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Origin of Joy's Law in the context of Near-Surface Convection on the Sun

Hannah Schunker, Asha Lakshmi K

TL;DR

The paper investigates the origin of Joy's law in the Sun, highlighting that near-surface convection likely governs active-region tilt rather than deep-seated flux tubes. It synthesizes observational results showing that the average tilt increases during emergence and that the tilt–latitude relation is primarily reflected in the north–south extent, consistent with a near-surface, Coriolis-influenced mechanism rather than a pre-emergent, deep geometric writhe. The review notes that the tilt follows a $\sin \gamma$–$\sin \lambda$ dependence and that the cycle amplitude anti-correlates with the average tilt, implying a role in dynamo regulation. It calls for more high-cadence, high-resolution surface magnetic field observations and refined statistics to disentangle the contributions of near-surface flows, flux transport, and non-linear tilt/latitude quenching to the solar cycle.

Abstract

Joy's law is a well-established statistical property of solar active regions that any theory of active region emergence must explain. This tilt angle of the active region away from an east-west alignment is a critical component for converting the toroidal magnetic field to poloidal magnetic field in some leading dynamo theories, and observations show they are important for the reversal of the sign of the global solar magnetic dipole. This review aims to synthesise observational results related to the onset of Joy's law, placing them within the broader context that describes active region emergence as a largely passive process occurring near the surface of the Sun.

Origin of Joy's Law in the context of Near-Surface Convection on the Sun

TL;DR

The paper investigates the origin of Joy's law in the Sun, highlighting that near-surface convection likely governs active-region tilt rather than deep-seated flux tubes. It synthesizes observational results showing that the average tilt increases during emergence and that the tilt–latitude relation is primarily reflected in the north–south extent, consistent with a near-surface, Coriolis-influenced mechanism rather than a pre-emergent, deep geometric writhe. The review notes that the tilt follows a – dependence and that the cycle amplitude anti-correlates with the average tilt, implying a role in dynamo regulation. It calls for more high-cadence, high-resolution surface magnetic field observations and refined statistics to disentangle the contributions of near-surface flows, flux transport, and non-linear tilt/latitude quenching to the solar cycle.

Abstract

Joy's law is a well-established statistical property of solar active regions that any theory of active region emergence must explain. This tilt angle of the active region away from an east-west alignment is a critical component for converting the toroidal magnetic field to poloidal magnetic field in some leading dynamo theories, and observations show they are important for the reversal of the sign of the global solar magnetic dipole. This review aims to synthesise observational results related to the onset of Joy's law, placing them within the broader context that describes active region emergence as a largely passive process occurring near the surface of the Sun.

Paper Structure

This paper contains 7 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Importance of Joy's Law for the solar cycle
  3. Importance of convection for active region emergence
  4. Origins of Joy's Law
  5. Onset of Joy's Law
  6. Uncertainties in measuring Joy's law
  7. Summary

Figures (2)

  • Figure 1: Number of active regions with visual emergence time interval relative to the nominal emergence time interval Schunkeretal2016. About half of the active regions in the sample can be visually identified as emerging up to 2 days before the nominal emergence time.
  • Figure 2: Average motion of the polarities during active region emergence. The red and blue data points closest to the centre are averages of the polarity locations at the visual emergence time (as defined in Fig. \ref{['fig:VET']}), and then subsequent time intervals are further and further apart. The tilt angle grows in time as the polarities move further apart Schunkeretal2019Schunkeretal2020.