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Laboratory for Atmospheric and Space Physics

Influence of precipitating electrons on stratospheric ozone

September 24, 2012

One of the most studied natural forcings of stratospheric O3 is that due to solar UV flux variations. Despite the large number of studies devoted to the subject, questions still remain with regard to the calculated and observed sensitivity [S = (% change in O3} / (% change in UV flux near 200 nm)] of O3 to UV flux changes during the 11-year solar cycle. Detrended and deseasonalized long-term O3 data near 2 mbar suggest that S{11y} is approximately 0.91 while 2-D models suggest a value of S{11y} for the 11-year solar cycle which ranges between 0.36 to 0.55. Model results also suggest the value of S{27d} for the 27-day solar rotation period should be essentially the same as that for the 11-year solar cycle and analysis of observations to derive this parameter yield results which are, in fact, in reasonable accord with the model results, with values falling between 0.38 and 0.51. Such a discrepancy creates difficulties when attempting to differentiate between natural and anthropogenic causes of O3 change and when attempting to validate model simulations. We have addressed the issue of why S{27d} derived from observations is substantially less than S{11y} derived from observations, and why model-derived S{11y} are in disagreement with S{11y} derived from observations.

Observational studies carried out during the SAMPEX mission have confirmed (1) that fluctuations of energetic electron precipitation (EEP) from the auroral and outer trapping regions of the magnetosphere are related to the 11-year solar cycle through variations in the high speed solar wind streams, (2) that EEP can lead to significant increases in odd nitrogen in the lower thermosphere and mesosphere, and (3) that NO2 formed from such events can be transported into the stratosphere. We have also reported results of simulations which suggest that significant variations in stratospheric NOy (and NO2) may result from this solar-terrestrial coupling during the solar cycle. Relative to the above noted issue, we have hypothesized that the discrepancy in the results of S{11y} stem from not taking into account the fluctuations of NOy (and NO2) due to EEP in assessing the O3 changes that occur during the solar cycle, i.e. investigators have attributed O3 changes due to NOy variations to solar UV effects leading to an erroneously large value of S{11y}. We have tested this hypothesis using well-validated 2-D model simulations. The results support this explanation and appear to resolve this long-standing discrepancy. If proper account is taken of NOy (and NO2) changes, the values of S{11y} calculated from simulations and observations should be approximately the same, in accord with theory. A test of the hypothesis using only data awaits an adequately long, drift-free time series of NO2 data.

Figure 1 illustrates the agreement between the SAGE II NO2 column and the 2-D simulation used in the study. Comparisons are made by interpolating the model simulation (with energetic electron precipitation, EEP) to the latitudes and times of all the SAGE II measurements over the three year period. The agreement is quite good with an average absolute error of 12.3% over the three year period.

Figure 2a. The solid line illustrates the simulated average NO2 mixing ratio at 2 mbar from -40 to 40 degrees latitude when no EEP is included (left scale). The dot-dashed line (right scale) shows the ratio of the simulations with EEP to that for no EEP. Increasing levels of NO2, due to EEP, during the approach to solar minimum are evident.

Figure 2b. The same as figure 2a except for ozone. With the inclusion of EEP, it is seen that additional reductions of ozone of 2-3% occur. Previous analyses of solar UV and ozone data have included this ozone reduction with the ozone reductions due to the declining solar UV fluxes during the approach to solar minimum conditions. This, apparently, is what has led to the unexpectedly large sensitivity of ozone to solar UV variations which is inconsistent with many model simulations and which has been an important and outstanding issue for a number of years. Properly accounting for the effects of the increases in NO2 due to EEP in the data analysis brings the solar UV-ozone sensitivity as derived from both simulations and data analysis into agreement.

A paper entitled “Calculated Upper Stratospheric Effects of Solar UV Flux and NOy Variations During the 11-Year Solar Cycle” by L. B. Callis, M. Natarajan, and J. D. Lambeth has been submitted to GRL.


Contributed by Lin Callis, LaRC

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