ION-CYCLOTRON HEATING IN THE DAYSIDE MAGNETOSPHERE

Observations of waves and particles obtained by the Freja satellite at altitudes around 1700 km in the dayside high-latitude magnetosphere a re used to study ion energization. We find that ions, including O+, du ring several events of intense ion energization can be heated perpendi cularly to the geomagnetic field to mean energies df up to about 20 eV via the process of cyclotron resonance by broadband waves around the ion gyrofrequencies. There is a good correlation between such broadban d waves and the ion energization. The waves show no spectral features at the O+ gyrofrequency. The observed wave amplitudes are used as an i nput to a Monte Carlo simulation to obtain the observed ion energies. The waves around the ion gyrofrequencies may be generated either by fi eld-aligned electrons or by nonlinear processes transferring energy fr om waves with lower frequencies. Not only the mean energy but also the shape of the particle distribution agrees with the cyclotron resonanc e heating mechanism. Other mechanisms, such as heating by lower hybrid waves or by a slowly varying electric field, are investigated but are found to be less important than cyclotron heating in this region of s pace.