Impact of solar irradiance and geomagnetic activity on polar NOx, ozone and temperature in WACCM simulations

N Tartaglione,T Toniazzo,Y Orsolini,O.H Otterå

doi:10.1016/j.jastp.2020.105398

Abstract

The response of the atmosphere to solar irradiance and geomagnetic activity is analyzed in experiments with the Whole Atmosphere Community Climate Model (WACCM) under idealized forcings. Four experiments are carried out combining high (H) and low (L) solar radiative forcing with high (7) and low (3) geomagnetic activity: H7 (with high radiative forcing and high geomagnetic activity), H3, (high/low), L7 (low/high), and L3 (low/low). The comparison between these experiment is used to assess the effects of solar radiative forcing and geomagnetic activity mainly on the stratosphere. A two-step Monte Carlo-based statistical test, which defines an impact score, is used to assess statistically significant impacts on regional scales, on pressure levels, for a few key model variables, like NOx, ozone, and temperature.Under low solar forcing (L7/L3), a statistically significant relationship between geomagnetic activity and NOx is found in both hemispheres and for all seasons. An equally strong relationship is lacking for ozone and temperature when analyzing these fields on isobaric levels. A statistically significant impact on stratospheric ozone is only seen in austral winter and spring. However, vertical cross sections show statistically significant impact on temperature and ozone mainly in the southern hemisphere (SH) during austral winter and the following spring.Significant and persistent signals in both SH NOx and ozone concentrations are only produced when the effect of high solar forcing is added to high geomagnetic activity (H7). In this case, statistically significant differences are also found for mesospheric temperatures, ozone and NOx. This latter result appears also under low geomagnetic activity as a result of solar forcing alone, suggesting that solar irradiance significantly affects NOx, ozone and stratospheric temperatures and, in some seasons, even tropospheric temperature.In summary, geomagnetic activity primarily affects NOx and ozone concentrations in the SH. Solar maximum conditions can reduce the amount of NOx in the stratosphere because of higher ozone production. Thus, we conclude that correlations between changes in solar irradiance and geomagnetic activity are important with respect to their effects on the atmosphere. In particular, geomagnetic activity can modulate atmospheric ozone concentrations and other associated stratospheric and tropospheric variables under conditions of high solar activity.

Highlights

The Earth’s atmosphere is subject to both continuous energetic electron precipitation (EEP) and more intense precipitation events associated with solar activity, like solar proton events
We investigate the respective roles of solar spectral irradiance and geomagnetic activity (GA) in affecting ozone, nitrogen oxides (NOx) and tempera ture through numerical experiments with steady forcing using the Whole Atmosphere Community Climate Model (WACCM)
The impact in the northern hemisphere (NH) is more limited than in the SH and the statistically significant differences appear to be confined to small areas. These results suggest that the in fluence of GA influence on NOx, ozone and temperature depends on solar irradiance (SI) conditions

Summary

Introduction

The Earth’s atmosphere is subject to both continuous energetic electron precipitation (EEP) and more intense precipitation events associated with solar activity, like solar proton events. EEP encompasses medium- and high-energy electrons and auroral electrons. EEP events are less energetic but more frequent than solar proton events. Their injection into the atmosphere is steered by geomagnetic activity (GA), which is driven by high-speed solar wind streams and coronal mass ejections (Myllys et al, 2015). In the present observational period, GA follows a rough cycle, somewhat delayed in phase with respect to the 11-year solar cycle (Fraser-Smith, 1972; Echer et al, 2004; Du, 2011)

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