MLT sector is almost linearly related to AE, with the global peak moving from pre-midnight to post-midnight as geomagnetic activity increases. Ratios between the Lyman-Birge-Hopfield-long and -short models allow the average energy to be calculated. Predictions from the FTA and two other auroral models were compared to the measurements by the Defense Meteorological Satellite Program Special Sensor Ultraviolet Spectrographic Imagers (SSUSI) on March 17, 2013. Among the three models, the FTA model specified the most confined patterns with the highest energy flux, agreeing with the spatial and temporal evolution of SSUSI measurements better and predicted auroral power (AP) better during higher activity levels (SSUSI AP > 20 GW). The Fuller-Rowell and Evans (1987) and FTA models specified very similar average energy compared with SSUSI measurements, doing slightly better by ~1 keV than the OVATION Prime model. [ABSTRACT FROM AUTHOR]

O2 aurora is one kind of important molecular aurorae that is not fully understood yet. It is hard to be investigated due to the contamination by nightglow. In this work, we studied O2 aurora based on the observations of O2 emission at 1.27 μm from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument during the nighttime over 18 years. The horizontal structure of O2 auroral zone is given for the first time, and the vertical profile of O2 auroral volume emission rate is obtained after removing O2 nightglow contamination. The O2 auroral intensity and the peak height and peak volume emission rate of the vertical profile averaged over the O2 auroral zone are studied in detail. For Kp Levels 1–5, the O2 auroral peak height varies between 104 and 112 km and is negatively correlated with the auroral intensity and peak volume emission rate. The O2 auroral intensity varies between 0.14 and 5.97 kR, and the peak volume emission rate varies between 0.97 × 102 and 41.01 × 102 photons cm−3 s−1. Furthermore, the dependences of the O2 aurora on geomagnetic and solar activities are analyzed. The O2 auroral intensity and peak volume emission rate exponentially increases with increasing Kp index, whereas the peak height decreases with increasing Kp index. The O2 auroral intensity and peak volume emission rate under solar minimum condition are larger than those under solar maximum condition. The peak height under solar minimum condition is lower than that under solar maximum condition. Plain Language Summary: Aurora is one of the most magnificent phenomena in the Earth's atmosphere. The types of aurora are abundant. Among a wide variety of aurorae, O2 aurora is one kind of important molecular aurorae. However, we still do not understand it clearly. It is hard to study O2 aurora using observations from ground and by nadir sounding due to the strong interference by O2 airglow. The limb sounding by satellite can provide a global distribution of the profiles of atmospheric emissions. This is beneficial for separating O2 aurora from O2 airglow and investigating vertical distribution of O2 aurora. We study O2 aurora based on the excellent limb sounding of O2 emission at 1.27 μm by the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) satellite. O2 aurora is derived after removing the interference from nightglow, and statistical structures of O2 auroral zone at different geomagnetic activity levels are displayed for the first time. We find that the structures of the O2 aurora change with geomagnetic and solar activities. The results provide further understanding of the horizontal and vertical structures of O2 aurora. The results are helpful for the verification of the excitation mechanisms of O2 aurora and the development and validation of O2 auroral models. Key Points: Statistical structures of nighttime O2 auroral zone are given for the first time by using TIMED/SABER limb observations over 18 yearsPeak heights of vertical profiles of O2 auroral emission are between 104 and 112 km for Kp Levels 0–5O2 auroral peak emission rate (peak height) increases (decreases) with geomagnetic activity and decreases (increases) with solar activity [ABSTRACT FROM AUTHOR]

Increased ionization in the auroral oval leads to the absorption of high‐frequency radio waves in the auroral zone, or auroral absorption. Auroral absorption is typically characterized by global geomagnetic activity indices, such as the Kp index. In this paper the hourly range of the magnetic field (HR) is examined as an alternative to the 3‐hr Kp index for describing the dynamic and localized features of auroral absorption represented by the hourly range of absorption (HRA). Kp, magnetometer, and riometer data were examined for a 3‐year period for stations spread across typical auroral latitudes. A general linear relationship was shown to exist between Kp and LOG10(HRA) for Kp < 4; for Kp ≥ 4 the correlation was weaker. A stronger linear correlation was demonstrated between LOG10(HRA) and LOG10(HR) for HR > 50 nT, characterized by a correlation coefficient of R = 0.63. Increased variability in the relationship between HRA and Kp was attributed to the following factors: the variability of the magnetic field within the 3‐hr window characterized by the Kp index, which was better represented by a 1‐hr HR; the dependence of the Kp index on subauroral magnetic data, which is not subject to the geomagnetic variations typically experienced within the auroral region; and reduced statistics for Kp > 6. Key Point: Hourly range of the magnetic field is stronger than Kp for characterizing auroral absorption [ABSTRACT FROM AUTHOR]

The onset of substorms is associated with bursty enhancements of Alfvén wave power throughout the magnetotail. While impossible to assess the total Alfvén wave power in the entire magnetotail, we have instead monitored waves that are funneled into the auroral acceleration region, in order to assess the temporal evolution of Alfvén wave power above the nightside auroral zone in relation to substorm phases. The substorms were grouped by three conditions: nonstorm periods, storm periods, and all (unconditioned) periods. Using superposed epoch analysis, we found that the global magnetohydrodynamic Alfvén wave power increased significantly at onset for all three conditions, while a power decrease to preonset values occurred within 2 hr. Specifically, the peak inflowing power during the expansion phase was 5.7 GW for unconditioned substorms, 5.6 GW for nonstorm‐time substorms, and 7.8 GW for storm‐time substorms. These results correspond to power increases with respect to preonset values of 138%, 366%, and 200%, respectively. Additional analysis in relation to the aurora was performed for nonstorm‐time substorms only. During the expansion phase, about 50% of the Alfvén wave power over the entire nightside auroral zone is collocated with the auroral bulge region. Furthermore, the total inflowing Alfvén wave power over the entire nightside auroral zone is 17% of the conjugate auroral power, while the inflowing power over the auroral bulge region is 32% of the conjugate aurora. However, allowing for a finite absorption efficiency inside the auroral acceleration region, the likely average Alfvénic contributions to the aurora are approximately 10% and 18%, respectively. Key Points: We calculated the rise and fall of global Alfvén wave power above the AAR throughout the substorm cycleAlfvén wave power significantly increases after onset over the entire nightside auroral zone and especially over the auroral bulgeAlfvén wave power can account for low‐altitude Alfvénic electron power and for significant fraction of aurora during the expansion phase [ABSTRACT FROM AUTHOR]

