Abstract. On the basis of field-aligned currents (FACs) and Hall currents derived from high-resolution magnetic field data of the Swarm constellation, the average characteristics of these two current systems in the auroral regions are comprehensively investigated by statistical methods. This is the first study considering both current types determined simultaneously by the same spacecraft in both hemispheres. The FAC distribution, derived from the novel Swarm dualspacecraft approach, reveals the well-known features of Region 1 (R1) and Region 2 (R2) FACs. At high latitudes, Region 0 (R0) FACs appear on the dayside. Their flow direction, up or down, depends on the orientation of the interplanetary magnetic field (IMF) B y component. Of particular interest is the distribution of auroral Hall currents. The prominent auroral electrojets are found to be closely controlled by the solar wind input, but we find no dependence of their intensity on the IMF B y orientation. The eastward electrojet is about 1.5 times stronger in local summer than in winter. Conversely, the westward electrojet shows less dependence on season. As to higher latitudes, part of the electrojet current is closed over the polar cap. Here the seasonal variation of conductivity mainly controls the current density. During local summer of the Northern Hemisphere, there is a clear channeling of return currents over the polar cap. For positive (negative) IMF B y a dominant eastward (westward) Hall current circuit is formed from the afternoon (morning) electrojet towards the dawn side (dusk side) polar cap return current. The direction of polar cap Hall currents in the noon sector depends directly on the orientation of the IMF B y . This is true for both signs of the IMF B z component. Comparable Hall current distributions can be observed in the Southern Hemisphere but for opposite IMF B y signs. Around the midnight sector the westward substorm electrojet is dominating. As expected, it is highly dependent on magnetic activity, but it shows only little response to season and IMF B y polarity. An important finding is that all the IMF B y dependences of FACs and Hall currents practically disappear in the dark winter hemisphere.

We present multimission observations of field‐aligned currents, auroral oval, and magnetopause crossings during the 17 March 2015 magnetic storm. Dayside reconnection is expected to transport magnetic flux, strengthen field‐aligned currents, lead to polar cap expansion and magnetopause erosion. Our multimission observations assemble evidence for all these manifestations. After a prolonged period of strongly southward interplanetary magnetic field, Swarm and AMPERE observe significant intensification of field‐aligned currents. The dayside auroral oval, as seen by DMSP, appears as a thin arc associated with ongoing dayside reconnection. Both the field‐aligned currents and the auroral arc move equatorward reaching as low as ~60° magnetic latitude. Strong magnetopause erosion is evident in the in situ measurements of the magnetopause crossings by GOES 13/15 and MMS. The coordinated Swarm, AMPERE, DMSP, MMS and GOES observations, with both global and in situ coverage of the key regions, provide a clear demonstration of the effects of dayside reconnection on the entire magnetosphere.

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The relationship between high‐latitude ionospheric currents (Hall current and field‐aligned current) and thermospheric wind is investigated. The 2‐D patterns of horizontal wind and equivalent current in the Northern Hemisphere derived from the CHAMP satellite are considered for the first time simultaneously. The equivalent currents show strong dependences on both interplanetary magnetic field (IMF) By and Bz components. However, IMF By orientation is more important in controlling the wind velocity patterns. The duskside wind vortex as well as the antisunward wind in the morning polar cap is more evident for positive By. To better understand their spatial relation in different sectors, a systematic superposed epoch analysis is applied. Our results show that in the dusk sector, the vectors of the zonal wind and equivalent current are anticorrelated, and both of them form a vortical flow pattern for different activity levels. The currents and zonal wind are intensified with the increase of merging electric field. However, on the dawnside, where the relation is less clear, antisunward zonal winds dominate. Plasma drift seems to play a less important role for the wind than neutral forces in this sector. In the noon sector, the best anticorrelation between equivalent current and wind is observed for a positive IMF By component and it is less obvious for negative By. A clear seasonal effect with current intensities increasing from winter to summer is observed in the noon sector. Different from the currents, the zonal wind intensity shows little dependence on seasons. Our results indicate that the plasma drift and the neutral forces are of comparable influence on the zonal wind at CHAMP altitude in the noon sector.