Location of Pc 1-2 waves relative to the magnetopause

Abstract

Spacecraft-borne and ground-based magnetometers frequently detect magnetospheric micropulsations in the period range 0.2-10 s, termed Pc 1-2, and attributed to electromagnetic ion cyclotron waves driven by temperature anisotropy (T⊥ > T∥). Previous surveys of Pc 1 occurrence locations have been limited to L ≤ 9. We present AMPTE/IRM observations of the distribution of Pc 1 waves out to the magnetopause, for a limited region of MLT = 10-14. The probability of wave occurence Pwav is large (> 0.15) between L = 7-12, peaking at L = 8-10 (Pwav ∼ 0.25). When the L-value is normalized to the magnetopause position Lmp, however, the highest probabilities of Pc 1 wave occurence are close to the magnetopause, with Pwav ∼ 0.25 for Lnorm Ξ L/Lmp = 0.8-1.0. These results are consistent with increased convective growth rate at large L and with the greater effect of magnetosphere compression close to the magnetopause. On the other hand, we only directly observe magnetic field compression for at most about 25% of the wave events.