Description: The daytime ionospheric E region hosts a vortex of horizontal equivalent currents driven by the wind dynamo in each hemisphere: the solar quiet current system. Differences in the dynamo actions at conjugate points in the two hemispheres drive interhemispheric currents (interhemispheric field‐aligned currents [IHFACs]). Despite the long history of investigations on this topic, only a few studies have reported a latitude dependence of the IHFACs. In this study we make use of the magneticfield observations from the European Space Agency's Swarm constellation to address the latitude dependence of IHFACs in depth. At low latitudes (〈35° magnetic latitude), the statistics generally agree with previous findings from other satellite missions. However, the polarity of currents changes at magnetic latitude of about ±35°.The magnetic latitude dependence of IHFACs can be reproduced by the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model, when the lower boundary of the model (~30 km)is properly constrained with meteorological reanalysis data. As for the seasonal variations, the midlatitude (〉35° magnetic latitude) IHFAC climatology during equinoxes is generally similar to that of June solstice while that of December solstice exhibits stand‐alone behavior.

Description: We report the first observation of plasma density oscillations coherent with magnetic Pc1 waves. Swarm satellites observed compressional Pc1 wave activity in the 0.5–3 Hz band, which was coherent with in situ plasma density oscillations. Around the Pc1 event location, the Antarctic Neumayer Station III (L ~ 4.2) recorded similar Pc1 events in the horizontal component while NOAA‐15 observed isolated proton precipitations at energies above 30 keV. All these observations support that the compressional Pc1 waves at Swarm are oscillations converted from electromagnetic ion cyclotron (EMIC) waves coming from the magnetosphere. The magnetic field and plasma density oscillate in‐phase. We compared the amplitudes of density and magnetic field oscillations normalized to background values and found that the density power is much larger than the magnetic field power. This difference cannot be explained by a simple magnetohydrodynamic (MHD) model, although steep horizontal/vertical gradients of background ionospheric density can partly reconcile the discrepancy.

Description: 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.

Description: 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.

Description: - Abstract - Introduction - Satellites, instruments, and data processing methods - Results - Discussion - Summary and conclusion - Acknowledgements - References

Description: In the equatorial F region exist upward/downward electric currents, which are generally attributed to dynamo action of thermospheric zonal wind. Despite the long history of their observations since the 1970s, the return path has not been thoroughly investigated. In this study, we revisit the magnetic eld data of the Challenging Minisatellite Payload (CHAMP) to address the statistical distribution of the return currents. In addition to the F region dynamo currents near the dip equator, we have identied offequatorial bands with the reverse polarity. Both equatorial and offequatorial bands ip signs around 1600 magnetic local time (MLT) and are the weakest during June solstice. These similarities suggest that the offequatorial currents are tied to the equatorial F region dynamo and provide the return paths. The offequatorial return currents have the following characteristics. First, they are mostly conned within ±20° in magnetic latitude (MLAT) at CHAMP altitudes, which corresponds to 〈1,300km apex height. Second, the peak locations of the equatorial dynamo and the offequatorial return currents are zonally displaced from each other in terms of MLT and longitude. It implies that zonal currents in the topside F region participate in the current closure. Third, the return currents exhibit multiple zonal bands (beyond |MLAT| 〉 20°) near dusk during combined equinoxes, whose origin is currently unknown.