Ion temperature in the ionosphere obtained from cyclotron damping of proton whistlers

Abstract

In this paper we discuss a method of determining ion temperature in the ionosphere from cyclotron damping of proton whistlers. These whistlers are dispersed forms of lightning impulses, observed by satellites as propagating left-hand polarized (ion cyclotron) waves. It is found that the amplitude of the proton whistler decreases abruptly at a wave frequency slightly below the proton gyrofrequency at the satellite. Of several factors considered, it is found that only cyclotron damping can satisfactorily explain this abrupt cutoff. The theory of cyclotron damping of proton whistlers is developed, and the difference between the wave frequency at cutoff and the proton gyrofrequency at the satellite is related to the proton temperature. Sample proton temperatures in the ionosphere are determined, using proton whistlers observed by the Injun 3 and Alouette 1 satellites. The temperatures found are comparable to those obtained previously by other experimenters. The method of observing ion temperatures developed here has inherent advantages in that the parameters measured are independent of electron temperature and ion composition.