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* Project leaders: J. Emmert (USA), L. Qian (USA) 
* Project members: L. Alfonsi (Italy), A. Elias (Argentina), M. Mlynczak (USA), S. Nossal (USA), H. Schmidt (Germany), S.-R. Zhang (USA)

==Introduction==
The Earth's thermosphere (~90–800 km) is primarily heated by solar far and extreme ultraviolet (FUV and EUV) irradiance, and the primary cooling mechanism is downward conduction to the lower thermosphere and radiative cooling by CO2, NO, and O(3p) [Roble, 1995]. Increasing CO2 concentrations are therefore expected to result in enhanced cooling and consequent contraction of the thermosphere. Roble and Dickinson [1989] predicted that a doubling of CO2 would result in a 40% reduction in mass density at a height of 400 km. An overview of theoretical and empirical studies of mesospheric and thermospheric climate change is given by Laštovička et al. [2008]. 

Model studies [Qian et al., 2008] and satellite measurements [Emmert et al., 2008] suggest that the density trend in the upper thermosphere depends on solar activity and is larger for solar minimum conditions than for solar maximum conditions. One important mechanism of this dependency is that CO2 cooling is dominant during solar minimum, whereas NO cooling becomes relatively more important during solar maximum [Mlynczak et al., 2010]. Another mechanism is that density scale heights are smaller during cooler solar minimum conditions, so that further contraction of the thermosphere has a larger relative effect on density at a fixed height.

Figure 1 shows measured density trends at 400 km, as a function of the F10.7 solar activity index, derived by Keating et al. [2000], Marcos et al. [2005], and Emmert et al. [2008], along with corresponding trends from theoretical simulations by Akmaev [2006] and Qian et al. [2006]. There is fairly good quantitative agreement among the empirical results. The theoretical trends from Qian et al. [2006] agree well with the observed trends in the interval 120 < F10.7 < 160, but the predicted trends at solar minimum are considerably smaller than the observed trends. Other mechanisms besides enhanced CO2 cooling may therefore be contributing to the observed solar minimum trends. The theoretical trend at 200 km computed by Akmaev et al. [2006], which represents moderate solar activity and includes the non-negligible effects of middle atmosphere ozone and water vapor trends, is much stronger than the observed trends under similar solar EUV conditions. 

Given the solar-cycle dependence of thermospheric long-term trends, it is reasonable to expect that the ionospheric long-term trends will also depend on solar activity. Qian et al. [2008, 2009] simulated the effects on the ionosphere of predicted CO2 increases from 2000 to 2100, under solar minimum and solar maximum conditions. The effect on F region parameters was larger under solar minimum conditions, similar to the solar-cycle dependence of secular change in the thermosphere. The solar cycle dependence of the response of E region parameters was less pronounced. To date, there has been no comprehensive assessment of the effect of the solar cycle on observed ionospheric trends.
The goal of this CAWSES-II project is to understand how the response of the thermosphere/ionosphere to anthropogenic forcing differs between solar minimum and other phases of the solar cycle.

[[File:trend.png|center|Fig. 1 Summary of observed and simulated thermospheric density trends at a height of 400 km, as a function of the F10.7 solar activity index. From Emmert et al. [2008]]]

Figure 1. Summary of observed and simulated thermospheric density trends at a height of 400 km, as a function of the F10.7 solar activity index. From Emmert et al. [2008].

==Research Plans==
1) Compare CO2 and NO cooling rates from TIMED/SABER measurements with those predicted by TIME-GCM model simulations. Adjust the model cooling rates to match the data and determine the impact on thermospheric and ionospheric structure during the recent solar minimum.

2) Analyze ionosonde parameters (from many stations) and Millstone Hill incoherent scatter radar (ISR) data for trends as a function of solar cycle. Compare with results from TIME-GCM model simulations.

3) Compare HAMMONIA historical simulations (which assimilate lower atmospheric data) with TIME-GCM simulations of prescribed CO2 and CH4 increases, with particular focus on the conditions of the recent solar minimum.

