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