Speaker
Description
We present a novel method for quantifying the eect that individual active regions have on the large-scale solar magnetic field as their magnetic flux spreads across the photosphere. This is achieved by combining the surface flux transport (SFT) model with a recent vector sum method.
We simulated the evolution of individual active regions of solar cycle 24 with the SFT model and used the vector sum to represent their state with a dipole vector at every Carrington rotation (CR). Due to the linearity, the sum of the resulting dipole vector time series is equivalent to the series calculated from the full SFT simulation. The dipole vector representation makes it straightforward to quantify the eect of each active region on the large-scale field (for example, using a dot product). Because the magnitude of the large-scale dipole vector closely follows the open solar flux (OSF) from the PFSS model, these eects also serve as a proxy for their OSF contributions.
Using these methods, we analyzed the rapid increase of the large-scale solar magnetic field in late 2014 that culminated in the OSF peak of solar cycle 24 in CR2157. We identified the most consequential active regions at the time of the OSF peak and found that 6 out of 7 of these regions emerged within a narrow longitude band in the southern hemisphere around Carrington longitude 240°. Some of these regions emerged already half a year prior to the peak OSF.
By randomizing the longitudinal locations of active regions in the SFT simulation, we find strong evidence that during the declining phase of the solar cycle 24 the longitudinal distribution of active regions was not random, but some of the active regions had a tendency to emerge at longitudes where their equatorial dipole components reinforced the existing large-scale field. Additionally, we found that taking into account the equatorial component of the solar dipole reduces the well-known degeneracy between the diusion and meridional flow amplitude parameters when optimizing the parameters of the SFT model based on the axisymmetric field alone. The reason is the opposite eect that diusion has on the axial and equatorial components of the solar magnetic field.
Our study demonstrates an eicient framework for studying the eects that the initially localized active region magnetic fields have on the global solar magnetic field as they spread across the photosphere. These methods are readily applicable for studying both the historical solar observations as well as the ongoing solar cycle 25.