ULF waves at comets halley and Giacobini-Zinner: Comparison with theory

Abstract

Because the Doppler shift occurring in ion-cyclotron resonance is equal and opposite to that associated with the flow of the solar wind plasma past the observer, one should be able to monitor the amplitude of waves due to the cyclotron resonant instability by examining the amplitude of waves near the gyrofrequency of the respective ions as measured in the spacecraft frame. This expectation has been checked by using one-dimensional electromagnetic hybrid simulations of two ion pickup. This simplifies the comparison of theoretical predictions of wave growth with the observations. Distant from both comets the wave amplitudes are very weak. As the comet is approached the wave amplitude grows. These observations agree qualitatively with the simulations which predict amplitudes that depend on the properties of the injected beams and the local injection rate.