Substorm‐associated VLF emissions with frequency drift observed in the premidnight sector

Abstract

Characteristics of premidnight substorm‐associated VLF emissions are examined on the basis of 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) as well as ISIS satellite measurements. Two remarkable events have been analyzed, and the following main properties have emerged: (1) the emissions observed are of hiss type and are excited mainly within the plasmapause, (2) the emission activity is preceded by development of two subsequent substorms, and (3) the emission frequency increases sharply at the first stage of the events and is followed by its subsequent gradual decrease. These properties of the temporal evolution of the wave spectrum are interpreted in terms of a quasi‐linear electron cyclotron instability model for wave excitation. The initial frequency increase is believed to be due to a combined effect of L shell drift of energetic electrons injected during the first substorm and decrease of a large‐scale convection electric field during the development of the events. The subsequent frequency decrease may be related to the additional injection of energetic electrons due to the development of the second substorm.