Solar Induced Variations of Odd Nitrogen and Ozone in the Stratosphere and Lower Mesosphere: Analysis of UARS HALOE Data

Several sensitivity studies using 3D chemistry climate models have suggested that solar-induced changes in odd nitrogen are important for solar cycle ozone variations. This is possible since odd nitrogen is the leading source of ozone catalytic losses at most altitudes in the stratosphere. Here, we investigate solar-induced variations of odd nitrogen and associated ozone variations using UARS HALOE data over the 1991-2003 period. Specifically, a multiple regression statistical model is applied to 3-month averages of these data at and above 10 hPa where measurements of odd nitrogen are sufficiently numerous. As a measure of solar forcing, we consider separately both the MgII solar UV index and the auroral Ap index. The former is a measure of solar UV forcing of ozone and also correlates well with the occurrence frequency of major solar proton events. The latter is a good proxy for magnetospheric energetic electron precipitation that influences the production rate of odd nitrogen in the thermosphere and mesosphere. Consistent with earlier studies, evidence is obtained for a decadal odd nitrogen variation at the highest available latitudes (50-70 degrees) that projects positively onto the solar cycle. At these latitudes, annual averages of HALOE odd nitrogen correlate best with the Ap index and solar wind plasma speed indicating a dominant source in the form of energetic electron precipitation followed by downward transport during the polar night. At the same high latitudes in the upper stratosphere (1 hPa), interannual variations of odd nitrogen correlate negatively with simultaneous HALOE ozone measurements. Thus, at high latitudes, particle precipitation induced odd nitrogen variations can significantly impact stratospheric ozone concentrations on interannual time scales. At latitudes lower than 50 degrees, statistically significant solar induced odd nitrogen variations occur only near and above the stratopause in the tropics. This low-latitude response is apparently caused primarily by increased photolysis of NO under solar maximum conditions. Throughout most of the rest of the stratosphere, no statistically significant response is obtained. This implies that decadal variations of odd nitrogen, regardless of their source, played no major role in the solar cycle variation of ozone at middle and low latitudes during the 1991-2003 period.