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

Evolution of Dayside
Magnetopause Reconnection
Exhaust Regions and FTE
Genesis: THEMIS Observations
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
University of Colorado, Boulder, CO, USA
2SSL, University of California, Berkeley, CA, USA
3IGEP, Technische Universitat, Braunschweig, Germany
4CETP, Velizy, France
Contact: eriksson@lasp.colorado.edu
acknowledgment: ISAS/JAXA “Conjunction Event Finder” http://www.darts.isas.jaxa.jp/stp/cef/cef.cgi
2130-2330 UT
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
Gradual dynamic pressure increase
(1 to 1.5 nPa) at TH-B magnetopause
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.
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).
Walen relation satisfied at TH-B and TH-C
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.
Predicted proton distributions just inside (left)
and outside the magnetopause (right).
Cowley, Rev.Geophys., 1982
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.
• 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
• 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.
• 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.
• 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.
• 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.
• The laminar TH-C magnetopause exhaust and its
magnetic field configuration is quite reminiscent of socalled 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 nearperfect 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.
• 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].