EEP in 2002–2012

Solar cycle 23 (SC23) was one of the longest cycles since 1847 and exhibited large variation in solar (UV radiation) and geomagnetic activity (solar storms, energetic particle precipitation). In 2003, during the declining phase of SC23, the majority of the days were geomagnetically disturbed. In contrast, the deep solar minimum that occurred in 2009 showed the lowest activity since the beginning of the Twentieth century. The current solar cycle (SC24) is so far the weakest cycle in the last 100 years. For this period, EEP events were strongest and most frequent during the transition between SC23 maximum and the following minimum (Fig. 1a). Almost 75% of all major EEP events (major=daily mean electron precipitation count rate exceeding 150 counts s−1) in the 2002–2012 period occurred between 2003 and 2006. The occurrence of solar proton events (SPEs) peaked during high solar activity (red numbers in Fig. 1a).

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 O 3 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 O 3 loss (%) for each set of available satellite measurements (MLS, SABER and GOMOS). Full size image

Overview of ozone-depleting events

We now consider the 60 major EEP events that occurred between 2002 and 2012. For these, the satellite measurements show EEP-induced ozone loss occurring consistently in both hemispheres (Fig. 1b,c). The maximum relative ozone depletion during EEP events occurs at altitudes between 70 and 78 km and varies from 5 to 90% across the events. The average response is 50, 37 and 24% for the Global Ozone Monitoring by Occultation of Stars (GOMOS), Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) and Microwave Limb Sounder (MLS) ozone observations, respectively. The differences between the average responses is partially because of the different vertical resolutions of the observations but is also connected to data availability for some events. For example, the MLS data do not cover the period 2002–2004 that contained multiple extremely strong and long-lasting EEP events.

Short-term ozone depletion

The response of mesospheric ozone to EEP is immediate; however, the magnitude and duration of the depletion can differ depending on both the characteristics of the event as well as the season (Fig. 2a–c). During strong EEP events lasting more than 5 days, for example, 03/2003, 11/2003 and 01/2005, significant ozone depletion of up to 90% is seen at altitudes 75–80 km, with the impact reaching down to 60 km altitude. Over the 60- to 80-km altitude range, these events are comparable to the effects of large SPEs. Shorter EEP events (1–5 days, Supplementary Fig. 1a,b) usually affect altitudes between 65 and 80 km with maximum O 3 decreases of 70%. The effect of EEP is typically more pronounced during the wintertime (Fig. 2a, Supplementary Fig. 1b), as the EEP-HO x production is then relatively larger when compared with the background HO x production by photodissociation of water vapour. Of the 60 EEP events, the one on 9–23 November 2003 caused the strongest ozone depletion (Fig. 2a). The event lasted 15 days, with major forcing on 10 of those days and occurred right after Halloween 2003 SPE event. This EEP event had ozone depleted by maximum of 92%, a day after the strongest EEP forcing on 11 November. Although in principle the ozone depletion caused by the Halloween SPE could influence the EEP event period, the GOMOS observations (Fig. 2a) as well as observations at higher latitudes from the MIPAS and SCIAMACHY instruments7,8,9 show that, in agreement with modelling, the mesospheric ozone recovered from the effects of the SPE event by 7–8 November, before the strong EEP forcing is observed.

Figure 2: Magnitude of the short-term EEP effects on mesospheric ozone. (a–c) O 3 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: O 3 loss at different altitudes. Full size image

Superposed epoch analysis

To assess the sensitivity and robustness of our results, we carried out a superposed epoch analysis of the 60 largest EEP events (Fig. 2d–i). All SPE periods that could possibly affect the results were excluded from the analysis. The ozone depletion coincides closely with EEP increases and can last from 3 to 10 days, depending on the EEP duration. As MLS does not cover years 2003–2004, during which many strong and long-lasting EEP events occurred, the average O 3 loss is weaker than in GOMOS and SABER data (Fig. 2h–i). The maximum loss of ozone occurs between 70 and 78 km altitude with magnitudes varying from 10 to 30%, depending on the number and strength of EEP events, instrument resolution and atmospheric conditions. A similar superposed epoch analysis for a randomly selected data set (Supplementary Fig. 2a–f) shows no negative response in ozone. The increasing trend in percentage difference in O 3 in the random epoch analysis is caused by a seasonal bias in the observation data sets. This is particularly evident for GOMOS that generally has poorer coverage during summer periods. The superposed EEP events shown in Fig. 2 include this underlying trend effect but still show decreases in O 3 percentage change nevertheless. The average ozone loss because of EEP (Fig. 2d–i) is clearly larger than the 95% confidence range for the random data set (Supplementary Fig. 2g–i). Finally, to address the seasonal variability, we carried out superposed epoch analysis separately for three different seasons: winter, summer and spring/autumn (Supplementary Fig. 3). The results confirm that, for the same EEP forcing, the ozone loss during the winter period is typically more pronounced, for example, stretching over a wider altitude range, than in the summer and autumn/spring seasons.

Long-term ozone variability

Although the duration of the forcing for individual EEP events is only a few days, the high frequency of the events during active years (Fig. 1a) is enough to cause variability in mesospheric ozone on solar cycle timescales (Fig. 3a–c). Determining EEP-related ozone anomaly as a function of year or solar cycle is not straight forward because the temporal distribution of EEP events does not smoothly vary across the solar cycle. For example, the majority of the strong EEP events were observed during the declining phase of SC23, with a peak in year 2003 (Fig. 1). Instead, we can look at the EEP impact by contrasting periods of maximum and minimum EEP activity, which is then an indication of the maximum variability during the solar cycle. On the basis of the strength and frequency of the EEP (Fig. 3a,b, Subplots), for GOMOS and SABER we selected wintertime 2003 and 2008–2009 as maximum and minimum EEP periods, respectively. For MLS data, because they do not cover 2003, we selected year 2005 to represent the EEP maximum (Fig. 3c, Subplot). Before the analysis, we carefully removed SPE-influenced periods from all data sets. For example, in November 2003 (Fig. 2a) we excluded days 1–8 which, according to previously published satellite observations of ozone7,8,9, were affected by the Halloween 2003 SPE. The wintertime ozone values are much smaller during the EEP maximum than during the EEP minimum. The largest differences, ~21% for GOMOS (Fig. 3a) and 34% for SABER (Fig. 3b), are observed at the altitudes of 70–80 km that are known to be most strongly affected by EEP. For MLS, the difference between years 2005 and 2009 is smaller (~9%), which is consistent with weaker forcing in 2005 compared with 2003 (Fig. 3, Subplots). Note that the ozone anomalies during the EEP maximum and minimum years are outside the 95% confidence range of the climatological mean from 2002 to 2012 (Fig. 3).