Magnetic pileup boundary and field draping at Comet Halley

Abstract

The approach of the Rosetta S/C to Comet Churyumov-Gerasimenko in 2014 reactivates the interest in the plasma interaction of the solar wind with the cometary coma. In preparation for the upcoming S/C observations and the start of outgassing of the cometary nucleus, we reinvestigate the magnetic field data from the Vega-1 S/C at the flyby of Comet Halley (1986), in search of the magnetic pileup boundary and increase of field line draping. The magnetic pileup boundary has been identified as a common feature for unmagnetized bodies with an induced magnetosphere. This boundary marks the outer edge of the magnetic pileup region, also known as the magnetic barrier region, in which the magnetic field is strong and highly draped. Initially, the magnetic field draping around Comet Halley was clearly identified from the Vega-1 magnetometer data through reversal of the field component in direction to the Sun at closest approach. Here, a detailed analysis is performed in regions further upstream in the magnetosheath. The Vega-1 high resolution magnetometer data on the in- and outbound leg but inside the bow wave are reinvestigated in search for the magnetic pileup boundary as an indicator for the outer edge of the magnetic barrier. The magnetic field pileup region is studied using the correlation between the field component towards the Sun and the radial component in an aberrated cometocentric frame; this technique proved very successful for Mars and also for comets Giacobini-Zinner and Halley in the case of Giotto observations. We can clearly identify the different regimes in the magnetic field data, on the in- and outbound leg of the orbit. Waves just within the newly determined magnetic pileup region have properties different from mirror mode waves, whereas waves observed out of the magnetic pileup boundary are confirmed as mirror mode. The boundaries found at Comet Halley prove that also the detailed structure of the interaction of unmagnetized bodies with an atmosphere with the solar wind is valid for active comets, but with larger space scale.