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Earth, Planets and Space, 73 (1), ISSN:1343-8832, ISSN:1880-5981

Simultaneous observations from a 630 nm all-sky airglow imager, GNSS-TEC receivers, and an ionosonde are used to investigate the role of E- and F-region coupling on the generation of medium-scale traveling ionospheric disturbances (MSTIDs). The primary observations are OI 630 nm airglow images taken by an all-sky imager in Kühlungsborn (54.07°N; 11.46°E, 53.79°N Mlat.), a site in northern Germany. Out of 226 nights of observations, MSTIDs were found only in 18 nights, demonstrating the low occurrence rate over Kühlungsborn. We focused on four MSTIDs events: two during the vernal equinox and two during summer. Coincident measurements of detrended GNSS-TEC supported the presence of MSTIDs during the selected events, and simultaneous observations from the ionosonde in Juliusruh (54.60°N, 13.4°E, 54.02°N Mlat.) showed sporadic-E (Es) layer and spread-F activity in the E- and F-region, respectively. We observed the onset of the observed MSTIDs to be around the 15°–20°E longitude and 60–45°N latitude belts. Additionally, we found that in each case, the onset of MSTIDs coincides with the presence of an Es layer with sporadic-E trace is observed (foEs) exceeding 4 MHz. This suggests that an Es layer with foEs ≥ 4MHz was a source of the generation of these MSTIDs. Altitude of the Es layer could be another important factor in generating MSTIDs. The Es layer should exist at an altitude where Hall conductivity is large, as happened in the present study.

The relation between the plasmapause (PP) and various ionospheric phenomena, such as the midlatitude ionospheric trough (MIT) has been studied for decades. More recently, it was found that the equatorward boundary of small-scale field-aligned currents (SSB) and the PP are also closely coupled. In spite of prolonged efforts many details of these relationships, as well as the mechanisms responsible for them remain poorly understood. ESA's Swarm mission in conjunction with magnetospheric missions (RBSP, Arase, and THEMIS) provides an unprecedented opportunity to study these relationships on a global scale and over an extended period. Swarm delivers observations of MIT, the associated sub-auroral electron temperature enhancement (SETE), as well as SSB, while PP crossings can be inferred from in-situ magnetospheric electron density measurements. In this study, we use 7 years of Swarm observations and PP positions from 2014 to 2017 to address some of the open questions. We confirm that MIT/SETE and PP are directly coupled, however only in the nighttime. Their correlation remains high after post-dawn, however, with an increasing, MLT-dependent time lag. Afternoon MIT observations were found conjugated with a plasmaspheric plume. The correlation between SSB and PP is also high and they intersect each other near MLT midnight. Our results confirm the scenario that the PP is formed on the night side, and propagates to the dayside by co-rotating with the Earth and suggest that the plasma is transported from the depleted ionospheric/dense plasmaspheric stagnation region also westward/sunward forming the afternoon MIT/narrow plumes, respectively.

In this study, we perform the first comprehensive comparison of ion density (Ni) in the topside ionosphere measured by the Langmuir probe (LP) and faceplate (FP) of the thermal ion imager on board Swarm satellites. Our results show a systematic difference between the LP and FP derived Ni values, and the systematic difference shows prominent dependences on solar flux, local time, and season. Although both Ni datasets show generally good linear regression with electron density (Ne) measurements from the incoherent scatter radar (ISR) located at Jicamarca, the Ni derived from LP shows an additional dependence on the solar flux, while such a dependence cannot be seen in the FP-derived Ni. We suggest that the solar flux dependence of LP-derived Ni is related to the ion compositions change at Swarm altitude, which has not been properly accounted for in the LP processing algorithm. More light ions (e.g., H+), diffusing down from the plasmasphere to the Swarm altitude, seem to cause the overestimation of Ni from LP during low solar activity. A linear relation between the Swarm LP-derived Ni and ISR Ne is derived, and such a function is recommended to be implemented into further updates of the Swarm LP plasma density data.

The equatorial electrojet (EEJ) is an important manifestation of ionospheric electrodynamics. Day-to-day changes of the EEJ result from E-region dynamo processes that are primarily driven by highly...

Ionospheric plasma irregularities can be successfully studied with the Swarm satellites. Parameters derived from the in-situ plasma measurements and from the topside ionosphere total electron content provide a comprehensive dataset for characterizing plasma structuring along the orbits of the Swarm satellites. The Ionospheric Plasma IRregularities (IPIR) data product summarizes these parameters and allows for systematic studies of ionospheric irregularities. IPIR has already been used in investigations of structuring and variability of ionospheric plasma. This report provides a detailed description of algorithms behind the IPIR data product and demonstrates its use for ionospheric studies.

The nature of the variability of the Total Electron Content (TEC) over Europe is investigated during 2009 and 2019 Northern Hemisphere (NH) SSW events in this study by using a combination of Global Navigation Satellite System (GNSS) based TEC observations and Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM) simulations. To simulate the SSW effects in TIE-GCM, the dynamical fields from the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) simulations of 2009 and 2019 SSWs are specified at the TIE-GCM lower boundary. The observed and simulated TEC are in overall good agreement and therefore the simulations are used to understand the sources of mid-latitude TEC variability during both SSWs. Through comparison of TIE-GCM simulations with and without geomagnetic forcing, we find that the TEC variability during the 2019 SSW event, was predominantly geomagnetically forced, while for the 2009 SSW, the major variability in TEC was accounted for by the changes in vertically propagating migrating semidiurnal solar (SW2) and lunar (M2) tides. By comparing the TIE-GCM simulations with and without the SW2 and M2 tides, we find that these semidiurnal tides contribute to urn:x-wiley:21699380:media:jgra56728:jgra56728-math-000120%–25% increase in the quiet background TEC.

Based on magnetic field data from the Swarm satellite constellation advanced estimates of field‐aligned and radial currents at middle and low latitudes can be derived. Detailed results have been obtained for low‐latitude (14°–35° magnetic latitude) interhemispheric field‐aligned currents related to the solar quiet (Sq) current system. The continuous data set of five years allows resolving the dependences on local time, season, and longitude. We confirm the known current flow from the southern to the northern hemisphere around June solstice. Unexpected results are obtained for the months following December. Stratospheric sudden warming events are suggested as a reason for that. These phenomena are known to amplify lunar tides and atmospheric planetary waves. Furthermore, we investigated the mean characteristic of the meridional current systems connected to the wind‐driven F region dynamo above the magnetic equator. Typical features of radially downward currents around noon and upward currents in the evening sector could be confirmed. From a detailed analysis of the connected field‐aligned current distribution we deduced that the mean altitude of the dynamo region is higher in the evening than around noon. And it appears also at greater heights in the western hemisphere than in the eastern. Special current configurations are encountered in the longitude range containing the South Atlantic Anomaly. Here summer‐like conditions are prevailing through all seasons. This infers that the ionospheric conductivity is significantly enhanced in this region of weak magnetic field strength.

