Evidence of potential thermospheric overcooling during the May 2024 geomagnetic superstorm

TL;DR

This study investigates thermospheric density and NO radiative cooling during the May 2024 geomagnetic superstorm, revealing potential overcooling effects driven by NO emission despite increased solar EUV flux.

Contribution

It provides the first detailed analysis of thermospheric overcooling and NO cooling flux during the May 2024 superstorm, highlighting the importance of NO cooling in storm recovery.

Findings

Post-storm thermospheric density depletion of -23% observed.

Thermospheric NO radiative cooling flux reached an all-time high during the storm.

Thermospheric density enhancement observed at 105-110 km altitude during overcooling.

Abstract

During intense geomagnetic storms, the rapid and significant production of NO followed by its associated infrared radiative emission in lower thermosphere contributes crucially to the energetics of the upper atmosphere. This makes NO infrared radiative cooling a very important phenomenon which needs to be considered for accurate density forecasting in thermosphere. This study reports the investigation of variations in thermospheric density, and NO radiative cooling during the recent geomagnetic superstorm of May 2024. A very rare post-storm thermospheric density depletion of about -23% on May 12 was observed by Swarm-C in northern hemisphere in comparison to the prestorm condition on May 9. This overcooling was observed despite the continuous enhancement in solar EUV (24-36 nm) flux throughout the event. The thermospheric NO infrared radiative emission in the recovery phase of the storm seems to be the plausible cause for this observed post-storm density depletion. The TIMED/SABER observed thermospheric density between 105 and 110 km altitude shows an enhancement during this thermospheric overcooling. Our analysis also suggests an all time high thermospheric NO radiative cooling flux up to 11.84 ergs/cm2/sec during May 2024 geomagnetic superstorm, which has also been compared with famous Halloween storms of October 2003.