ULF Field Fluctuations in a "Quiet" Subsolar Magnetosheath

Abstract

We present the analysis of plasma and field data acquired form the AMPTE/IRM satellite for ULF wave fluctuations in the subsolar magnetosheath under a ¡§quiet¡¨ condition. A total of seven magnetosheath crossings, representing twelve hours of observations, were selected for statistical analysis. The statistical analysis results indicated that mirror-like compressional fluctuations are typical of the quiet-state subsolar magnetosheath. It is also found that mirror-like fluctuations with typical frequencies between 10 mHz and 80 mHz (or 0.02 fH + <= f <= 0.2fH +, where f is the wave frequency and fH + the local proton gyro frequency) are associated with 0.1 <= (T/T-1) <= 0.6 and 1 <= B <= 10 magnetopause plasma, where Bp is the proton beta and T and T are the perpendicular and parallel proton temperature, respectively. We also found that the condition for mirror instability, T/T > 1+B-1, is generally not satisfied in the entire region of the magnetosheath, especially near the magnetopause. This may indicate that temperature anisotropy is reduced due to the development of mirror waves as suggested by Price et al. [1986] and Lee et al. [1988].

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