LANDAU DAMPING OF MAGNETOSPHERICALLY REFLECTED WHISTLERS

Unducted VLF signals produced by lightning activity can form a populat ion of magnetospherically reflected (MR) whistlers in the inner magnet osphere. It has been suggested recently that in the absence of signifi cant attenuation such waves could merge into a broadband continuum wit h sufficient intensity to account for plasmaspheric hiss. To test this conjecture we have evaluated the path-integrated attenuation of MR wh istlers along representative ray paths using the HOTRAY code. Using a realistic plasma distribution modeled on in situ data, we find that th e majority of MR waves experience significant damping after a few tran sits across the equator. This is primarily due to Landau resonance wit h suprathermal (0.1-1 keV) electrons. The attenuation is most pronounc ed for waves that propagate through the outer plasmasphere; this can r eadily account for the infrequent occurrence of multiple-hop MR waves for L greater than or equal to 3.5. Selected waves that originate at i ntermediate latitudes (15 degrees less than or equal to lambda less th an or equal to 35 degrees) and whose ray paths are confined to the inn er plasmasphere may experience up to 10 magnetospheric reflections bef ore substantial attenuation occurs. These waves should form the popula tion of observed MR waves. Wave attenuation becomes more pronounced at higher frequencies; this can account for the absence of multiple-hop waves above 5 kHz. Weakly attenuated MR waves tend to migrate outward to the L shell, where their frequency is comparable to the equatorial lower hybrid frequency. The enhanced concentration of waves due to a m erging of ray paths would produce a spectral feature that rises in fre quency at lower L. This is quite distinct from the reported properties of plasmaspheric hiss, which maintains a constant frequency band thro ughout the entire plasmasphere. Furthermore, in the absence of mode co nversion, waves below 500 Hz, which often form an important if not dom inant part of the spectral properties of hiss, are unable to escape fr om the topside ionosphere in the whistler mode. Consequently, we concl ude that unducted lightning signals cannot account for the origin of p lasmaspheric hiss.