1 / 30

THEMIS: Tail Reconnection triggering substorm onset

THEMIS: Tail Reconnection triggering substorm onset.

carol
Download Presentation

THEMIS: Tail Reconnection triggering substorm onset

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. THEMIS: Tail Reconnectiontriggering substorm onset • Vassilis Angelopoulos, James P. McFadden, Davin Larson, Charles W. Carlson, Stephen B. Mende, Harald Frey, Tai Phan, David G. Sibeck, Karl-Heinz Glassmeier, Uli Auster, Eric Donovan, Ian R. Mann, I. Jonathan Rae, Christopher T. Russell, Andrei Runov, Xu-Zhi Zhou, Larry Kepko Also: Thanks to Sabine Frey for the orbits/designand Sergey Apatenkov for SCW modeling • Mission overview, and the substorm problem • Overview and event selection • Event #1 2008-02-26 (reconnection onset) • Other events (2008-02-16, 22: reconnection onset) • Counter example: 2008-01-29 • Substorm Picture Revis(it)ed • Remaining questions

  2. CurrentDisruption Z Reconnection AuroralEruption X X Y Introduction: Definitions Substorms, the three main elements Courtesy: Pulkkinen & Wiltberger X Y Courtesy: H. Frey

  3. dJ dJ dJ FP FC FB The current disruption region: Ballooning Roux et al., 1985 • Three plasma forces in equilibrium: • FCurvature+FPlasma+FBfield =0 • Assume dV undulation • If dFp> dFC it grows • Plasma expelled • Linear instability criterion is: • 0 < kC < kP(2+b)/4 • kC, kP, kB : inverse scale sizes • b<2 and kC low enough: • 6<R<10 RE • Mode stable in linear MHD regime… • …but unstable in the non-linear (right) ,in the Hall-MHD and fully kinetic regimes Earth Tail Horton et al., 2005

  4. The current disruption region:A zoo of instabilities WHERE: fci = proton gyrofrequency; fLH = lower hybrid frequency Lcs= current sheet thickness; Lpx= pressure gradient scale size ri = proton gyroradius; vA = Alfvén speed

  5. The reconnection (Rx) regionThe phenomenon • Rapid release of magnetic energy into plasma energy • Performed by acceleration and heating of outflow plasma • Rate depends on local Alfvén speed and outflow capability • Proposed at the Sun and later in the magnetosphere • Plasma resistivity alone is too low to explain fast onset, but… • Petschek showed that shock formation around Rx linemakes the rate insensitive to the value of the resistivity

  6. Electron skin depth: Ion skin depth: Electron beta: The reconnection (Rx) region…the phenomenon • Reconnection rate depends on external flow coupling to whistler waves • Whistlers control reconnection rate as they mediate electron physics to ion outflow • Rate insensitive to simulation model as long as whistler mode is accounted for • Hall MHD is adequate for rate regardless of term is responsible for collisionless resistivity Electronpressure Electron inertia Hall term Alfvén waves Whistler waves Dispersive Alfvén waves Pritchett and Coroniti, 2001

  7. A simpler task: Distinguish BetweenTwo Models for Substorm Onset Magnetic Reconnection: Explosion at 25RE Current Disruption: Implosion at 10RE

  8. : Ground Based Observatory THEMIS Mission elements Probe conjunctions along Sun-Earth line recur once per 4 days over North America. … while THEMIS’s space-based probes determine onset of Current Disruption and Reconnection each within <10s. Ground based observatories completely cover North American sector; determine auroral breakup within 1-3s …

  9. EFIs EFIa SCM ESA BGS SST Operations UCB FGM Tspin=3s Instrument I&T UCB Ground Mission overview D2925-10 @ CCAS Release Probe instruments: ESA: ElectroStatic Analyzer(coIs: Carlson and McFadden)SST: Solid State Telescopes (coI: Larson)FGM: FluxGate Magnetometer(coIs: Glassmeier, Auster & Baumjohann)SCM: SearchCoil Magnetometer (coI: Roux) EFI: Electric Field Instrument (coI: Bonnell) Encapsulation & launch Mission I&T UCB

  10. P1 P2 P3 P4 P5 TH-B TH-C TH-D TH-E TH-A 2007-03-23 2007-06-03 Launch=2007-02-17 YGSE 2007-07-15 XGSE 2007-08-30 2007-12-04 First 10 months Angelopoulos, 2008 Space Sci. Rev. Submitted

