Observing Radio Signals of Auroral Origin (Invited Tutorial)

For decades, scientists have deployed instruments similar to amateur radio receiving systems to investigate radio signals of auroral origin observable at ground level in the Arctic and Antarctic. These naturally emitted signals fall into roughly four categories: (1) auroral "roar" occurs in relatively narrow ~100-kHz-wide bands centered around approximately 2.8, 4.2, 5.6, and 6.8 MHz, with center frequency depending on location of observation; (2) auroral "burst" has typical bandwidth 1 MHz occurring between about 1.5 and 4.5 MHz, and usually lasting only a few minutes; (3) auroral "hiss" is a broadband emission extending up to 1 MHz, also of short duration; and (4) "auroral kilometric radiation," also known as AKR, occurs in the frequency range 100-900 kHz and is observed often from spacecraft but much more rarely at ground level and primarily in Antarctica. Most of these signals are optimally received at locations 100-500 km poleward of the aurora. Most of the scientific studies have exploited Arctic and Antarctic research facilities, but there are potential observing locations accessible to intrepid amateurs who have mobile equipment. There are a number of unanswered questions about these signals, including possible existence of other types or sub-types, which could be targets of citizen science.