Highly anisotropic distributions of energetic electrons and triggered VLF emissions

Bell et aE. [2000] have reported an exciting set of new observations, made near the equatorial plane of the plasmasphere on the L = 3.4 flux tube, of hiss emissions between 4.0 and 5.6 kHz, highly anisotropic energetic electr ons, and unusual VLF triggering effects at 10.2 kHz. Here we show that thes e data as a whole can be considered as the first experimental evidence of t he existence of a special type of distribution function of energetic electr ons with a step-like feature in the velocity component parallel to the magn etic field. Such a shape of velocity distribution can be crucial for explan ation of triggered VLF emissions and chorus. Assuming the distribution to b e step-like, we can readily explain the spatial distribution of energetic e lectrons along the field line observed by Bell ct aE. [2000], and also stro ng wave amplification in the triggering process. We discuss how the step in the velocity distribution is maintained by the hiss emission. The frequenc y gap between the hiss band and the triggered signal is connected with the excitation of the quasi-electrostatic mode near the upper frequency edge of the hiss band.