10:45 AM - 12:15 PM
[PEM12-P34] Variability of the effective recombination coefficient during energetic particle precipitation events
Keywords:Atmospheric ionization, Solar X-rays
Energetic particle precipitation (EPP) is one of the major sources of energy deposition into the Earth's middle atmosphere. It is essential to accurately reproduce the energy deposition from observable electron density profiles for a quantitative understanding of the EPP impact on the atmosphere. The effective recombination coefficient, αeff, represents an efficiency of electron loss due to collisions with ions in an equation of the time evolution of the electron density, dNe/dt = q - αeff Ne2, where Ne is the electron density and q is ionization rate by EPP. The coefficient is known to vary with the ion density ratio to the electron density. The purpose of the study is to re-examine the variability of this coefficient and to discuss the limitation of how accurately we can determine the EPP energy inputs.
In this study, the altitude profiles of ionization rate q were calculated with a particle transport simulation, PHITS, using EPP fluxes observed by satellites as the input. The coefficient αeff was estimated from the calculated ionization rate and electron densities obtained from the EISCAT radar observations in Tromsø (Norway) and Longyearbyen (Svalbard). We found 8 events of X-ray enhancements associated with X-class solar flares in 1982-2022 in which at least one of the EISCAT radars was in operation and a solar zenith angle was < 90° at the radar site. We estimated the coefficient for an altitude range of 60-100 km, since enhanced X-rays during solar flares deposit most of their energies at the altitudes. The estimatied coefficient αeff agrees with the values estimated in previous studies (Gledhill, 1986) within an error of ~30 %. In this presentation, we will briefly discuss the validity and predictability of the variability in αeff, including the effect of the ion density ratio estimated using a chemical model.
In this study, the altitude profiles of ionization rate q were calculated with a particle transport simulation, PHITS, using EPP fluxes observed by satellites as the input. The coefficient αeff was estimated from the calculated ionization rate and electron densities obtained from the EISCAT radar observations in Tromsø (Norway) and Longyearbyen (Svalbard). We found 8 events of X-ray enhancements associated with X-class solar flares in 1982-2022 in which at least one of the EISCAT radars was in operation and a solar zenith angle was < 90° at the radar site. We estimated the coefficient for an altitude range of 60-100 km, since enhanced X-rays during solar flares deposit most of their energies at the altitudes. The estimatied coefficient αeff agrees with the values estimated in previous studies (Gledhill, 1986) within an error of ~30 %. In this presentation, we will briefly discuss the validity and predictability of the variability in αeff, including the effect of the ion density ratio estimated using a chemical model.