When are energetic electrons producing NO directly in the upper stratosphere?

Abstract

Compositional NOx changes caused by energetic electron precipitation (EEP) at a specific altitude are called the EEP direct effect. Changes co-dependent on vertical transport are referred to as the EEP indirect effect. The relative importance of EEP’s direct and indirect effect on NO and its subsequent impact on ozone and dynamic changes remain unresolved. The challenges are partly due to inadequate particle measurement and the relative scarcity of NO observations over the polar MLT region. Moreover, lower production rates in the mesosphere make it challenging to determine EEP’s direct impact on NO since small in-situ enhancements cannot be easily distinguished from the descending NO-rich air masses in the winter hemisphere. In this study, the uncertainty of the EEP observations is bypassed by exclusively identifying events applying NO-observations from the SOFIE instrument on board the AIM satellite. SOFIE daily averaged data from 2007 to 2014 is used to create a climatology based on the mean of the lower half of the data (lower 50 percentile mean). A direct EEP-produced NO-event at 90 km (“90km-event”) is identified when the NO density surpasses the climatology by 100%. If the NO density exceeds 25% above the climatology at 80, 70, 60, and 50 km, the event qualifies as a “50km-event”. By contrasting the 90km and 50km events, the characteristics of the solar wind and geomagnetic indices, as well as observed electron fluxes from POES, are studied. The goal is to unravel when EEP can produce NO directly in the upper stratosphere. The result will contribute to developing a parameterization of EEP from the radiation belt that includes both the direct and indirect impact of EEP to decipher the total EEP effect on the ozone and atmospheric dynamics.