17:15 〜 18:45
[PEM15-P15] Effect of the mirror force on the ionization rate due to energetic electron precipitation:
Comparison between Monte Carlo simulations and observational results
キーワード：高エネルギー電子降下、ミラー効果
Energetic Electron Precipitation (EEP) causes various phenomena such as auroral emissions and changes in the atmospheric composition through collisions with the neutral atmosphere. It is essential to understand how the precipitating electrons with various pitch angles ionize the atmosphere for quantitative understanding of the EEP's effect on the atmosphere. However, some fundamental processes, such as propagation in the atmosphere and the secondary electron production of precipitating electrons, are still poorly understood. Recently, the importance of the magnetic mirror effect for atmospheric ionization is suggested by numerical simulation in Katoh et al., (2023). One of the main results of the paper is that when the mirror force was included, an electron which had more energy than 100keV and a pitch angle of 70 degrees, produced an electron density 1/10 times greater at low altitude than if it had been ignored. Then, the purpose of our study is to verify the results of the numerical simulation by investigating observationally the effect of the magnetic mirror force on the atmospheric ionization.
In this study, we used the Monte Carlo simulation developed by Katoh et al., (2023) to calculate ionization rate in the atmosphere by EEP from 400 km altitude above Tromsø, Norway. First, we searched for events which EISCAT Tromsø VHF or UHF radar observed the altitude profile of electron density of the atmosphere and ELFIN satellite was under observation of electrons above them simultaneously. Second, we used number flux distributions of electrons resolved into energy and pitch angle observed by ELFIN satellite as initial conditions of the simulation to calculate the altitude profile of ionization rate. Third, using the recombination coefficient of Gledhill (1986), the electron density was estimated from the ionization rate obtained by numerical simulation under the assumption that the time variation of the electron density is small. Finally, we compared and verified the electron density data estimated and observed by the EISCAT Tromsø VHF/UHF radar.
We had 25 simultaneously observed events between September 2018 to September 2022, and we found 5 events had some enhancement of electron density at low altitude seemed to be made by EEP. The most significant difference among the simultaneous events was seen in the event on the morning of December 16, 2021, during the polar night. In this event, because most of the electrons had a large pitch angle, relatively many electrons were reflected by mirror force before colliding with the atmosphere. Therefore, numerical simulation showed that the electron density was about 40% smaller if we include the mirror force than that without the mirror force in the reliable altitude range, between 70 and 85 km. On the other hand, the event at night of December 9, 2020, showed the least difference whichever we included the mirror force or not. In this event, much more electrons went into the atmosphere than electrons reflected above the atmosphere even if mirror force was included as electrons had almost isotropic pitch-angle distribution. Therefore, little difference in electron density was calculated in the reliable altitude range.
We compared these simulation electron density results with EISCAT radar data. In the former event, the electron density distribution including the effect of the mirror force tends to be closer to the electron density height distribution derived from the EISCAT radar. However, there were some differences between the numerical simulation results and the observed results for some events. It is therefore important to improve and validate the simulation code to get closer to the actual atmospheric ionization rate, and to investigate the cases (time of day, geomagnetic activity level, solar activity level, etc.) where the mirror force effect is likely to occur significantly.
In our presentation, we will show the results our improved simulation, especially the differences in results due to the introduction of secondary electrons, which is generated by first incident electron, and we will discuss the effects of secondary electrons.
In this study, we used the Monte Carlo simulation developed by Katoh et al., (2023) to calculate ionization rate in the atmosphere by EEP from 400 km altitude above Tromsø, Norway. First, we searched for events which EISCAT Tromsø VHF or UHF radar observed the altitude profile of electron density of the atmosphere and ELFIN satellite was under observation of electrons above them simultaneously. Second, we used number flux distributions of electrons resolved into energy and pitch angle observed by ELFIN satellite as initial conditions of the simulation to calculate the altitude profile of ionization rate. Third, using the recombination coefficient of Gledhill (1986), the electron density was estimated from the ionization rate obtained by numerical simulation under the assumption that the time variation of the electron density is small. Finally, we compared and verified the electron density data estimated and observed by the EISCAT Tromsø VHF/UHF radar.
We had 25 simultaneously observed events between September 2018 to September 2022, and we found 5 events had some enhancement of electron density at low altitude seemed to be made by EEP. The most significant difference among the simultaneous events was seen in the event on the morning of December 16, 2021, during the polar night. In this event, because most of the electrons had a large pitch angle, relatively many electrons were reflected by mirror force before colliding with the atmosphere. Therefore, numerical simulation showed that the electron density was about 40% smaller if we include the mirror force than that without the mirror force in the reliable altitude range, between 70 and 85 km. On the other hand, the event at night of December 9, 2020, showed the least difference whichever we included the mirror force or not. In this event, much more electrons went into the atmosphere than electrons reflected above the atmosphere even if mirror force was included as electrons had almost isotropic pitch-angle distribution. Therefore, little difference in electron density was calculated in the reliable altitude range.
We compared these simulation electron density results with EISCAT radar data. In the former event, the electron density distribution including the effect of the mirror force tends to be closer to the electron density height distribution derived from the EISCAT radar. However, there were some differences between the numerical simulation results and the observed results for some events. It is therefore important to improve and validate the simulation code to get closer to the actual atmospheric ionization rate, and to investigate the cases (time of day, geomagnetic activity level, solar activity level, etc.) where the mirror force effect is likely to occur significantly.
In our presentation, we will show the results our improved simulation, especially the differences in results due to the introduction of secondary electrons, which is generated by first incident electron, and we will discuss the effects of secondary electrons.