A comparison between VEGA 1, 2 and Giotto flybys of comet 1P/Halley: implications for Rosetta

Abstract

Abstract. Three flybys of comet 1P/Halley, by VEGA 1, 2 and Giotto, are investigated with respect to the occurrence of mirror mode waves in the cometosheath and field line draping in the magnetic pile-up region around the nucleus. The time interval covered by these flybys is approximately 8 days, which is also the approximate length of an orbit or flyby of Rosetta around comet 67P/Churyumov-Gerasimenko. Thus any significant changes observed around Halley are changes that might occur for Rosetta during one pass of 67P/CG. It is found that the occurrence of mirror mode waves in the cometosheath is strongly influenced by the dynamical pressure of the solar wind and the outgassing rate of the comet. Field line draping happens in the magnetic pile-up region. Changes in nested draping regions (i.e. regions with different B x directions) can occur within a few days, possibly influenced by changes in the outgassing rate of the comet and thereby the conductivity of the cometary ionosphere.

Two-point observations of low-frequency waves at 67P/Churyumov-Gerasimenko during the descent of PHILAE: comparison of RPCMAG and ROMAP

“…These waves and their generation mechanism are clearly distinct from waves observed at other comets (Volwerk et al, 2014;Glassmeier et al, 1989).…”

Mass-loading, pile-up, and mirror-mode waves at comet 67P/Churyumov-Gerasimenko

“…It is, however, unclear if an increase in ion density can actually lead to such a strong increase in magnetic field strength through increased hang-up. Volwerk et al (2014) posited that a decrease in ion density at comet 1P/Halley could be the reason for the disappearance of the nested draped magnetic field between the flybys of Vega 1 and Vega 2. However, it is also quite possible that the increase in magnetic field strength and the increase in ion density are generated by an external source in the solar wind.…”

“…far away from the x direction, whereas the clock angle ξ varies strongly because of the large oscillations in the magnetic field components, the largest of which are also visible in the cone angle. As there is neither undisturbed solar wind data, nor real undisturbed field around the comet, the draping analysis as proposed by and applied to comet 1P/Halley (see also Delva et al, 2014;Volwerk et al, 2014) cannot be applied. However, the magnetic field direction and behaviour can be looked at in hedgehog-plots, as in Fig.…”

“…at comet 1P/Halley (see e.g. Schmid et al, 2014;Volwerk et al, 2014). The instability criterion for MM waves is given by:…”

“…This could imply that the freshly mass-loaded magnetospheric magnetic field is mirror-mode unstable (see e.g. Hasegawa, 1969;Tsurutani et al, 1999;Schmid et al, 2014;Volwerk et al, 2014). As the resolution of the plasma data is too low to check the pressure balance over the MM structures, the magnetic-field-only method by Lucek et al (1999) is used to investigate the data for MM waves.…”