ULF waves at comets Halley and Giacobini‐Zinner: Comparison with simulations

This paper compares observations and numerical simulations of magnetic fluctuations near the proton and water group ion cyclotron frequencies as a function of distance from the comets Giacobini‐Zinner and Halley. Because the Doppler shift of the ion cyclotron resonance is equal and opposite to that associated with the flow of the solar wind plasma past the observer, one can monitor the amplitude of waves due to different cyclotron resonant instabilities 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 comparison of the data to results of a one‐dimensional electromagnetic hybrid simulation of two‐ion pickup. Distant from both comets the wave amplitudes are very weak. But as the comet is approached, the wave amplitude grows. These observations are consistent with computer simulations which predict amplitudes that depend on the properties of the injected beams and the local injection rate.