L. Holt

Simulation of energetic particle precipitation effects during the 2003–2004 Arctic winter

Energetic particle precipitation (EPP) during the 2003–2004 Arctic winter led to the production and subsequent transport of reactive odd nitrogen (NOx = NO + NO2) from the mesosphere and lower thermosphere (MLT) into the stratosphere. This caused NOx enhancements in the polar upper stratosphere in April 2004 that were unprecedented in the satellite record. Simulations of the 2003–2004 Arctic winter with the Whole Atmosphere Community Climate Model using Specified Dynamics (SD-WACCM) are compared to satellite measurements to assess our understanding of the observed NOx enhancements. The comparisons show that SD-WACCM clearly displays the descent of NOx produced by EPP but underestimates the enhancements by at least a factor of four. Comparisons with NO measurements in January and February indicate that SD-WACCM most likely underestimates EPP-induced NO production locally in the mesosphere because it does not include precipitation of high energy electrons. Comparisons with temperature measurements suggest that SD-WACCM does not properly simulate recovery from a sudden stratospheric warming in early January, resulting in insufficient transport from the MLT into the stratosphere. Both of these factors probably contribute to the inability of SD-WACCM to simulate the stratospheric NOx enhancements, although their relative importance is unclear. The work highlights the importance of considering the full spectrum of precipitating electrons in order to fully understand the impact of EPP on the atmosphere. It also suggests a need for high-quality meteorological data and measurements of NOx throughout the polar winter MLT.

A multi tracer analysis of thermosphere to stratosphere descent triggered by the 2013 Stratospheric Sudden Warming

Arctic winter observations in 2013 by the Solar Occultation for Ice Experiment (SOFIE) show significant transport from the lower-thermosphere to the stratosphere of air enriched in nitric oxide, but depleted in water and methane. The transport is triggered by the Stratospheric Sudden Warming (SSW) on 11 January and is continuously tracked for over 3 months. Ultimately, evidence for lower thermospheric air is seen at 40 km in mid-April. Area integrated nitric oxide (NO) fluxes are compared with previous events in 2004, 2006, and 2009, to show that this event is the second largest in the past 10 years. The SOFIE data are combined with a meteorological analysis to infer descent rates from 40 to 90 km. The descent profile initially peaks near 75 km, shifting downward by approximately 5 km per 10 days. Our work demonstrates the utility of SOFIE tracer measurements in diagnosing vertical transport from the stratosphere to the edge of space.

Hemispheric distributions and interannual variability of NOy produced by energetic particle precipitation in 2002–2012

We investigate the interannual variability and hemispheric differences of reactive odd nitrogen produced by energetic particle precipitation (EPP-NOy) and transported into the stratosphere and lower mesosphere during polar winters in 2002–2012. For this purpose, EPP-NOy amounts derived from observations of the Michelson Interferometer for Passive Atmospheric Sounding by means of a tracer correlation method have been used. Southern hemispheric (SH) seasonal maximum EPP-NOy amounts transported below the 0.02 hPa level range from 0.5GM to 2.5GM in the 2009 and 2003 winters, respectively. Northern hemispheric (NH) amounts were typically 2–5 times smaller, with the exception of the 2003/2004 winter. This interhemispheric asymmetry is primarily caused by a reduction of the mesospheric descent rates in NH midwinter, as opposed to the SH. Hemispherically integrated NOy fluxes through given pressure levels reach up to 0.07GM/day at 0.1 hPa. A multilinear regression of the EPP-NOy evolution to the Ap index of the preceding months indicates that a large fraction of the SH interannual variability of EPP-NOy (excluding direct contributions by solar protons) can be linked to geomagnetic activity variations. This relationship holds throughout the winter and at all vertical levels where EPP-NOy is present. In the NH, a similar correlation is found until midwinter, however, breaking down afterward above 2 hPa in years with elevated stratopause occurrence. As an exception, EPP-NOy amounts in the Arctic winter 2004/2005 were much higher than in other NH winters with similar geomagnetic activity. We attribute this behavior to the unusually stable polar vortex in that winter, otherwise typical for the SH.