Direct radar observations of TIDs in the D- and E-regions

We have observed Traveling Ionospheric Disturbances (TIDs) in the night-time D- and E-regions using a 2.66 MHz imaging Doppler interferometer radar. TIDs were observed in two distinct ways. In the first, the TID was manifested as discrete traveling surges, with average spacings of 54 min. The D-region surges were so well defined that they could be tracked as they passed close to overhead by using the phase differences across the antenna arrays. A velocity of 135 m s^?1 to the south was measured, giving a horizontal wavelength of 440 km typical of medium scale TIDs. The direction of phase travel relative to the horizontal was ?6° (i.e. downwards). These observations were made during a night of extraordinary OH infrared mesopause structure activity made visible by the presence of a total lunar eclipse. In the second type of TID observation, we show the Doppler interferometer analysis of ripples on the under surface of sporadic-E layers taken on two nights of significant OH infrared and OI 5577 Å wave activity. The TIDs were observed to propagate at speeds of 120–300 m s^?1, with directions predominately toward the southwest, again typical of medium scale TIDs. These results show definite wave effects on MF radar returns and thus suggest that the measurement of mesospheric bulk winds with MF radars should be approached with some caution. Comparison of the TID characteristics with the OH structure characteristics show that the TIDs travel faster than the OH structures, have longer apparent horizontal wavelengths and generally travel in the opposite direction.