The near-polar orbit satellites of Swarm mission provide a good opportunity to investigate the conjugacy of equatorial plasma irregularities (EPIs) since their trajectories at low latitudes are basically aligned with fixed geographical longitude. However, the Swarm in situ electron density occasionally shows EPIs at only one hemisphere at this longitude. In this study, we provide detailed analysis of such EPI events from the in situ electron densities and onboard global positioning system (GPS) measurements of Swarm low pair satellites, and simultaneous GPS data from two geomagnetically conjugate ground stations at the Africa longitudes. The result indicates that when Swam in situ electron density sometime shows EPIs at only one hemisphere, the GPS scintillations are still observed from the Swarm onboard receiver and by the two conjugate ground stations. It implies that the EPIs should generally elongate along the geomagnetic flux tube. More than two-year statistic results show that the onset time of scintillation in the northern station is on average 16 and 18 min earlier than that in the southern station for September equinox and December solstice in 2015, while for March equinox in 2016 the onset time of scintillation of northern station is about 11 min later than that of southern station, which indicates the asymmetry features of EPIs along the flux tube. Further analysis of nearly three-year GPS data from two conjugate stations at the Asia longitudes, we find that during solar maximum years the local sunset time plays an important role for causing the difference of onset time of scintillation between two conjugate stations.

The equatorial electrojet (EEJ) is a band of zonal electric current flowing along the magnetic equator in the dayside E-region ionosphere. The direction of the EEJ is often eastward but sometimes westward. The mechanism for generating westward EEJ is not fully understood. This study examines the relationship between the eastward/westward EEJ and equatorial neutral winds using simultaneous observations of the EEJ from the European Space Agency's Swarm satellite mission and thermospheric winds from the Michelson Interferometer for Global High-resolution Thermospheric Imaging on NASA's Ionospheric Connection Explorer mission during December 2019–January 2021. Significant differences are found in the average zonal wind profiles between times of eastward and westward EEJ. The EEJ intensity correlates negatively (R = −0.54) and positively (R = 0.58) with the eastward wind velocities at ∼110 and ∼140 km, respectively. The results suggest that the modulation of the zonal electric field by the equatorial zonal wind plays a role in producing the westward EEJ.

In the past decades researchers have revealed links between a series of sub-auroral ionospheric phenomena and the plasmapause (PP) dynamics, such as the mid-latitude ionospheric trough (MIT) and the associated sub-auroral temperature enhancement (SETE), the light-ion trough (LIT), the sub-auroral ion drift (SAID) or the more intense sub-auroral polarisation stream (SAPS), and most recently, the inner boundary of small-scale field-aligned currents (SSFACs). Most of these phenomena can be directly observed by the Swarm constellation of ESA at LEO. Thus, Swarm presents a unique opportunity to study the relations between them and also their relation to the PP dynamics.In a recent Swarm DISC project, PRISM (Plasmapause Related boundaries in the topside Ionosphere as derived from Swarm Measurements), three new products have been developed. Two products characterise the MIT (and the associated SETE). The MITx_LP utilises the Langmuir probe measurements of electron density and temperature, while the MITxTEC product derives the MIT properties from GPS TEC observations. The third product, PPIxFAC provides information on the location and the main characteristics of the equatorial boundary of SSFACs, and it also includes a proxy for the location of the PP at MLT midnight.In this presentation we introduce the above Swarm L2 products, present the results of a comparative study aiming at revealing their mutual relations and also their dynamic coupling to the PP. Then we demonstrate how the observations of all these ionospheric phenomena combined can be used to develop an improved proxy for monitoring the PP dynamics at LEO as one of the goals of our new ESA-funded project PLASMA.

The reestimates of thermospheric winds from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) accelerometer measurements were released in April 2019. In this study, we compared the new-released GOCE crosswind (cross-track wind) data with the horizontal winds measured by four Fabry-Perot interferometers (FPIs) located at low and middle latitudes. Our results show that during magnetically quiet periods the GOCE crosswind on the dusk side has typical seasonal variations with largest speed around December and lowest speed around June, which is consistent with the ground-FPI measurements. The correlation coefficients between the four stations and GOCE crosswind data all reach around 0.6. However, the magnitude of the GOCE crosswind is somehow larger than the FPIs wind, with average ratios between 1.37-1.69. During geomagnetically active periods, the GOCE and FPI derived winds have a lower agreement, with average ratios of 0.85 for the Asian station (XL) and about 2.15 for the other three American stations (PAR, Arecibo and CAR). The discrepancies of absolute wind values from the GOCE accelerometer and ground-based FPIs should be mainly due to the different measurement principles of the two techniques. Our results also suggested that the wind measurements from the XL FPI located at the Asian sector has the same quality with the FPIs at the American sector, although with lower time resolution.

In this study, we investigated the scale sizes of equatorial plasma irregularities (EPIs) using measurements from the Swarm satellites during its early mission and final constellation phases. We found that with longitudinal separation between Swarm satellites larger than 0.4°, no significant correlation was found any more. This result suggests that EPI structures include plasma density scale sizes less than 44 km in the zonal direction. During the Swarm earlier mission phase, clearly better EPI correlations are obtained in the northern hemisphere, implying more fragmented irregularities in the southern hemisphere where the ambient magnetic field is low. The previously reported inverted-C shell structure of EPIs is generally confirmed by the Swarm observations in the northern hemisphere, but with various tilt angles. From the Swarm spacecrafts with zonal separations of about 150 km, we conclude that larger zonal scale sizes of irregularities exist during the early evening hours (around 1900 LT)., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe; 1112

