The Radial Propagation Characteristics of the Injection Front: A Statistical Study Based On BD-IES and Van Allen Probes Observations
Electron flux measurements outside geosynchronous orbit (GSO) obtained by the BeiDa Imaging Electron Spectrometer instrument on board a 55° inclined GSO satellite and inside GSO obtained by the Van Allen Probes are analyzed to investigate the temporal and spatial evolutions of the substorm injection region. In 1 year data started from October 2015, 63 injection events are identified. First, our study shows that the injection signatures can be detected in a large radial extent in one single event, for example, from L ∼ 4.1 to L ∼ 9.3. Second, injection onset times are derived from the energy dispersion of particle injection signatures of each satellite. The difference of the onset times among satellites reveals that the injection boundary, termed as "injection front" in this paper, can propagate both earthward and tailward with a speed varying from a few km/s to ∼100 km/s. Third, evolutions of the upper-cutoff magnetic moments (μuc) of injected electrons are analyzed, upon which the injection events are classified into two categories. In one category, the μuc observed by two radially separated satellites are equal taking into account the error caused by the finite width of energy channels, whereas in the other category, μuc at lower L shells are smaller than those at higher L shells.