Characteristics of nighttime medium-scale traveling ionospheric disturbances observed over Alaska

We focused on propagating wave-like structures which frequently appeared in the O I 630.0 nm airglow images obtained by an all-sky imager installed at Poker Flat Research Range (65.1 degrees N, 147.4 degrees W, magnetic latitude 65.6 degrees) in Alaska. These events are classified as medium-scale traveling ionospheric disturbances (MSTIDs) on the basis of the wavelength and propagating speed. The unique points of our observation are (1) the two-dimensional structures of MSTIDs in the subauroral region are revealed; (2) the horizontal direction of the electric field can be estimated from the motion of the Evening Co-rotating Patch (ECP) aurora appearing simultaneously; and (3) neutral winds are simultaneously observed by a Fabry-Perot spectrometer. Statistical study of these MSTID events from November 2001 to April 2002 indicates the following characteristics. The MSTIDs were observed almost every evening, and sometimes in the morning, but were not observed at midnight. Typical propagating speed, direction, and wavelength of the wave phenomena were about 135 m/s, southwestward, and about 250 km, respectively. When the MSTIDs appeared, northward neutral winds and westward plasma drifts were dominant. This indicates that the Perkins instability does not contribute to the development of MSTIDs in the Alaska region. The vertical wavelength analysis using the dispersion relation of gravity waves with the observed wave parameters and background winds suggests that the observed MSTIDs over Alaska are the atmospheric gravity waves (AGWs). As a case study, temporal variation of an MSTID event on 21 December 2001 is shown. Results of a ray tracing analysis of this event suggest that the AGWs come from the equatorward edge of the auroral oval.