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Development of the auroral absorption substorm: Studies of pre-onset phase and sharp onset using an extensive riometer network |
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| Authors: | H Ranta A Ranta |
| Institution: | Geophysical Observatory, SF-99600, Sodankylä, Finland Department of Environmental Sciences, University of Lancaster, Lancaster LA1 4YQ, England |
| Abstract: | The development of an auroral absorption substorm has been studied using riometer measurements in the northern hemisphere. In the events studied, the onset is preceded by an absorption bay which begins to develop before the onset. The bay may occur between L-values 3–19 and can cover as much as 150° of geomagnetic longitude, generally in the same longitudinal sector where the substorm breaks up and to the west of it. Whereas the substorm breaks up at or near the midnight meridian, the preceding bay may, in some geophysical conditions, appear in the afternoon sector. The preceding bay moves southward with a velocity between 60 and 600 ms?1, intensifying during the movement. This equatorward movement is consistent with an E × B drift in a cross-magnetotail electric field of between 0.5 and 1 mV m?1. The absorption at the onset exceeds that in the bay, and in the sector of break up the absorption shows a minimum just before the onset; to the west-of the break up the preceding bay continues its southward movement. In 14 cases studied, the sharp onset moved to the west with a velocity of 1–31 km s?1, median 6 km s?1. The onset was seen at higher L-values to the west than in the break-up sector. This applied also to the preceding bay. Whereas most onsets showed westward movement, in only about half of the cases studied was there movement towards the east. The injection area affected during the first minute of the onset was typically 1–2 L-value units, but as much as 30° of geomagnetic longitude. The onset later spread to cover 1–10 L-value units, and up to 130° of longitude. The contouring method used in the analysis of the data from the riometer is described in the Appendix. |
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