A Decade of Type 3 Radio Aurora Studies: Toward and Away from the EIC Interpretation.

Abstract

Auroral-zone VHF radars operating near 50 MHz often receive a type of coherent backscatter from ionospheric irregularities that is characterized by narrow spectra with mean Doppler velocities below the ion-acoustic velocity. Because these echoes are distinctly different from the familiar type 1 and type 2 echoes, they were deemed to represent a new class and were named type 3. In the mid 1980's, there evolved the idea that the plasma physical mechanism leading to auroral type 3 backscatter involves electrostatic ion cyclotron (EIC) waves. Further observations made in subsequent years seemed to support this interpretation. However, refined measurements and data analysis carriedd out in recent years, uncovered substantial disagreement between 50-MHz type 3 radar observations and predictions from EIC theories. The most obvious discrepancies between theory and observations include the Doppler frequency dependence of type 3 echoes on the radar frequency, their backscatter altitude, their magnetic aspect angle dependence, and their most common mean Doppler frequency. As a result, the idea of type 3 radio aurora being associated with EIC waves has lost support from the radio science community. Although work on finding an explanation for type 3 backscatter continues, no new interpretation has yet found wide support.