Storm time current distribution in the inner equatorial magnetosphere: THEMIS observations

Abstract

For the first time, the current density distribution in the inner equatorial magnetosphere ranging from 4 to 12 RE (RE is the Earth radius, 6371 km) has been obtained by using Time History of Events and Macroscale Interactions during Substorms (THEMIS) (P3, P4, and P5) three point magnetic measurements. This study mainly focuses on the storm events when the constellation of the three THEMIS spacecraft has relatively small separation distance. Two cases with different storm activities are first displayed to illustrate the effectiveness of the method. The inner magnetospheric equatorial current distribution ranging from 4 to 12 RE is shown through statistical analysis. The features of current density are separately analyzed for the storm main phase and the recovery phase. The statistical study reveals that with increasing radial distance the predominant ring current density reverses from Eastward (below r = 4.8 RE, where r is the geocentric radial distance) to Westward, but that the distribution behaves differently for the two phases of activity. During the main phase, both the westward and eastward current are enhanced by added signal and are more dynamic so that both radial profile and magnetic local time (MLT) structure is obscured. During the recovery phase, the radial profile of the westward current is smooth and peaks, then falls, between r = 5–7.5 RE showing some MLT dependence in this region. Beyond r = 7.5 RE, the current is lower and nearly constant and shows little MLT variation. The results also suggest that the change from eastward to westward current depends on the storm phase and hence storm activity.

Multi-spacecraft Current Estimates at Swarm

“…The curlometer can be applied to residual measured fields which result after first subtracting a zero-current model field from the measured magnetic field, however, and this minimises such effects. Indeed it is necessary in the high field regions of the magnetosphere and ionosphere (Yang et al 2016;Dunlop et al 2015). The use of a defined model field containing no current density is usually sufficient and of course it is usually desirable that any de-trending of the data in this way should not remove any real currents which may be present.…”

Strong southward and northward currents observed in the inner plasma sheet

“…The huge progress in satellite deployments makes it possible for direct observation of the inner-magnetosphere current system. It is believed that the magnetosphere and ionosphere are linked through a ring current (RC) and FACs (e.g., Le et al, 2004;Zhang et al, 2011). Therefore, many investigations are mainly focused on these two current systems, from high (e.g., Potemra, 1976, 1978;Wang et al, 2006;Dunlop et al, 2015a) to low latitudes (e.g., Vallat et al, 2005;Shen et al, 2014;Yang et al, 2016).…”

Strong Southward and Northward Currents Observed in the Inner Plasma Sheet

“…It is believed that the magnetosphere and ionosphere are linked through a ring current (RC) and field-aligned currents (FACs) (e.g., Le et al 2004;Zhang et al 2011). Therefore, many investigations are mainly focused on these two 25 current systems, respectively, from high latitude (e.g., Iijima and Potemra 1976;1978;Wang et al, 2006;Dunlop et al, 2015b) and low latitude (e.g., Vallat et al 2005;Shen et al 2014;Yang et al 2016). The region from low to middle latitude, which is the key area for the inner magnetosphere current link, however, has received less attention.…”