Modified Temperature-Anisotropy Instability Thresholds in the Solar Wind

The proton and electron temperature anisotropies in the solar wind are constrained by the instability thresholds for temperature-anisotropy-driven kinetic plasma instabilities. The modifications to the marginal instability conditions from accounting for the influence of damping connected with the collisional effects in the solar wind plasma are calculated for right- and left-handed polarized parallel propagating Alfvén waves and mirror and firehose fluctuations. These modifications provide tighter threshold constraints compared to the marginal thresholds but do not fully explain the observations at small values of the parallel plasma beta.

Figure 1

Anisotropy diagram for parallel propagating LH and RH polarized Alfvén waves and firehose fluctuations for the case of equal electron and proton temperature anisotropies resulting from the marginal (dotted lines) and modified (dashed lines) instability conditions, calculated for $h=2.1$. Unstable regions from the modified and marginal instability conditions are marked by “U” and “u,” respectively.

Figure 2

Modified threshold conditions calculated for $h=2.1$ for parallel propagating LH and RH polarized Alfvén waves, firehose (full lines), and mirror (dashed line) fluctuations in comparison with solar wind observations [4].

Figure 3

The same as Fig. 2 for $h=0.5$.