Comets provide another opportunity to discuss MHD turbulence and pickup ions. In this case the pickup ions are produced by photoionization or charge-exchange of water molecules evaporated from the comet's surface. Once again, the cometary particles are moving very slowly relative to the solar wind but are picked up by the solar wind with large gyrospeed and a varying energy with maximum value given by (12.?). The ring-beam distribution of pickup water ions then excites large amplitude MHD waves, as well as other higher frequency waves such as ion acoustic waves. Figure 12.16 [Glassmeier & Neubauer, 1993] show the input of power near the water ion cyclotron frequency and a turbulence cascade to smaller wavelengths.
Figure 12.16: Sample magnetic power spectrum measured by GIOTTO sunward of Comet Grigg-Skjellerup
[Glassmeier and Neubauer, 1993].
The dashed line shows the cyclotron frequency for water ions.