Morphology of the polar rain

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Abstract
Results of a statistical study of the occurrence and characteristics of the polar rain (Winningham and Heikkila, 1974) are reported. Precipitating electron data in the energy range from 50 eV to 20 keV from the SSJ/3 sensor on the polar‐orbiting DMSP/F2 satellite were used for the study. Intervals of clear polar rain were identified in all orbits in the 1‐year interval September 1977 to August 1978. The spectra from the intervals were binned in a two‐dimensional spatial array in geomagnetic latitude and magnetic local time. Separate arrays were maintained for different levels of magnetic activity, values of the components of the interplanetary magnetic field (IMF), and season. The average spectra were determined in each bin and were used to calculate the integral flux and average energy. Two‐dimensional maps of average energy and integral flux show significant large‐scale spatial variations that are roughly symmetric about an axis running prenoon to premidnight. Integral flux (average energy) decreases (increases) from the dayside to the nightside along this axis by a factor of almost 15 (3). This basic variation was maintained in all separations by magnetic activity, season, intensity of the polar rain, and sector structure, although small rotations in magnetic local time of the symmetry axis could be seen in some cases. The second most prominent variation is one that has been noted previously (Yeager and Frank, 1976; Meng and Kroehl, 1977); polar rain occurs preferentially in the northern (southern) cap for away (toward) IMF sectors. In the preferred cap the precipitation is stronger in the morning. Approximately 70% of the spectra occurred for Bz negative. The overall intensity of the polar rain increases and cools with increasing magnetic activity. The occurrence of polar rain falls within a circular region whose center is offset toward premidnight and whose radius increases with magnetic activity.