TAILWARD FLOWING ENERGETIC OXYGEN-ION BURSTS ASSOCIATED WITH MULTIPLEFLUX ROPES IN THE DISTANT MAGNETOTAIL - GEOTAIL OBSERVATIONS

An event of tailward flowing energetic (144 - 7959 keV) oxygen ion bur sts was observed in the distant magnetotail (X = -63, Y = +7, Z = -3.8 R(E)) on February 13, 1994. The observations were made with the HEP-L D spectrometer oil board the GEOTAIL spacecraft. The event was associa ted with magnetic field signatures characteristic of multiple flux rop es. During the event, which lasted from 1847 to 1907 UT, strong impuls ive increases in the oxygen flux were observed. From 1846 to 1900 UT t he proton counting rate also exhibited an increase, followed by a decr ease until the end of the oxygen event. The oxygen flux was confined t o a rather narrow range in polar and azimuthal angle (only 7 - 10% of 4 pi 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 i onosphere and the distant magnetotail. During the observed oxygen even t the ratio N-O/N-H+ is significantly higher than the ratios usually f ound in the center of the distant magnetotail. There is some evidence that the observed oxygen ions were more efficiently accelerated in thi s event than hydrogen and helium ions.