  11. TH-B TH-C TH-D TH-E TH-A P1 P2 P3 P4 P5 First year baseline orbit YGSE Dayside 12008-08-08 XGSE Tail 12008-02-02 Angelopoulos, 2008 Space Sci. Rev. Submitted

  12. Feb-26 Jan-29 1st Tail Neutral Sheet Crossings • Orbit was a North-South scan. As a result: • P1, P2 at 30 and 20 RE saw tailward flows/beams only when dZNS<1RE • When P1, P2 had 5RE >dZNS >1RE they observed classical NFTEs at onset • Earthward flows are more frequent than tailward (plasma sheet expansion) • Energetic particle beams were not observed (though we planned for them) • A number of events (Feb. 16 to Mar. 1) are amenable to timing 1st tail season (Jan-Mar 2008) Feb-26 Mar-01 Feb-22 Feb-18 Feb-14 Feb-10 Feb-6 Jan-29 Feb-2 Midnight Pre-midnight

  13. x Substorm study on Feb 26, 2008 Probe locations 2008-02-26 04:50UT P3 P1 P2 P4 P5 y

  14. First Event Second Event

  15. First Event Second Event

  16. Pi2 Signatures First Event Second Event

  17. First Event P1 P1 (TH-B) P2 (TH-C)

  18. Second Event Tail substorm signatures: Flows, Reconnection, Dipolarization

  19. Middle Panels: Transformation to • deHoffman-Teller frame • Bottom panels:Walen test of stress balance • Caveats: • Need to include energetic particle contribution to velocity • Aliasing from temporal variations

  20. Middle Panels: Transformation to • deHoffman-Teller frame • Bottom panels:Walen test of stress balance • Caveats: • Need to include energetic particle contribution to velocity • Aliasing from temporal variations • Note: When peak velocity considered • match to Alfven speed is better

  21. Combined substorm signaturesfor the second event: AE, Pulsations, Aurora

  22. TRx 2nd 2nd 1st 1st 3rd 3rd TAI TCD Timing of ground and space:

  23. A different opinion:2008-01-29 [Lui et al, 2008] P5 P1 P2 Velocity P3 P4 P5 P1 P2 Flows P3 P4

  24. P1 (TH-B) Detail 2008-01-29 Vz < 0 means Northward, Toward the neutral sheet Negative Bz impliestailward moving flux rope, near neutral sheet.

  25. 2008-Feb-16 event Liu et al., 2008

  26. 2008-Feb-22 event Gabrielse et al., 2008

  27. 2008-Mar-01 event Runov et al., 2008

  28. 2nd 2nd 2nd 1st 1st 1st 3rd 3rd 3rd Summary Note: this is not the classical time sequence: Aurora brightens before near Earth dipolarization This holds true on several events considered Outside-In model Inside out model Reconnection Aurora Current Disruption THEMIS finds:

  29. Caveats Caveats on Feb 26 event: • The plasma sheet was cold (Ti ~ 1keV) and dense (1/cc) • Communication speeds slow: perhaps reason for the long delays? • The first onset (0400 UT) might not be an isolated substorm (TBD). • Have requested Greenland mags and will collate KUUJ data Caveat on all 1st tail season events: • There have been no events with 4 probes exactly at the neutral sheet • No simultaneous observations of reconnection outflows and current disruption Remedy: • Second tail season • Reconfigured orbit to result in more neutral sheet encounters 2009 four-probe conjunctions: Nominal criteria: dzNS < 5 Re 63h+100h+93h=256h (comparable to 2008) Near neutral sheet: dzNS < 2 Re: 43h+ 95h+37h=175h Exactly at neutral sheet: dzNS < 1 Re: 30h+ 63h+10h=103h

  30. Open Questions • How does Rx communicate with and power aurora so fast (96s)? • What preconditions tail to reconnect? • Spontaneous tearing? • Driven by ballooning or KH instabilities and cross-scale coupling? • How can mapping be so distorted? • Need to model stretching using THEMIS data for validation (MHD, Tsyganenko) • Non-monotonic flux tube volume evolution vs. distance ought to be sources of current? • Consequences of tail stretching for tail stability, particle dynamics and wave growth?

More Related