Relations of VLF emissions to impulsive electron precipitation measured by EISCAT radar in the morning sector of auroral oval

Abstract

E-region electron density variations were observed above Tromsø, Norway, by the EISCAT UHF radar (69.59°N, 19.23°E) in the morning hours on 17 December 1990. The density variations have been compared with the characteristics of the VLF emissions seen in the frequency range of 0.3–3 kHz observed in Sodankyla, Finland (67.51°N, 26.33°E) and in Lovozero (Russia), (67.97°N, 35.08°E). Two different types of precipitation relating to VLF waves were found. The first type is in the form of precipitation impulses having only a few seconds duration repeating at intervals of some tens to a hundred seconds. The electrons were primarily produced by substorms which had commenced earlier in the midnight sector. The measured electron density profile suggests that the energy of the precipitating electrons is from below 20 to about 30 keV. Chorus emissions were simultaneously recorded showing properties which can be explained by the cyclotron instability in the magnetosphere. The second type of electron precipitation correlating with VLF waves has a duration of a few minutes. This precipitation is related to sudden impulses (SI) caused by a change in the solar wind pressure. The accompanying VLF waves had a frequency range from 1.5 to 2.5 kHz. The cyclotron instability together with the time depended magnetic field during the magnetospheric compression can explain the spectral and temporal characteristics of this kind of electron precipitation and VLF waves.