Azimuthal evolution of the aurora in tens of seconds around the expansive phase onset

Details Azimuthal evolution of the aurora in tens of seconds around the expansive phase onset Azimuthal evolution of the aurora in tens of seconds around the expansive phase onset

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsm21b..03l&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SM21B-03

Physics

2704 Auroral Phenomena (2407), 2744 Magnetotail, 2772 Plasma Waves And Instabilities (2471), 2790 Substorms

Scientific paper

Using data from the implementation phase (2004-2007) of the THEMIS All-Sky Imager (ASI) array, we carry out a study of the temporal evolution and fine structure of the substorm onset arc in the tens of seconds around expansive phase (EP) onset. The continent-wide array provides an unprecedented combination of mesoscale imaging with relatively high time resolution (3 second cadence), subject to the limitations imposed by viewing conditions. We have identified eight events in which we have excellent viewing of the onset aurora. Our results show the following salient features: 1. The initial brightening occurs along an azimuthally extended region of the arc. This brightening typically spans ~1 hour of MLT. In cases that the camera provides ideal coverage of the arc, the length of the brightening does not noticeably change during the first tens of seconds of EP. 2. Periodic structures along the arc emerge from ~10 s before the EP onset, and significantly brightens within ~10 s after the onset, constituting the most unambiguous and prominent feature from the optical auroral aspects of the substorm EP onset. Those periodic "wave" structure, on its initial formation and brightening, are usually well aligned (<15 deg) with the L-shell. The wavelength is about 40-130 km. These structures either remain stationary or propagate east or west during the initial brightening. 3. About 20-30 s after the onset, the brightening arc is no longer aligned with the L-shell. Titled structure, typically appears as a "bifurcation" of the initial onset arc, starts to develop and leads the poleward progression of the substorm auroras. We interpret these results as follows. First, the auroral breakup is on field lines connected to a region in the CPS that is unstable over an extended azimuthal range, but in a limited radial range (the latter a conclusion based on contemporaneous riometer observations). Second, the instability grows very rapidly across this initially unstable region but the region itself does not grow in size for tens of seconds after the initial onset. Third, the alignment between the onset-associated periodic structures and the magnetic L-shell gives strongly hints that they are controlled by near-Earth magnetotail dynamics. If that is true, then waves associated with the breakup in the CPS appear to have no preferred propagation direction. We finish by discussing the implications of these results for substorm onset mechanism theories.

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