Both ground‐ and satellite‐based airglow imaging have significantly contributed to understanding the low‐latitude ionosphere, especially the morphology and dynamics of the equatorial ionization anomaly (EIA). The NASA Global‐scale Observations of the Limb and Disk (GOLD) mission focuses on far‐ultraviolet airglow images from a geostationary orbit at 47.5°W. This region is of particular interest at low magnetic latitudes because of the high magnetic declination (i.e., about ‐20°) and proximity of the South Atlantic magnetic anomaly. In this study, we characterize an exciting feature of the nighttime EIA using GOLD observations from October 5, 2018 to June 30, 2020. It consists of a wavelike structure of a few thousand kilometers seen as poleward and equatorward displacements of the EIA‐crests. Initial analyses show that the synoptic‐scale structure is symmetric about the dip equator and appears nearly stationary with time over the night. In quasi‐dipole coordinates, maxima poleward displacements of the EIA‐crests are seen at about ± 12° latitude and around 20 and 60° longitude (i.e., in geographic longitude at the dip equator, about 53°W and 14°W). The wavelike structure presents typical zonal wavelengths of about 6.7 × 103 km and 3.3 × 103 km. The structure's occurrence and wavelength are highly variable on a day‐to‐day basis with no apparent dependence on geomagnetic activity. In addition, a cluster or quasi‐periodic wave train of equatorial plasma depletions (EPDs) is often detected within the synoptic‐scale structure. We further outline the difference in observing these EPDs from FUV images and in situ measurements during a GOLD and Swarm mission conjunction., Key Points: Characteristics of a wavelike structure in the nighttime equatorial ionization anomaly are reported using GOLD far‐ultraviolet observations. The structure is symmetric about the dip equator, appears stationary with time over the night, and is highly variable on a day‐to‐day basis. A cluster or quasi‐periodic wave train of equatorial plasma depletions is often detected within the synoptic structure., Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

Providing accurate measurement of the magnetic field intensity and its vector components is one of the primary objectives of the China Seismo‐Electromagnetic Satellite (CSES). The high precision magnetometer (HPM) payload assembled on CSES is designed to achieve this goal. In this study, the data format, naming convention, and content of the CSES HPM Level 2 scientific data products are introduced, as a reference for users who are interested in this data set. In particular, flags for potential magnetic field disturbances from the platform and payloads are discussed. Possible scientific applications are also outlined. A preliminary validation of the data is conducted through comparison with magnetic data from the ESA’s Swarm constellation, and the result demonstrates that the HPM data of CSES are of good quality. Taking the intense geomagnetic storm that occurred on August 25–26, 2018 as an example, the magnetic field variations and the expansion of the field‐aligned currents (FACs) during this storm are discussed. We finally show that the CSES HPM data can be used to derive a satellite‐derived index equivalent to the Dst index, which agrees well to the index during this event. Our analysis thus suggests a high scientific potential of the HPM data.

Both ground- and satellite-based airglow imaging have significantly contributed to our understanding of the low-latitude ionosphere, especially of the morphology and dynamics of the equatorial ioni...

Electric currents flowing in the terrestrial ionosphere have conventionally been diagnosed by low-earth-orbit (LEO) satellites equipped with science-grade magnetometers and long booms on magnetically clean satellites. In recent years, there are a variety of endeavors to incorporate platform magnetometers, which are initially designed for navigation purposes, to study ionospheric currents. Because of the suboptimal resolution and significant noise of the platform magnetometers, however, most of the studies were confined to high-latitude auroral regions, where magnetic field deflections from ionospheric currents easily exceed 100 nT. This study aims to demonstrate the possibility of diagnosing weak low-/mid-latitude ionospheric currents based on platform magnetometers. We use navigation magnetometer data from two satellites, CryoSat-2 and the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO), both of which have been intensively calibrated based on housekeeping data and a high-precision geomagnetic field model. Analyses based on 8 years of CryoSat-2 data as well as ~ 1.5 years of GRACE-FO data reproduce well-known climatology of inter-hemispheric field-aligned currents (IHFACs), as reported by previous satellite missions dedicated to precise magnetic observations. Also, our results show that C-shaped structures appearing in noontime IHFAC distributions conform to the shape of the South Atlantic Anomaly. The F-region dynamo currents are only partially identified in the platform magnetometer data, possibly because the currents are weaker than IHFACs in general and depend significantly on altitude and solar activity. Still, this study evidences noontime F-region dynamo currents at the highest altitude (717 km) ever reported. We expect that further data accumulation from continuously operating missions may reveal the dynamo currents more clearly during the next solar maximum. - Abstract - Introduction - Satellites, instruments, and data processing methods - Results - Discussion - Summary and conclusion - Acknowledgements - References

The conjugate measurements of a detached SAR arc on 28 March 2017 are analyzed using observations from the Arase satellite. Swarm and GNSS‐TEC data show that the electron density decreased and the electron temperature increased in the ionosphere above the SAR arc. The observed plasmas and electromagnetic fields suggest that Coulomb collision is the most plausible mechanism for the SAR‐arc generation.

The magnetic equator in the Brazilian region has moved over 1,100 km northward since 1957, passing the geomagnetic observatory Tatuoca (TTB), in northern Brazil, around 2013. We recovered and processed TTB hourly mean values of the geomagnetic field horizontal (H) component from 1957 until 2019, allowing the investigation of long‐term changes in the daily variation due to the influence of secular variation, solar activity, season, and lunar phase. The H day‐to‐day variability and the occurrence of the counter electrojet at TTB were also investigated. Until the 1990s, ionospheric solar quiet currents dominated the quiet‐time daily variation at TTB. After 2000, the magnitude of the daily variation became appreciably greater due to the equatorial electrojet (EEJ) contribution. The H seasonal and day‐to‐day variability increased as the magnetic equator approached, but their amplitudes normalized to the average daily variation remained at similar levels. Meanwhile, the amplitude of the lunar variation, normalized in the same way, increased from 5% to 12%. Within the EEJ region, the occurrence rate of the morning counter electrojet (MCEJ) increased with proximity to the magnetic equator, while the afternoon counter electrojet (ACEJ) did not. EEJ currents derived from CHAMP and Swarm satellite data revealed that the MCEJ rate varies with magnetic latitude within the EEJ region while the ACEJ rate is largely constant. Simulations with the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model based on different geomagnetic main field configurations suggest that long‐term changes in the geomagnetic daily variation at TTB can be attributed to the main field secular variation.

It is now well accepted that the ionosphere and thermosphere are sensitive to forcing from the lower atmosphere (troposphere-stratosphere) owing mainly to the progress that have been made in the last decade in understanding the vertical coupling mechanisms connecting these two distinct atmospheric regions. In this regard, the studies linking the upper atmosphere (mesosphere-lower thermosphere-ionosphere) variability due to sudden stratospheric warming (SSW) events have been particularly important. The change of stratospheric circulation due to SSW events modulate the spectrum of vertically upward propagating atmospheric waves (gravity waves, tides, and planetary waves) resulting in numerous changes in the state of the upper atmosphere. Much of our understanding about the upper atmospheric variability associated due to the SSWs events have been gained by studying the 2008/2009 SSW event, which occurred under extremely low solar flux conditions. Recently another SSW event in 2018/2019 occurred under similar low solar flux conditions. In this study we simulate both these SSW events using Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) and present the findings by comparing the ionospheric and thermospheric response to both these SSW events. The tidal characteristics of the semidiurnal solar and lunar tides and the thermospheric composition for both these SSW events are compared and the causes of varying responses are investigated.

