Solar active region scaling laws revisited

Abstract

The systematic variation of solar active region (AR) properties with their magnetic flux has been the subject of numerous studies but the proposed scaling laws still vary rather widely. A correct representation of these laws and the deviations from them is important for modelling the source term in surface flux transport and dynamo models of space climate variation, and it may also help constrain the subsurface origin of active regions. Here we determine active region scaling laws based on the recently constructed ARISE active region data base listing bipolar ARs for cycle 23, 24 and 25. For the area $A$, pole separation $d$ and tilt angle $γ$ we find scalings against magnetic flux $Φ$ and heliographic latitude $λ$. Residuals from these relations are also modelled. These scaling relations are recommended for use in space climate research for the modelling of future data or missing past data, as well as for the identification of candidate rogue ARs. We confirm that the tilt angle distribution of non-Hale ARs shows a significant excess at low tilts (anti-Hale ARs). In contrast to earlier studies we show that neither the anti-Hale ARs nor non-Hales in general follow Joy's law: instead, their tilt angle distribution is best represented by vanishing mean tilt. These results are most easily reconciled with a scenario where the AR flux loops originate in the deep convective zone or below, gaining tilt during their rise under the action of the Coriolis force. A small fraction of the loops is subjected to extreme, intermittent torques resulting in either very large tilts or anti-Hale orientation. Anti-Hale ARs are suggested to be fully curled `XO-loops', and their excess is caused by a simple mechanical effect, as the contact of their legs increases resistance against further deformation by the torque.

Figures (14)

• Figure 1: Histogram of the total flux $\Phi$ of active regions on linear (left) and logarithmic (right) scale
• Figure 2: Plot of active region area $A$ vs. magnetic flux $\Phi$, with power law fits to the medians of the binned data (blue circles with error bars). The dashed line corresponds to the optimal fit; the dotted line shows a linear relationship $A\sim\Phi$ for comparison. Lighter background is a scatterplot of the individual points.
• Figure 3: Histogram of the residuals from the fit in Fig. \ref{['fig:A_Phi']}. The solid line is a Gaussian fit.
• Figure 4: Plot of the polarity separation $d$ vs. logarithm of active region flux $\Phi$. Median values are plotted for each bin (blue circles with error bars). The dashed line is a linear (i.e. logarithmic) fit. The dotted line shows the scaling $d\sim\Phi^{1/2}$ for comparison. Lighter background is a scatterplot of the individual points.
• Figure 5: Histogram of the fractional residuals $d/\langle d\rangle$ relative to the linear fit in Fig. \ref{['fig:d_Phi']}. The solid curve is a lognormal fit.