Location of Pc 1-2 waves relative to the magnetopause

Spacecraft-borne and ground-based magnetometers frequently detect magnetospheric micropulsations in the period range 0.2-10s, termed Pc 1-2, and attributed to electromagnetic ion cyclotron waves driven by temperature anisotropy (Tperpendicular to > Tparallel to). Previous surveys of Pc 1 occurrence locations have been limited to L less than or equal to 9. We present AMPTE/IRM observations of the distribution of Pc I waves out to the magnetopause, for a limited region of MLT = 10-14. The probability of wave occurrence P-wav is large (> 0. 15) between L = 7-12, peaking at L = 8-10 (P-wav, 0.25). When the L-value is normalized to the magnetopause position L-mp, however, the highest probabilities of Pc 1 wave occurrence are close to the magnetopause, with P-wav,,, - 0.25 for L-norm equivalent to L/L-mp = 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.