What controls the magnetic geometry of M dwarfs?

TL;DR

This study investigates how the local Rossby number influences the magnetic field geometry of M dwarfs, linking observational data with dynamo models to understand magnetic diversity.

Contribution

It provides observationally motivated diagnostics connecting the Rossby number to magnetic field geometries in M dwarfs, highlighting a possible bistability at low Rossby numbers.

Findings

Transition from dipolar to multipolar fields linked to Rossby number threshold

Late M dwarfs may exhibit dynamo bistability at low Rossby numbers

Different differential rotation amplitudes inferred for dynamo branches

Abstract

Context: observations of rapidly rotating M dwarfs show a broad variety of large-scale magnetic fields encompassing dipole-dominated and multipolar geometries. In dynamo models, the relative importance of inertia in the force balance -- quantified by the local Rossby number -- is known to have a strong impact on the magnetic field geometry. Aims: we aim to assess the relevance of the local Rossby number in controlling the large-scale magnetic field geometry of M dwarfs. Methods: we explore the similarities between anelastic dynamo models in spherical shells and observations of active M-dwarfs, focusing on field geometries derived from spectropolarimetric studies. To do so, we construct observation-based quantities aimed to reflect the diagnostic parameters employed in numerical models. Results: the transition between dipole-dominated and multipolar large-scale fields in early to mid M dwarfs is tentatively attributed to a Rossby number threshold. We interpret late M dwarfs magnetism to result from a dynamo bistability occurring at low Rossby number. By analogy with numerical models, we expect different amplitudes of differential rotation on the two dynamo branches.