Electromagnetic ion cyclotron waves in the terrestrial plasma depletion layer: Effects of possible differential speeds between thermal H+ and He2+ ions

The terrestrial plasma depletion layer (PDL) is characterized by electromagnetic ion cyclotron waves (EICWs) [Anderson et al., 1991, 1994; Anderson and Fuselier, 1993]. These are of two types, continuous (CON), where the wave power transverse to the local mean field decreases continuously with frequency, and bifurcated (BIF), where a diminution in wave activity occurs between two activity peaks [Anderson et al., 1994]. Linear kinetic theory of plasma waves has been applied successfully to explain many features of the observed spectra [Denton et al., 1993; Gary et al., 1993]. In this paper we complement these theoretical studies by examining the effects a possible relative motion between the thermal H+ and He2+ populations has on the ion cyclotron activity, illustrating theoretical predictions by comparison with observed CON and BIF spectra. We find that even a modest differential speed (Uα ≡ | vα| − | vp |∼ 0.1 vA, where vA is the local Alfvén speed) can regulate the presence, location, and strength of the diminution in wave power often observed between the upper (∼ 0.55fp \u3c f \u3c fp) and lower (0.2 fp \u3c f \u3c∼ 0.40 fp) local maxima in wave activity. Using parameters appropriate to the CON spectrum, we find that when Uα = 0, there is a frequency interval centered around 0.45 fp, where the waves are damped, in disagreement with CON observations. With Uα, negative (i.e., the alpha particles are slower), the damping is enhanced and the damped frequency interval broadens, accentuating the discrepancy with the observations. When, however, Uα is small but positive, the previously damped frequency interval becomes an unstable region where wave growth rates are smaller with respect to peak values. Thus, although the activity is reduced there, it is not fully damped. By raising the positive values of Uα further, the bifurcation disappears and the spectrum becomes continuous. This may thus explain CON observations. For the BIF observation we find that a linear theory which does not incorporate inter-ion species drift predicts a frequency gap (i. e., a region where there are no waves, damped or otherwise) centered at ∼ 0.45 fp, in addition to further intervals where the waves are damped. A small positive Uα (∼ 0. 07 vA) eliminates the frequency gap. A further, small increase in Uα eliminates the damped frequency intervals. We also compare the effects of the relative positive drift between H+ and He2+ ions with those obtained by raising the proton beta. In summary, we suggest that an H+ – He2+ relative drift may further help explain features of the bifurcated and continuous wave spectra observed by the CCE spacecraft in the PDL