Tailward flowing energetic oxygen ion bursts associated with multiple flux ropes in the distant magnetotail: GEOTAil observations

Abstract

An event of tailward flowing energetic (144–7959 keV) oxygen ion bursts was observed in the distant magnetotail (X=−63, Y=+7, Z=−3.8 RE) on February 13, 1994. The observations were made with the HEP‐LD spectrometer on board the GEOTAIL spacecraft. The event was associated with magnetic field signatures characteristic of multiple flux ropes. During the event, which lasted from 1847 to 1907 UT, strong impulsive increases in the oxygen flux were observed. From 1846 to 1900 UT the proton counting rate also exhibited an increase, followed by a decrease until the end of the oxygen event. The oxygen flux was confined to a rather narrow range in polar and azimuthal angle (only 7–10% of 4π was occupied). This implies a streaming distribution or beam‐like structure. Comparison of the particle flow angles with the polar and azimuthal angles of the magnetic field indicates that the ion beam may have been embedded in flux ropes, which may be connecting the polar ionosphere and the distant magnetotail. During the observed oxygen event the ratio is significantly higher than the ratios usually found in the center of the distant magnetotail. There is some evidence that the observed oxygen ions were more efficiently accelerated in this event than hydrogen and helium ions.