Long-term mesospheric record of EPP-IE NO measured by Odin/SMR

Journal article, 2023

Due to the long lifetime of nitric oxide (NO) in darkness conditions, during polar winter, the NO produced by energetic particle precipitation (EPP) in the mesosphere and lower thermosphere (MLT) can descend, via the middle atmospheric residual circulation, to the lower mesosphere and stratosphere, where it is involved in catalytic destruction of ozone (O3). This process is known as energetic particle precipitation indirect effect (EPP-IE). There are still significant uncertainties on the estimated amount of EPP-IE NO. To improve such estimations, we measure the total winter flux of EPP-IE NO descending through three isentropic levels in the mesosphere, that is 2600 K, 3300 K and 4000 K, based on 15 years of NO nighttime observations from the Sub-Millimetre Radiometer (SMR) on board Odin satellite. At the moment, Odin/SMR is the only instrument ensuring a global coverage of mesospheric NO observations within a few days and this is the first time EPP-IE NO has been quantified using its NO data set. Moreover, such an estimate had never been calculated for the most recent winters which are included in this study. In our method we calculate the median nighttime NO inside the polar vortex during the month prior to the descent of NO-rich air; this value is assumed as a background, produced by N2O oxidation, to be subtracted from the daily median nighttime concentrations inside the vortex; the result of this subtraction is then multiplied by the area of the vortex and the descent rate to obtain the flux; finally these daily quantities and integrated to calculate the total NO flux for each winter. We thus calculated the total EPP-IE NO flux through the mentioned levels for Northern Hemisphere (NH) winters between 2006–07 and 2020–21 and for Southern Hemisphere (SH) winters between 2007 and 2012. The total winter EPP-IE NO fluxes presented in this study are consistent with the quantities presented in similar studies. NH winters 2008–09, 2012–13 and 2018–19 are the ones presenting the highest NO fluxes at all levels. They are winters characterised by sudden stratospheric warmings followed by elevated stratopause (SSW-ES) events. The measured fluxes vary between 490 and 1000 Mmol at 4000 K, 310 and 720 Mmol at 3300 K, 270 and 510 Mmol at 2600 K. All other NH and SH winters are characterised by sensibly lower values than SSW-ES winters. The fluxes from these more dynamically quiet winters vary from winter to winter following a trend similar to the one of geomagnetic activity, as indicated by the variations in Ap index. These results indicate that the variability in the total EPP-IE NO fluxes is dominated by dynamics during the SSW-ES winters, whereas during the remaining winters it is geomagnetic activity that plays a major role.