11:00 〜 11:15
[PEM11-11] 惑星間空間磁場を作りだしているオープンフラックスと太陽表面磁場の関係
★招待講演
キーワード：惑星間空間磁場、オープンフラックス、黒点、太陽活動周期
Space weather is the phenomenon that the solar activity influences the Earth and interplanetary space. To understand the background environment of space weather, it is important to focus on the magnetic field between the solar surface, corona, and interplanetary space as the basic structure of the heliosphere. The global magnetic field structure of the Sun changes over the 11-year solar cycle, and the open magnetic flux extending from the coronal holes produces the interplanetary magnetic field (IMF). During the solar minimum, the solar activity is low and a unipolar magnetic field extends from the coronal holes in the polar regions. During the solar maximum, active regions make the magnetic field structure complex, and coronal structures are changed, affecting the IMF evolution over the solar cycle. However, it is not enough to understand the connection between the open flux and the IMF. The open flux extrapolated from the photosphere is underestimated by a factor of 2 to 5 compared with the observed IMF near the Earth (the open flux problem; Linker et al., 2017, Wallace et al., 2019).
In this study, we focus on the observational fact that the sunspot peak is 7 months later than the sunspot peak. Our objective is to elucidate the cause of the delay and understand the evolution of the open flux extending from the solar surface. We investigate which component of the solar magnetic field produces the IMF evolution over Cycle 24 by comparing the evolution of the solar magnetic field with that of IMF, and simulate how open flux varies with the sunspot characteristics. We use the Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO) for magnetograms with the surface flux transport model to simulate the sunspot diffusion, and the potential field source surface (PFSS) model to extrapolate the coronal field, which we decompose into the components (l, m) of the spherical harmonic function.
As a result, it was found from the observational analysis that the peak of the solar dipole flux (l=1) is, similar to the IMF, delayed by 7 months from the sunspot maximum, and the equatorial dipole flux (l, m)=(1, ±1) is dominant during the solar maximum. Our simulations revealed that the larger the tilt angle of the sunspot and the higher the latitude at which the sunspot is located, the more the open flux increases, and when the sunspot has a tilt angle of 4 degrees or more, the open flux evolution is delayed by several months relative to the solar surface.
These results suggest that the equatorial dipole during the solar maximum and the nondipole flux during the solar maximum are keys to attacking the open flux problem. The generation mechanism of the IMF from the solar magnetic field during solar maximum is inferred as the following; i) solar active regions appear at low latitudes in the photosphere; ii) the magnetic fields of the active regions diffuse toward the polar regions; iii) the difference in the latitude of leading and following sunspots causes the magnetic field lines to be stretched in the longitude direction due to differential rotation and supergranulation; iv) the open flux is increased; and v) the IMF near Earth is increased. This process may be the reason why the peak of the IMF is seven months later than the solar maximum. During the solar minimum, it is suggested that the stable axial dipole component (l,m) = (1, 0) forms the global dipole field structure, and the variation in IMF is produced by the variation in the nondipole component (l≧2) from 2017 to 2021. We will discuss applying this suggestion to space weather prediction in this talk.
In this study, we focus on the observational fact that the sunspot peak is 7 months later than the sunspot peak. Our objective is to elucidate the cause of the delay and understand the evolution of the open flux extending from the solar surface. We investigate which component of the solar magnetic field produces the IMF evolution over Cycle 24 by comparing the evolution of the solar magnetic field with that of IMF, and simulate how open flux varies with the sunspot characteristics. We use the Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO) for magnetograms with the surface flux transport model to simulate the sunspot diffusion, and the potential field source surface (PFSS) model to extrapolate the coronal field, which we decompose into the components (l, m) of the spherical harmonic function.
As a result, it was found from the observational analysis that the peak of the solar dipole flux (l=1) is, similar to the IMF, delayed by 7 months from the sunspot maximum, and the equatorial dipole flux (l, m)=(1, ±1) is dominant during the solar maximum. Our simulations revealed that the larger the tilt angle of the sunspot and the higher the latitude at which the sunspot is located, the more the open flux increases, and when the sunspot has a tilt angle of 4 degrees or more, the open flux evolution is delayed by several months relative to the solar surface.
These results suggest that the equatorial dipole during the solar maximum and the nondipole flux during the solar maximum are keys to attacking the open flux problem. The generation mechanism of the IMF from the solar magnetic field during solar maximum is inferred as the following; i) solar active regions appear at low latitudes in the photosphere; ii) the magnetic fields of the active regions diffuse toward the polar regions; iii) the difference in the latitude of leading and following sunspots causes the magnetic field lines to be stretched in the longitude direction due to differential rotation and supergranulation; iv) the open flux is increased; and v) the IMF near Earth is increased. This process may be the reason why the peak of the IMF is seven months later than the solar maximum. During the solar minimum, it is suggested that the stable axial dipole component (l,m) = (1, 0) forms the global dipole field structure, and the variation in IMF is produced by the variation in the nondipole component (l≧2) from 2017 to 2021. We will discuss applying this suggestion to space weather prediction in this talk.