Two‐step evolution of auroral acceleration at substorm onset

Abstract

[1] The sudden formation of parallel electric fields in the magnetosphere-ionosphere (M-I) coupling system is essential to complete substorm onset. From this standpoint, we focus substorm ignition on field-aligned acceleration by studying the dynamical behavior of auroral kilometric radiation. Field-aligned auroral acceleration shows a distinct two-step evolution at substorm onset: the activation of low-altitude acceleration (h ∼ 4000-5000 km) which corresponds to auroral initial brightening and the subsequent abrupt breakout of high-altitude acceleration (h ∼ 6000-12,000 km) which corresponds to auroral breakup. Cases when only low-altitude acceleration (first-step evolution) is activated are pseudosubstorms. This indicates that the second evolution of field-aligned acceleration divides full substorm from pseudosubstorm. The statistical relationship between the plasma flow burst in the plasma sheet and its response to the M-I coupling region shows that about 65% of flow bursts cause pseudobreakup/initial brightening (low-altitude acceleration) and one third of them develops into full substorm (low-altitude and highaltitude accelerations), while the magnitude of flow velocity does not necessarily distinguish between pseudobreakup and full substorm. This suggests that some plasma flow bursts originate field-aligned current which first enhance low-altitude acceleration, and the increasing field-aligned current induces second acceleration above the preexisting low-altitude acceleration as a consequence of current/current-driven instabilities. In this sense, the substorm is finally ignited in the auroral M-I coupling region.

Azimuthal auroral expansion associated with fast flows in the near-Earth plasma sheet: Coordinated observations of the THEMIS all-sky imagers and multiple spacecraft

“…[20] The multistep activations around the auroral breakup (with intervals of several minutes) have been discussed in previous studies [e.g., Kadokura et al, 2002;Morioka et al, 2010;Saito et al, 2010]. They concluded that the stepwise evolution of auroral breakup was likely common.…”

Energy conversion regions as observed by Cluster in the plasma sheet

“…In a statistical investigation, Morioka et al [2010] showed that the generation of fieldaligned currents and accelerated auroral electrons in the auroral current circuit is tightly coupled to flow burst in the plasma sheet. Indeed, 65% of the flow bursts observed from Geotail data appeared to correspond to the generation of auroral kilometric radiation (AKR) in the auroral acceleration region within the M-I coupling region [Morioka et al, 2010]. Moreover, recent investigations by Marghitu et al [2010] and O.…”

Universal time control of AKR: Earth is a spin‐modulated variable radio source

“…Figure 2 shows the frequency-time spectrogram (f-t diagrams) of AKR observed by GEOTAIL and AL/AU index on 5 December 1995. The arrows in the f-t diagram show the transient AKR with sudden frequency expansion to lower frequencies accompanied with substorm onset [e.g., Anderson et al, 1998;Morioka et al, 2010]. The frequency expansion indicates that the AKR source region extended up to 14,000 km of altitude (see the right-side ordinate).…”

“…The frequency expansion indicates that the AKR source region extended up to 14,000 km of altitude (see the right-side ordinate). Morioka et al, [2010] showed that this AKR breakup appears at auroral breakup and is composed of two source regions: high-altitude (typically 6000-12,000 km) and lowaltitude (typically 3000-5000 km) AKR sources. This sudden frequency expansion of AKR is termed transient AKR in this paper.…”

“…The auroral plasma cavity is also the place where auroral electrons are energized along auroral field lines [e.g., Ergun et al, 1998;Strangeway et al, 2001]. Therefore, it is natural to consider that source regions of AKR are almost identical to the auroral particle acceleration region [Morioka et al, 2008[Morioka et al, , 2010, and such source regions are similar among magnetized planets [Zarka, 1998].…”