11:00 〜 11:15
[PCG18-08] Modeling diffuse auroral emissions at Mars: Contribution of precipitating protons and mirror effects of crustal fields
キーワード：火星、ディフューズオーロラ、Solar energetic particles
Solar Energetic Particle (SEP) and the Imaging UltraViolet Spectrograph (IUVS) instruments aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft have discovered new type of diffuse aurora that spans across nightside Mars, which resulted from the interaction of Solar Energetic Particles (SEPs) with the Martian atmosphere [Schneider et al., 2015]. Previous models suggested that 100 keV monoenergetic electron precipitation should be the origin of the low altitude (~ 60 km) peak of the limb emission, however, no models were able to reproduce the observed emission profiles by using the observed electron energy spectrum [e.g., Haider et al., 2019]. Previous auroral emission models did not take into account the contribution of MeV proton precipitation, although MeV proton can penetrate down to ~ 70 km altitude as well [e.g., Jolitz et al., 2017]. This study aims to model SEP-induced diffuse auroral emissions by both electrons and protons. In order to constrain the possible sources of the diffuse auroral emissions, we focus on the different magnetic mirror behaviors of the precipitating electrons and protons in the presence of crustal magnetic fields.
We have developed a Monte-Carlo collision and transport model of precipitating electrons and protons with magnetic fields on Mars. We calculated limb intensity profile of CO2+ ultraviolet doublet (UVD) excited by precipitating electrons and protons with energies ranging 100 eV - 100 keV and 100 eV - 5 MeV, respectively, during December 2014 SEP event and September 2017 SEP event by using electron and proton fluxes observed by MAVEN/SEP, SWEA and SWIA.
At the SEP event in December 2014, we found that calculated intensity at peak altitude of the CO2+ UVD due to proton precipitation is comparable to those by electron precipitation. On the other hand, inferred peak altitudes of the CO2+ UVD limb intensities are 80 km for protons and 110 km for electrons, respectively. In this event, we concluded that the MeV protons mainly contribute to reproduce the proton-induced CO2+ UVD emission, while the few keV electrons contribute to the electron-induced CO2+ UVD emission. At the SEP event in September 2017, the calculated peak limb intensity of CO2+ UVD due to proton precipitation is four times larger than that due to electron precipitation. Peak altitudes of the CO2+ UVD limb intensities are 60 km and 70 km for protons and electrons, respectively. In this event, 5 MeV protons contribute the most to the proton-induced CO2+ UVD emission and 100 keV electrons contribute the most to the electron-induced CO2+ UVD emission.
We compared our model results with the observations of CO2+ UVD limb intensity obtained in Schneider et al. (2015) and Schneider et al. (2018) for December 2014 SEP event and September 2017 SEP event, respectively. At the SEP event in December 2014, our result of total CO2+ UVD limb intensity of electron and proton contributions is 3 times larger than the observation. As for the peak altitude, our result is 80 km, which is 10 km higher than the observed peak at 70 km. In the SEP event in September 2017, our result of total CO2+ UVD limb intensity of electron and proton contributions is 1.5 times larger than the observation. As for the peak altitude, our result is 60km, which is almost identical to the observation. We concluded that our model results are almost consistent with observations.
We have also investigated the effect of crustal field on the emission of CO2+ UVD. The CO2+ UVD emissions due to precipitating electrons are depleted by a factor of 10 in the region of open crustal field and disappeared in the region of closed and parallel crustal field, whereas the emissions due to precipitating protons do not change significantly due to the difference in their gyro radii and their relations to the crustal field spatial scale. Further observations of diffuse aurora in the crustal field region are needed to constrain the origin of diffuse aurora on Mars.
We have developed a Monte-Carlo collision and transport model of precipitating electrons and protons with magnetic fields on Mars. We calculated limb intensity profile of CO2+ ultraviolet doublet (UVD) excited by precipitating electrons and protons with energies ranging 100 eV - 100 keV and 100 eV - 5 MeV, respectively, during December 2014 SEP event and September 2017 SEP event by using electron and proton fluxes observed by MAVEN/SEP, SWEA and SWIA.
At the SEP event in December 2014, we found that calculated intensity at peak altitude of the CO2+ UVD due to proton precipitation is comparable to those by electron precipitation. On the other hand, inferred peak altitudes of the CO2+ UVD limb intensities are 80 km for protons and 110 km for electrons, respectively. In this event, we concluded that the MeV protons mainly contribute to reproduce the proton-induced CO2+ UVD emission, while the few keV electrons contribute to the electron-induced CO2+ UVD emission. At the SEP event in September 2017, the calculated peak limb intensity of CO2+ UVD due to proton precipitation is four times larger than that due to electron precipitation. Peak altitudes of the CO2+ UVD limb intensities are 60 km and 70 km for protons and electrons, respectively. In this event, 5 MeV protons contribute the most to the proton-induced CO2+ UVD emission and 100 keV electrons contribute the most to the electron-induced CO2+ UVD emission.
We compared our model results with the observations of CO2+ UVD limb intensity obtained in Schneider et al. (2015) and Schneider et al. (2018) for December 2014 SEP event and September 2017 SEP event, respectively. At the SEP event in December 2014, our result of total CO2+ UVD limb intensity of electron and proton contributions is 3 times larger than the observation. As for the peak altitude, our result is 80 km, which is 10 km higher than the observed peak at 70 km. In the SEP event in September 2017, our result of total CO2+ UVD limb intensity of electron and proton contributions is 1.5 times larger than the observation. As for the peak altitude, our result is 60km, which is almost identical to the observation. We concluded that our model results are almost consistent with observations.
We have also investigated the effect of crustal field on the emission of CO2+ UVD. The CO2+ UVD emissions due to precipitating electrons are depleted by a factor of 10 in the region of open crustal field and disappeared in the region of closed and parallel crustal field, whereas the emissions due to precipitating protons do not change significantly due to the difference in their gyro radii and their relations to the crustal field spatial scale. Further observations of diffuse aurora in the crustal field region are needed to constrain the origin of diffuse aurora on Mars.