11:30 AM - 11:45 AM
[PEM09-10] Decomposing the global magnetic field of the Sun to understand the evolution of interplanetary magnetic field
Keywords:Sun: magnetic field, Sun: heliosphere, Sun: corona, Sun: photosphere, Interplanetary magnetic field
The solar magnetic structure changes over eleven years during the solar cycle. During solar minimum, the magnetic field shows a dipole structure and the "open" flux mainly from the polar regions produces the interplanetary magnetic field (IMF). During the solar maximum, the solar magnetic structure which concentrates the active regions in the low-mid latitudes is complicated and the source of the open flux is not clear. It is important to understand the connection of the magnetic field between the Sun and interplanetary space because it is the basis for understanding space weather, i.e. how the Sun affects the Earth and the entire heliosphere. However, there is a significant problem in that the extrapolated open flux from the Sun cannot properly estimate the IMF near the Earth. This is called the "open flux problem", and the open flux is underestimated by a factor of 2 to 5 compared with actual in situ IMF observations. Furthermore, it is not clear how the solar magnetic fields produce the evolution of IMF.
In this study, we focus on long-term variations of the open flux and IMF. We set the goal to connect the global magnetic fields of the Sun and IMF. We investigate which components of the solar magnetic field produce the IMF evolution by comparing the evolution of the IMF with that of the solar magnetic field. We extrapolate the coronal magnetic structures from the solar photospheric magnetic field observed by Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO) with the potential field source surface (PFSS) model from May 2010 to October 2021. Then, the coronal magnetic field is decomposed into components (l, m) by spherical harmonic function, where the component with l=1 is the dipole flux and the components with l>1 are the non-dipole flux and compared with IMF.
As a result, we find that the IMF increases rapidly and peaks in November 2014, which is seven months later than the solar maximum in April 2014. The evolution of the dipole flux (l=1), which is dominated by the equatorial dipole flux (l, m)=(1,±1), shows a similar trend in the same period. Then the IMF decreases until June 2020 in the solar minimum and then increases. The axial dipole flux (l, m)=(1, 0) is stable and the variation trend of non-dipole flux (l>1) is in agreement with IMF during the period. Our results suggest that we need to focus on the solar equatorial dipole flux during solar maximum and the non-dipole flux during solar minimum to solve the open flux problem. The evolution process from the solar magnetic field to the IMF during solar maximum is inferred; i) solar active regions emerge at low latitudes in the photosphere; ii) the magnetic fields of the active regions diffuse toward the polar regions; iii) the coronal magnetic field line is stretched in the longitude direction by differential rotation and supergranulation; iv) the open flux is increased; 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 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 non-dipole component (l>1) from 2017 to 2021.
In this study, we focus on long-term variations of the open flux and IMF. We set the goal to connect the global magnetic fields of the Sun and IMF. We investigate which components of the solar magnetic field produce the IMF evolution by comparing the evolution of the IMF with that of the solar magnetic field. We extrapolate the coronal magnetic structures from the solar photospheric magnetic field observed by Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO) with the potential field source surface (PFSS) model from May 2010 to October 2021. Then, the coronal magnetic field is decomposed into components (l, m) by spherical harmonic function, where the component with l=1 is the dipole flux and the components with l>1 are the non-dipole flux and compared with IMF.
As a result, we find that the IMF increases rapidly and peaks in November 2014, which is seven months later than the solar maximum in April 2014. The evolution of the dipole flux (l=1), which is dominated by the equatorial dipole flux (l, m)=(1,±1), shows a similar trend in the same period. Then the IMF decreases until June 2020 in the solar minimum and then increases. The axial dipole flux (l, m)=(1, 0) is stable and the variation trend of non-dipole flux (l>1) is in agreement with IMF during the period. Our results suggest that we need to focus on the solar equatorial dipole flux during solar maximum and the non-dipole flux during solar minimum to solve the open flux problem. The evolution process from the solar magnetic field to the IMF during solar maximum is inferred; i) solar active regions emerge at low latitudes in the photosphere; ii) the magnetic fields of the active regions diffuse toward the polar regions; iii) the coronal magnetic field line is stretched in the longitude direction by differential rotation and supergranulation; iv) the open flux is increased; 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 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 non-dipole component (l>1) from 2017 to 2021.