Electron temperature anisotropy instabilities: Whistler, electrostatic andz mode

An electron temperature anisotropy T-perpendicular to e/T parallel to e > 1 leads to excitation of three distinct instabilities in collisionless plasm as at frequencies below the electron cyclotron frequency \Omega(e)\ (Here p erpendicular to and parallel to denote directions relative to the backgroun d magnetic field B-o.). Linear Vlasov theory is used to study these growing modes, with emphasis on the scaling of the temperature anisotropy at insta bility threshold. If the electron plasma frequency omega(e) is greater than \Omega(e)\ and electrons are sufficiently hot, the whistler is the unstabl e mode with smallest anisotropy threshold; this electromagnetic mode has ma ximum growth rate at propagation parallel to B-o. At omega(e) > 0.5\Omega(e )\, an electrostatic electron anisotropy instability can arise at propagati on oblique to B-o; this mode may have the smallest threshold for sufficient ly cool electrons and omega(e) similar to \Omega(e)\. And T-perpendicular t o e/T parallel to(e) > 1 drives the z mode unstable at omega(e) < \Omega(e) \; this electromagnetic mode also has maximum growth rate at parallel propa gation and is the persistent instability at omega(e) less than or similar t o 0.5\Omega(e)\ The results are discussed in connection with observations f rom the polar and auroral regions of the terrestrial magnetosphere.