Title:  "Unexplained Ground Level Radio Signals During Solar Flares"

Abstract:  Fifteen years ago during certain solar flares, ground-level radio
receivers in Arctic and Antarctic regions detected radio signals, which did not match any of
the many known radio and ionospheric effects of solar flares. Only a handful of examples
were observed over several years in the late 1990's. They seemed correlated with the onset of type III
radio emissions, and there was some evidence that they correlated with hard (up to 15 MeV)
photons from the flares. From about that time, spacecraft studies of solar flares have
identified a new type of radio emission, denoted type III-D, which has some characteristics
in common with the mysterious ground-level signals. These observations open up a new
possibility to explain the ground-level signals, though many problems
and difficulties remain, especially regarding the mechanisms by which the signals penetrate
the ionosphere.