4) Use historical HAMMONIA model assimilations/simulations to estimate lower thermospheric temperature and composition anomalies since 1967, and thereby constrain the interpretation of height-dependent orbit-derived upper thermospheric density data. From the density data, estimate exospheric temperature trends as a function of solar cycle and compare with ion temperature trend results derived from the Millstone Hill ISR data. Investigate to what extent the estimated lower thermospheric anomalies can explain the unusually low upper thermospheric densities and temperatures of the recent solar minimum.

5) Compare height-dependent orbit-derived thermospheric density data and inferred exospheric temperature with those predicted by TIME-GCM model simulations of CO2 and CH4 increases. Assess, via sensitivity tests, which factors might have contributed to the anomalously low densities and temperatures of the recent solar minimum.

6) Analyze ground based observations of geocoronal Balmer-alpha emissions to determine whether there is a detectable trend in atomic hydrogen (an increase in upper thermospheric and geocoronal H has been predicted to occur as a result of CH4 increases). From model simulations, estimate how H trends might be modulated by the solar cycle. 

==References==
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#Emmert, J. T., J. M. Picone, and R. R. Meier (2008), Thermospheric global average density trends, 1967–2007, derived from orbits of 5000 near-Earth objects, Geophys. Res. Lett., 35, L05101, doi:10.1029/2007GL032809.
#Emmert, J. T., J. L. Lean, and J. M. Picone (2010), Record-low thermospheric density during the 2008 solar minimum, Geophys. Res. Lett., 37, L12102, doi:10.1029/2010GL043671.
#Keating, G. M., R. H. Tolson, and M. S. Bradford (2000), Evidence of long term global decline in the Earth's thermospheric densities apparently related to anthropogenic effects, Geophys. Res. Lett., 27, 1523–1526.
#Laštovička, J., R. A. Akmaev, G. Beig, J. Bremer, J. T. Emmert, C. Jacobi, M. J. Jarvis, G. Nedoluha, Yu. I. Portnyagin, and T. Ulich (2008), Emerging pattern of global change in the upper atmosphere and ionosphere, Ann. Geophys., 26, 1255–1268.
#Marcos, F. A., J. O. Wise, M. J. Kendra, N. J. Grossbard, and B. R. Bowman (2005), Detection of a long-term decrease in thermospheric neutral density, Geophys. Res. Lett., 32, L04103, doi:10.1029/2004GL021269.
#Mlynczak, M. G., et al. (2010), Observations of infrared radiative cooling in the thermosphere on daily to multiyear timescales from the TIMED/SABER instrument, J. Geophys Res., 115, A03309, doi:10.1029/2009JA014713.
#Qian, L., R. G. Roble, S. C. Solomon, and T. J. Kane (2006), Calculated and observed climate change in the thermosphere, and a prediction for solar cycle 24, Geophys. Res. Lett., 33, L23705, doi:10.1029/2006GL027185.
#Qian, L., S. C. Solomon, R. G. Roble, and T. J. Kane (2008), Model simulations of global change in the ionosphere, Geophys. Res. Lett., 35, L07811, doi:10.1029/2007GL033156.
#Qian, L., A. G. Burns, S. C. Solomon, and R. G. Roble (2009), The effect of carbon dioxide cooling on trends in the F2-layer ionosphere, J. Atmos. Solar-Terr. Phys., 71, 1592–1601.
#Roble, R. G. (1995), Energetics of the mesosphere and  thermosphere, The Upper Mesosphere and Lower Thermosphere, Geophys. Monogr. Ser., 87, 1–21.
#Roble, R. G., and R. E. Dickinson (1989), How will changes in carbon dioxide and methane modify the mean structure of the mesosphere and thermosphere?, Geophys. Res. Lett., 16, 1441–1444.
#Solomon, S. C., T. N. Woods, L. V. Didkovsky, J. T. Emmert, and L. Qian (2010), Anomalously low solar extreme-ultraviolet irradiance and thermospheric density during solar minimum, Geophys. Res. Lett., 37, doi:10.1029/2010GL044468, in press.