The Earth's core magnetic field model Mag.num was the parent model for the GFZ IGRF 13 candidate submission. The model is based on geomagnetic ground observatory and Swarm satellite observations. Epochs 2020.0 and beyond were not covered by the data available at the time of submission and our results were based on predictions. In this study, we investigate the effect of the more recent available data on our results of the 2020.0 epoch and the predicted secular variation by generating an updated Mag.num version. We especially focus on the spatial and temporal patterns of the local geomagnetic field minimum of the South Atlantic Anomaly (SAA). Recently, global geomagnetic field models have shown that an additional, although shallow, secondary minimum at Earth's surface has developed since around 2005. The location and significance of the secondary minimum and of the saddle point between the two minima are assessed also in view of the respective differences among the candidate models.

In this study, we present global climatological distributions of ionospheric plasma irregularities based on measurements by the Swarm satellites. These first global statistics obtained by direct, in situ measurements of plasma variations with Swarm confirm the presence of three main regions of strong ionospheric irregularities: the magnetic equator extending from postsunset to early morning, in the auroral ovals (from dayside cusp to nightside), and inside the polar caps. At equatorial latitudes, ionospheric irregularities form two bands of enhanced plasma fluctuations centered around ±10° magnetic latitude. Due to different plasma processes, ionospheric irregularities at high and low latitudes show different distributions. Though the averaged intensity of plasma irregularities is weaker at equatorial latitudes than at high latitudes, the occurrence rate of significant plasma fluctuations (corresponding to extreme events) is much higher at the equator than that at high latitudes. Equatorial irregularities display clear seasonal and longitudinal variations; that is, they are most prominent over South America during the December solstice and are located over Africa during the June solstice. The magnitude of ionospheric irregularities at all latitudes is strongly controlled by the solar activity. Ionospheric irregularities become significantly weaker after 2016 during the current declining phase of solar activity. The interplanetary magnetic field Bz modulates the occurrence of ionospheric irregularities at both high and low latitudes.

An exceptionally strong stationary planetary wave with Zonal Wavenumber 1 led to a sudden stratospheric warming (SSW) in the Southern Hemisphere in September 2019. Ionospheric data from European Space Agency's Swarm satellite constellation mission show prominent 6-day variations in the dayside low-latitude region at this time, which can be attributed to forcing from the middle atmosphere by the Rossby normal mode “quasi-6-day wave” (Q6DW). Geopotential height measurements by the Microwave Limb Sounder aboard National Aeronautics and Space Administration's Aura satellite reveal a burst of global Q6DW activity in the mesosphere and lower thermosphere during the SSW, which is one of the strongest in the record. The Q6DW is apparently generated in the polar stratosphere at 30–40 km, where the atmosphere is unstable due to strong vertical wind shear connected with planetary wave breaking. These results suggest that an Antarctic SSW can lead to ionospheric variability through wave forcing from the middle atmosphere.

Sections PDFPDF Tools Share Abstract Data assimilation aims to blend incomplete and inaccurate data with physics‐based dynamical models. In the Earth's radiation belts, it is used to reconstruct electron phase space density, and it has become an increasingly important tool in validating our current understanding of radiation belt dynamics, identifying new physical processes, and predicting the near‐Earth hazardous radiation environment. In this study, we perform reanalysis of the sparse measurements from four spacecraft using the three‐dimensional Versatile Electron Radiation Belt diffusion model and a split‐operator Kalman filter over a 6‐month period from 1 October 2012 to 1 April 2013. In comparison to previous works, our 3‐D model accounts for more physical processes, namely, mixed pitch angle‐energy diffusion, scattering by Electromagnetic Ion Cyclotron waves, and magnetopause shadowing. We describe how data assimilation, by means of the innovation vector, can be used to account for missing physics in the model. We use this method to identify the radial distances from the Earth and the geomagnetic conditions where our model is inconsistent with the measured phase space density for different values of the invariants urn:x-wiley:jgra:media:jgra55451:jgra55451-math-0001 and urn:x-wiley:jgra:media:jgra55451:jgra55451-math-0002. As a result, the Kalman filter adjusts the predictions in order to match the observations, and we interpret this as evidence of where and when additional source or loss processes are active. The current work demonstrates that 3‐D data assimilation provides a comprehensive picture of the radiation belt electrons and is a crucial step toward performing reanalysis using measurements from ongoing and future missions.

This paper examines the response of the upper atmosphere to equatorial Kelvin waves with a period of ∼3 days, also known as ultrafast Kelvin waves (UFKWs). The whole atmosphere model Ground‐to‐topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) is used to simulate the UFKW events in the late summer of 2010 and 2011 as well as in the boreal winter of 2012/2013. When the lower layers of the model below 30‐km altitude are constrained with meteorological data, GAIA is able to reproduce salient features of the UFKW in the mesosphere and lower thermosphere as observed by the Aura Microwave Limb Sounder. The model also reproduces ionospheric response, as validated through comparisons with total electron content data from the Gravity field and steady‐state Ocean Circulation Explorer satellite as well as with earlier observations. Model results suggest that the UFKW produces eastward‐propagating ∼3‐day variations with zonal wavenumber 1 in the equatorial zonal electric field and F region plasma density. Model results also suggest that for a ground observer, identifying ionospheric signatures of the UFKW is a challenge because of ∼3‐day variations due to other sources. This issue can be overcome by combining ground‐based measurements from different longitudes. As a demonstration, we analyze ground‐based magnetometer data from equatorial stations during the 2011 event. It is shown that wavelet spectra of the magnetic data at different longitudes are only in partial agreement, with or without a ∼3‐day peak, but a spectrum analysis based on multipoint observations reveals the presence of the UFKW.

