Helliwell was the author of one book , " Whistlers and Related Ionospheric Phenomena , " and more than 90 scientific papers . Three of his papers deserve special mention . One of these reported on low frequency emissions associated with the 1989 Loma Prieta earthquake . Another describes how injection of a low frequency radio signal into the magnetosphere causes a radio quieting of the natural noise that 's associated with these frequencies . Another paper showed that the alternating current power grids in North America ( 60 Hz ) and Europe ( 50 Hz ) affect auroral chorus emissions
1989 Loma Prieta earthquake
1989 Loma Prieta earthquake
Robert Helliwell