Correlation of solar energetic protons and polar cap absorption

This study shows the results of a model of polar cap absorption events (PCA s) using solar energetic proton flux as an input. The proton data are recor ded by the Charged Particle Measurement Experiment (CPME) on board the IMP 8 satellite and are collected by the Applied Physics Laboratory at Johns Ho pkins University. The IMP 8 satellite orbits the Earth at distances between 30 and 35 Earth radii, which places it in the solar energetic particle env ironment throughout most of its orbit. It has been shown in previous studie s that these solar energetic particles have direct and immediate access to the polar atmosphere [Reid, 1970]. Our model shows that the majority of the ionization resulting from the influx of solar energetic protons occurs in the altitude range from similar to 50-90 km. Excess ionization at these alt itudes causes enhanced absorption of cosmic HF radio waves. The levels of a bsorption used for comparison in this study were measured directly by the r iometer at South Pole station, Antarctica. The results show a very strong c orrelation between the incident proton flux and measured path-integrated co smic HF radio noise absorption for significant events, involving absorption s greater than 1.0 dB. For absorption levels lower than this it is obvious that other phenomena dominate. For HF radio waves the primary contributors to PCA are protons with energies near 20 MeV. This study extends the correl ated observations of interplanetary particles and PCA throughout a 9-year p eriod. The close quantitative agreement between the measured and calculated values of absorption supports the validity of the assumptions and supposit ions made by this model. The data also suggest a method by which the path-i ntegrated cosmic noise absorption may be used to probe the E and D layers o f the ionosphere to determine the effective ion-electron recombination coef ficients within these regions.