HEATING OF O+ IONS IN THE CUSP CLEFT - DOUBLE-CYCLOTRON ABSORPTION VERSUS CYCLOTRON-RESONANCE

The process of double-cyclotron absorption, where an ion simultaneously absorbs two waves whose frequencies sum to the ion gyrofrequency, can heat ions perpendicular to the ambient magnetic field. It has been suggested that this process may play a role in the production of ion conics. The low-energy oxygen (O+) distributions and the electric spectral densities observed by DE 1 in the cusp/cleft during an O+ conic event are used as inputs to a Monte Carlo simulation of the heating of the O+ ions due to double-cyclotron absorption of waves near the ion gyrofrequency. The results are compared with the observed O+ conics and with the results of simulations of ion heating via the process of cyclotron resonance involving left-hand polarized waves near the ion gyrofrequency. The simulations show that for any particular event cyclotron resonance can probably heat ions over a much greater altitude range than double-cyclotron absorption is capable of doing. They also demonstrate the much stronger dependence of the heating due to double-cyclotron absorption on local intensifications of the spectral density, and they indicate that a relatively narrow region of intense wave activity may be sufficient for this heating process to produce O+ conics with energies comparable to those observed. Heating via double-cyclotron absorption may therefore be important for relatively local heating of O+ ions. However, cyclotron resonance heating by waves around the gyrofrequency is probably more important in the majority of O+ conic events in the cusp/cleft.