Geotail electron observations in association with intense bursts of electron cyclotron harmonic waves in the dayside magnetosphere

摘要

Along the magnetopause skimming orbit on October 17, 1992, the Geotail spacecraft observed intense bursts of electron cyclotron harmonic waves called "totem pole" (TP) emissions which occurred sporadically and intermittently in time in the dayside magnetosphere. The present paper focuses on the analysis of the electron environment associated with the TP emissions. Field analysis shows that, because of the occurrence of an intense Pc5 pulsation, there was E*B drift flow which was switching its direction in time near the magnetopause. The Geotail/comprehensive plasma instrumentation data show that the electron density fluctuated corresponding to the drift flow variation. Overall, TP emissions tend to occur during a transient time interval including a time when the density of cold electrons below 100 eV takes a minimum value. In the interval from 2225 to 2235 UT, TP emissions were well correlated with the density decrease of the cold electrons. In the interval including a stagnation time of the flow switching from the Sun-duskward to the tail-dawnward direction, the density of the cold electrons became minimum, and simultaneously, TP emissions were observed. Contrary to the decrease of the cold electrons, the electron density increased at energies higher than 100 eV in association with the TP emissions. Namely, the TP emissions were observed when the density ratio of the hot electrons to the cold ones (n_h_c) increased. In the interval from 2252 to 2302 UT, TP emissions were found during a transition time of the cold electron density. We can conclude that the sporadic and intermittent signature of TP emissions can be due to the change of the electron environment driven by the intense field pulsation.