The present Letter puts forth a possible explanation for the outstanding problem of measured proton temperature anisotropy in the solar wind at 1 AU apparently being regulated by the mirror and oblique fire-hose instabilities. Making use of the fact that the local magnetic field intensity near 1 AU undergoes intermediate-scale temporal variations, the present Letter carries out the quasilinear analysis of the temperature anisotropy-driven instabilities with a time-varying local $B$ field, assuming arbitrary initial temperature ratios and parallel betas. It is found that the saturated states in (${\beta }_{\parallel }$, $T_{\perp }/T_{\parallel }$) space are bounded by the mirror and oblique fire-hose instabilities, which is superficially similar to the observation.

• Received 9 October 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.071103