Electron Instabilities Driven by Beaming Electrons with Anisotropic Temperatures in High-beta Plasmas

Electron beams are ubiquitous in the heliosphere and dominate the electron heat flux. Large heat fluxes can excite microinstabilities, which are widely regarded as the primary mechanism regulating heat transport in the solar wind. Since electrons often exhibit temperature anisotropy, it is essential to clarify how anisotropy modifies beam-driven instabilities. Here we investigate the combined effects of electron streaming and temperature anisotropy on instability excitation. We find a general scenario: for slow-streaming beams, the parallel whistler heat flux instability transitions to an antiparallel whistler instability as anisotropy increases; for fast-streaming beams, the oblique whistler heat flux instability transitions to either an antiparallel whistler instability or an antiparallel left-hand Alfvén instability; and for superfast-streaming beams, the dominant mode is the electron beam/acoustic heat flux instability, which is largely insensitive to anisotropy. Furthermore, we identify a new whistler instability that arises from the interplay between streaming and temperature anisotropy.