17:15 〜 18:45
[PEM13-P04] Effects of intrinsic magnetic field strength on the magnetic storms based on global inner magnetospheric simulations
キーワード：環電流、内部磁気圏、ドリフト運動論モデル、固有磁場
The Earth's intrinsic magnetic field has decreased by ~30% over the past 2000 years [Olson and Amit, 2006]. The decrease may change the near-Earth space environment and its variation such as the magnetic storms. A previous study examined the influence of the intrinsic magnetic field on auroral substorms using global MHD simulations [Ebihara et al., 2020]. However, in the inner magnetosphere, which is crucial for magnetic storm development, kinetic processes cannot be ignored. Therefore, it remains unclear how differently magnetic storms and ring current develop under different intrinsic magnetic field conditions.
To investigate the magnetic storms under a weak intrinsic magnetic field condition, we conducted global inner magnetospheric simulations. The model used in this study is the magnetosphere-ionosphere coupled model [Yamakawa et al., 2023], which combines GEMSIS-RC [Amano et al., 2011] and GEMSIS-POT [Nakamizo et al., 2012] together with a separate cold plasma fluid simulation for plasmasphere. GEMSIS-RC models ring current particles in the inner magnetosphere and solves the 5-D drift-kinetic equation and Maxwell equations self-consistently. GEMSIS-POT is a 2-D potential solver for the height-integrated ionosphere. In this study, we applied R1-FAC to the ionosphere to be consistent with Ebihara et al. [2020]. The temperature and pressure of the plasma sheet were derived from the results of previous global MHD simulations [Ebihara et al., 2020]. We conducted simulations for three cases: the present Earth (Case 1), a planet with a weak (2/3 of Case 1) intrinsic magnetic field and high ionospheric conductivity (Case 2), and with the weak magnetic field and standard ionospheric conductivity (Case 3). Case 1, 2, 3 correspond to Run 1, 3, 5 in Ebihara et al. [2020], respectively.
The development of the ring current was investigated in detail for every case. We assessed the intensity of the magnetic storms by calculating the Sym-H index using the Dessler-Parker-Sckopke equation. First, the results show that the magnetic storm intensities turned out to be in the following order: Case 3 > Case 2 > Case 1. Under the weak magnetic field cases, i.e., in Cases 2 and 3, ions from the nightside can approach closer to the Earth and form the ring current forms at lower L values than in Case 1. Consequently, the storm intensity measured by Sym-H becomes larger. In Case 3, low ionospheric conductivity strengthens the convection electric field and allows ions to approach even closer to the planet, which makes the storm intensity larger than in Case 2. We also found that the magnetic storms develop more rapidly in Cases 2 and 3 than in Case 1. The ring current under weak magnetic fields develops quickly in azimuthal direction because of the shorter azimuthal distance around the planet at the closer ring current location. In summary, it is implied that a planet with a weak intrinsic magnetic field experiences intense and rapid development of the magnetic storms.
References:
Amano, T., et al. (2011), J. Geophys. Res., 116, A02216, doi:10.1029/2010JA015682.
Ebihara, Y., and T. Tanaka (2021). J. Geophys. Res., 126, e2020JA028009. doi:10.1029/2020JA028009
Nakamizo, A., et al. (2012), J. Geophys. Res., 117, A09231, doi:10.1029/2012JA017669.
Olson, P., and H. Amit (2006), Naturwissenschaften, 93, 519-542, doi:10.1007/s00114-006-0138-6.
Yamakawa, T., et al., (2023), J. Geophys. Res., 128, e2023JA031638. doi:10.1029/2023JA031638.
To investigate the magnetic storms under a weak intrinsic magnetic field condition, we conducted global inner magnetospheric simulations. The model used in this study is the magnetosphere-ionosphere coupled model [Yamakawa et al., 2023], which combines GEMSIS-RC [Amano et al., 2011] and GEMSIS-POT [Nakamizo et al., 2012] together with a separate cold plasma fluid simulation for plasmasphere. GEMSIS-RC models ring current particles in the inner magnetosphere and solves the 5-D drift-kinetic equation and Maxwell equations self-consistently. GEMSIS-POT is a 2-D potential solver for the height-integrated ionosphere. In this study, we applied R1-FAC to the ionosphere to be consistent with Ebihara et al. [2020]. The temperature and pressure of the plasma sheet were derived from the results of previous global MHD simulations [Ebihara et al., 2020]. We conducted simulations for three cases: the present Earth (Case 1), a planet with a weak (2/3 of Case 1) intrinsic magnetic field and high ionospheric conductivity (Case 2), and with the weak magnetic field and standard ionospheric conductivity (Case 3). Case 1, 2, 3 correspond to Run 1, 3, 5 in Ebihara et al. [2020], respectively.
The development of the ring current was investigated in detail for every case. We assessed the intensity of the magnetic storms by calculating the Sym-H index using the Dessler-Parker-Sckopke equation. First, the results show that the magnetic storm intensities turned out to be in the following order: Case 3 > Case 2 > Case 1. Under the weak magnetic field cases, i.e., in Cases 2 and 3, ions from the nightside can approach closer to the Earth and form the ring current forms at lower L values than in Case 1. Consequently, the storm intensity measured by Sym-H becomes larger. In Case 3, low ionospheric conductivity strengthens the convection electric field and allows ions to approach even closer to the planet, which makes the storm intensity larger than in Case 2. We also found that the magnetic storms develop more rapidly in Cases 2 and 3 than in Case 1. The ring current under weak magnetic fields develops quickly in azimuthal direction because of the shorter azimuthal distance around the planet at the closer ring current location. In summary, it is implied that a planet with a weak intrinsic magnetic field experiences intense and rapid development of the magnetic storms.
References:
Amano, T., et al. (2011), J. Geophys. Res., 116, A02216, doi:10.1029/2010JA015682.
Ebihara, Y., and T. Tanaka (2021). J. Geophys. Res., 126, e2020JA028009. doi:10.1029/2020JA028009
Nakamizo, A., et al. (2012), J. Geophys. Res., 117, A09231, doi:10.1029/2012JA017669.
Olson, P., and H. Amit (2006), Naturwissenschaften, 93, 519-542, doi:10.1007/s00114-006-0138-6.
Yamakawa, T., et al., (2023), J. Geophys. Res., 128, e2023JA031638. doi:10.1029/2023JA031638.