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 electrons, and unusual VLF triggering effects at 10.2 kHz. Here we show that these data as a whole can be considered as the first experimental evidence of the existence of a special type of distribution function of energetic electrons with a step-like feature in the velocity component parallel to the magnetic field. Such a shape of velocity distribution can be crucial for explanation of triggered VLF emissions and chorus. Assuming the distribution to be step-like, we can readily explain the spatial distribution of energetic electrons along the field line observed by Bell ct aE. [2000], and also strong wave amplification in the triggering process. We discuss how the step in the velocity distribution is maintained by the hiss emission. The frequency 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.