Recent THEMIS and coordinated GBO measurements of substorm expansion onset: Do we finally have an answer? Larry Kepko NASA/Goddard Space Flight Center 2011 ILWS Science Workshop Beijing, China

ICS-9, Seggau, Austria  45 minute discussion: “What is a substorm?” “[...] the community is more interested in traveling than in working, especially since the same ideas about substorm processes are being debated today as were being debated twenty years ago. One cynic has even suggested a conspiracy theory of substorms, that the principals have agreed not to solve the problem in order to keep the field alive.” - Chris Russell, 1994.

A substorm is a global process by which the magnetosphere unloads stored energy, arising from imbalanced convection. IMF turns South Magnetopause rate Tail rate ~20 min Substorm

Eastward electrojets Westward electrojets & substorm current wedge But the solar wind doesn’t always play nice.

Distilling the substorm problem to an inside- out/outside-in controversy ignores the question of what is happening all the time ‣ How do CD models account for non-substorm activations, that appear to have similar temporal sequence as substorm onsets? Does the magnetosphere generate events with same characteristics (aurora, Pi2, flows) etc., in 2 different manners?

What is a substorm? In the question of what is or is not a substorm, there is (often) an underlying, tacit disagreement: Or: Isolated flow bursts, pseudo-breakups and substorms are a continuum of the same underlying physical process(es), and differ only in scale (region) or magnitude/intensity Isolated flow bursts, pseudo-breakups and substorms are fundamentally different phenomena.

Distant X-line Transition region Earth Equatorward boundary Open/closed boundary Dipolarization Pi2 & SCW white light Expansion Traditional NENL NENL Flow Nothing? Time Auroral Zone Magnetosphere

Ionosphere Magnetosphere White light ASI 8 Rx Auroral onset region The reconnection site and auroral onset are not magnetically connected

SDO AIA 094SDO AIA 304 SST ESA 24 hours SDO AIA 4500

0523: activity 0526: arc forms 0530:07 - WL Beads 0530:31 - Poleward Exp Quick Review We observe a pre- onset, equatorward moving diffuse auroral patch Kepko et al., 2009 GRL

Distant X-line Transition region Earth Equatorward boundary Open/closed boundary Dipolarization Pi2 & SCW 6300 diffuse aurora (this talk) white light Expansion NENL Flow Traditional NENL Time Auroral Zone Magnetosphere

Distant X-line Transition region Earth Equatorward boundary Open/closed boundary Pi2 & SCW Instability growth ??? Poleward expansion Rarefaction Flow Expansion NENL Flow impact effects ? NENL Flow Current Disruption (CD) Time Auroral Zone Magnetosphere (further) dipolarization Two impulses NENL CD 6300 white light

Two drivers for the ballooning believers: 1) Theory & modeling (always appealing!) 2) Auroral beads at onset

RaysBeadsExpansionGrowth Beads occur just after flow impact & after geo SCW perturbations

Distant X-line Transition region Earth Equatorward boundary Open/closed boundary (further) dipolarization Pi2 & SCW??? Poleward expansion Rarefaction Flow Expansion NENL Flow impact effects ? white light NENL Flow Time Auroral Zone Magnetosphere Instability growth Current Disruption (CD) 6300

Azimuthal motion Distant X-line Transition region Earth Equatorward boundary Open/closed boundary (further) dipolarization Pi2 & SCW Flow ??? Instability growth ??? Poleward expansion Rarefaction Flow Expansion NENL Flow impact effects ? white light Nishimura/Lyons (triggered CD) Three impulses Time Auroral Zone Magnetosphere

Azimuthal motion Distant X-line Transition region Earth Equatorward boundary Open/closed boundary (further) dipolarization Pi2 & SCW Flow ??? Instability growth ??? Poleward expansion Rarefaction Flow Expansion NENL Flow impact effects ? white light Nishimura/Lyons (triggered CD) Three impulses Time Auroral Zone Magnetosphere 1. What are the ground effects of the first flow impact? 2. What are the ground effects of the second flow impact? 3. When do particle injections occur? 4. Two-stage dipolarization?

Conclusions Majority of THEMIS substorm papers support NENL picture Dozens of papers showing flows prior to auroral onset, consistent temporal sequence of events Nishimura/Lyons has support (but lacking in situ observations) Almost no magnetospheric evidence for CD (theory & ASI) Still lack a theory of auroral arcs NENL (flow burst) model explains the continuum of activity Other models may require different modes Coordinated measurements has revolutionized substorm studies

Backup

White light auroral onset Gillam THEMIS ASI South North EastWest reflection

J || E J Generation of 5577 emissions ‣ Current system of the flow burst ‣ Flow diversion, magnetic shear ahead e.g., Chen & Wolf (1993); Nakamura et al. (2001); Kepko et al. (2004) Similar cartoon for Nishimura

Distant X-line Transition region Earth Equatorward boundary Open/closed boundary Dipolarization Pi2 & SCW 6300 diffuse aurora (this talk) white light Expansion NENL Flow Traditional NENL Time Auroral Zone Magnetosphere

White Light Similar response as Å Responds to low energy electrons (eV - 1 keV) 5577 Å Responds to both protons and electrons (>1 keV) 4278 Å Responds to medium energy electrons (keV and up) Gillam multi-spectral all sky imager “Energetic”