Modulation of NTC frequencies by Pc5 ULF pulsations:Experimental test of the generation mechanismand magnetoseismology of the emitting surface

摘要

Nonthermal continuum (NTC) radiation is believed to be emitted by the conversion ofan electrostatic wave into an electromagnetic one, which takes place at the Earth'smagnetic equator. It is generally accepted that the frequency of the electrostatic wave atthe source meets a local characteristic frequency placed in between two multiples ofthe electron cyclotron frequency, fce, which results in emission of a narrow band frequencyelement. In an event on 14 August 2003, we compare oscillations of the central frequencyof distinct NTC frequency elements observed from Cluster orbiting near perigee, withsimultaneous Pc5 Ultra Low Frequency (ULF) pulsations in the magnetic field observedfrom the same platform. The latter magnetic perturbations are interpreted asmagnetohydrodynamic poloidal waves, where fundamental and second harmonic modescoexist. The NTC oscillation and the fundamental wave have similar periods, but arephase shifted by a quarter of period. From the correlation between both signals, and theproximity of the NTC source (localized via triangulation) with Cluster, we infer thatthe poloidal perturbations are spatially uniform between the source and the satellites. Fromthe phase shift between signals, we conclude that the electrostatic wave which convertsinto NTC is mainly governed by the plasma density, affected by movements of themagnetic field lines. Furthermore, we demonstrate that the observations can be used toperform a magnetoseismology of the emitting surface. The results show a steepening ofthe plasmapause density profile near the satellites, which can be responsible for thegeneration of NTC emission.