Springtime effects in the mesosphere and ionosphere observed at northern midlatitudes

Nighttime volume emission rates and rotational temperatures, obtained from simultaneous observations of molecular oxygen and hydroxyl airglow at Almaty (43.25°N, 76.92°E) and Sierra-Nevada (37.2°N, 356.7°E), along with ionospheric density derived from foF2 in the vertical sounding ionograms over Almaty are analysed to study the variability and coupling of parameters observed in the upper mesosphere and ionosphere during the period of February – April, 2000. Ionospheric critical frequency measurements and airglow observations by the Mesopause Rotational Temperature Imager (MORTI) at Almaty and the Spectral Airglow Temperature Imager (SATI) at Sierra-Nevada Observatories show an increase in long-period planetary wave (PW) activity from the end of February until the middle of March, 2000. Very good agreement was found in the temporal variations of emission rates and rotational temperatures from March 1–15, 2000 measured at the Almaty and Sierra-Nevada sites. Similar perturbations could also be seen in the ionospheric critical frequency (ΔfoF2) obtained as a difference between current foF2 values and an ionospheric background level. The perturbations observed have been interpreted employing the Met office stratospheric model results. Latitudinal structure of a quasi 5-day wave was identified, for which the first-symmetric-mode amplitude and symmetric behaviour of phase are in good agreement with theoretical prediction. The analysis of the Met office stratospheric data indicate the presence of westward-propagating PW with periods of ∼5 and 10 days during the period of interest. The temporal correlation between planetary scale oscillations observed in the datasets examined (ionospheric, optical and meteorological) suggest dynamical coupling with the stratosphere. A negative disturbance in ΔfoF2 of ∼25% observed 1 day before a sharp increase in the MORTI mesospheric rotational temperature registered on March 10 at Almaty, is also discussed in the context of the possible stratosphere/mesosphere/ionosphere coupling.