Magnetic spectral signatures in the Earth's magnetosheath and plasma depletion layer

Correlations between plasma properties and magnetic fluctuations in the subsolar magnetosheath downstream of a quasi‐perpendicular shock have been found and indicate that mirror and ion cyclotronlike fluctuations correlate with the magnetosheath proper and plasma depletion layer, respectively ( Anderson and Fuselier , 1993). We explore the entire range of magnetic spectral signatures observed from the AMPTE/CCE spacecraft in the magnetosheath downstream of a quasi‐perpendicular shock. The magnetic spectral signatures typically progress from predominantly compressional fluctuations, δ B ∥ /δ B ⊥ ≈ 3, with F/F p <0.2 ( F and F p are the wave frequency and proton gyrofrequency, respectively) to predominantly transverse fluctuations, δ B ∥ /δ B ⊥ ≈ 0.3, extending up to F p . The compressional fluctuations are characterized by anticorrelation between the field magnitude and electron density, n e , and by a small compressibility, C e ≡ (δn e /n e ) 2 (B/δB ∥ ) 2 ≈ 0.13, indicative of mirror waves. The spectral characteristics of the transverse fluctuations are in agreement with predictions of linear Vlasov theory for the H + and He 2+ cyclotron modes. The power spectra and local plasma parameters are found to vary in concert: mirror waves occur for β ∥p (β ∥p ≡ 2µ 0 n p kT ∥p /B 2 ) ≈ 2, A p ≡ T ⊥p /T ⊥ ‐ 1 ≈ 0.4, whereas cyclotron waves occur for β ∥p ≈ 0.2 and A p ≈ 2. The transition from mirror to cyclotron modes is predicted by linear theory. The spectral characteristics overlap for intermediate plasma parameters. The plasma observations are described by A p = 0.85β ∥p −0.48 with a log regression coefficient of −0.74. This inverse A p ‐ β ∥p correlation corresponds closely to the isocontours of maximum ion anisotropy instability growth, γ m /Ω = 0.01, for the mirror and cyclotron modes. The agreement of observed properties and predictions of local theory suggests that the spectral signatures reflect the local plasma environment and that the anisotropy instabilities regulate A p . We suggest that the spectral characteristics may provide a useful basis for ordering observations in the magnetosheath and that the A p ‐ β ∥p inverse correlation may be used as a beta‐dependent upper limit on the proton anisotropy to represent kinetic effects.