On Making Magnetic-Flux-Rope $\Omega$ Loops for Solar Bipolar Magnetic Regions of All Sizes by Convection Cells

TL;DR

This paper proposes a convection-cell-based model for forming magnetic flux-rope $ m f extOmega$ loops that emerge as bipolar magnetic regions of all sizes, supported by observations and simulations, offering an alternative to the standard magnetic buoyancy scenario.

Contribution

It introduces a novel convection-driven mechanism for flux-rope formation, avoiding the need for extremely strong initial magnetic fields in the convection zone.

Findings

Supports the idea that convection cells create $ m f extOmega$ loops from horizontal fields.

Explains the size and flux of large bipolar magnetic regions.

Provides insights into hemispheric helicity rule violations.

Abstract

We propose that the flux-rope $\Omega$ loop that emerges to become any bipolar magnetic region (BMR) is made by a convection cell of the $\Omega$-loop's size from initially-horizontal magnetic field ingested through the cell's bottom. This idea is based on (1) observed characteristics of BMRs of all spans ($\sim$ 1000 km to $\sim$ 200,000 km), (2) a well-known simulation of the production of a BMR by a supergranule-size convection cell from horizontal field placed at cell bottom, and (3) a well-known convection-zone simulation. From the observations and simulations, we (1) infer that the strength of the field ingested by the biggest convection cells (giant cells) to make the biggest BMR $\Omega$ loops is $\sim$ 10$^3$ G, (2) plausibly explain why the span and flux of the biggest observed BMRs are $\sim$ 200,000 km and $\sim$ 10$^{22}$ Mx, (3) suggest how giant cells might also make "failed-BMR" $\Omega$ loops that populate the upper convection zone with horizontal field, from which smaller convection cells make BMR $\Omega$ loops of their size, (4) suggest why sunspots observed in a sunspot cycle's declining phase tend to violate the hemispheric helicity rule, and (5) support a previously-proposed amended Babcock scenario for the sunspot cycle's dynamo process. Because the proposed convection-based heuristic model for making a sunspot-BMR $\Omega$ loop avoids having $\sim$ 10$^5$ G field in the initial flux rope at the bottom of the convection zone, it is an appealing alternative to the present magnetic-buoyancy-based standard scenario and warrants testing by high-enough-resolution giant-cell magnetoconvection simulations.