Magnetic field dependence of bipolar magnetic region tilts on the Sun: Indication of tilt quenching

TL;DR

This study investigates how bipolar magnetic region tilts on the Sun depend on magnetic field strength, revealing a nonlinear tilt quenching effect that impacts solar magnetic field generation models.

Contribution

It provides observational evidence of tilt quenching related to magnetic field strength, linking empirical data with the thin flux tube model of BMR formation.

Findings

Bimodal distribution of BMR magnetic field strength.

Joy's law slope increases then decreases with magnetic field strength.

Systematic decrease in tilt scatter with increasing magnetic field.

Abstract

The tilt of bipolar magnetic region (BMR) is crucial in the Babcock--Leighton process for the generation of the poloidal magnetic field in Sun. Based on the thin flux tube model of the BMR formation, the tilt is believed to be caused by the Coriolis force acting on the rising flux tube of the strong toroidal magnetic field from the base of the convection zone (BCZ). We analyze the magnetic field dependence of BMR tilts using the magnetograms of Michelson Doppler Imager (MDI) (1996-2011) and Helioseismic and Magnetic Imager (HMI) (2010-2018). We observe that the distribution of the maximum magnetic field ($B_{\rm max}$) of BMRs is bimodal. Its first peak at the low field corresponds to BMRs which do not have sunspots as counterparts in the white light images, whereas the second peak corresponds to sunspots as recorded in both types of images. We find that the slope of Joy's law ($\gamma_0$) initially increases slowly with the increase of $B_{\rm max}$. However, when $B_{\rm max} \gtrsim 2$ kG, $\gamma_0$ decreases. Scatter of BMR tilt around Joy's law systematically decreases with the increase of $B_{\rm max}$. The decrease of observed $\gamma_0$ with $B_{\rm max}$ provides a hint to a nonlinear tilt quenching in the Babcock--Leighton process. We finally discuss how our results may be used to make a connection with the thin flux tube model.