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)

Research Area

• Magnetotail and Plasma Sheet (MPS)

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% 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 630.0 nm 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.

Timeliness

It has now been five years since the initial work of Nishimura et al. [2010] and Lyons et al. [2010], and the group has now expanded their work with numerous follow-on studies. There is therefore sufficient evidence that something of interest is occurring; yet there is yet to be a consensus on interpretation of the data. There is intense scientific interest in the community on this topic, and the discussions have reached a critical stage. Also this focus group will provide a stage to discuss initial results from a new campaign of THEMIS-MMS-ground coordination in year 2015-2017. New redline (6300 Å) data from highly sensitive cameras will also be coming online this winter, and will be available for the next GEM summer workshop in 2015. A focus group on this topic couldn’t be more timely or appropriate for GEM.

Relation to existing focus groups

Currently there exists no FG where the type of activity described here could occur. This FG will work with the “Scientific Magnetic Mapping & Techniques” FG, as well as the “Tail-inner magnetosphere interactions” FG, from the standpoint of ‘bubbles’, and "Magnetic Reconnection in the Magnetosphere" FG. Additionally, once the observable features are better quantified, the results could be fed into the “Geospace Systems Science" FG for better integration with the global models and systems-science methodologies that are the focus of that group.

Goals and deliverables

The primary goal of the focus group is to determine if, when, and under what conditions, localized flux tubes convecting across the polar cap lead to depleted entropy flow bursts within the plasmasheet that can trigger the reconfiguration of the nightside magnetosphere that is known as a substorm. The result of the FG will be a rigorous community assessment of the role of pre-onset polar cap flows and auroral zone streamers (as proxies for magnetospheric flows) on magnetospheric reconfiguration, as well as on localized tail reconnection. In addition to a series of published research papers, the results will be communicated directly to the GGCM modelers, as these new observations provide new constraints and metrics that the models will have to match.

Expected Activities

This will primarily be an intensive data analysis focus group, with immersive discussions and very few presentations. We will invite a small number of speakers (both from GEM and CEDAR) to present their results on selected events, and ask several experienced community members to join as panelists for facilitating discussions with audience. We will also encourage modelers to present simulation results for assisting interpretation of observations and for obtaining physical pictures. One goal is to return to the original GEM model of active data analysis, and we may try a ‘two-screen’ approach to improve interactions and discussions.

A primary objective of the proposed focus group is to reach consensus on the significance of and how to interpret auroral streamers and polar patches, and their associated flow structures, in the context of substorm development. For the first two years we anticipate a few short presentations to provide the framework, then the team will work on identifying a subset of events to analyze in detail (this will also occur well before each summer workshop) – starting first with isolated substorms, then moving onto intensifications embedded in strong activity. In later years, similarities and differences of substorm-time and nonsubstorm-time sequences will be discussed.