Chorus wave generation near the dawnside magnetopause due to drift shell splitting of substorm-injected electrons

We study the relationship between the electron injection and the chorus waves during a substorm event on 23 March 2007. The chorus waves were detected at high geomagnetic latitude (\~70\textdegreeS) Antarctic observatories in the range of 0600\textendash0900 h in magnetic local time (MLT). Electrons drifting from the injection event were measured by two LANL spacecraft at 0300 and 0900 MLT. The mapping of auroral brightening areas to the magnetic equator shows that the injection occurred in an MLT range of 2200\textendash2400. This estimate is consistent with observations by the THEMIS A, B, and D spacecraft (which were located at 2100 MLT and did not observe electron injections). Our backward model tracing from the magnetic equator near the dawnside magnetopause (which magnetically connects to the Antarctic observatories) also supports the deduced injection region. Since chorus waves are believed to be generated through the electron cyclotron instability by an anisotropic temperature distribution, we examine, by performing forward model tracing, whether the electrons injected during this substorm form a pancake-like pitch angle distribution when they arrive near the dawn-side magnetopause. We find that the onset time of the modeled pitch angle anisotropy is consistent with that of the observed chorus waves. We conclude that the development of the anisotropy is due to particle drift shell splitting.