By comparing data from 530 nightside auroral zone crossings of the three Swarm satellites with images from THEMIS ground‐based all‐sky imagers (ASIs), we find that 154 arc systems, containing two or more arcs, host regions of fluctuating magnetic and electric fields (Alfvén waves) up to 8 Hz. Many cases exhibit a standing wave pattern as seen by comparing the amplitude ratio and phase difference of electric and magnetic field Fourier components. The largest peak of Alfvénic cross‐power spectral density (CPSD, which is similar to Poynting flux) within a large‐scale current system tends to be narrow in latitude (about 50–100 km) and its magnitude increases with an increasing number of current sheets in the field‐aligned current (FAC) system. The peak of CPSD tends to be located on the poleward side of the current system but can extend as equatorward as the boundary between the Region 1 and Region 2 FACs. CPSD tends to peak either within downward current sheets (40% of cases) or on the boundary between downward and upward FACs (37%). Only 23% of peaks occur within upward current sheets, indicating that there is only a weak association with discrete auroral arcs, a finding which is confirmed by comparison with ASI images. These findings demonstrate that Alfvénic fluctuations occur over a much wider range of locations and conditions than previously reported. Key Points: Roughly one third of nightside auroral zone crossings reveal regions of strong Alfvén wave activity up to 8 HzAlfvénic regions can occur in upward and downward FACs and at the boundary between themAlfvénic regions occur over a much wider range of locations and conditions than previously reported [ABSTRACT FROM AUTHOR]

High‐latitude ionospheric plasma flows often contain embedded localized velocity enhancements with associated plasma density structures. Localized strong flows are significant energy sources to the thermosphere that are not contained in present empirical models of plasma convection. In this study, Global Ionosphere‐Thermosphere model, a nonhydrostatic model with flexible resolution, has been utilized to evaluate the influence of such localized strong enhancements of forcing on the global dynamics of the upper atmosphere. Specifically, three idealized configurations of flow burst have been examined, one flow burst with the typical spatial scale seen in the auroral zone, two neighboring flow bursts with the same spatial scale, and one flow burst with doubled spatial scale. The comparison between the cases with and without the flow burst illustrates that one regular‐sized flow burst at midnight can cause the neutral density to increase by 5% and the horizontal wind to increase by ∼30 m/s at 300 km altitude. The associated enhancement in the electron precipitation can increase total electron content by 15%(1 TECU) during the first 15 min. Two simultaneous flow bursts can cause a rich wave structure with multiple wavelengths in traveling atmospheric disturbances due to wave‐wave interactions. A flow burst with doubled spatial size can increase the impact on the ionosphere/thermosphere by 2 times or even more. The outcome of this study illustrates substantial local and nonlocal responses of the ionosphere‐thermosphere system to the mesoscale energy and momentum inputs from the magnetosphere. Key Points: One regular‐sized flow burst at midnight can cause 5% neutral density increase and 30 m/s horizontal wind increase at 300 km altitudeTwo simultaneous flow bursts can cause a rich wave structure in traveling atmospheric disturbances (TADs)A flow burst with larger spatial size can very efficiently increase the impact on the ionosphere/thermosphere [ABSTRACT FROM AUTHOR]

The present study investigates the dependence of the local auroral electrojet (AEJ) intensity on solar illumination by statistically examining northward geomagnetic disturbances in the auroral zone in terms of the solar zenith angle χ. It is found that on the dayside, both westward and eastward electrojets (WEJ and EEJ) are more intense for smaller χ, suggesting that the solar extreme ultraviolet‐induced conductance is the dominant factor for the AEJ intensity. On the nightside, in contrast, the χ dependence of the AEJ intensity, if sorted solely by the magnetic local time, apparently depends on the station longitude and hemisphere. However, if additionally sorted by the dipole tilt angle ψ, a consistent pattern emerges. That is, although χ and ψ are correlated, the solar zenith angle and dipole tilt angle have physically different effects on the AEJ intensity. The nightside AEJ, especially the WEJ, tends to be more intense for smaller |ψ|. Moreover, whereas the WEJ is statistically more intense when the ionosphere is dark, the EEJ is more intense when it is sunlit. The preference of the WEJ for the dark ionosphere prevails widely in magnetic local time from premidnight to dawn, and therefore, it cannot be attributed to the previously proposed processes of the preferred monoenergetic or broadband auroral precipitation in the dark ionosphere. Instead, it may be explained, at least morphologically, in terms of the conductance enhancement due to the diffuse auroral precipitation, which is also prevalent from premidnight to dawn and is more intense in the dark hemisphere. Key Points: The nightside westward electrojet (WEJ) is more intense when the ionosphere is dark, and the eastward electrojet (EEJ) when it is sunlitBoth WEJ and EEJ are more intense when the dipole tilt is smaller, and this effect is comparable to the effect of solar illuminationThe preference of the WEJ intensity for the dark ionosphere likely reflects that of diffuse e‐ precipitation and ionospheric conductance [ABSTRACT FROM AUTHOR]

