A polar rain index is constructed using Defense Meteorological Satellite Program particle data from 1977 to the present. It is determined from the average polar rain flux precipitating in a confined region (in corrected geomagnetic and local time coordinates) near the magnetic pole. The index is computed separately for each hemisphere on a daily basis. Descriptions are given of (1) the algorithm used to automatically select intervals of polar rain, (2) the in-flight cross normalization of the various satellite detectors, and (3) the spectral corrections for background and noise. Using the index, three long-period (tens of days to years) variations are found in the polar rain: (1) variation with interplanetary magnetic field (IMF) sector. (2) variation with season, and (3) variation with solar cycle.

After removing the seasonal variation, a comparison of the adjusted polar rain flux in each hemisphere can be used to monitor IMF sector variations on a daily basis. These data show the evolution from a stable, double sector structure, lasting almost 2 years prior to solar minimum, to a nearly unipolar structure during solar minimum. There are two aspects of the polar rain variation that are most prominent over the solar cycle. The first is a variation in the yearly averaged polar rain flux that correlates best with variations in the yearly averaged magnitude of the interplanetary magnetic field, or with combinations of the average magnetic field to some power divided by the average solar wind density (e.g., the Alfv¿n speed). The second is that the hemispheric difference in polar rain number flux decreases markedly during solar minimum. ¿American Geophysical Union 1990