Abstract
Abstract. It is generally believed that field-aligned currents (FACs) and the ring current (RC) are two dominant parts of the inner magnetosphere. However, using the Cluster spacecraft crossing the pre-midnight inner plasma sheet in the latitudinal region between 10 and 30∘ N, it is found that, during intense geomagnetic storms, in addition to FACs and the RC, strong southward and northward currents also exist which should not be FACs because the magnetic field in these regions is mainly along the x–y plane. Detailed investigation shows that both magnetic-field lines (MFLs) and currents in these regions are highly dynamic. When the curvature of MFLs changes direction in the x–y plane, the current also alternatively switches between being southward and northward. To investigate the generation mechanism of the southward and northward current, we employed the analysis of energetic particle flux up to 1 MeV. For energetic particles below 40 keV, observations from Cluster CIS/CODIF (Cluster Ion Spectrometry COmposition and DIstribution Function analyzer) are used. However, for higher-energy particles, the flux is obtained by extrapolations of low-energy particle data through Kappa distribution. The result indicates that the most reasonable cause of these southward and northward currents is the curvature drift of energetic particles.
Highlights
Abundant current systems existing in the Earth’s magnetosphere play a very important role in energy transformation in different regions (Kuijpers et al, 2014)
It can be seen that the x and y components of the magnetic field have the largest fluctuations, which is consistent with the results obtained from Figs. 2 and 3
It is found that the ULF wave covering a range of frequencies spanning from 4 to 10 mHz can be observed, which is consistent with the typical current density variation in the ∼ 2–4 min period
Summary
Abundant current systems existing in the Earth’s magnetosphere play a very important role in energy transformation in different regions (Kuijpers et al, 2014). Through simulations and observations, numerous studies have shown that the inner-magnetosphere currents have a more complicated structure and distribution than originally thought. In the low latitude, the magnetic-field geometry can be altered significantly into a tail-like shape during storm time (Tsyganenko et al, 2003). One or multiple banana currents can exist in the inner magnetosphere, which makes the link of the current systems more complicated (Liemohn et al, 2013). Field-aligned currents (FACs) have more sophisticated structures except for the known large-scale region-1 and region-2 currents (Mishin et al, 1997; Dunlop et al, 2015a, b). More work is still needed to reveal the true nature of these current systems
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