Energetic particle precipitation (EPP) increases the production of odd hydrogen (HOX) species in the mesosphere, which catalytically destroy ozone (O3) in sunlight. Hence, the EPP‐HOX impact on the tertiary O3 maximum (TOM) depends on a complex geometry of a geographic‐oriented TOM, geomagnetic‐oriented auroral zone, producing short‐lived HOX species, and a destruction process depending on the solar zenith angle (SZA). Particle observations from the Medium Energy Proton and Electron Detectors telescopes aboard the Polar Orbiting Environmental Satellites, and hydroxyl (OH) and O3 mixing ratios from Aura microwave limb sounder (MLS) are used to investigate the potential limitations of using the MLS observations to study EPP‐OH impact on the TOM in the Northern Hemisphere. Our results show limited overlap between the auroral zone and the TOM at twilight conditions. A composite analysis indicates O3 mixing ratio decrease over the auroral zone lagged by ∼1 day compared to the maximum energetic electron precipitation (EEP)‐OH impact. Hence, MLS is predominantly observing a lagged and lower estimate of the response of O3 to EEP‐OH at SZA > 95°. The EEP impact region within the TOM is smaller than the overlap region, strongly modulated by the background atmospheric dynamics. The results, although limited by the satellites viewing conditions, imply that the importance of EEP upon O3 mixing ratio is strongly influenced by the background atmosphere, both in terms of chemistry and dynamics. Multisatellite observations at different solar local times are required to separate the direct from the lagged EEP‐OH impact on O3. Key Points: MLS provides a unique opportunity to study the direct impact of energetic particle precipitation on mesospheric hydroxyl (OH) and ozone (O3)There is limited overlap between the auroral zone and the tertiary O3 maximum at twilight conditionsThe importance of energetic electron precipitation on the tertiary O3 maximum is strongly governed by the background atmosphere [ABSTRACT FROM AUTHOR]

This study presents observations of the two‐dimensional velocities and field‐aligned currents of magnetospheric field line resonances. Knowledge of the two‐dimensional plasma flows is essential for determining the field‐aligned current structure of the field line resonance and for understanding the role that field line resonances play in the generation of some auroral arc forms. In this work the full velocity vectors are determined from a high‐resolution fit to the Super Dual Auroral Radar Network (SuperDARN) line‐of‐sight data. Two discrete field line resonance events are presented, one at 4.0 mHz (4‐ to 5‐min period) and the other at 0.6 mHz (30‐min period). Band‐pass filtering the north‐south and east‐west components of the total convection velocity vectors reveals the two‐dimensional velocity field specific to an individual field line resonance. The field line resonance velocities exhibit periodic vorticity, which correspond to periodic poleward moving bands of enhanced field‐aligned current. Assuming a constant ionospheric Pedersen conductance of 10 S, peak field‐aligned current values are estimated to be 2–4 μA/m2. The field‐aligned currents alternate between upward and downward directed and peak in magnitude ~0.5–1° poleward and/or equatorward of the peak in field line resonance flow velocity. A Swarm satellite overpass occurs directly over the 0.6‐mHz field line resonance. The SuperDARN field‐aligned current profile along the Swarm trajectory is found to be similar in magnitude and structure to that determined from Swarm‐A and Swarm‐C in situ measurements. Key Points: Two‐dimensional velocities of a field line resonance are determined from a high‐resolution fit to SuperDARN line‐of‐sight observationsField line resonance flows exhibit periodic variations in vorticity, which generate periodic poleward propagating field‐aligned currentsEstimates of field‐aligned currents generated by the field line resonance are consistent with Swarm measurements of field‐aligned current [ABSTRACT FROM AUTHOR]

We have used all‐sky imaging to relate different types of auroral oval disturbances to large‐scale traveling ionospheric disturbances (LSTIDs). We selected eight nights with good all‐sky imaging and Global Positioning System total electron content coverage, including five non–storm time periods with isolated initiations of geomagnetic activity and three storm main phase periods with continuous activity. Periods with LSTIDs generally started and stopped with initiation and cessation of activity. We found evidence that individual LSTIDs often show 1‐1 correspondence with identifiable auroral disturbances, disturbances either being related to a substorm onset or to auroral streamers without a substorm. Since substorm ground magnetic depressions are directly related to the electric fields and electron precipitation of auroral streamers, we hypothesize that streamers may be the primary drivers of individual nightside LSTIDs with or without a substorm. Additionally, we found evidence that (1) LSTIDs detection is more likely near the longitude range of the initiating disturbance than further away, (2) the orientation of LSTID phase fronts depends on location relative to disturbance longitude, and (3) disturbance ionospheric current and magnetic latitude may influence whether a given disturbance leads to a detectable LSTID. Numerous LSTIDs (10 to 12 over 7‐ to 8‐hr periods) were detected during southward interplanetary magnetic field periods of coronal mass ejection storm main phases, the vast majority reflecting streamers in the absence of substorms. Less LSTIDs were seen during the one examined high‐speed‐stream storm. We have also found evidence that omega band disturbances may drive interesting TIDs that are distinct from the LSTIDs driven by the substorm and streamer disturbances. Key Points: All‐sky imaging was used to relate auroral oval disturbances to large‐scale traveling ionospheric disturbances (LSTIDs)Streamers may be the primary drivers of individual nightside LSTIDs, with or without a substormPossible LSTID longitude, orientation, and strength connections suggested [ABSTRACT FROM AUTHOR]

