Semi-annual variation of excited hydroxyl emission at mid-latitudes

Ground-based observations show a phase shift in semi-annual variation of excited hydroxyl (OH∗) emissions at mid-latitudes (43∘ N) compared to those at low latitudes. This differs from the annual cycle at high latitudes. We examine this behaviour by utilising an OH∗ airglow model which was incorporated into a 3D chemistry–transport model (CTM). Through this modelling, we study the morphology of the excited hydroxyl emission layer at mid-latitudes (30–50∘ N), and we assess the impact of the main drivers of its semi-annual variation: temperature, atomic oxygen, and air density. We found that this shift in the semi-annual cycle is determined mainly by the superposition of annual variations of temperature and atomic oxygen concentration. Hence, the winter peak for emission is determined exclusively by atomic oxygen concentration, whereas the summer peak is the superposition of all impacts, with temperature taking a leading role.

Semi-Annual Variation of Excited Hydroxyl Emission at Mid-Latitudes

Ground-based observations show a phase shift in semi-annual variation of excited hydroxyl (OH*) emissions at mid-latitudes (43° N) compared to those at low latitudes. This differs from the annual cycle at high latitudes. We examine this behaviour utilising an OH* airglow model which was incorporated into the 3D chemistry-transport model (CTM). Through this modelling, we study the morphology of the excited hydroxyl emission layer at mid-latitudes (30° N–50° N), and we assess the impact of the main drivers of its semi-annual variation: temperature, atomic oxygen, and air density. We found that this shift in the semi-annual cycle is determined mainly by the superposition of annual variations of temperature and atomic oxygen concentration. Hence, the winter peak for emission is determined exclusively by atomic oxygen concentration, whereas the summer peak is the superposition of all impacts, with temperature taking a leading role.

Comparison between seasonal variations in tidal and internal gravity wave activity as derived from observations at Maimaga and Tiksi

Since 2015, simultaneous observations of temperature of the high-latitude mesopause (87 km) have been made at Maimaga (63.04° N, 129.51° E) and Tiksi (71.58° N, 128.77° E) stations. These stations record spectra with Shamrock (Andor) photosensitive infrared spectrographs detecting the OH (3, 1) band in the near-infrared region (about 1.5 μm). We analyze temperature data obtained in observation seasons from 2015 to 2017. Standard deviations of temperature σ from its mean values are taken as characteristics of wave activity at night. We have obtained standard temperature deviations corresponding to internal gravity waves (IGW) (σgw) and tidal waves (σtd). Mean night rotational temperatures of hydroxyl emission almost coincide, and seasonal variations of gravity and tidal waves have a similar form during two seasons of simultaneous observations at Tiksi and Maimaga.

Comparison between seasonal variations in tidal and internal gravity wave activity as derived from observations at Maimaga and Tiksi

Since 2015, simultaneous observations of temperature of the high-latitude mesopause (87 km) have been made at Maimaga (63.04° N, 129.51° E) and Tiksi (71.58° N, 128.77° E) stations. These stations record spectra with Shamrock (Andor) photosensitive infrared spectrographs detecting the OH (3, 1) band in the near-infrared region (about 1.5 μm). We analyze temperature data obtained in observation seasons from 2015 to 2017. Standard deviations of temperature σ from its mean values are taken as characteristics of wave activity at night. We have obtained standard temperature deviations corresponding to internal gravity waves (IGW) (σgw) and tidal waves (σtd). Mean night rotational temperatures of hydroxyl emission almost coincide, and seasonal variations of gravity and tidal waves have a similar form during two seasons of simultaneous observations at Tiksi and Maimaga.