Evolution of Dayside Magnetopause Reconnection Exhaust Regions and FTE Genesis: THEMIS Observations

1. S. Eriksson1, J.T. Gosling1, V. Angelopoulos2, J.P. McFadden2, K.-H. Glassmeier3, A. Roux4, H.-U. Auster3, O. le Contel4, and R.E. Ergun1 1LASP, University of Colorado, Boulder, CO, USA 2SSL, University of California, Berkeley, CA, USA 3IGEP, Technische Universitat, Braunschweig, Germany 4CETP, Velizy, France Evolution of Dayside Magnetopause Reconnection Exhaust Regions and FTE Genesis: THEMIS Observations Contact: eriksson@lasp.colorado.edu

2. Cluster Geotail Cluster TC1 Themis Geotail Themis TC1 acknowledgment: ISAS/JAXA “Conjunction Event Finder” http://www.darts.isas.jaxa.jp/stp/cef/cef.cgi

4. 2130-2330 UT

5. Solar wind context: 08 June 2007 2100-0000 UT ACE solar wind data shifted to match TH-B clock angle. Wind shifted to match ACE (Bx, By, |B|). Rw=(257.5, 50.6, 22.7) Re (GSE) Ra=(233.9, -40.4, 10.3) Re (GSE) dR=95 Re Steady solar wind speed and IMF conditions. Gradual dynamic pressure increase (1 to 1.5 nPa) at TH-B magnetopause transition.

6. MP B D C E A

7. MP

8. MP

9. MP

10. Cluster 3 obs. (31 January 2001) of low-energy 10-100 eV dispersed cold ionospheric ions due to magnetopause motion electric field. Adiabatic H+ acceleration and deceleration [Sauvaud et al., AG, 2001]. Themis observed very similar signatures as the FTEs passed the s/c and the same process was likely responsible for the ion dispersion. However, Sauvaud et al. [2001] did not infer relative FTE motion, rather magnetopause boundary crossings.

15. Nakamura and Scholer [2000] hybrid simulation of dayside reconnection (guide field By>0). Expanding bulge assumed to be the signature of FTE. Hall fields appear at magnetosheath current sheet (red circle). A similar Hall field is observed at Themis, but for northward jet (not Southward as in simulation).

16. Walen relation satisfied at TH-B and TH-C TH-B TH-C

17. Some of the first observations of accelerated plasma due to reconnection. ISEE-1 (top) and ISEE-2 (bottom). [Sonnerup et al., 1981] THEMIS speed observations are very similar on 8 June 2007 with the addition of FTEs.

23. Predicted proton distri- butions just inside (left) and outside the magneto- pause (right). Cowley, Rev.Geophys., 1982

30. Russell and Elphic [1979] famous observations of FTEs (ISEE-1 and ISEE-2). Very similar to THEMIS FTEs on 8 June 2007. However, four THEMIS probes on either side of magnetopause provides new context for relation of FTEs to “quasi-steady” reconnection region.

37. Summary/Conclusions: • Four THEMIS probes (B,D,C,E) transition the postnoon subsolar magnetopause in a pearl-on-string configuration on 8 June 2007 2130-2330 UT when the IMF was southward and IMF By<0. TH-A was earthward of the magnetopause. Cowley [1982] predicts positive then negative bipolar BN for FTEs for these IMF conditions. • When TH-B observed northward reconnection jets and an FTE within this reconnection layer, then TH-D,C,E ~simultaneously observed the FTE inside the magnetosphere in agreement with Cowley [1982] bipolar prediction. Field strength decreases away from TH-B suggesting TH-B closest to FTE and that FTE was generated inside the boundary layer.

38. Summary/Conclusions: • When TH-D transitioned the magnetopause, it too observed an FTE in the magnetopause boundary layer that was observed in the magnetosheath by TH-B and inside the magnetosphere by TH-C,E. All four probes observed the positive-then-negative bipolar BN in agreement with Cowley [1982]. The strongest field strength was observed at TH-D with decreasing intensity away from TH-D suggesting this FTE, too, was generated within the boundary layer. • TH-C then transitioned a highly laminar reconnection exhaust with no clear FTEs being observed there. No FTE was observed at the other probes either.

39. Summary/Conclusions: • TH-E entered the boundary layer less than 5 min after TH-C left it. It did not observe a laminar field configuration as observed by TH-C. However, unlike the TH-C crossing, TH-E did observe a strong FTE that was observed in the magnetosheath by TH-C,D. TH-B may have observed the very edge of its compression region. • Unlike all other FTEs during this 2 hour period, the bipolar BN sequence related to the last TH-E magnetopause crossing seemed negative-then-positive with a clear southward plasma flow deflection suggesting a southern hemisphere connectedness (TBD). The magnetosheath conditions were steady, however.

40. Summary/Conclusions: • The axial FTE field appeared to be predominantly in the BM-direction when the FTE was observed within the boundary layer whereas dominated by the BL-direction when observed away from the magnetopause. • These THEMIS observations clearly connect the FTE genesis with the magnetopause boundary layer. A structured exhaust configuration was observed when the FTE was observed as opposed to the laminar TH-C event. What came first (FTE or structured exhaust) remains to be determined.

41. Summary/Conclusions: • The laminar TH-C magnetopause exhaust and its magnetic field configuration is quite reminiscent of so-called bifurcated current sheets bounding solar wind reconnection exhausts [e.g., Gosling et al., 2005]. Two well-defined current sheets was observed by TH-C. • The SCM power in the 48 Hz band showed a near-perfect correlation with reconnection exhausts at TH-B and TH-C. The same 48 Hz band enhancement was observed at TH-D and TH-E when they crossed the magnetopause boundary layer, suggesting that the subsolar reconnection site was active throughout the 2200-2335 UT period, despite the FTE observations.

42. Summary/Conclusions: • The 48 Hz band activity may be related to lower hybrid drift instability [e.g., Gary and Eastman, 1979; Cowley, 1982] which may be important for the generation of the magnetopause boundary layer formation. • The low-energy ion dispersion signatures observed in the magnetosphere at the time of the FTEs are very reminiscent of local ion acceleration caused by the passing FTE [e.g., Cluster observations by Sauvaud et al., 2001].