Jupiter's auroral X‐rays are rather mysterious, with an unknown driver, and several previous reports of individual cases of quasi‐periodic emission. In this work we revisit heritage X‐ray data sets from the 1990s to 2015 and apply robust significance testing of emerging quasi‐periodicities, seeking to understand the robustness and regularity of previously reported quasi‐periodic emissions. Our analysis incorporates the use of the Rayleigh test as an alternative to Lomb‐Scargle analysis or Fast Fourier Transforms, where Rayleigh is particularly suited to a time‐tagged data set of sparse counts such as is common for jovian X‐ray data. Furthermore, the analysis techniques that we present (including Rayleigh testing and Monte Carlo simulation) can be applied to any time‐tagged data set. The code to conduct such analysis is released as supplementary information to accompany this paper. The five most significant (p value <0.01) quasi‐periods from Jupiter's northern auroral region have periods ranging from ~8.0 to 45.96 min, and the two most significant (p value <0.01) quasi‐periods from the south have periods of ~14.1 and ~34.9 min. The selection of a restrictive hot spot source region seems to be critical for detecting quasi‐periodic emission, suggesting that the site of pulsations may be spatially localized. Periods vary from one Jupiter rotation to the next in one long observation, and the north and south are shown to pulse independently in another conjugate observation. These results have important implications for understanding the driver of jovian X‐ray emission. Key Points: Timing analysis of Chandra observations of Jupiter's X‐ray auroras from 1999 to 2015Statistically significant quasi‐periodicities in jovian auroral X‐rays are relatively rare and periods variable, even on successive planetary rotationsWe use Rayleigh testing and Monte Carlo simulation to search for statistically significant quasi‐periods in sparse, time‐tagged data [ABSTRACT FROM AUTHOR]

Abstract Based on 20 years-long data base of EISCAT incoherent scatter radar and IMAGE magnetometer observations in Scandinavia, we investigate statistically the ionospheric conductance variations in the dark nightside auroral zone. We focus on the relationship of precipitation-caused conductances with the variations of local equivalent current and global AL index, as well as on their dependence on the solar wind (SW) parameters. In terms of paired correlation, the main SW drivers for AL index and for the Pedersen and Hall conductances (Σ P and Σ H) are the SW merging electric field (characterized, e.g., with the Kan-Lee function, E k l ) and the solar wind velocity V s w . The relative importance of these SW drivers varies. Whereas E k l is the main driver of AL index, the role of V s w increases for the conductances so that it outruns the E k l as the main driver for the Hall conductance. Quantitatively this dependence is represented as Σ H = (7.7 * V + 1.75 * V 2) + (5.7 * E − 0.86 * E 2) − 6.1 Siemens, where E and V are E k l and V s w normalized with < E k l > = 0.79 mV/m and < V s w > = 429 km/s. The strongest influence of V s w is, however, observed for the Hall-to-Pedersen conductance ratio R H P = Σ H / Σ P , indicating solar wind velocity control of the electron acceleration. Physically the energization is a major factor which contributes to the large conductance values. On the nightside, local equivalent currents are significantly controlled by the local Hall conductance (CC = 0.78) and most of the equivalent current increase during active periods is due to the conductivity change. In that sense the AL index variations during active times are controlled by the Hall conductance variations which, to a large extent, are controlled by the processes of magnetospheric electron acceleration. Highlights • Statistical analyses of large data base of EISCAT conductances and equivalent currents in dark nightside ionosphere. • Hall conductance correlates better with SW velocity than with SW electric field. • Hall to Pedersen conductance ratio (indicating e-acceleration) primarily depends on V sw • Conductance changes control the variations of equivalent currents during active periods. [ABSTRACT FROM AUTHOR]

Nishimura et al. (2010) proposed a new plasma intrusion or preonset aurora scenario of substorm triggering. In this scenario, a substorm is triggered by a fast earthward flow generated at the distant neutral line which corresponds to a preonset auroral streamer or arc in the ionosphere propagating from the auroral poleward boundary to the initial auroral brightening site, i.e., "preonset aurora". In the present paper, we revisited three substorm events reported as being triggered by such a mechanism related to preonset auroras, based on THEMIS ground-based all-sky imager data. Unlike previous studies, we examined the arrival timing of the preonset aurora relative to the three steps of auroral onset arc development (initial brightening, enhancement of the wavelike structure, and poleward expansion) to make the role of the preonset aurora in the auroral steps clearer. Our detailed timing analysis found that preonset auroral streamers reached the auroral onset arc but away from the initial brightening site after initial brightening for two events, while no preonset aurora reaching the initial brightening site could be identified for the other event. This result suggests that the processes associated with auroral streamers are unlikely to affect at least initial brightening, even if we consider not only the presence and arrival timing and location of the auroral streamers but also the scale of the corresponding flow and flow vortices. We list a series of open questions for testing the preonset aurora scenario further in future studies. [ABSTRACT FROM AUTHOR]

The relationship between the magnetospheric field-aligned currents (FAC) monitored by the Swarm satellites and the magnetic activity PC index (which is a proxy of the solar wind energy incoming into the magnetosphere) is examined. It is shown that current intensities measured in the R1 and R2 FAC layers at the poleward and equtorward boundaries of the auroral oval are well correlated, the R2 currents being evidently secondary in relation to R1 currents and correlation in the dawn and dusk oval sectors being better than in the noon and night sectors. There is evident relationship between the PC index and the intensity of field-aligned currents in the R1 dawn and dusk layers: increase of FAC intensity in the course of substorm development is accompanied by increasing the PC index values. Correlation between PC and FAC intensities in the R2 dawn and dusk layers is also observed, but it is much weaker. No correlation is observed between PC and field-aligned currents in the midnight as well as in the noon sectors ahead of the substorm expansion phase. The results are indicative of the R1 field-aligned currents as a driver of the polar cap magnetic activity (PC index) and currents in the R2 layer. [ABSTRACT FROM AUTHOR]

