The Aurora Borealis are more commonly known as the "Northern Lights" and is caused by ionisation as a result of a flux of particles from the Sun trapped by the Earth's magnetic North Pole. There is a corresponding "Southern Lights" known as the Aurora Australis which occurs in the southern hemisphere. Aurora are generally restricted to the polar regions but the Aurora Borealis can occur as far south as the United Kingdom in rare circumstances.
The ionisation occurs is curtains which are visible to the naked eye. These support radio random wave reflection at 3 - 5 MHz and propagation in this range over long distances. VHF signals are returned in much the same direction as they were sent.
Auroral propagation produces a rapid flutter on signals which ranges from about 100 Hz to 2000 Hz. This makes it impractical for all but morse code.
