17:15 〜 19:15
[PEM13-P07] Effects of intrinsic magnetic field strength on ring current development and associated ULF wave excitation
キーワード：リングカレント、磁気嵐、固有磁場、ULF波動
Earth’s intrinsic magnetic field strength is one of the most critical parameters in geospace. It has decreased by ~30% over the past 2000 years [Olson and Amit, 2006]. The decrease may change the quasi-static state of the magnetosphere-ionosphere-thermosphere system [Cnossen et al, 2012]. It is also plausible that disturbances like substorms and magnetic storms may be affected. A previous study examined the dependency of auroral substorms on the intrinsic magnetic field by global MHD simulations [Ebihara et al., 2020]. However, the effect of intrinsic magnetic field strength on the development of magnetic storms is still unclear, because in the inner magnetosphere, kinetic processes, which are not included in the MHD approximation, are dominant.
We investigated the development of magnetic storms and ring current, using the magnetosphere-ionosphere coupled model [Yamakawa et al., 2023], which combines GEMSIS-RC [Amano et al., 2011] and GEMSIS-POT [Nakamizo et al., 2012]. In GEMSIS-RC, the 5D drift kinetic equation and Maxwell equations are solved self-consistently. GEMSIS-POT solves ionospheric electric potential for the height-integrated ionosphere. In the coupled model, distribution of plasmaspheric cold plasma is also evolved based on a continuity equation. Simulations were conducted 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, and 3 correspond to Run 1, 3, and 5 in Ebihara et al. [2020], respectively. For each case, we applied R1-FAC to the ionosphere and set the temperature and density of the plasma sheet according to the corresponding run.
We analyzed the ring current development in each case in detail. We calculated SYM-H index with the Dessler-Parker-Sckopke equation to compare the development of magnetic storms. First, the intensities of magnetic storms turned out to be in the following order: Case 3 > Case 2 > Case1. Second, we found that SYM-H index declines more rapidly in Cases 2 and 3 than in Case 1. This is because in the weak magnetic field cases, the ions' motion in the azimuthal direction is faster because the distance from the planet is smaller at the same magnetic field strength. In Case 3, ionospheric conductivity and electric convection are as strong as in Case 1, although the scale of the dipole is smaller, which allows ions to inject more efficiently and results in the most intense and rapidly developing storm. Furthermore, we detected the excitation of storm-time ULF (Ultra-Low Frequency) waves on the dayside in every case, as reported in a previous study [Yamakawa et al. 2023], although the spatial distribution differs in each case. The effects of intrinsic magnetic field strength on the ULF waves will also be discussed in detail, including their excitation mechanisms.
References:
Amano, T., et al. (2011), J. Geophys. Res., 116, A02216, doi:10.1029/2010JA015682.
Cnossen, I., et al. (2012), J. Geophys. Res., 117, A05302, doi:10.1029/2012JA017555
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.
We investigated the development of magnetic storms and ring current, using the magnetosphere-ionosphere coupled model [Yamakawa et al., 2023], which combines GEMSIS-RC [Amano et al., 2011] and GEMSIS-POT [Nakamizo et al., 2012]. In GEMSIS-RC, the 5D drift kinetic equation and Maxwell equations are solved self-consistently. GEMSIS-POT solves ionospheric electric potential for the height-integrated ionosphere. In the coupled model, distribution of plasmaspheric cold plasma is also evolved based on a continuity equation. Simulations were conducted 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, and 3 correspond to Run 1, 3, and 5 in Ebihara et al. [2020], respectively. For each case, we applied R1-FAC to the ionosphere and set the temperature and density of the plasma sheet according to the corresponding run.
We analyzed the ring current development in each case in detail. We calculated SYM-H index with the Dessler-Parker-Sckopke equation to compare the development of magnetic storms. First, the intensities of magnetic storms turned out to be in the following order: Case 3 > Case 2 > Case1. Second, we found that SYM-H index declines more rapidly in Cases 2 and 3 than in Case 1. This is because in the weak magnetic field cases, the ions' motion in the azimuthal direction is faster because the distance from the planet is smaller at the same magnetic field strength. In Case 3, ionospheric conductivity and electric convection are as strong as in Case 1, although the scale of the dipole is smaller, which allows ions to inject more efficiently and results in the most intense and rapidly developing storm. Furthermore, we detected the excitation of storm-time ULF (Ultra-Low Frequency) waves on the dayside in every case, as reported in a previous study [Yamakawa et al. 2023], although the spatial distribution differs in each case. The effects of intrinsic magnetic field strength on the ULF waves will also be discussed in detail, including their excitation mechanisms.
References:
Amano, T., et al. (2011), J. Geophys. Res., 116, A02216, doi:10.1029/2010JA015682.
Cnossen, I., et al. (2012), J. Geophys. Res., 117, A05302, doi:10.1029/2012JA017555
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.