Contrasting electron acceleration processes during two substorms

Abstract Although energetic particle injections during substorms have been observed near geosynchronous orbit since the late 1960s, how the injected particles are accelerated during a substorm is still not fully understood. Studies show that injections are associated with earthward propagating dipolarization fronts and suggest that particle acceleration can occur due to a combination of betatron acceleration during earthward convection and acceleration in the vicinity of magnetotail reconnection sites. Since observed energetic particle distributions vary from substorm to substorm, it is important to study acceleration mechanisms as a function of different substorm magnetotail configurations. In this paper, we discuss electron acceleration for two substorm events featuring different magnetotail configurations and particle acceleration processes. The study utilizes in situ observations, global MHD simulations, and large‐scale‐kinetic (LSK) simulations. The two substorms in question occurred on 15 February 2008 and 15 August 2001. The 15 February 2008 event was observed by the THEMIS spacecraft at x GSE  ~ −10 R E , while the 15 August 2001 event was observed by Cluster spacecraft at x GSE  ~ −20 R E . Earthward propagating dipolarization fronts are found in both the observations and the MHD simulations, which are driven by the solar wind conditions that existed during those events. The two simulations show that very different magnetotail configurations are formed for each event, with contrasting flows, magnetic reconnection configuration, and plasma sheet structure. We find that for the 15 February 2008 event reconnection is localized with a narrow region of high‐speed flow. During this event, the electron velocity distribution functions f ( v ) are found to be anisotropic with f ( v ⊥ ) >  f ( v || ), where || and ⊥ correspond to parallel and perpendicular to magnetic field. In contrast, for the 15 August 2001 event, there is a wide neutral line extending across the tail with low‐speed flows. For this event, the distribution functions are anisotropic with f ( v || ) >  f ( v ⊥ ), the opposite sense as the 15 February 2008 event. We are able to relate these differences to the different magnetotail configurations seen during each event. These differences lead to contrasting occurrence rates of nonadiabatic acceleration. The differences between the magnetotail configurations during these two substorms may result from differences in the location of dayside reconnection. On 15 August 2001 the reconnection occurs on the flanks of the magnetopause, while on 15 February 2008 the reconnection occurs nearer the subsolar point.