HR: 1340h
AN: SM23B-1399    [Abstracts]
TI: Occurrence characteristics of MF auroral radio emissions observed in Iceland
AU: * Sato, Y
EM: yuka@stpp1.geophys.tohoku.ac.jp
AF: Graduate School of Science, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai, 980- 8578, Japan
AU: Ono, T
EM: ono@stpp1.geophys.tohoku.ac.jp
AF: Graduate School of Science, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai, 980- 8578, Japan
AU: Sato, N
EM: nsato@nipr.ac.jp
AF: National Institute of Polar Research, 1-9-10, Kaga, Itabashi-ku, 173-8515, Japan
AU: Fujii, R
EM: rfujii@stelab.nagoya-u.ac.jp
AF: STEL, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan
AU: Miyaoka, H
EM: miyaoka@nipr.ac.jp
AF: National Institute of Polar Research, 1-9-10, Kaga, Itabashi-ku, 173-8515, Japan
AU: Kadokura, A
EM: kadokura@nipr.ac.jp
AF: National Institute of Polar Research, 1-9-10, Kaga, Itabashi-ku, 173-8515, Japan
AB: 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‚Ś breakup. We compared the 3f_ce 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 f_ce 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)f_ce 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.
DE: 2704 Auroral phenomena (2407)
DE: 2772 Plasma waves and instabilities (2471)
DE: 2788 Magnetic storms and substorms (7954)
DE: 6964 Radio wave propagation
DE: 7534 Radio emissions
SC: SPA-Magnetospheric Physics [SM]
MN: 2007 Fall Meeting