Silje Eriksen Holmen

The turbopause is the demarcation between atmospheric mixing by turbulence (below) and molecular diffusion (above). When studying concentrations of trace species in the atmosphere, and particularly long-term change, it may be important to understand processes present, together with their temporal evolution that may be responsible for redistribution of atmospheric constituents. The general region of transition between turbulent and molecular mixing coincides with the base of the ionosphere, the lower region in which molecular oxygen is dissociated, and, at high latitude in summer, the coldest part of the whole atmosphere.<br><br> This study updates previous reports of turbopause altitude, extending the time series by half a decade, and thus shedding new light on the nature of change over solar-cycle timescales. Assuming there is no trend in temperature, at 70° N there is evidence for a summer trend of  ∼  1.6 km decade<sup>−1</sup>, but for winter and at 52° N...

The mesopause region can be considered a “boundary region” between the neutral atmosphere, where atmospheric constituents and momentum are transported mainly by winds and turbulent eddies, and the ionosphere, where the main transport mechanism is molecular diffusion. In the mesopause, complex interactions between dynamics and photochemistry occur, and we are far from a complete understanding of these interactions. This thesis aims to better understand the processes responsible for the large temperature fluctuations we observe in the polar mesopause region, especially the effects of atmospheric circulation and wave activity from lower atmospheric layers. Investigations of trends have also been conducted. To carry out these investigations, we have derived and examined mesopause temperatures from two high-latitude locations: Tromsø (70◦N, 19◦E) and Longyearbyen (78◦N, 16◦E), and turbopause height only from Tromsø. A long-term change in turbopause height may be important for understandi...

The airglow hydroxyl temperature record from Longyearbyen, Svalbard, is updated with data from the last seven seasons (2005/2006–2011/2012). The temperatures are derived from ground-based spectral measurements of the hydroxyl airglow layer, which ranges from 76 to 90 km height. The overall daily average mesospheric temperature for the whole temperature record is 206 K. This is by 3 K less than what Dyrland and Sigernes (2007) reported in their last update on the temperature series. This temperature difference is due to cold winter seasons from 2008 to 2010. 2009/2010 was the coldest winter season ever recorded over Longyearbyen, with a seasonal average of 185 K. Temperature variability within the winter seasons is investigated, and the temperature difference between late December (local minimum) and late January (local maximum) is approximately 8 K.

This paper gives an update on the long-term trend in hydroxyl (OH*) airglow winter temperatures measured by a 1 m Ebert-Fastie spectrometer in Longyearbyen, Svalbard (78°N, 16°E), from 1983 to 2013. The temperatures are derived through synthetic fits of measured airglow spectra of the OH*(6-2) vibrational state. The dataset is corrected for seasonal variations by subtracting the mean climatology. Also, solar cycle dependence is investigated. A solar response coefficient of 3.6 K ± 4.0 K / 100 SFU is calculated from F10.7 cm solar radio flux data. After subtraction of the climatology and solar response, the remaining long-term trend is a near-zero trend, -0.2 K ± 0.5 K/decade. Trend analysis of monthly temperatures indicates positive trends for November, January and February and a negative trend for December, but the uncertainties are high.