UALT

The increase in anthropogenic greenhouse gases and the associated global change are well recognized problems. As space physicists, we ask two essential questions:  Does the greenhouse gas increase affect the upper atmosphere directly, including neutral species and charged particles?  Does the surface global change contribute indirectly to the upper atmospheric long-term change? Roble and Dickinson [1989] indicated using a General Circulation Model (GCM) simulation that doubling the CO₂ concentration in the thermosphere will cause ~50ºK increase in the thermospheric (exospheric) temperature as well as 50% reduction in neutral densities near 200-300 km, as a result of atmospheric contraction, the “falling sky”. The ionosphere, embedded in the dense neutral atmosphere environment, is expected to experience changes in plasma density and its height distribution.

Detecting long-term changes in the upper atmosphere has been an area of active research following the Roble and Dickinson [1989] study. Ionosonde-based trend detection provided initial evidence of the ionospheric cooling [e.g., Ulich and Turunen, 1997, and as summarized in Laštovička, et al., 2006]. Further compelling evidence of long-term upper atmospheric cooling has come from carefully calibrated datasets of the thermosphere and the ionosphere thermal status, including satellite drag data [e.g., Keating et al., 2000; Emmert et al., 2004, Marcos et al., 2005], which are strongly determined by neutral density, and incoherent scatter radar  (ISR) ion temperature data [Holt and Zhang, 2008; Zhang et al., 2013; Ogawa et al., 2015], which are strongly correlated with neutral temperature.

This project provides a new opportunity to review those existing observational and modeling results, and examine all possible causes for the long-term trends in the ionosphere and thermosphere. Specially, our project will focus on the following scientific topics that address the aforementioned outstanding and challenging questions.

1. 1.    Greenhouse gas increase and long-term trends. Thermosphere- ionosphere GCM modeling studies [Qian et al., 2006; Solomon et al., 2015] suggest that the observed long-term change in thermospheric density is mainly caused by the increase of CO₂ which act as an efficient cooling agent in the thermosphere. CO₂ is increasing throughout the atmosphere, but the recent satellite measurements of CO₂ showed that it is increasing faster above the mesopause than in the lower and middle atmosphere [Emmert et al., 2012; Yue et al., 2015]. This presents a challenge to understand thermospheric trends. Therefore a revisit to the greenhouse gas effect with recent observations and the state-of-the art modeling is highly needed.
2. 2.    Understanding the trend variability. Variability in the observed thermospheric density and ionospheric temperature trends is substantial, depending on, for example, the solar activity level, the time of day, altitude, and geomagnetic location [Zhang et al., 2013; Emmert et al., 2015; Ogawa et al., 2015]. Lower atmosphere trends have significantly different characteristics due to different driving forces [Lubken et al., 2013; Berger et al., 2015 ]. Further identifying variability dependencies and factors driving them remain to be an ongoing effort. This variability study can potentially result in new view on what is truly driving the long-term changes in the upper atmosphere.
3. 3.    Strong cooling from multiple ISR observations. Substantial differences exist between ISR-based temperature trends and greenhouse gas increase-driven temperature simulations, with the observed cooling being essentially much larger [e.g., Holt and Zhang, 2008], inconsistent with what would be expected based on neutral density trend measurements [Akmaev, 2012].  More and more ISR data from middle, subauroral, auroral and dayside cusp latitudes show similar stronger cooling trends well beyond the CO₂ effects, probably suggesting different or additional cooling agents, such as long-term changes in atmospheric wave activities [Oliver et al., 2013; Jacobi, 2014].
4. 4.    The impact of long-term changes in wave activity needs to be verified with substantial efforts, including both theoretical and observational investigations [Lastovicka, 2015; Oliver et al.,2015]. If they do exist in the lower atmosphere [Jacobi et al., 2008], they will have a fundamental impact on composition and density of the thermosphere via changes in eddy diffusion and the energy budget.
5. 5.    Effects of secular changes in solar and geomagnetic activity as well as the geomagnetic field. Prior studies have shown these influences [Lockwood et al., 2009; Lockwood, 2012; Yue et al., 2008; Cnossen et al., 2012; Elias et al.; 2010] . It is critical to evaluate their relative roles as compared to other drivers.

We propose to form an international team to work on these important aspects of the upper atmospheric long-term change.  Three specific research tasks are: (a) characterizing long-term trends with observations from in situ and ground-based instruments. This will be conducted by summarizing and reviewing all existing ionospheric and thermospheric trend results; (b) understanding various driving mechanisms and identifying the ones that can consistently account for key observations; (c) evaluating potential societal impacts of climate changes in the upper atmosphere.  The research task (b) is the central area of our effort for which we plan to examine drivers from below and above, specifically,