Analysis of the annual variation of the E-layer critical frequency median foE in the nighttime (22−02 LT) auroral zone by the data of several stations of the Northern Hemisphere has shown the median maximum in winter and minimum in summer, even though the summer contribution of solar radiation to foE is greater. Thus, a new phenomenon was discovered-an foE median winter anomaly in the nighttime auroral zone. Its amplitude (ratio of winter to summer foE figures) can reach 10-15%; however, this anomaly was weakly expressed and statistically insignificant at particular stations located in the auroral zone. The winter anomaly is more distinct for foE , the median of the E-layer critical frequency foE caused by the auroral source of atmospheric ionization, i.e., excluding the solar radiation contribution to foE. For foE , the amplitude of the winter anomaly can reach 15-20%. Based on the qualitative analysis, it has been found that foE winter anomaly is stipulated by the winter/summer asymmetry of energy flow of accelerated electrons, which induce discrete aurorae in the nighttime auroral zone. [ABSTRACT FROM AUTHOR]

The auroral event is defined as a period of various special aurora appearance. It is a good way to understand the interaction among the solar wind plasma, the terrestrial magnetic field, and the Earth’s atmosphere, since the motion of auroral event supplies a powerful tool to discover the underlying coherence among those natural phenomena. Therefore, studies on different types of auroral events have attracted growing attentions. In this study, an auroral event representation approach is proposed for automatic aurora classification. Specifically, 3D steerable filters are used to generate space-time oriented energies, which can simultaneously capture texture and motion of the auroral event. Then, n-ary fusion operator is used to combine oriented energies together to generate a unified representation, which also makes the representation has the property of gray-scale invariance. In order to capture roughly global shape information, a block partition scheme is used to further represent the auroral event. Finally, the performance of the representation is evaluated by the auroral event classification. The superior experimental results demonstrate the effective of our proposed auroral event representation method. [ABSTRACT FROM AUTHOR]

Seasonal variation of geomagnetic field around auroral zone is analyzed in terms of geomagnetic latitude, magnetic local time (MLT) and geomagnetic condition in this study. The study uses horizontal component (H) of geomagnetic field obtained from 6 observatories located in geomagnetic latitude of 57.8°N–73.8°N along 115°E longitudinal line. The results indicate that seasonal variations of geomagnetic field around auroral zone are different combinations of annual and semiannual variations at different latitudinal ranges. Both annual and semiannual variations show distinct MLT dependency: (1) At dayside auroral latitudes (around 72°N geomagnetic latitude), geomagnetic field shows distinct annual variation under both quiet and disturbed conditions. Furthermore, the annual component is mainly contributed by data of dusk sector. (2) At nightside auroral latitudes (around 65°N), geomagnetic field shows semiannual dominated seasonal variation. Under quiet conditions the annual component is comparable to the semiannual component, while under disturbed conditions, the semiannual component is twice as much as the annual component. Under quiet conditions, the semiannual component is mainly contributed by 1300–1400 MLT, while the annual component has two peaks: one is around 1100–1300 MLT and the other is around 2000–2200 MLT. Under disturbed conditions, the semiannual component is mainly contributed by data around midnight, while the annual component is mainly contributed by dusk sector. (3) At subauroral latitudes (around 60°N), annual variation is comparable to semiannual variation under both quiet and disturbed conditions. Both annual and semiannual components show similar MLT dependencies as that of nightside auroral latitudes. [ABSTRACT FROM AUTHOR]

Modulational instability analysis of Electron-acoustic (EA) rogue waves is carried out in a three-dimensional magnetized plasma holding two electron populations (nonthermal hot electrons following Cairns distribution and cold electron fluid) and stationary ions. A $(3+1)$ -dimensional nonlinear Schrödinger (NLS) equation that governs the evolution of the EA rogue waves in the current plasma system is derived through derivative expansion method. The existence domains for the first and second order rogue waves are investigated. Effects of the plasma parameters on the propagation of the first and second order rogue waves are examined analytically and numerically. Furthermore, the role of the different plasma parameters as well as the critical threshold of the obliqueness $\theta $ on the first and second order rogue waves for parameters regimes corresponding to dayside auroral zone plasma are emphasized. [ABSTRACT FROM AUTHOR]

A link between solar wind magnetic sector boundary (heliospheric current sheet) crossings by the Earth and the upper-level tropospheric vorticity was discovered in the 1970s. These results have been later confirmed but the proposed mechanisms remain controversial. Extratropical-cyclone tracks obtained from two meteorological reanalysis datasets are used in superposed epoch analysis of time series of solar wind plasma parameters and green coronal emission line intensity. The time series are keyed to times of maximum growth of explosively developing extratropical cyclones in the winter season. The new statistical evidence corroborates the previously published results (Prikryl et al., 2009). This evidence shows that explosive extratropical cyclones tend to occur after arrivals of solar wind disturbances such as high-speed solar wind streams from coronal holes when large amplitude magneto-hydrodynamic waves couple to the magnetosphere-ionosphere system. These MHD waves modulate Joule heating and/or Lorentz forcing of the high-latitude thermosphere generating medium-scale atmospheric gravity waves that propagate energy upward and downward from auroral zone through the atmosphere. At the tropospheric level, in spite of significantly reduced amplitudes, these gravity waves can provide a lift of unstable air to release the moist symmetric instability thus initiating slantwise convection and forming cloud/precipitation bands. The release of latent heat is known to provide energy for rapid development and intensification of extratropical cyclones. [ABSTRACT FROM AUTHOR]

