AM Broadcast Signals Observed at South Pole

For many years, Dartmouth College has operated radio receivers at the Amundsen-Scott South Pole Station, primarily at 100-5000 kHz (LF through lower HF). The primary purpose is to measure radio noise of natural auroral origin, but beacon and broadcast bands are received as a by-product. South Pole has a unique situation of six months of darkness/daylight; that is, a six month day-night cycle, but a 24-hour magnetic local time cycle. Broadcast band signals are received during the six months of darkness, but the local time dependence determined from low-resolution receivers was always a mystery, exhibiting peaks around both noon and midnight magnetic local time. Recent high resolution observations resolved the mystery, demonstrating that one of these local time peaks consists of Region 1 AM signals on 10-kHz spacings, and the other peak consists of Region 2 signals on 9-kHz spacings. The local time dependence results from the geographical distribution of the sources, combined with the position of the solar terminator. In some cases detailed geographical dependences produce observable propagation effects. The Region 1 signals are received around magnetic midnight and heavily affected by auroral activity, whereas the Region 2 signals are received during daytime aurora and are less variable. These interesting effects provide additional arguments for establishing a space-weather radio receiver at South Pole in the future, though they also argue for taking the effort to install a sufficiently sensitive antenna/pre-amplifier.