  The solar magnetic structure changes over the solar cycle. It has a dipole
structure during solar minimum, where the open flux extends mainly from the
polar regions into the interplanetary space. During maximum, a complex
structure is formed with low-latitude active regions and weakened polar fields,
resulting in spread open field regions. However, the components of the solar
magnetic field that is responsible for long-term variations in the
interplanetary magnetic field (IMF) are not clear, and the IMF strength
estimated based on the solar magnetic field is known to be underestimated by a
factor of 3 to 4 against the actual in-situ observations (the open flux
problem). To this end, we decomposed the coronal magnetic field into the
components of the spherical harmonic function of degree and order $(\ell, m)$
using the potential field source surface model with synoptic maps from SDO/HMI
for 2010 to 2021. As a result, we found that the IMF rapidly increased in
December 2014 (seven months after the solar maximum), which coincided with the
increase in the equatorial dipole, $(\ell, m)=(1, \pm1)$, corresponding to the
diffusion of active regions toward the poles and in the longitudinal direction.
The IMF gradually decreased until December 2019 (solar minimum) and its
variation corresponded to that of the non-dipole component $\ell\geq2$. Our
results suggest that the understanding of the open flux problem may be improved
by focusing on the equatorial dipole and the non-dipole component and that the
influence of the polar magnetic field is less significant.