The equatorial electrojet (EEJ) is an electric current that flows eastwards in the ionospheric E-region, along themagnetic equator. Occasionally, it reverses during morning and afternoon hours, leading to periods of westwardcurrent that are known as counter electrojet (CEJ) events. The EEJ/CEJ magnetic signal can be isolated fromother large-scale variations by using a pair of stations from the same longitude sector, one equatorial and other oflow-latitude, and taking the difference between their H components.Here, we present the first analysis of CEJ climatology and CEJ dependence on solar flux and lunar phasefor the Brazilian sector, based on an extensive ground-based data set for the years 2008 to 2017 from thegeomagnetic observatory Tatuoca (1.2◦S, 48.5◦W), and we compare it to the results found for Huancayoobservatory(12.0◦S, 75.3◦W) in the Peruvian sector.We found a predominance of morning CEJ events for both sectors. The afternoon CEJ occurrence rate inthe Brazilian sector is twice as high as in the Peruvian sector. The afternoon CEJ occurrence rate strongly dependson season, with maximum rates occurring during the northern-hemisphere summer for the Brazilian sector andduring the northern-hemisphere winter for the Peruvian sector. Significant discrepancies between the two sectorsare also found for morning CEJ rates during the northern-hemisphere summer. These longitudinal differences arein agreement with a CEJ climatology derived from contemporary Swarm satellite data and can be attributed in partto the well-known longitudinal wave-4 structure in the background EEJ strength that results from nonmigratingsolar tides and stationary planetary waves. Simulations with the Thermosphere-Ionosphere-ElectrodynamicsGeneral Circulation Model show that the remaining longitudinal variability of CEJ rates during northern summercan be explained by the effect of migrating tides in the presence of the varying geomagnetic field in the SouthAtlantic Anomaly.

The lower atmospheric forcing effects on the ionosphere are particularly evident during extreme meteorological events known as sudden stratospheric warmings (SSWs). During SSWs, the polar stratosphere and ionosphere, two distant atmospheric regions, are coupled through the SSW-induced modulation of atmospheric migrating and nonmigrating tides. The changes in the migrating semidiurnal solar and lunar tides are the major source of ionospheric variabilities during SSWs. In this study, we use 55 years of ground-magnetometer observations to investigate the composite characteristics of the lunar tide of the equatorial electrojet (EEJ) during SSWs. These long-term observations allow us to capture the EEJ lunar tidal response to the SSWs in a statistical sense. Further, we examine the influence of solar flux conditions and the phases of quasi-biennial oscillation (QBO) on the lunar tide and find that the QBO phases and solar flux conditions modulate the EEJ lunar tidal response during SSWs in a similar way as they modulate the wintertime Arctic polar vortex. This work provides first evidence of modulation of the EEJ lunar tide due to QBO. Plain Language Summary This study focuses on the vertical coupling between the polar stratosphere and equatorial ionosphere during sudden stratospheric warmings (SSWs). Extreme meteorological events such as SSWs induce variabilities in the ionosphere by modulating the atmospheric migrating and nonmigrating tides, and these variabilities can be comparable to a moderate geomagnetic storm. Observations and modeling studies have found that the changes in the migrating semidiurnal solar and lunar tides are a major source of ionospheric variabilities during SSWs. The equatorial electrojet (EEJ) is a narrow ribbon of current flowing over the dip equator in the ionosphere and is particularly sensitive to tidal changes. Long-term ground-magnetometer recordings have been used in this study to estimate the variations induced in EEJ during SSWs due to the lunar semidiurnal tide in a statistical sense. The wintertime Arctic polar vortex and the occurrence of SSWs are modulated by solar flux conditions and the phases of quasi-biennial oscillation. In this work, we find the first evidence of lunar tidal modulation of EEJ due to quasi-biennial oscillation during SSWs. Our findings will be useful in providing improved predictions of ionospheric variations due to SSWs. The aeronomy community will be the most impacted by this paper.

In this study, we developed an autodetection technique for the equatorial plasma depletions (EPDs) and their occurrence and depletion amplitudes based on in situ electron density measurements gathered by Swarm A satellite. For the first time, comparisons are made among the detected EPDs and their amplitudes with the loss of Global Positioning System (GPS) signal of receivers onboard Swarm A, and the Swarm Level-2 product, Ionospheric Bubble Index (IBI). It has been found that the highest rate of EPD occurrence takes place generally between 2200 and 0000 magnetic local time (MLT), in agreement with the IBI. However, the largest amplitudes of EPD are detected earlier at about 1900-2100 MLT. This coincides with the moment of higher background electron density and the largest occurrence of GPS signal loss. From a longitudinal perspective, the higher depletion amplitude is always witnessed in spatial bins with higher background electron density. At most longitudes, the occurrence rate of postmidnight EPDs is reduced compared to premidnight ones; while more postmidnight EPDs are observed at African longitudes. CHAMP observations confirm this point regardless of high or low solar activity condition. Further by comparing with previous studies and the plasma vertical drift velocity from ROCSAT-1, we suggest that while the F region vertical plasma drift plays a key role in dominating the occurrence of EPDs during premidnight hours, the postmidnight EPDs are the combined results from the continuing of former EPDs and newborn EPDs, especially during June solstice. And these newborn EPDs during postmidnight hours seem to be less related to the plasma vertical drift.

We report the concurrent observations of F-region plasma changes and field-aligned currents (FACs) above isolated proton auroras (IPAs) associated with electromagnetic ion cyclotron Pc1 waves. Key events on March 19, 2020 and September 12, 2018 show that ground magnetometers and all-sky imagers detected concurrent Pc1 wave and IPA, during which NOAA POES observed precipitating energetic protons. In the ionospheric F-layer above the IPA zone, the Swarm satellites observed transverse Pc1 waves, which span wider latitudes than IPA. Around IPA, Swarm also detected the bipolar FAC and localized plasma density enhancement, which is occasionally surrounded by wide/shallow depletion. This indicates that wave-induced proton precipitation contributes to the energy transfer from the magnetosphere to the ionosphere.

The effects of coupling between different layers of the atmosphere during Stratospheric Sudden Warming (SSW) events have been studied quite extensively in the past fewyears, and in this context large lunitidal enhancements in the equatorial ionosphere have also been widely discussed. In this study we report about the longitudinal variabilities in lunitidal enhancement in the equatorial electrojet (EEJ) during SSWs through ground and space observations in the Peruvian and Indian sectors. We observe that the amplification of lunitidal oscillations in EEJ is significantly larger over the Peruvian sector in comparison to the Indian sector. We further compare the lunitidal oscillations in both the sectors during the 2005-2006 and 2008-2009 major SSW events and during a non-SSW winter of 2006-2007. It is found that the lunitidal amplitude in EEJ over the Peruvian sector showed similar enhancements during both the major SSWs, but the enhancements were notably different in the Indian sector. Independent from SSW events, we have also performed a climatological analysis of the lunar modulation of the EEJ during December solstice over both the sectors by using 10years of CHAMP magnetic measurements and found larger lunitidal amplitudes over the Peruvian sector confirming the results from ground magnetometer observations. We have also analyzed the semidiurnal lunar tidal amplitude in neutral temperature measurements from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) at 110km and found lesser longitudinal variability than the lunitidal amplitude in EEJ. Our results suggest that the longitudinal variabilities in lunitidal modulation of the EEJ during SSWs could be related to electrodynamics in the E region dynamo.

