Effects of supernova-induced soft X-rays on middle- and upper-atmospheric nitric oxide and stratospheric ozone

Abstract. We provide a quantitative test of the recent suggestion (Brunton et al., 2023) that supernovae could significantly disrupt ozone layers of Earth-like planets through a multi-month flux of soft X-rays that produce ozone-destroying odd nitrogen (e.g., NO and NO2). Since soft X-rays do not directly penetrate down to the ozone layer, this effect would be indirect and require downward transport of NOx from the mesosphere. Mirroring previous studies of the indirect effects of energetic particle precipitation (EPP-IE), we call this the X-ray indirect effect (Xray-IE). We use the NCAR Thermosphere–Ionosphere–Mesosphere–Electrodynamics General Circulation Model (TIME-GCM) to simulate the production of NO and its transport into the stratosphere. We model the soft X-ray flux as if it were a multi-month-long solar flare and use our previously developed solar flare model to simulate the soft X-ray enhancement. Our results yield significant enhancement in stratospheric odd nitrogen, most dramatically in the Southern Hemisphere. The strongest global effects are seen in the upper stratosphere at pressure surfaces between 1–3 hPa (about 42–48 km), consistent with previous observations of the EPP-IE. We then use a detailed stratospheric photochemistry model to quantify the effects of this NOx enhancement on ozone. Widespread ozone reductions of 8 %–15 % are indicated; however, because these are limited to the upper edges of the ozone layer, the effects on the ozone column are limited to 1 %–2 %. We thus conclude that the effects of a multi-month X-ray event on biologically damaging UV radiation at the surface are also likely to be small.