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Annales Geophysicae


Spacecraft-borne and ground-based magnetome- ters 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 (T⊥ > T∥). Previous surveys of Pc 1 occur- rence 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 occurrence 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 occurrence are close to the magnetopause, with Pwav ∼ 0.25 for Lnorm ≡ L/Lmp = 0.8–1.0. These results are consis- tent 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.