Solar cycle signal in stratospheric ozone

5.4 Solar cycle signal in stratospheric ozone

It is clear from the discussion of Section 5.3 that ozone is produced by short wavelength solar ultraviolet radiation and destroyed by radiation at somewhat longer wavelengths. Because the amplitude of solar cycle variability is greater in the far ultraviolet (see Section 5.2) ozone production is more strongly modulated by solar activity than its destruction and this leads to a higher net production of stratospheric ozone during periods of higher solar activity. Observational records (Figure 29

a) suggest a peak in ozone response of about 2% over the solar cycle in the upper stratosphere, with a secondary maximum in the lower stratosphere, although the restricted length of the data series means that these results are not yet statistically robust. Ozone column (Figure 29

b) shows 0.5 – 4% higher values in ozone columns at 11-year cycle maximum relative to minimum.

Figure 29:

(a) Meridional cross section of solar cycle signal in ozone concentration from multiple regression of SAGE I and II data (% per 100 units of F_10.7 radio flux, a typical solar cycle amplitude is 130 of these units). Shading denotes that the fit is not statistically significant. (b) Latitudinal profile of the solar cycle variations in column ozone (atm cm) derived from vertically integrated SAGE I and II data (over 20–50 km), and three column ozone data sets (ground-based, SBUV, and merged TOMS/SBUV data). Error bars on the TOMS/SBUV curve denote 2*sigma uncertainty in the fit. From Randel and Wu (2007 ).

Solar activity is manifest not only in variations of the Sun’s emission of electromagnetic radiation but also in a range of other parameters. One of these is the occurrence and severity of coronal mass ejections which result in the emission of energetic particles, some of which reach the Earth. Precipitating electrons and solar protons follow the Earth’s magnetic field lines and so have greatest initial impact at high latitudes. They affect the nitrogen oxide budget of the middle atmosphere through the ionization and dissociation of nitrogen and oxygen molecules. NO catalytically destroys ozone, as discussed in Section 5.3, so that solar energetic particle events are associated with reductions in atmospheric ozone (see Figure 30 ). The highest energy particles penetrate well into the stratosphere and ozone depletion regions may propagate downwards and equatorwards over the period of a few weeks (Jackman et al., 2006 ). It is interesting to note that the effect of energetic particle events on ozone is in the opposite sense to that of enhanced ultraviolet irradiance. Particle events tend to occur more often when the Sun is in a declining phase of the solar cycle so the combined effect on ozone may be complex in its geographical, altitudinal and temporal distribution and may confuse regression analyses (such as shown in Figure 29 ) which use only radiative indicators of solar activity, especially at higher latitudes.

Figure 30:

Latitude-height section of percentage change in NO_y (top) and O₃ (bottom) calculated using a 2D model for November 1989, following the major solar proton event of October 1989. From Vlachogiannis and Haigh (1998 ).