Alfvénic turbulence in the solar wind flow during the approach to comet P/Halley

Summary

The Johnstone Plasma Analyser on Giotto operated almost continuously for two days before the encounter with comet Halley. Throughout this period it made observations of the proton and alpha particle distributions in the solar wind with 8 seconds time resolution. As the comet was approached, fluctuations were observed in all the primary bulk parameters, i.e. density, temperature and flow velocity, of both distributions at levels above the usual solar wind turbulence. We present here a survey of the data from a distance of 5 10⁶ km from the nucleus up to the cometary foreshock at 1.4 10⁶ km. Early in this period the variance was higher than in the solar wind on a similar day but the activity may have been associated with a crossing of the heliospheric current sheet. From a distance of 2.7 10⁶ km the power level gradually increased, as the mass loading of the solar wind increased, to levels well above normal solar wind turbulence. We estimate that up to 16% of the free energy associated with the implanted cometary ion distribution went into the wave energy, similar to the proportion which has been estimated by quasilinear theory. Although the solar wind convection time through the mass-loaded region is short compared with the development time for a shell distribution in the cometary ions estimated from numerical simulation, the observations indicate that the cometary ion distribution is still basically ring-like.