Amplification of electrostatic noise in cyclotron resonance with an adiabatic auroral beam

Abstract

The adiabatic evolution of the auroral beam in the convergent geomagnetic field gives rise to a region of ∂F/∂ν⊥ >0 in the electron distribution function. The altitude dependence of this source of free energy is examined, and its ability to drive electrostatic waves toward instability is considered. General linear properties of the possible instabilities are discussed, and a detailed numerical calculation based on a collisionless height dependent model of an auroral arc (Maggs and Lotko, this issue) is described. Of the three wave modes considered, the upper hybrid mode is optimally amplified at altitudes below about 1000 km. Oblique electron Bernstein modes may be weakly unstable at the lowest altitudes, and the whistler mode appears to be stable at all altitudes. Wave amplification is limited by a finite resonance time which is determined by refraction of parallel wavenumbers by the topside ionospheric density gradient. Slow beams originating at relatively high altitudes and nearly perpendicularly propagating waves with frequencies away from cyclotron harmonics are favored by this instability mechanism.