A comprehensive study of the ionospheric processes encountered during the superstorm which started on March 17th 2015 has been carried out using magnetometer, ionosonde, riometer, ionospheric tomography and an all-sky camera installed in the observatory of Sodankylä, Finland. The storm manifested a number of interesting features. From 12:00 on March 17 there was a significant decrease of critical frequencies foF2 and intensive sporadic Es layers were observed. During the disturbance, there was a lack of variation of the X-component of the magnetic field at times, but the absorption level measured by the riometer was high. A comparison of the electron density distributions for the quiet and disturbed days as shown in the tomography data were very different. Where results were available at the same times, the tomographic foF2 values coincided with the “real” foF2 values from the ionosonde. Where the ionosonde data was missing due to absorption, the tomographic foF2 values were used instead. The keograms from the all-sky camera showed that during disturbed days the aurorae manifested themselves as bright discrete forms. It was shown that the peaks of absorption due to particle precipitation seen by the riometer coincided in time with the brightenings of aurorae seen on the keograms. [ABSTRACT FROM AUTHOR]

The region-1 and region-2 boundary has traditionally been identified using data from a single spacecraft crossing the auroral region and measuring the large scale changes in the cross track magnetic field. With data from the AUTUMN, CANMOS, CARISMA, GIMA, DTU MGS, MACCS, McMAC, STEP, THEMIS, and USGS ground magnetometer arrays we applied a state-of-art technique based on spherical elementary current system (SECS) method developed by Amm and Viljanen (1999) in order to calculate maps of region-1 and region-2 current system over the North American and Greenland auroral region. Spherical elementary current (SEC) amplitude (proxy for vertical currents) maps can be inferred at 10 s temporal resolution, ~1.5° geographic latitude (Glat), and 3.5° geographic longitude (Glon) spatial resolution. We compare the location of the region-1 and region-2 boundary obtained by the DMSP spacecraft with the region-1 and region-2 boundary observed in the SEC current amplitudes. We find that the boundaries typically agree within 0.2°±1.3°. These results indicate that the location of the region-1 and region-2 boundary can reasonably be determined from ground magnetometer data. The SECS maps represent a value-added product from the magnetometer database and can be used for contextual interpretation in conjunction with other missions as well as help with our understanding of magnetosphere-ionosphere coupling mechanisms using the ground arrays and the magnetospheric spacecraft data. [ABSTRACT FROM AUTHOR]

The auroral oval concept radically changed the view that existed for a century in geophysics on the patterns in aurora planetary spatial-temporal distributions. The auroral zone, which is located around the geomagnetic pole as a continuous ring at a constant angular distance of ~23°, was replaced by the auroral oval in 1960. The auroral oval spatial position reflects the shape of the Earth's magnetosphere, which is compressed by the solar wind on the dayside and stretches into the magnetotail on the nightside. The oval is fixed relative to the direction toward the Sun and is located around the geomagnetic pole at altitudes of the upper atmosphere at an angular distance of ~12° at noon and ~23° at midnight. After an animated discussion over several subsequent years, the existence of the auroral oval was accepted by the scientific community as a paradigm of a new science, i.e., solar-terrestrial physics. The oval location indicates the zone where electron fluxes with energies varying from ~100 eV to ~20 keV precipitate into the upper atmosphere and is related to the structure of plasma domains in the Earth's magnetosphere. The paper describes the scientific studies that resulted in the concept of the auroral oval existence. It has been shown how this concept was subsequently justified in the publications by Y.I. Feldstein and O.B. Khorosheva. The issue of the priority of the auroral oval concept introduction into geophysics has been considered. The statement that the concept of the oval is an archaic paradigm of solar-terrestrial physics has been called into question. Some scientific fields in which the term auroral oval or simply oval was and is the paradigm have been listed. [ABSTRACT FROM AUTHOR]

Many independent observations have shown that resonant field line Pc5 pulsations (typical periods of about several minutes) are preferably excited inside the auroral oval. Consideration of the most evident interpretation schemes shows that there is no simple explanation of this effect. Here we consider theoretically the generation of toroidal Pc5 Alfvén waves by fluctuating magnetospheric field-aligned currents. It is shown that the Alfvén wave latitudinal structure has the same features as in the case of the classical field line resonance driven by external sources: a peak localized in latitude and a 180° phase shift across the resonant magnetic shell. This model suggests an additional mechanism of ultra-low-frequency (ULF) wave excitation which can operate at auroral latitudes. [ABSTRACT FROM AUTHOR]

Estimates of the past location of the auroral zone can aid archaeologists in the interpretation of documented light phenomena in the night sky; in particular, during periods without written records and in regions where aurorae are scarce today. Aurora occurrence largely depends on solar activity as well as the tilt and strength of the geomagnetic field. Here, we introduce a tool ( AUREST) that visually combines the time-dependent variations of these parameters to provide relative estimates of aurora occurrence in mid-latitude regions of the Northern Hemisphere over the past 10 000 years. Tests against historical aurora observations show good compatibility with AUREST. [ABSTRACT FROM AUTHOR]

Permanent variation in parameters affecting the key characteristics of the auroral ionosphere hinders the creation of a sufficiently accurate model for practical application within classical approaches describing quasi-stationary phenomena. The fractal approach for describing the properties of plasma in the auroral region has the following advantages: the results are versatile and the emergence of self-similar structures is nature-independent. Due to the self-similarity and fractality of ionospheric structures, it will suffice to have a few measurements within a characteristic scale; therefore, it is necessary to obtain a series of simultaneous measurements at intervals of tens of meters to tens of kilometers to describe the spatial and temporal distribution of inhomogeneities in the ionospheric plasma. Small and relatively low-cost satellites (the socalled cubesats) are supposed to be used to check whether the fractal approach can be applied to study the inhomogeneous structure of the ionosphere, including with artificial heating. The satellites should be located at different distances from each other to span the scales ranging from the inertial length of electrons to the inertial length of O ions. For each satellite, it is supposed to measure the variations in plasma density and electric and magnetic fields. [ABSTRACT FROM AUTHOR]

