Electron temperature anisotropy instabilities: Whistler, electrostatic and z mode

Abstract

An electron temperature anisotropy T⊥e/T‖e > 1 leads to excitation of three distinct instabilities in collisionless plasmas at frequencies below the electron cyclotron frequency |Ωe| (Here ⊥ and ‖ denote directions relative to the background magnetic field B0.). Linear Vlasov theory is used to study these growing modes, with emphasis on the scaling of the temperature anisotropy at instability threshold. If the electron plasma frequency ωe is greater than |Ωe| and electrons are sufficiently hot, the whistler is the unstable mode with smallest anisotropy threshold; this electromagnetic mode has maximum growth rate at propagation parallel to B0. At ωe > 0.5|Ωe|, an electrostatic electron anisotropy instability can arise at propagation oblique to B0; this mode may have the smallest threshold for sufficiently cool electrons and ωe ∼ |Ωe|. And T⊥e/T‖e > 1 drives the z mode unstable at ωe < |Ωe|; this electromagnetic mode also has maximum growth rate at parallel propagation and is the persistent instability at ωe ≲ 0.5|Ωe|. The results are discussed in connection with observations from the polar and auroral regions of the terrestrial magnetosphere.