FG: Testing Proposed Links between Mesoscale Auroral and Polar Cap Dynamics and Substorms

Focus Group Leaders

• Kyle Murphy (NASA GSFC)
• Toshi Nishimura (UCLA)
• Emma Spanswick (University of Calgary)
• Jian Yang (Rice University)

Term

• 2015-2019 (5 years)

Abstract

For decades the debate on substorms had centered on the ‘outside-in’ (reconnection) and ‘inside-out’ (current disruption) scenarios. However, recent ground-based observations have suggested a hybrid model, wherein the substorm is initiated by flows that may originate from the dayside, cross the polar cap, move through the open/closed boundary where they trigger localized reconnection, and then trigger substorm expansion after reaching the near-Earth transition region. This is radically different from the previous and intensely debated proposals. However, there are important disagreements in the community regarding both the interpretation of the ground-based (ASI, redline, and radar) observations and the proposed picture. Furthermore, the complete idea now includes the new proposal that flow structures from within the polar cap trigger localized tail reconnection, an idea which is of major importance for full understanding of tail reconnection, but which has not received much attention in the past. It is the purpose of this proposed focus group to bring together key players in this area to work together on the new questions of the auroral and magnetotail sequence leading to substorm onset, in a CDAW/GEM style format. Though our central focus is on substorms, we will also compare to non-substorm time phenomena because some key features are common in different levels of magnetic activity. Due to the structure of GEM as a working group style meeting, and the potential impact of this new paradigm on understanding the Geospace environment, GEM is the ideal venue through which to rigorously evaluate the newly proposed paradigms. Due to the importance of ground-based observations for the proposed topics, this focus group will encourage collaboration between GEM and CEDAR.

Topics

• How commonly do substorm precursors occur?

Nishimura et al. have suggested >90% (95?) of substorms exhibit a pre-onset streamer. Mende et al. [2011] examined a small subset of these events and found 65% were consistent the scenario, while Kepko recently presented at ICS-12 work based on visual examination of the entire onset list (~400 events) and suggested that at most 25% may be consistent with the scenario.

• What are similarities and differences of PBIs/streamers/plasma sheet flows during isolated substorms, active-time substorms and non-substorm times? The three events presented in the original Nishimura et al. [2010] paper occurred during quite active magnetospheric conditions. These types of events had been observed previously (Henderson), but were considered embedded substorms. A major question is whether these types of ‘embedded substorms’/intensifications are equivalent to other substorms.
• How does the asymmetric and bursty nature of plasma sheet transport and plasma sheet wave instability influence substorm onset and other types of auroral brightening? The pre-onset streamers that were observed prior to onset have a wide range of local time and universal time differences relative to substorm onset time. Validating (or not) these delays is critical for understanding both what leads to the underlying instability and, and for determining the validity of the association. On this latter point, it is well known that during active conditions auroral streamers are commonly observed. A critical question, then, can we quantify auroral or flow evidence for the connection of streamers to onset sufficiently to statistically determine the association.
• How do plasma sheet fast flows/streamers couple to lobe/polar cap phenomena? More recent work has included observations of polar cap flows observed in radar measurements and inferred from diffuse emissions observed by 6300 Å ASIs. These events appear to indicate that the white light pre-onset streamers observed in the THEMIS ASIs begin as polar cap patches and associated flows. There is thus the crucial question of whether the polar cap flows do indeed drive the flows bursts within the plasma sheet that are seen as streamers, both those during the growth phase flows and those that lead to pre-onset flows, and what conditions may determine whether or not a flow leads to an onset.