Stormtime substorm onsets: occurrence and flow channel triggering

Abstract

Bright auroral emissions during geomagnetic storms provide a good opportunity for testing the proposal that substorm onset is frequently triggered by plasma sheet flow bursts that are manifested in the ionosphere as auroral streamers. We have used the broad coverage of the ionospheric mapping of the plasma sheet offered by the highresolution THEMIS all-sky-imagers (ASIs) and chose the main phases of 9 coronal mass ejection (CME) related and 9 high-speed stream (HSS)-related geomagnetic storms, and identified substorm auroral onsets defined as brightening followed by poleward expansion. We found a detectable streamer heading to near the substorm onset location for all 60 onsets that we identified and were observed well by the ASIs. This indicates that substorm onsets are very often triggered by the intrusion of plasma with lower entropy than the surrounding plasma to the onset region, with the caveat that the ASIs do not give a direct measure of the intruding plasma. The majority of the triggering streamers are "tilted streamers, " which extend eastward as their eastern tip tilts equatorward to near the substorm onset location. Fourteen of the 60 cases were identified as "Harang streamers, " where the streamer discernibly turns toward the west poleward of reaching to near the onset latitude, indicating flow around the Harang reversal. Using the ASI observations, we observed substantially less substorm onsets for CME storms than for HSS storms, a result in disagreement with a recent finding of approximately equal substorm occurrences. We suggest that this difference is a result of strong non-substorm streamers that give substorm-like signatures in ground magnetic field observations but are not substorms based on their auroral signature. Our results from CME storms with steady, strong southward IMF are not consistent with the ~ 2-4 h repetition of substorms that has been suggested for moderate to strong southward IMF conditions. Instead, our results indicate substantially lower substorm occurrence during such steady driving conditions. Our results also show the much more frequent occurrence of substorms during HSS period, which is likely due to the highly fluctuating IMF.

Identification of Auroral Zone Activity Driving Large‐Scale Traveling Ionospheric Disturbances

“…During this period, the IMF was mostly steadily southward at −8 nT, and the SML index displayed numerous irregular variations as it ranged between −500 and −1,500 nT. As is typical for steady, strongly southward IMF conditions (Lyons et al, 2018), numerous strong auroral streamers throughout the period shown in Movie S7 but substorm were few, only one being observed (seen first at 0531 UT after an onset equatorward of the FOVs of ASIs). Only the substorm is identified in Figure 9, since there were far too many streamers to separate into quiet and streamer periods and to distinguish individual events.…”

“…The SML index reflects the fluctuating IMF becoming mostly northward from~0115 to 0245 UT and again from~0600 to 0730 UT. Despite more substorms (5) being identified in the ASI observations than during the above CME storms, as is typical for HSS storms (Lyons et al, 2018), less LSTIDs are identifiable in the ΔTEC measurements. This is most likely due to the lack of the continuing intense streamer activity that occurred during the CME storms.…”

Effects of Substorms on High‐Latitude Upper Thermospheric Winds

“…In addition to the auroral bulge, localized, transient auroral structures, such as poleward boundary intensifications (PBIs) (Lyons et al, 1999(Lyons et al, , 2000(Lyons et al, , 1998Zesta et al, 2002), streamers (Henderson et al, 1998;Nakamura et al, 1993;Rostoker et al, 1987), Harang auroras (Nishimura et al, 2010), etc., occur during substorms. As an integral part of substorm dynamics (e.g., Lyons et al, 2018;Nishimura et al, 2010Nishimura et al, , 2011, such auroral structures are associated with enhanced ionospheric flows (Gallardo-Lacourt et al, 2014;Lyons et al, 2010;Nishimura et al, 2014;Zou et al, 2014). Although the hour-long response of neutrals would normally preclude substantial response to such transient auroras, neutrals have been observed to exhibit clearly defined wind structures that mimic the plasma flows (Zou et al, 2018).…”

Prebreakup Arc Intensification due to Short Circuiting of Mesoscale Plasma Flows Over the Plasmapause

“…One of the classical features of auroral substorms is a sudden intensification of the prebreakup arc near the equatorward border of the auroral zone followed by the substorm breakup in a few minutes (Akasofu, ). The intensification starts (Lyons et al, , , ; Nishimura et al, , ) when the arc is approached by an equatorward moving auroral streamer—the ionospheric signature of earthward‐injected mesoscale plasma flows, MPFs (e.g., Haerendel, ; Sergeev et al, ). This suggests that the reconnection process initiates the breakup (Angelopoulos et al, ; Nishimura et al, ).…”