High-latitude thermospheric neutral density changes

Satellite studies at mid-latitudes have shown that the global thermosphere is gradually contracting. The rate of decrease in density at 400 km is estimated to range from 2% to 5% per decade at solar maximum and minimum. Modeling work gives a more conservative estimate on the thermospheric density decline, typically 1 to 2% per decade at 400 km. The cause of this contraction is linked to the long term increases in CO2 concentration which has resulted in warming of the troposphere. However, CO2 acts as a highly effective radiative coolant in the middle and upper atmosphere and it is believed that this cooling is responsible for the long-term contraction of the thermosphere. We propose to extract the long-term trend over 30 years from high-latitude, and 16 years from polar cap, radar data. We will compare our results to those from lower latitudes, likewise compare our results to those from model predictions of climate change.

A low-latitude satellite study showed that thermospheric neutral density near 400 km altitude is related to solar output flux. The solar flux is very well correlated to the 11-year solar activity cycle. Variations in thermospheric mass density of up to an order of magnitude occur throughout the solar cycle at low latitudes. Since no such study has been performed at high-latitudes or in the polar cap, we now propose to do so.

Global satellite drag measurements at mid- and high-latitudes have shown that large geomagnetic storms can result in dramatic enhancements of the atomic oxygen density of several 100% at altitudes near 400 km. Sometimes the enhancement can reach up to almost a factor of ten. In addition, significant changes in lower-thermospheric composition may occur, affecting the O/N2 ratio, which arise from atmospheric upwelling driven by Joule heating at high latitudes that circulates and redistributes atmospheric constituents globally. These substantial changes take place on time scales that range from a few hours to a few days. The thermosphere may also experience highly efficient cooling in the recovery stage of storms. This can even lead to 'overcooling' where densities after storms have been observed to be up to 36% below the quiet-time densities prior to a large geomagnetic storm. We propose to investigate the thermospheric response to geomagnetic storm forcing, in particular during the recovery phase. Of interest is the time to fully recovery, when the 'over-cooling' phenomenon occurs and how long it lasts, as well as how the history of geomagnetic activity prior to a storm affects the recovery phase and final outcome.

The proposal objectives can be achieved using a novel radar technique developed by the principle investigator which is either by passive observation or active experiment. Both require the NERC-funded EISCAT radar facilities located in northern Norway and Svalbard archipelago in the polar cap, where the Earth's magnetic field is near-vertical. When NERC took on responsibility for EISCAT in late 2009, it also inherited a valuable and unique long-term dataset, i.e. 30 years for the mainland EISCAT radar and 16 years for the polar cap EISCAT Svalbard Radar, which we fully intend to exploit now. In addition, by virtue of having the world's only high-latitude incoherent scatter radar co-located with an ionospheric high-power pumping facility (called the Heater), EISCAT is the only facility in the world that can execute the proposed active experiments to yield thermospheric neutral density.

Space weather-driven disturbances of the Earth's upper atmosphere can have negative effects on the aviation and power industries, on navigation accuracy, radio communications as well as many satellite-based services. The ultimate goal is to provide accurate and timely forecasts of space weather impacts via the UK Met Office's unified model to the affected industries so they may take the appropriate mitigating actions. Our main contribution will be to provide real observations of the thermosphere, both historic as well as recent, in order to calibrate the unified model. There are multiple societal and economic benefits to the proposed research, achieved through prior warning of the consequences of events that may cause technological performance degradation or, in the worst case, complete loss.

Low orbit satellite lifetime is intimately coupled to atmospheric drag, which is a direct function of its density. Obviously, the very high cost of building and launching satellites must be ameliorated over the operational lifetime. Likewise, the orbit of space debris, a hazard to operational spacecraft, is similarly affected by atmospheric density. At high altitudes the atmospheric density is highly variable, up to an order of magnitude, depending on geomagnetic and solar activity. Operators can mitigate atmospheric drag effects when it is especially high by either using thrusters or re-configuring the spacecraft to minimise drag. To do this, they need to know when to take action and for how long. Similarly, for safety reasons, spacecraft operators need to know where the debris is going as atmospheric density is changing. Our contribution will be to provide real observations of the thermosphere, both historic as well as recent, in order to calibrate the orbital models by providing them with an accurate density measurement in a variable atmosphere.

We will compare our observations of the long-term thermospheric contraction to models associated with climate change. In this manner, we expect to determine which fraction of the contraction rate may be associated with climate change due to anthropogenic activity. The cause of this contraction is linked to the long term increases in CO2 concentration resulting from human activity, which has resulted in warming of the troposphere. However, CO2 acts as a highly effective radiative coolant in the middle and upper atmosphere and it is believed that this cooling is responsible for the long-term contraction of the thermosphere. This information should prove useful to policy makers regarding the rate at which the move to low-carbon or carbon-free energy sources should occur.