Earth Planets Space, Vol. 54 (No. 1), pp. 31-44, 2002
A. Saito1,2, M. Nishimura3, M. Yamamoto3, S. Fukao3, T. Tsugawa4, Y. Otsuka5, S. Miyazaki6, and M. C. Kelley1
1304 Rhodes Hall, School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853-3801, U.S.A.
2On leave from Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
3Radio Science Center for Space and Atmosphere, Kyoto University, Uji 611-0011, Japan
4Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
5Solar-Terrestrial Environment Laboratory, Nagoya University, Aichi 442-8507, Japan
6Geographical Survey Institute, Ibaraki 305-0811, Japan
(Received October 31, 2000; Revised September 20, 2001; Accepted October 1, 2001)
Abstract: The nighttime traveling ionospheric disturbances (TIDs) and the F-region 3-m scale field-aligned irregularities were simultaneously observed with the MU radar and GEONET, a GPS network, during the FRONT (F-region Radio and Optical measurement of the Nighttime TID) campaign periods in May 1998 and August 1999. The vertical profile of electron density detected by the incoherent scatter observation of the MU radar clarified that ionized atmosphere on the bottomside of the ionospheric F-region was deeply modulated by TIDs, which would cause the variations of the 630 nm band airglow luminosity. The coherent echoes from the 3-m scale field-aligned irregularities were detected also on the bottomside of the F-region in the nights when TIDs were intense in amplitude and the ionosphere was uplifted. Two-dimensional structures of the field-aligned irregularities detected by the multi-beam observation of the MU radar revealed that the 3-m scale irregularities formed band-like structures and traveled to the southwest in several nights. Their wave vector and traveling velocity were coincident with those of the nighttime TIDs that were simultaneously detected by the TEC observation of GEONET. The intense Doppler velocities of the coherent echoes indicate that the polarization electric field is generated inside the TIDs. We consider that the horizontal gradient of the electric conductivity associated by TIDs and the vertical gradient of the conductivity on the bottomside of the F-region ionosphere generates the 3-m scale irregularities through the gradient-drift instability process. The anti-correlation of the occurrence rate of the F-region field-aligned irregularities to the solar activity would be caused by the anti-correlation of the amplitude of TIDs and of the vertical gradient of the Pedersen conductivity.