Fine structures and dynamics in auroral initial brightenings at substorm onsets

Details Fine structures and dynamics in auroral initial brightenings at substorm onsets Fine structures and dynamics in auroral initial brightenings at substorm onsets

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsm43a1698s&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #SM43A-1698

Other

2704 Auroral Phenomena (2407), 2790 Substorms

Scientific paper

The physics that causes auroral expansion during magnetospheric substorm are still unknown. In this presentation, we show four auroral initial brightenings focusing on fine-scale structures and their dynamics, which were observed at substorm onsets. The observations were carried out using all-sky TV cameras with a sampling rate of 30 Hz at Gillam (56.4N, 265.4E) and Fort Smith (60.0N, 248.1E), Canada during January 2- 15, 2008. Two substorms were observed at (Event 1) ~2148 LT and (Event 2) ~0130 LT on January 8 at Fort Smith. Although the initial brightening might start from beyond the field of view of all-sky cameras, developing edges of initial brightenings showed characteristic dynamics and structures, as follows. Event 1: Auroral expansion phase started after several auroral arcs were generated periodically (period ~15s) and propagated westward with velocities of 4-16 km/s which increase with time. Event 2: Initial brightening had peal-beading structure moving eastward with a velocity of ~5 km/s. These repetition features of aurora at substorm onsets indicate that physical process having same repetition periods derived field-aligned currents in the magnetosphere. Initial brightenings of the other two substorms started just in the field of views of all- sky cameras at (Event 3) ~0041 LT on January 7 at Fort Smith and at (Event 4) ~2155 LT on January 15 at Gillam. Characteristic expansion features in the longitudinal direction are seen, as follows. Event 3: Initial brightening had an arc-like structure. It developed eastward with a velocity of ~23 km/s at the first 10 seconds, and the velocity decreased to ~11 km/s at the next 10 seconds. Event 4: Initial brightening had ray- like structures. It developed westward with a velocity of ~20 km/s at the first 10 seconds, and the velocity decreased to ~12 km/s at the next 10 seconds. The velocity of 20-km/s in the ionosphere is roughly projected to be 400-km/s at 10 Earth Radii on the equatorial plane. This velocity is consistent with the typical velocity of high-speed ion bulk flow that is considered to be associated with substorm. From these four fine-scale structures and dynamics of initial brightenings at substorm onset, we will discuss possible plasma instabilities that triggered auroral expansion.

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