Occurrence characteristics of MF auroral radio emissions observed in Iceland

In order to study the generation and propagation processes of the MF auroral radio emissions (auroral roar and MF burst), a radio spectrograph ststem was installed at Husafell in Iceland (invariant latitude: 65.3deg). Within the observation period since late 2006, several MF auroral radio emissions have been detected. Based on our observation results, the polarization character of the MF bursts was consistent to the previous result by Shepherd et al. [1997], and identified that 3fce roar is L-O mode wave. It is suggested that auroral roar appears during magnetic storm recovery phase while MF burst is associated with aurora breakup. We compared the 3fce roar detected on May 23, 2007 with auroral image observed by the Polar/UVI and auroral particle observed by the DMSP/SSJ4 in the southern hemisphere. These image data show that the auroral oval has multilayer structure and the Husafell station is located between enhanced layers. In a region with relatively low energy electrons' precipitation (several 100 eV), MF radio waves can propagate to the ground because there is weak ionization in the D and E regions. Therefore, it is suggested that the auroral roar is generated by low energy electrons precipitating near the observation site and propagates downward to the ground. To understand the mechanism, the energy spectrum of precipitating electrons is the key parameter not only for plasma instability but also for ionization of low altitude region. This observation result shows important evidence. On the hypothesis that the frequency of auroral roar coincides with harmonics of fce in the source region, the observation frequencies of 3.1-3.5 MHz correspond to the altitude range of 550-880 km, and electron density should be more than 105/c.c. in this altitude range. It is less likely to occur in the polar ionosphere. When we introduce alternative hypothesis that the plasma instability occurs at (n+1/2)fce ESCH waves, the estimated altitude of the source region becomes 250-350 km. Thus, we have to reexamine the proposed generation mechanism of the MF auroral radio emissions.