Which Component of Solar Magnetic Field Drives the Evolution of Interplanetary Magnetic Field over the Solar Cycle?

Abstract

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 are 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 3–4 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 (ℓ, m) using the potential field source surface model with synoptic maps from SDO/HMI for 2010–2021. As a result, we found that the IMF rapidly increased in 2014 December (7 months after the solar maximum), which coincided with the increase in the equatorial dipole, (ℓ, m) = (1, ±1), corresponding to the diffusion of active regions toward the poles and in the longitudinal direction. The IMF gradually decreased until 2019 December (solar minimum) and its variation corresponded to that of the nondipole component ℓ ≥ 2. Our results suggest that the understanding of the open flux problem may be improved by focusing on the equatorial dipole and the nondipole component and that the influence of the polar magnetic field is less significant.

Formulating Mass-loss Rates for Sun-like Stars: A Hybrid Model Approach

“…A plausible explanation is a shift in magnetic morphology, where higher-order multipolar components become dominant (Garraffo et al 2016(Garraffo et al , 2018. This leads to reduced open magnetic flux (Yoshida et al 2023) and massloss rate as per Equation (39), subsequently lowering magnetic braking efficiency (Matt et al 2012;Réville et al 2015;Finley & Matt 2018). Our findings therefore support this hypothesis, although we need further investigations such as deriving a formula for open magnetic flux as a function of surface magnetic field and mass-loss rate.…”

“…However, the latest improvements in modeling stellar large-scale magnetic field structures, via global MHD simulations and Zeeman-Doppler Imaging (Folsom et al 2016(Folsom et al , 2018See et al indicate potential feasibility for deducing open magnetic flux (Ó Fionnagáin et al 2019;Airapetian et al 2021;Evensberget et al 2021Evensberget et al , 2022Evensberget et al , 2023. A comprehensive study of the relation between the Sun's surface and open magnetic flux (Yoshida et al 2023) could also be crucial in eliminating Φ open from Equation (39).…”