High frequency (HF) fields below 30MHz are terrestrial in origin and are caused be electric storms. They are known as 'static' or 'sferics' and are evident when one listens to LF or MF radio transmissions.

Above about 30MHz the fields are cosmic in origin.

The magnetosphere, ionosphere and atmosphere surrounding the earth affect the propagation of fields originating from distant sources, effectively forming a filter which permits only some parts of the spectrum of incident radiation to reach the earth's surface.