Plasma and magnetic field behavior across the magnetosheath near local noon

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Abstract
Using Active Magnetospheric Particle Tracer Explorers/Ion Release Module data from 10 magnetosheath crossings in the 11–13 LT sector, we have investigated the behavior of the proton temperature and the proton temperature anisotropy between the bow shock and the magnetopause. The proton anisotropy increases toward the magnetopause. Comparison with earlier predictions, however, shows that this increase is much more modest than was predicted by assuming the double adiabatic equations. It is shown that neither of the two double‐adiabatic equations holds separately. We conclude that energy is transferred between the perpendicular and the parallel component of the proton temperature. It is well established that instabilities such as the mirror and the ion‐cyclotron instability show such a behavior and lead to a decrease in the temperature anisotropy. Our data show that in the magnetosheath proper, the threshold for the onset of the mirror instability is always marginally met. We show that we can give an empirical relation between the temperature anisotropy and β⊥, as long as β⊥ is not less than 1.

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