MHD model calculations for the effect of interplanetary shocks on the plasma tail of a comet

Details MHD model calculations for the effect of interplanetary shocks on the plasma tail of a comet MHD model calculations for the effect of interplanetary shocks on the plasma tail of a comet

Feb 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...294..601w&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 294, no. 2, p. 601-614

Astronomy and Astrophysics

Astrophysics

26

Comet Tails, Interplanetary Medium, Magnetohydrodynamic Flow, Magnetohydrodynamic Waves, Oblique Shock Waves, Three Dimensional Models, Time Dependence, Comets, Ionization, Magnetic Fields

Scientific paper

The response of a cometary plasma tail to interplanetary shocks is studied by means of time dependent three dimensional magnetohydrodynamic (MHD) model simulations. Three major phenomena are identified which can be seen by ground based observations. First, the plasma compressed by the shock forms tail rays. Secondly, the shock excites a transversal standing wave in the tail. Consequently, the tail alternates between confined and diffused states. In particular the first echo of the shock induces a conspicuous narrowing of the tail. Thirdly, a cloud is formed which starts with an initial velocity of about 20 km/s at the coma. The cloud formation can be explained qualitatively as the result of the interaction of discontinuities. At the same time the coma is dimmed by a factor of about 2. These numbers depend on the magnetosonic Mach number of the interplanetary shock as is shown for examples of shocks with Mach numbers 1.4, 2 and 4. The cloud moves tailward with a constant acceleration. It is confined between a fast tenuous flow behind and a slow dense plasma in front. The phenomenology resembles closely the observed tail disconnection events. An oblique shock changes also the direction of the tail. If after a while an inverse shock follows a new tail is formed in the original direction. As a result the tail appears to be diagonally split. It is demonstrated that the mechanism of cloud formation does not rely on surplus ionization by charge exchange.

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