A new dynamo pattern revealed by the tilt angle of bipolar sunspot groups

TL;DR

This study reveals a new pattern in the tilt angles of bipolar sunspot groups, showing a consistent hemispheric polarity and a surprising opposite tilt in smaller ephemeral regions, providing insights into solar dynamo processes.

Contribution

The paper introduces a novel analysis of tilt angles across different bipole sizes, highlighting a distinct behavior in ephemeral regions that challenges existing dynamo models.

Findings

Large bipoles follow Joy's law with hemispheric tilt polarity.

Small bipoles exhibit opposite tilt angles to large bipoles.

Tilt angles of small bipoles are robust despite data processing variations.

Abstract

We obtain the latitude-time distribution of the averaged tilt angle of solar bipoles. For large bipoles, which are mainly bipolar sunspot groups, the spatially averaged tilt angle is positive in the Northern solar hemisphere and negative in the Southern, with modest variations during course of the solar cycle. We consider the averaged tilt angle to be a tracer for a crucial element of the solar dynamo, i.e. the regeneration rate of poloidal large-scale magnetic field from toroidal. The value of the tilt obtained crudely corresponds to a regeneration factor corresponding to about 10% of r.m.s. velocity of solar convection. These results develop findings of Kosovichev and Stenflo (2012) concerning Joy's law, and agree with the usual expectations of solar dynamo theory. Quite surprisingly, we find a pronounced deviation from these properties for smaller bipoles, which are mainly solar ephemeral regions. They possess tilt angles of approximately the same absolute value, but of opposite sign compared to that of the large bipoles. Of course, the tilt data for small bipoles are less well determined than those for large bipoles; however they remain robust under various modifications of the data processing.