OBSERVATIONS OF COMETARY PLASMA TAILS AND THE HELIOSPHERE NEAR SOLAR MAXIMUM

摘要

Measurements from the Ulysses spacecraft show that, except near solar maximum, the heliosphere has a relatively simple structure consisting of a fast, steady, low-density polar solar wind and a slow, highly variable, high-density equatorial solar wind containing the heliospheric current sheet (HCS). Cometary plasma tails mirror this structure in terms of their appearance, orientation, and occurrence of disconnection events (DEs). The simple bimodal structure does not hold for a short time around solar maximum when the HCS makes a small angle with the solar rotation axis and the solar wind velocity variation with solar latitude is essentially chaotic. Observations of comets LINEAR (C/2000 WM1) and Ikeya-Zhang (C/2002 C1) have helped determine the latitude structure near solar maximum. Their appearance, tail orientation, and location of DEs are consistent with an equatorial-type solar wind north of the HCS and a polar-type solar wind south of the HCS for the latitudes sampled. The comet observations sometimes imply locations of the HCS that differ from locations based on photospheric field measurements. Comets appear to be useful probes of the solar wind for solar minimum and maximum conditions.