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

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.

Figures (8)

• Figure 1: Temporal variations in IMF at 1 AU (blue solid line) and sunspot numbers (orange dotted line) during Cycle 23 and Cycle 24 (1-month running average). A characteristic increase in IMF is observed in the latter half of 2014 during Cycle 24. The sunspot number data is provided by WDC-SILSO, Royal Observatory of Belgium, Brussels (DOI: https://doi.org/10.24414/qnza-ac80), and the in-situ IMF data is obtained from the NASA/GSFC's OMNI database.
• Figure 2: Spatial distributions of two BMRs for the three cases of $\#$2. The orange sphere represents the Sun, the white and black circles depict sunspots with positive and negative polarity, respectively, and the red arrow indicates the direction of the magnetic field. (a) Placing two bipolar BMRs at the same longitude. The BMR in the southern hemisphere has a magnetic polarity opposite to that placed in the northern hemisphere. (b) Two bipolar BMRs, one of which is placed at a different longitude (180$^{\circ}$ apart) in a different hemisphere. (c) Two BMRs are placed in the southern hemisphere, one of which is placed apart 180$^{\circ}$ from the other.
• Figure 3: Time evolution of the amount of the magnetic flux in the source surface (a) and the ratio of the open flux at the source surface (b) when a small BMR (solid blue lines) or a large BMR (dashed red lines) is placed. All values are normalized by the initial value of the total unsigned magnetic flux in the photosphere.
• Figure 4: The open flux footpoints for configuration #1. The upper panel shows the open flux footpoints when a small BMR is placed, and the lower panel shows that when a large BMR is placed. The black and white regions represent negative and positive open area.
• Figure 5: Time evolution of the amount of the magnetic flux in the source surface (a) and the ratio of the open flux at the source surface (b). The blue, red, and green lines represent the result for Configuration #2 (a), (b), and (c) in Figure \ref{['fig:sun_tilt']}, respectively. All values are normalized by the initial value of the total unsigned magnetic flux in the photosphere.