Generation of auroral kilometric radiation by electron horseshoe distributions

High time resolution of rocket and satellite electron distribution functions within the source region of auroral kilometric radiation display a characteristic crescent shaped or horseshoe distribution. Such distribution functions are created by a field aligned electron beam moving into an increasing magnetic field, conservation of the first adiabatic invariant causes an increase of their pitch angle. This produces a broad region on the distribution function where partial derivative f(e)/partial derivative v(perpendicular to)> 0, and is a possible source of free energy leading to radio wave emission by the cyclotron maser instability, which is more efficient than the conventional loss-cone maser instability. The stability of these electron horseshoe distribution functions is examined for right-hand extraordinary mode (R-X mode) radiation close to the electron cyclotron frequency propagating perpendicular to the magnetic field. (C) 2000 American Institute of Physics. [S1070-664X(00)00907-1].