The spatial-temporal variations in aurora and VLF emissions during an weak intensification in the auroral zone morning sector on December 30, 2011, have been analyzed. The event was accompanied by a negative bay (~70 nT) in the X component of the magnetic field at ground stations in northern Scandinavia. At the recovery phase of this bay, the precipitation zone moved and VLF emission frequency simultaneously increased over ten minutes, which may indicate that waves and precipitating electrons had a common source. VLF noise bursts in the 600-1000 Hz band with a characteristic modulation scale of ~10 s and the corresponding aurora intensifications localized in the ~100 km region were observed during the following ten minutes, which also confirms that recorded waves are related to electron precipitation. This correspondence of the pulsating aurora periods and VLF noise modulation has been revealed for the first time. The role of VLF wave generation processes during the cyclotron interaction with electrons in the magnetosphere and the propagation of these waves from the magnetosphere to the observation point are discussed. [ABSTRACT FROM AUTHOR]

The National Oceanic and Atmospheric Administration provides four space weather data products targeted at satellite operators: the energetic proton flux, energetic electron flux and ambient magnetic field at the location of a Geostationary Operational Environmental Satellite spacecraft, and the global Kp index. Kp is a global blended measure of the disturbance of the geomagnetic field at mid-latitude ground stations. Elevated levels of the Kp index are associated with magnetic storms and substorms, which have been correlated with spacecraft surface charging. However, a review of the Kp index during recurring solar array circuit failures showed no evidence that they might be caused by surface charging and discharging. Kp in this case was absolutely misleading. Two other sources of data, ground station magnetograms from high-latitude observatories near the auroral arc and X-ray emissions from the atmosphere in the auroral regions, demonstrate that severe substorm activity was occurring at the affected spacecraft immediately preceding nearly every instance of circuit failure. Based on these results, new space weather data products to show the risk of surface charging in geostationary earth orbit are proposed. [ABSTRACT FROM PUBLISHER]

Nonlinear shock wave structures in collisionless unmagnetized viscous plasma comprised of fluid of cold electron and nonisothermal hot electrons obeying superthermal electron distribution and ions in stationary state are examined. For nonlinear electron acoustic shock waves, a reductive perturbation method was applied to deduce the Burger equation in terms of first order potential. When the shock wave amplitude was enlarged, the steepness and the velocity of the wave sidetrack from Burger equation. We have to resume our calculations to obtain the Burger-type equation with higher order dissipation. The collective solution for the resulting equations has been given by the renormalization method. The effects of spectral index, the ratio of the initial equilibrium density of cold electron to hot electrons β, and the kinematic viscosity coefficient on the broadband electrostatic shock noise in aurora are also argued. [ABSTRACT FROM AUTHOR]

Context. Saturn's polar upper atmosphere exhibits significant auroral activity; however, its impact on stratospheric chemistry (i.e. the production of benzene and heavier hydrocarbons) and thermal structure remains poorly documented. Aims. We aim to bring new constraints on the benzene distribution in Saturn's stratosphere, to characterize polar aerosols (their vertical distribution, composition, thermal infrared optical properties), and to quantify the aerosols' radiative impact on the thermal structure. Methods. Infrared spectra acquired by the Composite Infrared Spectrometer (CIRS) on board Cassini in limb viewing geometry are analysed to derive benzene column abundances and aerosol opacity profiles over the 3 to 0.1 mbar pressure range. The spectral dependency of the haze opacity is assessed in the ranges 680-900 and 1360-1440 cm-1. Then, a radiative climate model is used to compute equilibrium temperature profiles, with and without haze, given the haze properties derived from CIRS measurements. Results. On Saturn's auroral region (80°S), benzene is found to be slightly enhanced compared to its equatorial and mid-latitude values. This contrasts with the Moses & Greathouse (2005, J. Geophys. Res., 110, 9007) photochemical model, which predicts a benzene abundance 50 times lower at 80°S than at the equator. This advocates for the inclusion of ion-related reactions in Saturn's chemical models. The polar stratosphere is also enriched in aerosols, with spectral signatures consistent with vibration modes assigned to aromatic and aliphatic hydrocarbons, and presenting similarities with the signatures observed in Titan's stratosphere. The aerosol mass loading at 80°S is estimated to be 1-4 × 10-5 g cm-2, an order of magnitude less than on Jupiter, which is consistent with the order of magnitude weaker auroral power at Saturn. We estimate that this polar haze warms the middle stratosphere by 6 K in summer and cools the upper stratosphere by 5 K in winter. Hence, aerosols linked with auroral activity can partly account for the warm polar hood observed in Saturn's summer stratosphere. [ABSTRACT FROM AUTHOR]

Cosmic noise absorption (CNA) at high latitudes is a typical manifestation of enhanced precipitation of energetic charged particles during the course of a magnetospheric substorm. Present analysis demonstrates the energetic particles precipitate to the high latitude ionosphere during substorms, affecting upper and lower regions of the ionosphere simultaneously. Previous studies have reported that intense and short-lived CNA events associated with substorms are mostly observed in the midnight sector of the auroral oval. In the current study, we have examined such type of CNA events predominantly occurring during 0000–0600 UT (2300–0500 MLT) at an Indian Antarctic station Maitri (corrected geomagnetic (CGM) coordinates 62.59°S, 53.59°E), which is located at the equatorward edge of the auroral oval. Absorption events related to isolated substorm and storm-time substorms exhibit distinct features in terms of their intensity and extent in latitude and longitude. Our study suggests that the maximum intensity of CNAs depends on the interplanetary conditions, such as, the solar wind speed, southward component of IMF Bz , and duskward component of IEF Ey . Moreover, the role of duskward component of IEF Ey is more noteworthy than other interplanetary parameters. [ABSTRACT FROM AUTHOR]