The polar ionosphere is often characterized by irregularities and fluctuations in the plasma density. We present a statistical study of ionospheric plasma irregularities based on the observations from the European Space Agency's Swarm mission. The in situ electron density obtained with the Langmuir probe and the total electron content from the onboard global positioning system receiver are used to detect ionospheric plasma irregularities. We derive the irregularity parameters from the electron density in terms of the rate of change of density index and electron density gradients. We also use the rate of change of total electron content index as the irregularity parameter based on the global positioning system data. The background electron density and plasma irregularities are closely controlled by the Earth's magnetic field, with averaged enhancements close to the magnetic poles. The climatological maps in magnetic latitude/magnetic local time coordinates show predominant plasma irregularities near the dayside cusp, polar cap, and nightside auroral oval. These irregularities may be associated with large‐scale plasma structures such as polar cap patches, auroral blobs, auroral particle precipitation, and the equatorward wall of the ionospheric trough. The spatial distributions of irregularities depend on the interplanetary magnetic field (IMF). By filtering the irregularity parameters according to IMF By, we find a clear asymmetry of the spatial distribution in the cusp and polar cap between the Northern (NH) and Southern Hemispheres (SH). For negative IMF By, irregularities are stronger in the dusk (dawn) sector in the NH (SH) and vice versa. This feature is in agreement with the high‐latitude ionospheric convection pattern that is regulated by the IMF By component. The plasma irregularities are also controlled by the solar activity within the current declining solar cycle. The irregularities in the SH polar cap show a seasonal variation with higher values from September to April, while the seasonal variation in the NH is only obvious around solar maximum during 2014–2015.

Transverse Pc1 waves propagating from magnetospheric source regions undergo mode conversion to the compressional mode in the ionosphere due to the induced Hall current. Mode converted Pc1 waves propagate across the magnetic field through the ionospheric waveguide. This process is called Pc1 wave ducting (PWD). PWDs have been observed by magnetometers on both ground and low Earth orbit satellites over a wide latitudinal and longitudinal range. In this work, we present the statistical analysis results of PWD exploiting Swarm satellites from 2015 to 2019. Spatial distributions show that the PWDs are mainly observed over the South Atlantic Anomaly longitudes, possibly due to the high Hall conductivity and F-region density, and at subauroral/auroral latitudes (  50 70 MLAT). The occurrence rate of PWD increases with increasing AE and | SYM-H | indices. Seasonal dependence shows that PWD exhibits a high occurrence rate during equinox and local summer while local winter hosts only a low occurrence. The asymmetry between summer and winter can be explained by the ionospheric plasma density. The high occurrence rate in equinox may result from intense geomagnetic activity during the equinox, probably due to the Russell-McPherron effect. From our statistical analysis, we conclude that the occurrence of PWD is controlled by both ionospheric plasma conditions and geomagnetic activity, and that the mode conversion and PWD occur more efficiently as plasma density increases.

Earth’s magnetic field results from various internal and external sources. The electric currents in the ionosphere are major external sources of the magnetic field in the daytime. High-resolution magnetometers onboard low-Earth-orbit satellites such as CHAMP and Swarm can detect small-scale currents in the nighttime ionosphere, where plasma density gradients often become unstable and form irregular density structures. The magnetic field variations caused by the ionospheric irregularities are comparable to that of the lithospheric contribution. Two phenomena in the nighttime ionosphere that contribute to the magnetic field variation are presented: equatorial plasma bubble (EPB) and medium-scale traveling ionospheric disturbance (MSTID). EPB is formed by the generalized Rayleigh–Taylor instability over the dip equator and grows nonlinearly to as high as 2000 km apex altitude. It is characterized by deep plasma density depletions along magnetic flux tubes, where the diamagnetic effect produced by a pressure-gradient-driven current enhances the main field intensity. MSTID is a few hundred kilometer-scale disturbance in the midlatitude ionosphere generated by the coupled electrodynamics between the ionospheric $E$ and $F$ regions. The field-aligned currents associated with EPBs and MSTIDs also have significant signatures in the magnetic field perpendicular to the main field direction. The empirical discovery of the variations in the magnetic field due to plasma irregularities has motivated the inclusion of electrodynamics in the physical modeling of these irregularities. Through an effective comparison between the model results and observations, the physical process involved has been largely understood. The prediction of magnetic signatures due to plasma irregularities has been advanced by modeling studies, and will be helpful in interpreting magnetic field observations from satellites.

High-precision magnetic measurements taken by LEO satellites (flying at altitudes between 300 and 800 km) allow for studying the ionosphericand magnetospheric processes and electric currents that causes only weak magnetic signature of a few nanotesla during geomagnetic quiet conditions. Of particular importance for this endeavour are multipoint observationsin space, such as provided by the Swarm satellite constellation mission, inorder to better characterize the space-time-structure of the current systems. Focusing on geomagnetic quiet conditions, we provide an overview of ionospheric and magnetospheric sources and illustrate their magnetic signatureswith Swarm satellite observations.

The Swarm constellation has been used to estimate zonal currents in the topside F-region ionosphere at about 500 km. Near-simultaneous magnetic field measurements from two altitudes but the same meridian are used for the current density calculations. We consider the period 15 February to 23 June 2014 for deriving a full 24-hour local time coverage of the latitudinal distribution over ±50° in magnetic latitude. Intervals with close orbital phasing at the two heights are considered, which repeat every 6 days. From such days seven successive orbits are used where the epochs of equator crossings differ by less than 2 minutes. Deduced current densities are predominantly eastward (about 20 nA/m2) on the dayside and westward (about 10 nA/m2) on the nightside. A number of different drivers contribute to the observed total current. We identified the gravity-driven eastward current as the most prominent at low latitudes. Eastward currents in the northern hemisphere are clearly stronger than in the south. This is attributed to the proximity of our study period to June solstice, when the solar radiation is stronger in the north. In addition, inter-hemispheric winds from the northern (summer) to the southern (winter) hemisphere contribute. They cause eastward currents in the north and westward in the south. We find a relatively large variability of the zonal currents both in space and time. The standard deviation is at least twice as large as the mean value of current density. This large variability is suggested to be related to gravity wave forcing from below.

In this study we investigate the three-dimensional structure of low-latitude plasma blobs using multi-instrument and multisatellite observations of the Swarm constellation. During the early commissioning phase the Swarm satellites were flying at the same altitude with zonal separation of about 0.5∘ in geographic longitude. Electron density data from the three satellites constrain the blob morphology projected onto the horizontal plane. Magnetic field deflections around blobs, which originate from field-aligned currents near the irregularity boundaries, constrain the blob structure projected onto the plane perpendicular to the ambient magnetic field. As the two constraints are given for two noncoplanar surfaces, we can get information on the three-dimensional structure of blobs. Combined observation results suggest that blobs are contained within tilted shells of geomagnetic flux tubes, which are similar to the shell structure of equatorial plasma bubbles suggested by previous studies.

