[P-EM28_29PO1] Magnetosphere-Ionosphere Coupling

[PEM28-P06] Magnetic latitude and MLT dependence of the bandwidth of MF/HF auroral radio emissions in the topside ionosphere

*Masataka SAKAI¹, Atsushi KUMAMOTO¹, Yuto KATOH¹, Yuka SATO² (1.Department of Geophysics, Tohoku University, 2.National Institute of Polar Research)

In the ionosphere, auroral radio emissions are generated by precipitating auroral particles. Previous studies reported that the MF/HF auroral radio emissions emitted from the bottomside ionosphere were observed on the ground. The narrowband emissions are called auroral roar, and the broadband emissions are called MF burst. On the other hand, Sato et al. [2010] showed the spectrum and polarization of two events of MF/HF radio emissions observed in the topside ionosphere by the Akebono satellite. Based on the event studies, they suggested that the observed narrowband emissions are generated by the mode conversion of UHR waves enhanced in the auroral ionosphere where the upper hybrid frequency matches the harmonics of the electron cyclotron frequency as suggested for generation mechanism of the auroral roar observed on the ground [Weatherwax et al., 1995; Yoon et al., 1998; Weatherwax et al., 2002]. In this study, we have focused on broadband emissions observed in the topside ionosphere which are similar with broadband MF burst observed on the ground. We analyzed MF/HF broadband emissions (with wider bandwidth of > 0.5 kHz) observed by the Akebono satellite. Because it is difficult to observe broadband emissions on the ground and in the topside ionosphere at the same time, we have performed statistical analysis. We found that the bandwidth of the MF/HF emissions was larger in the high latitude and in the dusk side. The bandwidth of the MF/HF emissions was greater than 1 MHz in higher geomagnetic latitude than 70 degree in the sector from 12 to 24 MLT. Previous studies suggested that the MF bursts observed on the ground were generated by the mode conversion of upper hybrid waves stimulated by the energetic auroral electrons [e.g. Sato et al., 2008]. Therefore, we can expect that the bandwidth of MF bursts depend on the generation processes of upper hybrid waves, mode conversion processes of upper hybrid waves, and propagation processes of converted electromagnetic waves in the auroral ionosphere.