Plasma conditions in auroral roar source regions inferred from radio and radar observations

A medium frequency interferometer (MFI) has been established in Sondrestrom , Greenland, to measure the direction of arrival (DOA) of auroral roar emis sions at 2.5-3.0 MHz. Combined with electron density data from the Sondrest rom incoherent scatter radar (ISR), these DOA data provide a powerful test of the most-cited generation mechanism of auroral roar emissions. This mech anism requires that the relationship between the electron plasma frequency (fee) and the electron gryofrequency (f(ce)) in the source regions is f(pe) (2) = (n(2) - 1) f(ce)(2), where n = 2, 3 is the harmonic number of the emi ssion. For each of five auroral roar events studied, to within experimental uncertainty this frequency-matching condition is met throughout the source region of the emissions inferred from the MFI and ISR data. Furthermore, r ay-tracing analysis indicates that signals at the roar frequency are able t o propagate from the source region to the location of the MFI receiver.