On the relationship of energetic particle precipitation andmesopause temperature

Abstract. Energetic Particle Precipitation (EPP) has the potential to change the neutral atmospheric temperature at the mesopause region. Recent results, however, are inconsistent leaving the mechanism and the actual effect still unresolved. Here we have searched for electron precipitation events and investigated a possible correlation between D region electron density enhancements and simultaneous neutral temperature changes. The rotational temperature of the exited hydroxyl (OH) molecules is retrieved from the spectrum of the OH airglow. The electron density is monitored by the EISCAT Svalbard radar from the International Polar Year (IPY) in 2007–2008, when the EISCAT Svalbard radar was run continuously, until February 2019. Particle precipitation events are characterized by rapid increases in electron density by a factor of 4 at an altitude range of 80–95 km, which overlaps with the nominal altitude of the OH airglow layer. The OH airglow measurements and the electron density measurements are co-located. Most of our 8 electron precipitation events are associated with a temperature decrease of 10–50 K. Only one event was related to temperature change less than 10 K. We interpret the results in terms of the change in the chemical composition in the mesosphere. Due to EPP ionisation the population of excited OH at the top of the airglow layer decreases. As a consequence, the airglow peak height changes and the temperatures are probed at lower altitudes, providing inconsistent temperature responses. This is in agreement with conclusions of earlier studies, but is, for the first time, constructed from electron precipitation measurements as opposed proxies. The EPP related temperature change recovers very fast, typically within 30 minutes. We therefore further conclude that this type of particle precipitation events would only have a significant impact on the longer-term heat balance in the mesosphere if the lifetime of the precipitation was much longer than that of a typical EPP event found in this study.