Larry Kepko NASA/Goddard Space Flight Center

1. 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

2. ICS-9, Seggau, Austria - 2008 • 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.

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

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

5. 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?

6. What is a substorm? In the question of what is or is not a substorm, there is (often) an underlying, tacit disagreement: • 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 Or: • Isolated flow bursts, pseudo-breakups and substorms are fundamentally different phenomena.

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

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

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

10. Quick Review 0523:15 - 6300 activity 0526:47 - 5577 arc forms 0530:07 - WL Beads 0530:31 - Poleward Exp We observe a pre-onset, equatorward moving diffuse auroral patch Kepko et al., 2009 GRL

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

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

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

14. Growth Rays Beads Expansion Beads occur just after flow impact & after geo SCW perturbations

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

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

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

18. 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

19. Backup

20. White light auroral onset North West East reflection South Gillam THEMIS ASI

22. J|| 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

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

24. Gillam multi-spectral all sky imager White Light Similar response as 5577 4278 Å Responds to medium energy electrons (keV and up) 5577 Å Responds to both protons and electrons (>1 keV) 6300 Å Responds to low energy electrons (eV - 1 keV) “Energetic”