Influence of non‐Bi‐Maxwellian distribution function of solar wind protons on the ion cyclotron instability

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Abstract
Satellite measurements of the thermal anisotropy of solar wind protons have shown that the particle velocity distributions at 1 AU are not adequately described by a bi‐Maxwellian distribution function. On the other hand, the stability criteria derived from a bi‐Maxwellian distribution require T⊥ > T∥, which is, however, hardly ever observed. A modified axisymmetric distribution function for solar wind protons is used here to calculate the stability and growth rate of the ion cyclotron instability in a warm slow speed solar plasma at 1 AU. Cyclotron wave growth is found also in the case T∥ > T⊥.