Characteristics of dawnside mid-latitude VLF emissions associated with substorms as deduced from the two-stationed direction finding measurement

Abstract

Mid-latitude VLF emissions are found to take place in close association with geomagnetic substorms at two local times; one at dawn and the other in the pre-midnight. The characteristics of dawnside VLF emissions are examined in the present paper, based on the data obtained by our VLF/ELF campaign during November 1978 to February 1979 by means of the direction finding measurements at two stations, Brorfelde in Denmark ( L∼3) and Chambon-la-Foret in France ( L∼2). Six remarkable events have been analyzed and the following properties have emerged; (i) the occurrence is highly peaked at dawn time, L.T. ⋍ 06.00, (ii) the VLF emissions observed are identified as being mainly of the hiss-type, (iii) the region of maximum intensity corresponding to the latitude of generation region is estimated to be inside the plasmapause, based on the direction finding results, and (iv) the VLF emissions are found to exhibit a regular frequency drift; the frequency is increasing with local time for all the events, but one. It is shown that all of the above features can be understood within the quasi-linear model of cyclotron instability of the electrons injected in association with geomagnetic substorms. In particular, the property (iv) is related to the energy dispersion of the electrons drifting from the midnight sector of their initial injection toward the generation region of VLF emissions (dawn sector).