Page "Ionosphere" Paragraph 8

During solar proton events, ionization can reach unusually high levels in the D-region over high and polar latitudes.

Such very rare events are known as Polar Cap Absorption ( or PCA ) events, because the increased ionization significantly enhances the absorption of radio signals passing through the region.

In fact, absorption levels can increase by many tens of dB during intense events, which is enough to absorb most ( if not all ) transpolar HF radio signal transmissions.

Such events typically last less than 24 to 48 hours.