In this study we investigate a dayside, midlatitude plasma depletion (DMLPD) encountered on 22 May 2014 by the Swarm and GRACE satellites, as well as ground-based instruments. The DMLPD was observed near Puerto Rico by Swarm near 10 LT under quiet geomagnetic conditions at altitudes of 475–520 km and magnetic latitudes of ∼25°–30°. The DMLPD was also revealed in total electron content observations by the Saint Croix station and by the GRACE satellites (430 km) near 16 LT and near the same geographic location. The unique Swarm constellation enables the horizontal tilt of the DMLPD to be measured (35° clockwise from the geomagnetic east-west direction). Ground-based airglow images at Arecibo showed no evidence for plasma density depletions during the night prior to this dayside event. The C/NOFS equatorial satellite showed evidence for very modest plasma density depletions that had rotated into the morningside from nightside. However, the equatorial depletions do not appear related to the DMLPD, for which the magnetic apex height is about 2500 km. The origins of the DMLPD are unknown, but may be related to gravity waves.

In order to analyse the sensitivity of the equatorial ionospheric current system, i.e. the solar quiet current system and the equatorial electrojet, to solar cycle variations and to the secular variation of the geomagnetic main field, we have analysed 51 years (1935–1985) of geomagnetic observatory data from Huancayo, Peru. This period is ideal to analyse the influence of the main field strength on the amplitude of the quiet daily variation, since the main field decreases significantly from 1935 to 1985, while the distance of the magnetic equator to the observatory remains stable. To this end, we digitised some 19 years of hourly mean values of the horizontal component (H), which have not been available digitally at the World Data Centres. Then, the sensitivity of the amplitude ΔH of the quiet daily variation to both solar cycle variations (in terms of sunspot numbers and solar flux F10.7) and changes of the geomagnetic main field strength (due to secular variation) was determined. We confirm an increase of ΔH for the decreasing main field in this period, as expected from physics based models (Cnossen, 2016), but with a somewhat smaller rate of 4.4% (5.8% considering one standard error) compared with 6.9% predicted by the physics based model.

Magnetic and electric field observations from the European Space Agency Swarm mission are used to report the direction of electromagnetic energy flux associated with equatorial plasma depletions. Contrary to expectations, the observations suggest a general interhemispheric Poynting flux rather than concurrent flows at both hemispheres toward or away from the equator. Of high interest is a particular behavior noticed over the region with the largest variation in the magnetic declination. This is a Poynting flux flowing mainly into the southern magnetic hemisphere about between 60 degrees W and 30 degrees E and into the northern magnetic hemisphere between 110 degrees W and 60 degrees W. The abrupt change in the flow direction at 60 degrees W is suggested to be caused by an asymmetry between the hemispheres on the ionospheric conductivity, likely due to the influence of thermospheric winds and the presence of the South Atlantic Anomaly.

We report on simultaneous observations of electromagnetic ion cyclotron (EMIC) waves associated with traveling convection vortex (TCV) events caused by transient solar wind dynamic pressure (P-d) impulse events. The Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft located near the magnetopause observed radial fluctuations of the magnetopause, and the GOES spacecraft measured sudden compressions of the magnetosphere in response to sudden increases in Pd. During the transient events, EMIC waves were observed by interhemispheric conjugate ground-based magnetometer arrays as well as the GOES spacecraft. The spectral structures of the waves appear to be well correlated with the fluctuating motion of the magnetopause, showing compression-associated wave generation. In addition, the wave features are remarkably similar in conjugate hemispheres in terms of bandwidth, quasiperiodic wave power modulation, and polarization. Proton precipitation was also observed by the DMSP spacecraft during the wave events, from which the wave source region is estimated to be 72 degrees-74 degrees in magnetic latitude, consistent with the TCV center. The confluence of space-borne and ground instruments including the interhemispheric, high-latitude, fluxgate/induction coil magnetometer array allows us to constrain the EMIC source region while also confirming the relationship between EMIC waves and the TCV current system. National Science Foundation (NSF) [AGS-1547252]; NSF [PLR-1543364, PLR-1247975, PLR-1443507, PLR-1341677, PLR-1341493] The work at New Jersey Institute of Technology was supported by National Science Foundation (NSF) grant AGS-1547252. The work at Virginia Tech was supported by NSF grant PLR-1543364. The fluxgate magnetometer projects at SPA and AGOs are supported by NSF grants PLR-1247975 and PLR-1443507, respectively, to New Jersey Institute of Technology. The induction coil magnetometer projects at STF, IQA, SPA, and AGOs are supported by NSF grants PLR-1341677 to the University of New Hampshire and PLR-1341493 to Augsburg College. The authors would like to thank the following persons/institutes for providing data: the THEMIS team for fluxgate magnetometer and electrostatic analyzer data; the NOAA GOES team for fluxgate magnetometer data; the DMSP team for SSJ particle data; Lorne McKee at Natural Resources Canada (NRCan) and INTERMAGNET for fluxgate magnetometer data from IQA, which is operated by NRCan; the Technical University of Denmark National Space Institute (DTU Space) for the fluxgate magnetometer data (STF); the British Antarctic Survey (BAS) team for fluxgate data (HBA); and the AGO project team for operations of the magnetometer stations. All data used in this study can be obtained from the following data repositories and tools: OMNI, ACE, and THEMIS data from the NASA Coordinated Data Analysis Web (CDAWeb) at http://cdaweb.sci.gsfc.nasa.gov and Space Physics Environment Data Analysis Software (SPEDAS); GOES magnetometer data from the data archive at National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Information (NCEI, previously known as NGDC) (http://satdat.ngdc.noaa.gov/sem/goes/data/new_full/); DMSP particle data from the DMSP Online Spectrogram Viewer Tool at http://sd-www.jhuapl.edu/Aurora/spectrogram/index.html; IQA fluxgate data from the Intermagnet (www.intermagnet.org); STF fluxgate magnetometer data from the DTU database (https://ftp.space.dtu.dk/data/Ground_magnetometers/Adjusted/); HBA fluxgate data from the BAS Data Access System (http://psddb.nerc-bas.ac.uk); and SPA and AGO fluxgate and induction coil magnetometer data from the New Jersey Institute of Technology database (www.antarcticgeospace.org).

