Active regions and the large-scale magnetic field of solar cycle 24

Abstract

Most of the intracyclic variability in the large-scale solar magnetic field comes from the equatorial dipole component of the solar magnetic field. The equatorial dipole component is highly sensitive to the longitude distribution of the active regions. We quantify the effect of individual active regions on the large-scale solar magnetic field of the solar cycle 24. We study the effect of the longitude distribution of active regions on the strength of the large-scale dipole component. We used a surface flux transport (SFT) model to simulate the evolution of individual active regions and quantified their effect on the large-scale magnetic field using the recently developed vector sum method. We took advantage of the longitudinal translational invariance of the SFT model and compared the observed solar cycle 24 to the 10 000 simulations of the solar cycle 24 using randomized longitudinal source locations, but otherwise identical flux emergence. We find that taking into account both the axial and equatorial components of the vector sum characterizing the global solar magnetic field sets better constraints on the parameter space of the SFT model than, for example, using the axial dipole moment alone as an optimization metric. We studied the maximum of cycle 24 and identified the recurrent and localized flux emergence in the southern hemisphere as the main culprit behind the rapid strengthening of the large-scale magnetic field in late 2014. We find that during the declining phase of the solar cycle, the strength of the large-scale magnetic field stayed above the median level of randomized simulations (p < 0.027) for 42 subsequent. This indicates that the longitudinal distribution of active regions is not random and, rather, that it demonstrates a tendency for some regions to emerge at longitudes where their equatorial components reinforce the large-scale equatorial field.

Figures (11)

• Figure 1: Vector sum magnitude, latitude and longitude. Upper panel: PFSS OSF ($l_{max}~=~50, R_{SS}=~2.5R_\odot$, blue) and the absolute value of the vector sum (orange) from the HMI synoptic magnetograms. Middle panel: Vector sum latitude. Lower panel: Vector sum longitude.
• Figure 2: Axisymmetric (left panel) and nonaxisymmetric (right panel) component of the PFSS OSF ($l_{max}~=~50, R_{SS}=~2.5R_\odot$, blue) and the vector sum (orange). The vector sum components correspond to projections of the vector sum along ($\Phi\sin{\lambda}$, axial) and perpendicular ($\Phi\cos{\lambda}$, equatorial) to solar rotation axis. The PFSS OSF components correspond to PFSS expansion using only axisymmetric (m=0) and nonaxisymmetric components ($m\neq0$).
• Figure 3: Best-fit simulations. Left column: Best-fit simulations based on the total vector sum. Middle column: The best-fit simulations based on the axial component. Right column: Best-fit simulations based on the equatorial component. First row: Total vector sum. Middle row: Axial component. Lower row: Equatorial component. Blue color shows the vector sum from HMI magnetograms and orange from the SFT simulations.
• Figure 4: Effect of SFT parameters on the axial and equatorial components. Left panel: Axial component. Right panel: Equatorial component. Color shows the mean deviation between the SFT simulation and HMI vector sum components. The best-fit parameters $\eta~=~350~\mathrm{km^2/s}$ and $u_0~=~11$ m/s obtained using the total vector sum is marked with a cross.
• Figure 5: Equatorial components of active regions. (a) Equatorial component at CR2157 vs. time of emergence. (b) Evolution of equatorial components of seven active regions which had strongest equatorial component at CR2157. (c) Projections of the active region equatorial components along the large-scale field at CR2157. (d) Evolution of contributions of seven active regions which had strongest equatorial component at CR2157. Colors mark the seven active regions that had the strongest equatorial component at CR2157 and correspond to same active regions in all panels.