Polar Cap Absorption (Pca):
An anomalous condition of the polar ionosphere whereby hf and vhf (3 - 300 mhz) radiowaves are absorbed, and lf and vlf (3 - 300 khz) radiowaves are reflected at lower altitudes than normal. In practice, the absorption is inferred from the proton flux at energies greater than 10 mev, so that pcas and proton events are simultaneous. Transpolar radio paths may still be disturbed for days, up to weeks, following the end of a proton event.
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