Frequency shifts observed in the Alouette II cyclotron harmonic plasma resonances

Abstract

The cyclotron harmonic plasma resonances observed by Alouette II were investigated using a technique that yielded frequency measurements with an accuracy of the order of 1 kHz. The resonances observed near f H and 2f H experience frequency shifts of the order of +2 percent and +0.5 percent, respectively, when f N /f H ≈3 where f N is the electron plasma frequency and f H is the electron cyclotron frequency. The resonances observed above the upper hybrid frequency approach the true frequency values of nf H as n increases; beyond about n=5 the observed cyclotron harmonic resonant frequences are consistent with a single value of f H which agrees with model magnetic field calculations within the estimated 0.2 percent reliability of this field. The frequency shifts observed previously for these resonances on the Alouette I data are attributed to magnetic contamination from the Alouette I spring steel, antennas. A comparison of the earlier Alouette I results with the-present Alouette II results indicates that the bulk of the original excitation volume associated with the higher cyclotron harmonic resonances is confined to a region considerably less than one antenna-length in radius from the antenna, that the size of this region decreases with increasing harmonic number n; and that it increases significantly as the radiating antenna becomes parallel to the direction of the earth's magnetic field.