Thermal magnetic fluctuations of whistlers in a Maxwellian plasma

Abstract

Thermal fluctuations have been measured with a magnetic loop antenna inside a large afterglow plasma in the regime of whistler waves (f≲fce≂30 MHz≪fpe≂3000 MHz; Ar, 2×10−4 Torr, 1 m diam×2.5 m length). The magnetic fluctuations B̃(ω) exhibit a 1/f-like spectrum for whistlers (f<fce), no resonant enhancement at the electron cyclotron frequency fce, and a flat spectrum in the evanescent regime (fce<f≪fpe). Thus the observed fluctuations are neither described by blackbody radiation laws (B̃∝ω) nor by cyclotron emission (lines at nfce), but resemble the decaying Alfvénic fluctuation spectrum calculated by Cable and Tajima [Phys. Rev. A 46, 3413 (1992)].