Plasma flow inside Comet P/Halley

Abstract

The plasma flow parameters (bulk velocity, number density, temperature) have been computed from the Giotto JPA data for the solar wind protons and for the water group cometary implanted ions. The decrease of the plasma bulk speed due to mass loading is followed from the bow shock at 1.16 x 10 to the 6th km from the nucleus down to well inside the cometopause (around 180,000 km). Inside the cometopause and for 60,000 km the plasma flow speed is constant at 23 km/s. The proton flow is decoupled from the water ions flow starting at 2200 GRT, i.e. 500,000 km. A possible interpretation of such observation in terms of center of mass switching from solar wind protons to cometary ions is discussed. The flow stabilization after the cometopause is discussed in terms of the critical ionization velocity mechanism. The observed flow parameters are compared with published MHD computer models.