Temporal Evolution of Sunspot Groups and Increase in the Open flux During Solar Maximum in Cycle 24

TL;DR

This study investigates how the evolution of bipolar magnetic regions and their interactions during solar maximum in Cycle 24 lead to significant increases in open magnetic flux, affecting the interplanetary magnetic field.

Contribution

The paper introduces a simulation-based analysis of how BMR characteristics and configurations influence open flux evolution during solar maximum, highlighting the role of equatorial dipoles and large sunspot groups.

Findings

Open flux doubled during late 2014 solar maximum.

Large sunspot groups and specific configurations facilitated flux expansion.

Interactions with coronal holes significantly affected flux variations.

Abstract

The evolution of the global solar magnetic field directly impacts the interplanetary magnetic field (IMF). During the solar maximum of Cycle 24, the monthly averaged IMF strength doubled over five Carrington rotations in late 2014. To understand the physical origin of this increase, we investigate the temporal evolution of open magnetic flux resulting from the emergence and decay of bipolar magnetic regions (BMRs). Using surface flux transport and potential field source surface models, we simulated how BMR characteristics, spatial distributions, and interaction with background magnetic fields affect open flux evolution. Our simulation confirmed that the relative configuration of BMRs can either inhibit open flux expansion via closed loops or promote it through favorable connections. The increase in open flux is primarily driven by the equatorial dipole component, which is enhanced by differential rotation acting on tilted BMRs. These behaviors suggest that large open field structures develop from equatorial dipole components formed by these stretched BMRs. We attribute the rapid IMF increase in 2014 (Carrington rotations 2152-2157) to the combination of the following three factors: (1) a specific sunspot configuration that facilitated the expansion of the southern coronal hole; (2) the emergence of a giant sunspot group (active region 12192) with high magnetic intensity; and (3) the diffusion of these regions, which reinforced the global magnetic field. These results imply that rapid open flux variations during solar maximum are governed not only by the characteristics of emerging BMRs but also by their interaction with pre-existing large coronal holes.