Energetic oxygen ion bursts in the distant magnetotail as a product of intense substorms: Three case studies

On the basis of Geotail high energy particle ‐ low energy particle detector (HEP‐LD) observations, this paper reports on three energetic (144 – 4000 keV) oxygen burst sequences detected in the distant tail (X gse = −40 to −66 R E ) and their relation to substorm signatures. Those energetic oxygen ion bursts lasted only 20 to 30 min and exhibited strong beam‐like structures. Two of the events (at about 1000 and 1900 UT on February 13, 1994) occurred in tailward flowing plasma after the flow direction changed from earthward to tailward; geostationary and ground based observations detected intense substorm activity during these periods, and the local magnetic field component B z assumed predominantly negative values. The occurrence of a magnetic field with southward polarity and oxygen bursts embedded in tailward flowing plasma is consistent with the basic signatures of reconnection (formation of a neutral line) in the magnetotail. The third energetic oxygen burst with earthward flow was observed relatively close to Earth on August 27, 1993, X= −40 R E . No reversal in the plasma flow direction was seen, and the magnetic field polarity was essentially positive throughout the event. This is consistent with a “near‐Earth” neutral line that had formed beyond X=−40 R E . We conclude that (1) a large amount of heavy ions from the ionosphere can be transferred to the distant tail and accelerated to high energies during substorm activity and that (2) these oxygen O + ions from the polar ionosphere can be considered as “tracer ions” in the substorm dynamical process.