Spectral characteristics of low‐frequency plasma turbulence upstream of comet P/Halley

Abstract

Spectral characteristics of low‐frequency plasma turbulence in a cometary environment have been determined using cross‐spectral analyses of magnetic field and plasma measurements made on board the Giotto spacecraft upstream of comet P/Halley. Two upstream regions have been identified, where the mean solar wind magnetic field was approximately parallel and perpendicular to the solar wind flow velocity direction, respectively. The two regions also differ with respect to magnetic field lines having been disconnected (connected) to the cometary bow shock in the quasi‐parallel (quasi‐perpendicular) regions. In the quasi‐perpendicular region the mean magnetic field and solar wind flow direction as well as the Giotto velocity relative to the comet are approximately mutually perpendicular to each other. Thus there is no Doppler shift between the observations made in the spacecraft and plasma frame of reference for parallel propagating waves in the quasi‐perpendicular region. In both regions the turbulence spectrum can be described by a power law with a spectral index steeper than that of a Kolmogoroff‐Obukov spectrum. In the quasi‐parallel region the observed turbulence clearly exhibits an Alfvénic character with dynamic alignment between magnetic field and flow found throughout the frequency range (1‐100 mHz) analyzed. Wave propagation is sunward for all spectral components, at least in the quasi‐parallel region. The turbulence cascade observed is probably driven by right‐hand resonant pickup water group ion instabilities in the quasi‐parallel region, while in the quasi‐perpendicular region, left‐hand ion cyclotron ring or Alfvén/ion anisotropy instabilities might be a possible driving mechanism. The polarization of the pump wave in the quasi‐parallel region is almost linear, while at higher frequencies almost linear as well as right‐ and left‐handed polarization is found. In the quasi‐parallel, region evidence for water group ion cyclotron harmonics has been found with the first harmonic exhibiting a slow mode character and all harmonics almost propagating along the mean magnetic field. In the quasi‐perpendicular connected region, no clear evidence has been found for the existence of backstreaming protons, reflected off the cometary bow shock, and associated upstream wave activity.