Transresonant electron acceleration

Helliwell and Bell have suggested synchronous acceleration of electrons by electromagnetic waves in the whistler range in the geomagnetic field. The acceleration of trapped electrons by electromagnetic waves is here generalized to include nonsynchronous or transresonant acceleration. It is shown that whistlers will scatter the electron velocity, by an amount inversely proportional to the square root of the time rate of change of the whistler frequency, during the time that the whistler frequency is passing over the electron gyrofrequency. Only electrons with initial energy above about 102 ev can see the Doppler-shifted whistler frequency at the gyrofrequency and be affected by this transresonant acceleration. Thermal electrons experience no effect. Current observation of whistlers in the outer geomagnetic field is too sketchy to allow a determination of transresonant acceleration rates, so that the relative importance, as compared, for example, with the strong-whistler synchronous acceleration of Helliwell and Bell, cannot be assessed. The importance of whistler acceleration can be established when the whistler frequency spectrum and recurrence rate are known throughout the geomagnetic field.