Temperature variations and trends at 90 km height in the Arctic atmosphere

Temperature variations and trends at 90 km height in the Arctic atmosphere

Top image: Airglow and a meteor penetrating Earth’s atmosphere. Photo: International Space Station/NASA Earth Observatory

Silje Eriksen Holmen has investigated temperatures in the middle atmosphere over Longyearbyen and Tromsø and has found that temperature varies periodically according to atmospheric wind patterns and waves. Holmen will defend her PhD thesis at UNIS on 19 December 2016.

13 December 2016
Press release from the University Centre in Svalbard/Birkeland Centre for Space Science and UiT The Arctic University of Norway

The mesopause region is located at 80 to 100 km height in the atmosphere. At such high altitudes, it is difficult to perform direct measurements of basic meteorological parameters like for instance temperature. Weather balloons do not reach that high. Rockets can give us insights into what the weather is like at high altitudes, but they only provide us with a snapshot of the state of the atmosphere. If we want to know what the temperature in the mesopause is on a regular basis, we therefore often use indirect methods, meaning that we measure something that we know is related to temperature and calculate temperature from this.

Two ways of acquiring temperatures from the mesopause region are from the intensity of airglow emissions and from meteors burning up in the atmosphere. Airglow is emissions of light emanating from chemical reactions in the upper atmosphere. It is not a well-known phenomenon among most people, because the emitted light is mostly from the infrared, which we cannot see with the naked eye from Earth’s surface. The intensity of airglow is related to atmospheric temperature. A spectrometer located at the Kjell Henriksen Observatory in Longyearbyen has been providing airglow temperatures for more than three decades.

The Kjell Henriksen Observatory outside Longyearbyen. Photo: Eva Therese Jenssen/UNIS

The top of the Kjell Henriksen Observatory outside Longyearbyen. Photo: Eva Therese Jenssen/UNIS

Meteors are small pieces of rock or metal travelling in outer space. Some of them enter Earth’s atmosphere, where they are slowed down by the increasing aerodynamic pressure. As they reach the mesopause region, the air drag becomes so large that the meteors evaporate. A few of them we can observe as shooting stars from the ground, but most of them we are not able to see. A radar on the ground, like the meteor radar located in Tromsø, can send out radio waves towards the trail that forms behind the evaporated meteor. The radar measures how long time it takes for the trail to spread out, and this is related to temperature of the surrounding air.

Why care about temperature in the mesopause region?
This part of the atmosphere is located far away from where we live. Studies of the mesopause region are often conducted simply because we want to know more about the world we live in. In addition, we know that the different parts of the atmosphere are connected. By learning more about the mesopause region we learn more about the whole atmospheric system, and processes in the mesopause may possibly influence weather on the ground.

Temperature variability governed by waves and winds in the atmosphere
In her PhD work, Silje Eriksen Holmen has looked into how temperature in the mesopause region varies over both short and long time periods. Temperatures were obtained from a spectrometer and radar located in Tromsø and Longyearbyen. Temperatures obtained from Tromsø vary periodically, corresponding with wind patterns and activity by waves generated in the lower atmosphere. Temperatures over Longyearbyen show a slightly different variation. This may indicate that differences in local conditions play a role, for instance the location of mountains relative to the measurement sites, which again impacts the generation of atmospheric waves.

Temperature trends at approximately 90 km height over Tromsø and Longyearbyen were also investigated. The Longyearbyen winter trend is near-zero from 1983 to 2013, while the Tromsø winter and summer trends from 2003 to 2014 are negative and near-zero, respectively. The response of solar variability to temperatures from Longyearbyen was investigated. The response was found to be on the same level as results from other high-latitude locations. This result contributes to questions of strategic importance in context with Earth’s climate.

Dissertation
Silje Eriksen Holmen will defend her PhD thesis “Trends and variability in polar mesopause region temperatures attributed to atmospheric dynamics and solar variability” at UNIS on Monday 19 December at 12:30. She will give a trial lecture entitled “From PMSE to PMWE, related phenomena or not? Our current understanding” at 10:15 the same day.

Both lectures will take place in the auditorium Lassegrotta in Svalbard Science Centre.

Supervisors have been Professor Fred Sigernes (UNIS) and Professor Chris Hall, Tromsø Geophysical Observatory. The committee consists of Dr. Frank Mulligan, Senior Lecturer, National University of Ireland, Maynooth (1. opponent), Dr. Anja Strømme, Senior Advisor, Norwegian Space Centre (2. opponent), and Adjunct Professor Ulf-Peter Hoppe, UiT The Arctic University of Norway (Leader of the committee).

Silje Eriksen HolmenAbout the candidate:
Silje Eriksen Holmen (33) is from Fredrikstad, Norway. After completing her MSc degree in meteorology at the University of Oslo / The University Centre in Svalbard in 2009, she worked as a forecasting meteorologist at the Norwegian Meteorological Institute in Tromsø for three years. In 2012, Holmen started her PhD studies at UNIS / Birkeland Centre for Space Science and UiT The Arctic University of Norway.

Contact information:
E-mail: silje.e.holmen@gmail.com