In this paper, we analyzed the variations of TEC along a latitudinal East Euro-African chain, during the storm of April 5, 2010. We observed a large asymmetry between the two hemispheres. We detected the presence of a TID in the Northern hemisphere on April 5. The propagation time of the TID from high to low latitudes and the speed of the TID was determined. On April 5, 6 and 7, we observed a decrease of the TEC and changes of the NO + in the Northern hemisphere. This depletion is caused by the large-scale thermospheric wind disturbances due to Joule heating dissipation in the auroral zone. [ABSTRACT FROM AUTHOR]

One year of high rate dual frequency GPS phase rate variation statistics at three Canadian auroral locations are compared with geomagnetic field variations observed at co-located magnetic observatories. The geomagnetic variations are characterized at the Canadian Space Weather Forecast Centre by the hourly range of magnetic field for northward, eastward and vertical downward directions. The hourly mean of GPS phase rate statistics are derived from 1-Hz GPS dual frequency phase measurements which are continuously being monitored at the Canadian Geodetic Survey of Natural Resources Canada. Analysis reveals correlation coefficients of about 0.7–0.8 between these two hourly indices. The highest correlation is found between hourly mean phase rate variations and hourly ranges of the magnetic field vertical component. This can be attributed to changes in the position of the auroral electrojet during rapid expansions of the auroral oval during periods of auroral substorms. Apart from allowing studies on local ionospheric irregularities, these results indicate that GPS phase rate measurements in the auroral zone are responding to the geomagnetic field variations which could be used as representative proxy measures for scintillation occurrence in the region. Analysis of GPS phase rate statistics also revealed station specific features due to non-ionospheric effects. [ABSTRACT FROM AUTHOR]

In order to study instabilities caused by inhomogeneities of the electric field and plasma density in the auroral zone, numerical algorithms are developed and numerical simulations are performed for different conditions in the background plasma. To this end, a nonlocal dispersion relation for a given type of wave is analyzed. It is shown that the dispersion relation has unstable solutions in a wide range of frequencies and wavenumbers. These solutions manifest themselves in satellite observations as a broadband spectrum of electrostatic perturbations. Two mechanisms of broadband noise generation related to the gradients of the density and electric field are compared. [ABSTRACT FROM AUTHOR]

We have explored the use of COSMIC data to provide valuable scientific information on the ionospheric impacts of energetic particle precipitation during geomagnetic storms. Ionospheric electron density in the E region, and hence ionospheric conductivity, is significantly altered by precipitating particles from the magnetosphere. This has global impacts on the thermosphere-ionosphere because of the important role of conductivity on high latitude Joule heating. Two high-speed stream (HSS) and two coronal mass ejection (CME) storms are examined with the COSMIC data. We find clear correlation between geomagnetic activity and electron density retrievals from COSMIC. At night 10 time local times, the number of profiles with maximum electron densities in the E layer (below 200 km altitude) is well correlated with geomagnetic activity. We interpret this to mean that electron density increases due to precipitation are captured by the COSMIC profiles. These "E layer dominant ionosphere" (ELDI) profiles have geomagnetic latitudes that are consistent with climatological models of the auroral location. For the two HSS storms, that occurred in May of 2011 and 2012, a strong hemispheric asymmetry is observed, with nearly all the ELDI profiles found in the southern, less sunlit, hemisphere. Stronger aurora and precipitation have been observed before in winter hemispheres, but the degree of asymmetry deserves further study. For the two CME storms, occurring in July and November of 2012, large increases in the number of ELDI profiles are found starting in the storm's main phase but continuing for several days into the recovery phase. Analysis of the COSMIC profiles was extended to all local times for the July 2012 CME storm by relaxing the ELDI criterion and instead visually inspecting all profiles above 50° magnetic latitude for signatures of precipitation in the E region. For nine days during the July 2012 period, we find a signature of precipitation occurs nearly uniformly in local time, although the magnitude of electron density increase may vary with local time. The latitudinal extent of the precipitation layers is generally consistent with auroral climatology. However, after the storm main phase on 14 July 2012, the precipitation tended to be somewhat more equator ward than predicted by the climatology (by about 5-10° latitude). We conclude that, if analyzed appropriately, high latitude COSMIC profiles have the potential to contribute to our understanding of MI coupling processes and extend and improve existing models of the auroral region. [ABSTRACT FROM AUTHOR]

Solar-terrestrial physics, like any other scientific field, has evolved and developed by replacing older theories with newer theories. Unfortunately, each generation of young researchers tends to learn naturally only the latest, and perhaps the most popular theory and believes that it is the only useful one to pursue. Therefore, they do not necessarily realize that in the past the theory they chose had struggled to reach its presently acceptable state, and that eventually it might be replaced with a new theory. Two generations of scientists or in some subjects even more generations tend to be guided by one particular idea or theory. Thus, among us (namely, one or two generations) a high degree of agreement occurs, both on the theoretical assumptions and on the problem to be solved within the framework provided by the theory. Such an idea or theory was termed paradigm by Kuhn (1970). The purpose of this article is to describe several examples of the transition of paradigms and ideas in the subjects of solar-terrestrial physics. The examples are subjects that experienced a paradigm change after prevailing in the field for a few generations and also some that are perhaps on the verge of the transition. The chosen subjects are (1) Stormer's single particle theory to Chapman's plasma theory (1907-1963), (2) the auroral zone to the auroral oval (1860-1971), (3) the closed to open magnetosphere (1931-1971), (4) the current system controversies (1918-1963) and (1964-present), (5) the fixed pattern concept to the concept of auroral/magnetospheric substorms (1935-1982), (6) the importance of the interplanetary magnetic field (IMF) in the development of geomagnetic storms (1905-1966), (7) the ring current: solar wind protons to oxygen ions from the ionosphere (1933-1977), (8) the storm-substorm controversy (1963-present), (9) substorm onset (1964-present), (10) solar flares (1958-present) and (11) sunspots (1961-present). [ABSTRACT FROM AUTHOR]