Model studies of short‐term variations induced in trace gases by particle precipitation in the mesosphere and lower thermosphere

Abstract

The 3‐D global chemistry and transport model (3dCTM) was used to investigate NO, OH, and O3 from January 2002 to May 2010 between 60 km and 133 km. Their daytime and nighttime mean zonal means (55°–75° geomagnetic latitude) were analyzed with respect to short‐term variations associated with particle precipitation. The corresponding ionization rates were derived from the 3‐D atmospheric ionization module Osnabrück (AIMOS), which is based on particle flux measurements. The trace gas variations with respect to their background were investigated by using a superposed epoch analysis. The 27 day signature associated with particle precipitation is found in NO, while it is only indicated in OH and O3 during winter. A varying solar spectrum associated with the 11 year solar cycle causes modifications of this signal up to 10%, while the main patterns are conserved. Published observations show a clear 27 day signal, qualitatively agreeing with the model results at altitudes >70 km except for O3 in Northern Hemisphere winter. Further differences occur with respect to the magnitude of the trace gas variations, primarily attributed to the different trace gas background and dynamical variations of the background atmosphere. Absolute OH variations are overestimated by the 3dCTM during winter, while the opposite is true for O3. These differences might originate from an unknown offset in AIMOS, incorrect chemical reaction rates, a different background of H2O and O3, and the model dynamics. However, their nonlinear relationship and their altitude of largest response are qualitatively captured in Southern Hemisphere winter.

NO<sub><i>y</i></sub> production, ozone loss and changes in net radiative heating due to energetic particle precipitation in 2002–2010

“…Relative ozone anomalies due to energetic particle precipitation at high southern latitudes (70-90 • S), calculated as the difference of model runs with to without particle impact (%). the background in HO x is higher, and therefore the relative increase in HO x due to the electron precipitation is rather smaller (Fytterer et al, 2015b(Fytterer et al, , 2016. The values simulated here are in the range of the observations, which show ozone losses of about 10 % averaged over one winter, and up to 90 % in individual strong events (Andersson et al, 2014b).…”

“…Mesospheric ozone loss has been observed to be related to strong energetic electron precipitation events (Andersson et al, 2014b) and also to be related to the 27-day cycle of the geomagnetic activity (Fytterer et al, 2015b). It is also predicted by model studies (Semeniuk et al, 2011;Fytterer et al, 2016;Arsenovich et al, 2016), and is likely related to the increase in HO x during electron precipitation events.…”

“…It is also predicted by model studies (Semeniuk et al, 2011;Fytterer et al, 2016;Arsenovich et al, 2016), and is likely related to the increase in HO x during electron precipitation events. It is restricted mainly to polar winter because during summer, Figure 12.…”

“…Variations in the density of NO x in the mesosphere and lower thermosphere related to geomagnetic activity as a proxy for auroral electron precipitation are reported based on observations, (e.g., by Kirkwood et al, 2015;Hendrickx et al, 2015;Sinnhuber et al, 2016). Mesospheric ozone loss and an increase in mesospheric OH have been observed to be related directly to increases in both electron fluxes Andersson et al, 2014a, b) and geomagnetic activity (Fytterer et al, 2015b). NO x from the high-latitude upper mesosphere and lower thermosphere can propagate downward during polar winter in the large-scale downwelling motions of the winter middle atmosphere.…”

Relative Importance of Nitric Oxide Physical Drivers in the Lower Thermosphere

“…We choose the auroral electrojet ( AE ) index as proxy for geomagnetic activity as it focuses on the polar regions and correlates better with NO concentrations in SOFIE observations than the planetary A p index (Hendrickx et al, ). The response in the NO peak density altitude layer, around 105 km, to geomagnetic activity is typically lagged by 1 day (Solomon et al, ; Hendrickx et al, ) but is dependent on altitude (Fytterer et al, ). Since we are interested in the NO response throughout the lower thermosphere, we use an AE value averaged over the past 2 days.…”