Dipolarization fronts and associated auroral activities: 2. Acceleration of ions and their subsequent behavior

Abstract

[1] We present case studies of THEMIS multipoint observations of ion distributions in the magnetotail plasma sheet at various locations upstream of earthward-propagating dipolarization fronts. Observations made near the neutral sheet show a characteristic signature, enhancements of earthward-moving ion fluxes about 30 s before dipolarization front arrival. In previous studies, this signature has been well explained as front-reflected ions confined to a region characterized by their gyroradii over the background B z field that coexist with the ambient population. However, at higher latitudes near the plasma sheet boundary layer, observations suggest that earthward-moving ions appear a few minutes earlier than at the central plasma sheet, indicating that the ions reflected at the same dipolarization front could access farther toward the Earth at higher latitudes. These observed phenomena, as also stated in our companion paper, are associated with transient intensifications of proton auroral brightness, which suggests a direct connection between magnetospheric and ionospheric signatures during geomagnetic disturbed conditions. We carry out numerical simulations and theoretical analysis of ion dynamics to interpret and reproduce these observations, to improve our understanding of interactions between earthward-propagating fronts and the ambient plasma in the near-Earth magnetotail, and to establish the proton auroral effects of dipolarization fronts.

Dipolarization fronts and associated auroral activities: 1. Conjugate observations and perspectives from global MHD simulations

“…This difference has been explained by Zhou et al [2010]. In our companion paper we will further discuss the almost unrestricted streaming of these high‐latitude ions along field lines toward the Earth [ Zhou et al , 2012]. In Figure 3g, we show the differential flux of ions at P5 versus pitch angles.…”

“…In our resistive MHD simulation, however, particle energization or plasma heating by DFs is not present because most dissipation occurs in regions where the resistivity is relatively large, e.g., the diffuse region of magnetic reconnection. We will further investigate energization of reflected ions by DFs in our companion paper using theoretical analysis and test‐particle simulations [ Zhou et al , 2012].…”

Energetic ions in dipolarization events

“…In a number of papers, Zhou et al [2010Zhou et al [ , 2011Zhou et al [ , 2012aZhou et al [ , 2014 have emphasized observations of energetic ions of a few tens of keV preceding the arrival of dipolarization fronts, coincident with an increase of the plasma density and flow velocity. They attributed these observations to ion reflection and acceleration at the propagating front and supported this view with test particle investigations in a simplified model.…”

Current reduction in a pseudo‐breakup event: THEMIS observations

“…It is noteworthy that the DF observed at THE preceded that observed at THA (Figure 5c) by several seconds, which is also consistent with the picture presented in the cartoon. The nonuniform compressional effect ahead of the DF between the plasma sheet boundary layer and the central plasma sheet may be caused by an ion DF-reflection process, which has been reported recently in observations and particle simulations [Zhou et al, 2012a[Zhou et al, , 2012b.…”