Instabilities driven by anisotropic strahl electrons in the inner heliosphere

Abstract We investigate the instabilities driven by anisotropic strahl electrons in the near-Sun solar wind. Motivated by Parker Solar Probe observations showing that the strahl population exhibits significant temperature anisotropy ($T_{\mathrm{es\parallel}}/T_{\mathrm{es\perp}}=A_{\mathrm{es}}$ up to 2) within 0.25 au, we adopt a two-component electron model with isotropic core and anisotropic strahl populations and solve the hot-plasma dispersion relation using the PDRK/BO code at a heliocentric distance of 30 $R_{\mathrm{S}}$. Our results show that strahl temperature anisotropy significantly suppresses the parallel whistler heat-flux instability (PW-HFI), which is traditionally regarded as the dominant mechanism for constraining the strahl drift velocity. When the anisotropy exceeds $A_{\mathrm{es}} \gtrsim 1.4$, PW-HFI is completely damped, and the oblique Alfv\'{e}n heat-flux instability (OA-HFI) emerges as the mode with the lowest excitation threshold. The lower-hybrid and electron-acoustic heat-flux instability (LH-HFI) are also suppressed, while OA-HFI growth rate increases modestly with $A_{\mathrm{es}}$. Our findings suggest that OA-HFI, driven primarily by core electrons, becomes the instability with the lowest excitation threshold when strahl temperature anisotropy is considered, potentially playing a role in constraining the strahl drift velocity in the inner heliosphere.