Comets in the solar wind
The plasma tail of a comet is shaped by the solar wind. An observer sees the comet motionless. But when one follows the comet for some time one notices that the tail is in violent motion. This is shown by a movie made by ESO during comet Halley's 1986 apparition.
The plasma tail reacts like a windsock to changes in the
solar wind. One could use it as an instrument which measures
(free of charge) the solar wind, if one could only decipher the
message, the tail is telling us. Comets are unfortunately
too far from satellites measuring the solar wind, so that
one never knows the conditions at the comet during the time of observation.
Model calculations on the computer offer an ideal means to study the reaction of the cometary tail to well specified solar wind events. Some time ago we had already studied how an interplanetary shock wave can tear off the tail.
Observational evidence indicates that tail disconnection can also occur when the comet crosses the heliospheric current sheet. At the current sheet the magnetic field changes its direction by 180 degrees. It can reconnect and the released energy could be used to disrupt the tail. But up to now computer experiments could not confirm this effect. On the other hand the current sheet is embedded in a region of slow, dense solar wind which is surrounded by domains of fast, tenuous solar wind. Therefore, the comet after crossing the current sheet meets soon again fast wind (a high speed stream). In the transition from slow to fast wind the flow changes its direction by about 10 degrees and the magnetic field is enhanced by a factor of 2. Model calculations show that by such a transition from slow to fast wind the plasma tail is disconnected.