Missing driver in the Sun-Earth connection from energetic electron precipitation impacts mesospheric ozone

Abstract

Energetic electron precipitation (EEP) from the Earth's outer radiation belt continuously affects the chemical composition of the polar mesosphere. EEP can contribute to catalytic ozone loss in the mesosphere through ionization and enhanced production of odd hydrogen. However, the long-term mesospheric ozone variability caused by EEP has not been quantified or confirmed to date. Here we show, using observations from three different satellite instruments, that EEP events strongly affect ozone at 60-80 km, leading to extremely large (up to 90%) short-term ozone depletion. This impact is comparable to that of large, but much less frequent, solar proton events. On solar cycle timescales, we find that EEP causes ozone variations of up to 34% at 70-80 km. With such a magnitude, it is reasonable to suspect that EEP could be an important part of solar influence on the atmosphere and climate system.

Figure 1. Signature of EEP in observed mesospheric ozone.

(a) Monthly mean ECRs (black bars), maximum proton flux >10 MeV (red numbers) in proton flux units (1 pfu=1 p cm⁻² sr⁻¹ s⁻¹) and sunspot number (SSN, grey area) between 2002 and 2012. (b,c) Maximum O₃ loss (%) at altitudes between 70 and 78 km in the Northern hemisphere (b) and Southern hemisphere (c) during 60 EEP events, with daily ECR >150 (counts s⁻¹). Numbers: the average O₃ loss (%) for each set of available satellite measurements (MLS, SABER and GOMOS).

Figure 2. Magnitude of the short-term EEP effects on mesospheric ozone.

(a–c) O₃ anomalies (%) for selected EEP events in the Northern hemisphere and in the Southern hemisphere derived from GOMOS (a), SABER (b) and MLS (c) observations. Black dashed lines: EEP event start end end; red dashed lines: SPE event start end end; black numbers: daily mean ECRs; red numbers: >10 MeV pfu. (d–i) Superposed epoch analysis for EEP events with daily ECR >150 (counts s⁻¹) showing ozone anomalies (%) and ECR (black lines) in the Northern hemisphere (d,f,h) and in the Southern hemisphere (e,g,i). White numbers: O₃ loss at different altitudes.

Figure 3. Magnitude of the long-term EEP effects on mesospheric ozone.

(a–c) Ozone anomalies (%) of deseasonalized daily means, averaged over the winter time. (a) November to February in the Northern hemisphere from GOMOS showing years 2003 (blue line) and 2008–2009 (red line). (b) November to February in the Northern hemisphere from SABER showing years 2003 (blue line) and 2008–2009 (red line). (c) May to August in the Southern hemisphere from MLS showing years 2005 (blue line) and 2009 (red line). Black lines: winter time climatology from 2002 to 2012; grey area: 95% confidence range of the climatological mean. Subplots: winter time average ECRs between 2002 and 2012.