Re-evaluation of Observations for the February 26, 2008 Substorm Reported to Show Tail Reconnection Triggering Substorm Expansion Onset

Details Re-evaluation of Observations for the February 26, 2008 Substorm Reported to Show Tail Reconnection Triggering Substorm Expansion Onset Re-evaluation of Observations for the February 26, 2008 Substorm Reported to Show Tail Reconnection Triggering Substorm Expansion Onset

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsm43a1722l&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #SM43A-1722

Physics

2704 Auroral Phenomena (2407), 2721 Field-Aligned Currents And Current Systems (2409), 2744 Magnetotail, 2790 Substorms

Scientific paper

A recent article from the THEMIS program reported that tail reconnection triggered a substorm expansion onset on February 26, 2008. We have re-examined the observations of that substorm with the following findings: (1) the conventional signatures of reconnection at P1 (at Xgsm = -21.5 Re) occurred later by ~1-2 min than the dipolarization activity onset at P4 (at Xgsm = -10.2 Re) and P3 (at Xgsm = -10.9 Re), indicating dipolarization onset in the near-Earth region preceded tail reconnection onset in the mid-tail; (2) the unconventional signatures used to indicate reconnection onset at P1 is subject to an alternative interpretation of plasma sheet thinning; (3) observations at P2 (at Xgsm = -17.2 Re) shortly after the claimed reconnection onset at P1 showed discrepancy from the expected reconnection signatures (positive excursion of Bz accompanied by tailward plasma flows); (4) the observed electron energy flux at P1 at reconnection onset was too weak to be responsible for the observed auroral intensity at Gillam, indicating that reconnection activity at P1 was not directly responsible for the auroral activity on the ground; (5) a substorm expansion started at ~0403 UT on that day and was centered around Iceland located in the midnight sector, suggesting the possibility that auroral activity at Gillam and reconnection onset at P1 might be related to substorm intensification at a later stage of this substorm. This re-evaluation shows that the reported event is not a compelling case of tail reconnection triggering substorm expansion onset. Furthermore, the reported case is more consistent with current disruption than with tail reconnection to be the trigger for the substorm expansion onset.

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