Improved reconstruction of the century-long solar magnetic field by incorporating morphological asymmetry in sunspots

Abstract

Accurately modeling the solar magnetic field is important for understanding long-term solar activity and space weather, but it is challenging due to limited observations, especially near the poles. The Surface Flux Transport (SFT) model simulates how magnetic flux moves across the solar surface and contributes to the polar field, but it parametrizes emerged sunspots as simple symmetric bipolar regions and needs improvement by including more realistic sunspot features. In this study, we reconstruct the century-long evolution of the solar magnetic field, including the polar regions, using an improved SFT model. We incorporate cycle-dependent morphological asymmetry between leading and following sunspots, along with observationally derived tilt angles and sunspot area data for a century (1913-2016), to better represent magnetic flux transport and investigate the impact of asymmetry on polar field development. To study morphological asymmetry, we consider two cases: first, a long-term asymmetry factor calculated from the ratio of leading and following sunspot areas spanning over a century; second, the temporal asymmetry factor observed during solar cycle 23 applied to every solar cycle. Our simulated magnetic flux transport with inclusion of morphological asymmetry for both cases gets improved compared to the no asymmetry case in terms of enhanced low and mid-latitude magnetic flux and matches closely with observations. The simulated polar fields with asymmetry also show a better agreement with polar field observations for most cycles, particularly in capturing the timing of the polar field reversals and the peak amplitude during solar minima, which has severe consequences in solar cycle prediction

Figures (13)

• Figure 1: A representative example of the morphology and magnetic configuration of a typical sunspot group. Panel-a displays a white-light image of the sunspots captured by the Helioseismic and Magnetic Imager (HMI) 4500 Å continuum channel, while Panel-b shows their photospheric line-of-sight (LOS) magnetic configuration as recorded by the HMI 6173 Å channel. In the magnetogram, the yellow and red colors represent the north (outward) polarity, whereas the green and blue colors indicate the south (inward) polarity. Credit: SDO/HMI 2012SoPh..275..207S
• Figure 2: Ratio of the 5-year running mean areas of umbra and pores of Following ($A_F$) and Leading ($A_L$) polarity in units of $\mu hm$ based on TLATOV2015835.
• Figure 3: Century-long evolution of photospheric magnetic field (Butterfly Diagram) for about 100 years (1913-2016) from our simulations. The upper panel shows the butterfly diagram without inclusion of morphological asymmetry (Case-1), the middle panel shows the butterfly diagram with long-term asymmetry variation in sunspots (Case-2), and the lower panel shows the butterfly diagram with inclusion of cycle-23 morphological asymmetry (Case-3) factor for all cycles.
• Figure 4: Comparison of the polar field over 100 years from the simulation, with and without the long-term temporally varying asymmetry factor, against observational polar field data from Kodaikanal Solar Observatory (KSO). The solid line represents the south pole, while the dotted line represents the north pole. The observed polar field from WSO is merged with KSO polar field data after 1976. The vertical grey dotted lines show the position of solar minima.
• Figure 5: Same as Figure \ref{['polar_field_kso']} but with observational polar field data from MWO (1913-1976) and WSO (1976-2016).