Global characteristics of auroral Hall currents derived from the Swarm constellation: dependences on season and IMF orientation

Abstract

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.

The Relationship of High‐Latitude Thermospheric Wind With Ionospheric Horizontal Current, as Observed by CHAMP Satellite

“…Collocated with the currents, strong westward wind appears. This is typical for the prevailing positive IMF B y condition (see Huang et al, 2017;Lühr et al, 2012). On the nightside the downward/upward R1/R2 FACs are accompanied by a strong westward electrojet.…”

Field‐Aligned and Horizontal Currents in the Northern and Southern Hemispheres From the Swarm Satellite

“…Hemispherical differences in horizontal currents have been discussed by Huang et al (2017) and Zhou and Lühr (2017). Huang et al (2017) note that the values of the auroral electrojets (effectively the east-west component of the DF current) derived from Swarm data are larger in the NH than in the SH during local summers. Zhou and Lühr (2017) use CHAMP satellite along-track component of magnetic field to estimate the net ionospheric currents during different seasons.…”

“…In the dawn (dusk) sector the R1 FACs flow into (out of) the ionosphere at latitudes in the poleward part of the auroral oval, while R2 FACs flow out of (into) the ionosphere at latitudes in the equatorward part of the auroral oval. The average pattern of FAC depends on magnetic activity, interplanetary magnetic field (IMF) direction and magnitude, and season (e.g., Anderson et al, 2008;Christiansen et al, 2002;Huang et al, 2017;Iijima & Potemra, 1978;Juusola et al, 2014;Kamide, 1982;Laundal et al, 2018;Papitashvili et al, 2002;Weimer, 2001). The average pattern of FAC depends on magnetic activity, interplanetary magnetic field (IMF) direction and magnitude, and season (e.g., Anderson et al, 2008;Christiansen et al, 2002;Huang et al, 2017;Iijima & Potemra, 1978;Juusola et al, 2014;Kamide, 1982;Laundal et al, 2018;Papitashvili et al, 2002;Weimer, 2001).…”

“…The R1 and R2 FACs, respectively, are associated with the magnetopause currents flowing on the outer layer of the plasma sheet and the divergence of the partial ring current in the inner magnetosphere (e.g., Cowley, 2000). The average pattern of FAC depends on magnetic activity, interplanetary magnetic field (IMF) direction and magnitude, and season (e.g., Anderson et al, 2008;Christiansen et al, 2002;Huang et al, 2017;Iijima & Potemra, 1978;Juusola et al, 2014;Kamide, 1982;Laundal et al, 2018;Papitashvili et al, 2002;Weimer, 2001).…”

Seasonal and Temporal Variations of Field‐Aligned Currents and Ground Magnetic Deflections During Substorms

“…While examining the low‐altitude FAC using single or multispacecraft missions has provided new insights into the small‐scale structuring of the FACs (e.g., Dunlop et al, ; Forsyth et al, ; Gjerloevet al, ; Lühr et al, ), the global FAC system has previously only been accessible through statistical averaging of multiple polar passes (e.g., Gjerloev & Hoffman, ; Iijima & Potemra, ; Juusola et al, , ; Huang et al, ; Papitashvili et al, ; Stauning, ; Stauning et al, ; Wang et al, ; Weimer, ). Since 2012, the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE; Anderson et al, , , ; Green et al, ; Waters et al, ) has provided near‐continuous estimates of the large‐scale polar FAC systems by combining data from the 66 satellites that comprise the Iridium constellation.…”