Traveling ionospheric disturbances observed in the OI 630‐nm nightglow images over Japan by using a Multipoint Imager Network during the FRONT Campaign

Pilot observations using a network of five all‐sky imagers (ASIs) were conducted during the new moon period of May 19–22, 1998 as part of the F ‐region Radio and Optical measurement of Nighttime TID (FRONT) campaign. The network observation enabled us to track propagation of medium‐scale traveling ionospheric disturbances (TIDs) in the OI 630‐nm nightglow over a distance of more than 2500 km. The TIDs were observed every night during the campaign period, but occurrence was limited from evening to midnight. They have horizontal wavelengths of 200–600 km, travel a horizontal distance of more than 1000 km, and last for more than three hours. In every case, the TIDs moved southwestward with a velocity of 83–137 m/s. Using dual‐site TID images, the altitude of the TID structures in the 630‐nm nightglow was calculated to be ∼260 km, which corresponds to the bottom side of the mid‐latitude ionospheric F layer.