In this study, we investigate the climatology of high-latitude total electron content (TEC) variations as observed by the dual-frequency Global Navigation Satellite Systems (GNSS) receivers onboard the Swarm satellite constellation. The distribution of TEC perturbations as a function of geographic/magnetic coordinates and seasons reasonably agrees with that of the Challenging Minisatellite Payload observations published earlier. Categorizing the high-latitude TEC perturbations according to line-of-sight directions between Swarm and GNSS satellites, we can deduce their morphology with respect to the geomagnetic field lines. In the Northern Hemisphere, the perturbation shapes are mostly aligned with the L shell surface, and this anisotropy is strongest in the nightside auroral (substorm) and subauroral regions and weakest in the central polar cap. The results are consistent with the well-known two-cell plasma convection pattern of the high-latitude ionosphere, which is approximately aligned with L shells at auroral regions and crossing different L shells for a significant part of the polar cap. In the Southern Hemisphere, the perturbation structures exhibit noticeable misalignment to the local L shells. Here the direction toward the Sun has an additional influence on the plasma structure, which we attribute to photoionization effects. The larger offset between geographic and geomagnetic poles in the south than in the north is responsible for the hemispheric difference.

The Earth’s magnetic field results from different sources in the Earth’s interior and in near Earth space. Commonly, when investigating solar-terrestrial interactions where the Earth’s internal field plays a major role, scientists concentrate on strong (tens to thousands of nanotesla) and rapid (seconds to days) magnetic field variations that are caused by currents in the ionosphere and magnetosphere when solar activity, and correspondingly the electric currents in Earth’s environment, are enhanced. However, for studying the internal sources of the geomagnetic field, originating in the core and crust, scientists use observations from so called “geomagnetic quiet” times, when external field variations are expected to be weak. However, even these weak variations impact internal field modelling, and incomplete knowledge of them hinders their separation. Difficulties arise in particular in characterizing the long term behaviour of external sources, e.g., seasonal and solar cycle variations of the magnetospheric ring current, polar convection currents or ionospheric dynamo currents driven by atmospheric tides, since they have amplitudes and spatial scales similar to those of the core field’s secular variation or the lithospheric field. Since such external currents are also present during geomagnetic quiet conditions they may result in biased core and crustal field models. An additional complexity arises from magnetic observations taken by satellites because of the movement of the platform, leading to a possible space-time ambiguity.

[1] It is commonly assumed that geomagnetic activity is symmetrical between interhemispheric conjugate locations. However, in many cases, such an assumption proved to be wrong. Especially in high-latitude regions where the magnetosphere and the ionosphere are coupled in a more complex and dynamic fashion, asymmetrical features in geomagnetic phenomena are often observed. This paper presents investigations of geomagnetic responses to sudden change in solar wind pressure to examine interhemispheric conjugate behavior of magnetic field variations, which have rarely been made mainly due to the difficulty of facilitating conjugate-point measurements. In this study, using magnetometer data from three conjugate stations in Greenland and Antarctica, solar wind pressure impulse events (>5 nPa in

[1] An isolated burst of 0.35 Hz electromagnetic ion cyclotron (EMIC) waves was observed at four sites on Svalbard from 0947 to 0954 UT 2 January 2011, roughly 1 h after local noon. This burst was associated with one of a series of ~50 nT magnetic impulses observed at the northernmost stations of the IMAGE magnetometer array. Hankasalmi SuperDARN radar data showed a west-to-east (antisunward) propagating vortical ionospheric flow in a region of high spectral width ~ 1–2° north of Svalbard, confirming that this magnetic impulse was the signature of a traveling convection vortex. Ground-based observations of the Hα line at Longyearbyen indicated proton precipitation at the same time as the EMIC wave burst, and NOAA-19, which passed over the west coast of Svalbard between 0951 and 0952, observed a clear enhancement of ring current protons at the same latitude. Electron precipitation from this same satellite indicated that the EMIC burst was located on closed field lines, but near to the polar cap boundary. We believe these are the first simultaneous observations of EMIC waves and precipitating energetic protons so near to the boundary of the dayside magnetosphere. Although several spacecraft upstream of Earth observed a steady solar wind and predominantly radial interplanetary magnetic field orientation before and during this event, data from Geotail (near the morning bow shock) showed large reorientations of the interplanetary magnetic field and substantial decreases in ion density several minutes before it, and data from Cluster (near the afternoon bow shock) showed an outward excursion of the bow shock simultaneous with it. These upstream perturbations suggest that a spontaneous hot flow anomaly, a bow shock related instability, may have been responsible for triggering this event, but do not provide enough information to fully characterize that instability.

[1] We use ground-based and satellite measurements to examine, for the first time, the characteristics of equatorial electrodynamic perturbations measured during the 2002 major and 2010 minor Southern Hemisphere sudden stratospheric warming (SSW) events. Our data suggest the occurrence of enhanced quasi 2 day fluctuations during the 2002 early autumnal equinoctial warming. They also show a moderately large multi-day perturbation pattern, resembling those during arctic SSW events, during 2002 late equinox, as the major SSW was weakening. We also compare these data with extensive recent results that showed the fundamentally important role of lunar semidiurnal tidal effects on low latitude electrodynamic perturbations during arctic SSW events.

Low Earth orbiting geomagnetic satellite missions, such as the Swarm satellite mission, are the only means to monitor and investigate ionospheric currents on a global scale and to make in situ measurements of F region currents. High-precision geomagnetic satellite missions are also able to detect ionospheric currents during quiet-time geomagnetic conditions that only have few nanotesla amplitudes in the magnetic field. An efficient method to isolate the ionospheric signals from satellite magnetic field measurements has been the use of residuals between the observations and predictions from empirical geomagnetic models for other geomagnetic sources, such as the core and lithospheric field or signals from the quiet-time magnetospheric currents. This study aims at highlighting the importance of high-resolution magnetic field models that are able to predict the lithospheric field and that consider the quiet-time magnetosphere for reliably isolating signatures from ionospheric currents during geomagnetically quiet times. The effects on the detection of ionospheric currents arising from neglecting the lithospheric and magnetospheric sources are discussed on the example of four Swarm orbits during very quiet times. The respective orbits show a broad range of typical scenarios, such as strong and weak ionospheric signal (during day- and nighttime, respectively) superimposed over strong and weak lithospheric signals. If predictions from the lithosphere or magnetosphere are not properly considered, the amplitude of the ionospheric currents, such as the midlatitude Sq currents or the equatorial electrojet (EEJ), is modulated by 10–15 % in the examples shown. An analysis from several orbits above the African sector, where the lithospheric field is significant, showed that the peak value of the signatures of the EEJ is in error by 5 % in average when lithospheric contributions are not considered, which is in the range of uncertainties of present empirical models of the EEJ., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 887