On the Relationship Between Energetic Electron Precipitation and Mesopause Temperature

Abstract: Energetic Particle Precipitation (EPP) can potentially change the neutral atmospheric temperatureat the mesopause region. Cresswell-Moorcock et al. (2013) used European IncoherentScatter Scientific Association (EISCAT) radar data to identify strong electron precipitationevents. Here we use a similar approach and search for electron precipitation events to investigatethe simultaneous and co-located neutral temperature measurements. The temperature ofthe excited hydroxyl (OH) molecules is representative for the neutral air temperature at theheight of the OH layer, assuming there is local thermodynamical equilibrium.We use electron density datasets from the EISCAT Svalbard radar ranging from a historicaldata set from the International Polar Year (IPY) in 2007-2008, when EISCAT Svalbard radarwas run continuously, up to February 2019. Following Cresswell–Moorcock et al. (2013)particle precipitation events are characterized by a rapid increase of the electron density by afactor of 5 at an altitude range of 80-100 km. To determine the neutral temperature, we useairglow data. Spectrometer measurements of OH airglow are collected at Kjell Henriksen-Observatory (KHO), only about 1 km away from the radar site. The neutral temperatures areaveraged over one hour and half an hour and are available since the IPY.The study shows different temperature responses to EPP, which are classified accordingly.Most events show an initial decrease in the order of 20 K. The temperature decrease maymean that the EPP ionisation changes the chemical composition in the mesosphere anddecreases the population of excited OH at the top of the layer. As a consequence, theairglow peak height changes and the temperatures are probed at lower altitudes. SporadicE-layers were additionally captured by the automatic routine to search for electron densityenhancements and were examined separately. The response of sporadic E-layers on themesopause temperature at the onset is comparable to the ones for EPP, even though thedecrease in temperature is smaller than for EPP.