Space Science Center, University of New Hampshire, Durham, NH, USA

Selecting a variety of interplanetary conditions observed by the Wind spacecraft, we present observations of polar cap precipitation made by the HYDRA instrument on the Polar spacecraft. Specifically, we inquire into the consequences of accepting the solar heat flux (strahl) origin of this precipitation. Examples of inter-hemisphere, dawn-dusk and night-day asymmetries are presented. Hemispherically symmetric polar rain occurs when the Earth is engulfed by a solar ejecta, and supports the idea that when this happens both polar caps are threaded by magnetic field lines which connect back to the Sun. However, even then, large differences in precipitation intensity over the same polar cap are observed when the north-south component (B_Z) of the solar ejecta field reverses polarity, highlighting basic differences between polar cap precipitation for B_Z north and south. Observations of localized, intense precipitation (polar showers) are related to strongly northward-pointing interplanetary magnetic fields. Intense and energetic precipitation filling the entire polar cap (polar squalls) may have an inner magnetospheric origin. The richness of the HYDRA data suggests various avenues of future study.

in The Solar Wind - Magnetosphere System 3, edited by J. K. Biernat, C. J. Farrugia, and D. F. Vogl, p. 245, Verlag der Österreichischen Akademie der Wissenschaften, Austria, 2000