Multidecadal Variability of Upper Atmospheric Hydrogen

Abstract:

Numerical models predict a whole atmosphere response to increases in greenhouse gases with impacts extending through the upper thermosphere. Hydrogen in the thermosphere is predicted to increase due both to carbon dioxide cooling of the upper atmosphere and to increases to the source species for hydrogen resulting from rising concentrations of atmospheric methane. The 11-year solar cycle is a major source of natural variability in the upper atmosphere and its impact must be understood to isolate potential signs of longer-term change in this region. Ground-based Fabry-Perot observations of Balmer α emissions taken from Northern mid-latitudes span multiple solar cycles, facilitating investigation of decadal scale variations, including natural variability in the hydrogen response to solar geophysical changes. The observations suggest a somewhat surprising increase in hydrogen emission intensity between the solar-maximum period of 1990-1991 (S.C. 22) and the near-solar-maximum period of 2000-2001 (S.C. 23), with the caveat that this is a limited data set and that there are calibration uncertainties [Nossal et al., 2019]. Solar activity was higher during the earlier solar maximum period. Thus, the apparent intensity increase is counter to previous midlatitude observations for which the observed intensity increases with higher solar activity. This increase was also not seen in comparison of intensities from three solar minima periods [Nossal et al., 2008]. Further, the apparent intensity increase is also likely of larger magnitude than model simulations would predict due to increases in greenhouse gases [Nossal et al., 2016, 2019]. We will discuss the extended Northern hemisphere hydrogen emission data set, including strategies for assessing uncertainty, as well as interpretation of these observations in the context of climate simulations with the NCAR Whole Atmosphere Community Climate Model-eXtended (WACCM-X).