Abstract
On January 10 and 11, 1997, a coronal mass ejection (CME) encountered the Earth's magnetosphere and produced a moderate geomagnetic storm (Dst = −84 nT). During this event, the GEOTAIL spacecraft was skimming the dayside magnetopause, and observed a number of magnetopause crossings in response to the varying solar wind dynamic pressure. Around 1200 UT on January 10, GEOTAIL was traveling in the vicinity of the magnetopause at 1500 LT. Energetic oxygen ion bursts were repeatedly observed by the HEP/LD spectrometer on board GEOTAIL. This was a period of strong southward IMF related to the magnetic cloud structure of the CME. Two of these events with clear energy dispersion effects lasted from 1050 to 1113 UT, on January 10. This can be explained by invoking a quasi‐steady layered structure of the energetic oxygen ions in the magnetosheath with the higher energy particles further away from the magnetopause than the lower energy ones. The observed energy dispersion is interpreted as a result of a crossing through this spatial layer due to the arrival of a filamentory pressure pulse in the solar wind. It is suggested that the existence of the structured oxygen layer is the result of particle leakage through the dayside magnetopause into magnetosheath.
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