Auroral X-ray bursts and ULF geomagnetic pulsations during magnetospheric substorms.

A series of Bermsstrahlung X-ray enhancements has been observed at a balloon altitude over Syowa Station, Antarctica, in conjunction with sharp negative bays during auroral substorms. X-ray bursts with pulsative fluctuations are associated with ULF geomagnetic pulsations, and the dominant frequency of the fluctuations is found in 0.2-0.4Hz which is close to the main frequency band of ULF pulsations. It is also shown that a one-to-one correspondence is obvious within the time difference of a few minutes between sharply-rising X-ray bursts and mid-latitude Pi2 geomagnetic pulsations. This fact suggests that Pi2 pulsations observed at middle latitude region are closely related to the energetic auroralzone electron precipitation.