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The role of the Gendrin mode of VLF propagation in the generation of magnetospheric emissions
Author(s) -
Helliwell Robert A.
Publication year - 1995
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/95gl02003
Subject(s) - physics , wave packet , electron , plasmasphere , computational physics , atomic physics , phase velocity , magnetosphere , magnetic field , instability , group velocity , optics , mechanics , quantum mechanics
Impulsive VLF wave packets and electron beams may be formed in the magnetosphere from the superposition of Gendrin mode components defined by the condition cos θ = 2 f f Hwhere θ=wave normal angle measured from the Earth's static magnetic fieldB ¯ o, f =frequency of the wave component and f H = electron gyrofrequency. The frequency range of the Gendrin mode spectrum is defined by f L < f < f H /2, where f L = lower hybrid resonance frequency ≈= f H/ 43 and f H i = proton gyrofrequency. Since the group ray velocity of all Gendrin components is given byυ G = c 2f H f Nwhere f N = plasma frequency, and is aligned withB ¯ oand since the longitudinal (parallel toB ¯ o ) component of the phase velocity is the same as υ G , a Gendrin wave packet will travel in a homogeneous medium without distortion, constituting a kind of soliton. An electron whose parallel velocity υ ∥ is close to υ G (i.e., near longitudinal resonance) may become trapped by the E ∥ of the Gendrin wave packet, giving up energy to the packet as it propagates in a region of decreasing υ G , such as a typical field line path approaching the magnetic equator. As the packet grows E ∥ increases causing more trapping and accordingly more wave growth, giving rise to an “Impulsive Wave Instability” (IWI). Wave packet growth is limited by wave loss due to drift away from the main packet of previously generated components that no longer satisfy the Gendrin condition. As the packet leaves the equator growth ceases, the packet collapses and the previously trapped electrons become transient electron beams that may themselves trigger more emissions. It is suggested that the impulsive VLF emissions observed by DE1 and other satellites may be caused by the IWI mechanism.