Two classes of medium-scale traveling ionospheric disturbances observed by an HF Doppler array

The daytime horizontal trace velocities (Vh) of medium-scale traveling ionospheric disturbances (TID) observed with a network of HF Doppler receivers in central Japan from September 17 to October 11, 1982 are analyzed. The TID with a specified wave period of 40 min are found to fall into two separate classes not previously distinguished; i.e., the first class has Vh less than or equal to 300 m/s with the propagation azimuth in a confined range of about + or - 40 deg around the southeast direction, whereas the other has Vh greater or equal to 400 m/s with the omnidirectional features. The results are discussed in the light of atmospheric gravity wave theory. It is concluded that while the first class can be identified with the internal gravity waves, the second class may be explicable in terms of the quasi-evanescent gravity waves inherent to the dissipative thermosphere.