Solving the Substorm Puzzle: THEMIS and FAST Observations

1. Title Putting Together the Pieces of the Substorm Puzzle using Observations from THEMIS and FAST J. P. McFadden Collaborators: The Entire THEMIS and FAST Teams

2. Is Current Disruption a viable candidate for substorms? Ignoring the issues of THEMIS onset timing for the moment, are there other reasons to reject CD? CD instabilities require high (Vi-Ve ~Vi >Vith~1400 km/s) cross tail flows associated with growth phase current sheet thinning. Are such flows observed? CD requires breaking the frozen in condition so that dipolarization occurs without inward plasma flow. Do the BBFs associated with dipolarization satisfy E=-VxB?

3. Dipolarizations Growth phase currents, Viy < 400 km/s Viy<400 km/s Growth Phase Currents

4. Currents Break Frozen In

5. BBFs generally satisfy E+VxB=0 on 3 second time scales

6. Dipolarization BBF are Frozen In

7. Caution – must include both ESA and SST ions in moments.

8. What Can THEMIS/FAST say about NENL Model? 1. How does the NENL model generate the substorm aurora? 2. What is going on during growth phase? Is there pre-existing structure? What is the source of the southern-most arc? 3. How does Substorm onset produce so much auroral structure? Does onset arise from a single NENL or is reconnection foamy? 4. What are the energy sources for FACs associated with arcs? 5. What are the dynamics of the Recovery Phase?

9. V B B B B E E E Dipolarization does not make aurora. No dipolarization in steady state!!! Electric fields associated with the dipolarization are inductive and do not map to the ionosphere. It is like a poloidal mode. Dipolarization makes BBFs but not aurora. Dipolarization does not necessarily produce FACs since E does not map to ionosphere. Dipolarization is similar to Drakes bubbles.

10. So where does all the substorm aurora come from?

11. Five Major Phenomena that Generate Auroral FACs • 1. Kinetic Alfven Waves (problem - most aurora are discrete arcs) • Large scale, 2-cell convection (associated with quite time arcs, not substorms) • IMF By induced field aligned twist (Ostgaard 2004,2005) produces a mismatch in reconnected field-line footpoints and associated FACs between ionospheres (this may play a role but Substorms happen when IMF By is small -- so can be major source of FACs.) • Toroidal Resonant Mode Oscillations – prime candidate. • 5. Interchange instability – prime candidate.

12. Substorm Growth Phase – Pre-Midnight During substorm growth phase, the latitude of Region I currents moves south of the open-closed polar cap boundary (indicated by ion precipitation) and becomes adjacent to the Region II current. Shown to the left is a pre-midnight FAST pass during growth phase as determined from AE.

13. Substorm Growth Phase – Post-Midnight High entropy flux tubes prior to substorm onset results in a return flow to the dayside with a small magnetic flux content. The return flow, which maps to the magnetopause at >10 Re on the flank, is restricted to ILAT less than ~67o. Shown to the left is a post-midnight FAST pass during growth phase as determined from AE. Return of magnetic flux is limited by dynamic pressure and high entropy flux tubes - pressure catastrophe

14. THEMIS observations during growth phase Growth Phase Plasmasheet appears uniform - no density structure. Plasma beta shows little variability other then slow trends caused by current sheet thinning

15. FAST Observations of Onset Arc (Mende et al., 2003) Onset arc at northern boundary between dense PS and tenuous PS. Ions fill the source cone. Arc is bounded by a pair of FACs.

16. Substorm Expansion The auroral expansion generally consists of pairs of FACs as determined from cross-track B. (note dipole field not subtracted) Auroral arcs are primarily inverted-V type, not Alfvenic. Expansion boundary is delineated by the transition between hot-dense and cooler-tenuous plasma. Downward currents, small-scale Alfvenic currents, and EMIC waves produce ion conic outflows.

17. Substorm Expansion East-west deflections consistent with electron flux indicating east-west aligned FAC sheets. A net current may be observed across the substorm expansion, but most currents close locally. The expansion is characterized by sharp transition from a dense to tenuous plasma sheet as the s/c moves northward.

18. After Substorm Expansion

19. Mid-tail NENL indicates “foamy” reconnection Multiple current sheet crossings in vicinity of NENL. Rapid variations in ion & electron flux. Variable density Bursty flows. NENL flow reversal Large variations in plasma beta.

20. THEMIS D substorm dipolarization Dipolarization Pseudo-breakup? Counterstreaming field-aligned e- BBF toroidal and poloidal oscillations Pressure increase Large variations in plasma beta can lead to interchange.

21. THEMIS E substorm dipolarization Dipolarization Pseudo-breakup? Counterstreaming field-aligned e- BBF toroidal and poloidal oscillations Pressure increase Large variations in plasma beta can lead to interchange.

22. Complex Azimuthal Structure during Dipolarization

23. Westward Traveling Surge with a Fold Fold in the arc produces a pair of FACs. The inverted-V arc has a dip in the middle where the FAC reverses. Is the WTS the upward current portion of the current wedge? Is the current wedge just the combined interchange vortices?

24. Toroidal Resonant Mode Oscillations or Folds?

25. Signatures of Interchange along Plasma Pause

26. What about aurora during recovery? PBI

27. PBIs appear to be Untwisting Equator-moving Arcs Downward current Upward E-field Flow FAST

28. Summary 1. Weak cross-tail flows associated with current sheet thinning and dipolarizations that exhibit frozen-in (E+VixB=0) plasma invalidate the CD model. 2. Substorm growth phase shows uniform, quiet PS while Region I FAC moves south until it is adjacent to Region II FAC. The narrow return flow channel is bordered by the southern-most arc. 3. Mid-tail NENL at substorm onset indicates “foamy” reconnection. BBFs reaching inner magnetosphere contain large variations in plasma beta that should lead to interchange instabilities. BBFs at dipolarization produce toroidal oscillations that likely produce aurora. 4. Substorm expansions observed at low altitudes contain numerous paired currents indicating local flow channels and vorticies. The northern boundary of the expansion is the transition between dense PS and tenuous PS. 5. Azimuthal expansion of dense PS away from midnight, due to a combination of interchange and return flow, creates a sharp N-S boundary between dense-plasma dipolar field lines and tenuous-plasma stretched field lines, with FACs and arcs at the boundary. 6. Recovery phase contains residual arcs, including PBIs, which appear to have locally closing FACs. This indicates that interchange instabilities and/or field-line twist (mis-matched field line footpoints in the northern and southern hemisphere), maintain aurora long after dipolarization is complete.