Polar Cap Absorption Events - Massive Short Wave Communications Blackouts

What Are the Levels of Solar Protons in near-Earth Space Right Now?

The Trigger: The Arrival of a Solar Proton Event

Occasionally the Sun blows clouds of very high velocity protons at the Earth (called a solar proton event). Solar protons can penetrate into the Earth's magnetosphere near the magnetic poles. Solar protons with energies up to about 10 million electron volts bombard the ionosphere, knocking electrons free during collisions with air particles and increasing the density of the ionosphere at low altitudes (in the D- and E-regions).

The Effects on Radio Waves

Short wave radio waves at relatively high frequencies (in the HF range) are absorbed by the increased particles in the low altitude ionosphere causing a complete black out of radio communications. This is called a Polar Cap Absorption (PCA) event. Lower frequency waves that would normally reflect off the low altitude ( D- or E-region) ionospheric layers now do so at lower than normal altitudes changing dramatically their propagation paths.

How Long Does a PCA Last?

PCA events are particularly devastating to radio communications because they can last for days depending on the size and location of the magnetic disturbance on the Sun that produces them.

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