Solar-Terrestrial Coupling through High Energy Particle Precipitation in the Atmosphere: a Norwegian contribution (HEPPA-Norway)

To understand the impact of the Sun on the Earth middle atmosphere, lower atmosphere and surface climate is the focus of major international scientific programmes. The importance of solar forcing relative to anthropenic forcing over various time scales from minutes to centuries is not fully elucidated. Previous studies have largely focused on solar irradiance, while the impact of energetic particles originating from auroral activity, solar storms and perturbations in the Earth radiation belt, is just starting to be explored. The chemical signatures of energetic particle precipitation (EPP) are clearly observed in satellite observations. EPP can produce nitrogen oxides (NOx) in-situ in the upper stratosphere during solar proton events, or else in the mesosphere and thermosphere during the pervasive low energy auroral electrons precipitation, and during more sporadic and higher energy electron precipitation events. EPP is the largest source of stratospheric NOx, a major ozone catalyst. The seasonal descent of NOx through the polar stratopause in winter is modulated by dynamical perturbations, such as stratospheric sudden warmings and elevated stratopause events, and hence varies considerably from year-to-year. A detailed analysis of a series of geomagnetic storms in April and May 2010 has been made, where satellite observations of nitric oxide from NASA's AIM satellite have been compared to the climate model WACCM. The study identified large discrepancies between the modelled and observed nitric oxide, which we believe is caused by an incorrect representation of electron precipitation in the model. By replacing the current representation with electron measurements from NOAA's POES-satellites, the modelled nitric oxide appears to be much closer to the observations by AIM. These geomagnetic storms have also been simulated with WACCM_D, where D stands for the D-region of the ionsophere, recently developed at NCAR in collaboration with the Finnish Meteorological Institute. The new model incorporates advanced mesospheric ion chemistry, which has allowed us to map fluctuations of key neutral and ionic species, and to highlight their important role in reproducing the chemical signatures of particle precipitation. Together, the newly corrected observational data, the improved representation of energetic electrons put into the WACCM model, and the inclusion of a sophisticated ion chemistry in the mesosphere, allow us to better quantify the direct and the indirect effects of particle precipitation on nitrogen oxides, as well as the broader impacts of the composition of the nitrogen species family.

In order to improve our understanding of the effect of energetic particle precipitation upon the chemistry and dynamics of the coupled mesosphere-stratosphere system, and ultimately upon surface climate, the project will rely on combined modeling and obse rvational studies. Using particle precipitation measures from polar-orbiting or geostationary satellites as well as ground-based observations, we will investigate how the middle atmospheric chemistry and temperature are influenced by different types of energetic particle precipitation events, and improve the spatial and temporal characterisation of energetic particle precipitation events. Using modeling studies of energetic particle precipitation with a chemistry-climate model (WACCM) including improv ed parametrisations of ionic chemistry and of energetic electron precipitation, we will evaluate the chemical-dynamical couplings in the upper and middle atmosphere, and their potential surface and climate impact at high latitudes will also be examined. T he model results will be benchmarked against satellite observations.