ハレー彗星周辺のプラズマ波動及び衝撃波からの電波

概要

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By Plasma Wave Probe (PWP) on board the Sakigake spacecraft, two discoveries have been made; the first is the emissions of the radio waves from the comet Halley and the second is the in-situ plasma waves expanded over the distance of 10^7km from the nucleus of the comet Halley. The discovered electromagnetic waves are called CKR (cometary kilometric radiation) because the emission frequency is within a range from 30kHz to 200kHz. The frequency made continuous changes suggesting the movement of the source position. The source of CKR are identified at the shock; the observed CKR spectra show, then, that the shock waves are moving corresponding to the variation of the solar wind condition and also the emissivity of the water vapour from the nucleus. The detected in-situ plasma waves consist of the whistler mode waves, waves associated with the electrostatic electron cyclotron instabilities and the electron acoustic waves. All these are enhanced due to the solar wind interaction with the cometary plasma; the ion pick up phenomena are one of the main cause for these plasma instabilities. The observations then indicate that the ion-pick up phenomena are expanded over 10^7km from the nucleus.