Authors: Minami Yoshida (The University of Tokyo, ISAS/JAXA), Toshifumi Shimizu (ISAS/JAXA, The University of Tokyo), Shin Toriumi (ISAS/JAXA)

The “open” magnetic field extending from the Sun forms the interplanetary magnetic field (IMF). To understand the structure of IMF in relation to the solar magnetic field, there are some problems. One of the most important problems is the “open flux problem,” in which the near-Earth magnetic field estimated from the solar magnetic field is underestimated by a factor of 2-5 compared with in-situ observations.
In this study, we focus on the characteristic long-term evolutions of the IMF to understand this problem. We decomposed the solar coronal magnetic field into its components (l, m) by spherical harmonic functions and compared them with the evolution of the IMF over the whole solar cycle 24. The results showed that the solar magnetic field and the IMF relationship were characterized separately for each phase of the solar cycle. We found that the IMF rapidly increased at the end of the solar maximum. This variation corresponded to that of the solar equatorial dipole flux, (l, m) = (1, ±1). It suggests that lower-order components, (l, m) = (1, ±1) increased owing to the longitudinal diffusion by differential rotation and supergranulation when active regions diffuse toward the polar regions, and they drive the IMF to increase. In the solar minimum phase, IMF decreased and then increased gradually. This trend corresponds to that of the nondipole flux (l ≥ 2), while the dipole component (l = 1) was stable. In conclusion, our results suggest that the solar magnetic field component at low latitudes is the key to IMF evolution over the solar cycle. To solve the open flux problem,  we need to focus on the low-latitude magnetic field of the Sun.