Long term behaviour of the MLT quasi-7-day wave at two radar-sites at northern polar latitudes

The activity of the mesospheric-lower-thermosphere (MLT) quasi-7-day wave at two northern polar meteor-radar sites is presented. The study is based on long term meteor wind observations over Resolute Bay (75°N, 95°W) (May1998 to Feb 2009) and Yellowknife (62.5°N, 114.3°W) (June 2002 to Oct 2008). The wave showed clear seasonal variations, with strong activity during winter, more modest occurrence in the equinoxes, and minimum strengths during summer. The period of the quasi-7-day wave was 7.4±0.3 days with little to no systematic seasonal variation in the wave period. The wave was characterized by increasing amplitude with increasing height and with downward phase propagation, suggesting that the wave had its origin at lower heights. The planetary wave (PW) activity was stronger in the zonal component than in the meridional component. The PW activity shows year to year variations, and appears to be sensitive to the background wind. The possible source mechanisms for the quasi-7-day wave are briefly discussed in the light of current understanding about planetary waves.

► This is the first report concerning the presence of the 7–8 day wave in northern polar MLT region.
► The quasi-7-day wave activity is highly sporadic in nature. In general, it is strongest during winter followed by the equinoxes and minimum during the summer.
► The wave activity is stronger in the zonal component than the meridional component. It appears to be sensitive to the background wind.
► Differences between Yellowknife and Resolute Bay suggest that local effects on the wave are important, suggesting gravity wave modulation.