Geometry of duskside equatorial current during magnetic storm main phase as deduced from magnetospheric and low-altitude observations
We present the results of a coordinated study of the moderate magnetic storm on 22 July 2009. The THEMIS and GOES observations of magnetic field in the inner magnetosphere were complemented by energetic particle observations at low altitude by the six NOAA POES satellites. Observations in the vicinity of geosynchronous orbit revealed a relatively thin (half-thickness of less than 1 RE) and intense current sheet in the dusk MLT sector during the main phase of the storm. The total westward current (integrated along the z-direction) on the duskside at r ~ 6.6 RE was comparable to that in the midnight sector. Such a configuration cannot be adequately described by existing magnetic field models with predefined current systems (error in B > 60 nT). At the same time, low-altitude isotropic boundaries (IB) of > 80 keV protons in the dusk sector were shifted ~ 4° equatorward relative to the IBs in the midnight sector. Both the equatorward IB shift and the current strength on the duskside correlate with the Sym-H* index. These findings imply a close relation between the current intensification and equatorward IB shift in the dusk sector. The analysis of IB dispersion revealed that high-energy IBs ( E > 100 keV) always exhibit normal dispersion (i.e., that for pitch angle scattering on curved field lines). Anomalous dispersion is sometimes observed in the low-energy channels (~ 30–100 keV). The maximum occurrence rate of anomalous dispersion was observed during the main phase of the storm in the dusk sector.