Measuring the magnetic fields in the chromospheres of low-mass stars

TL;DR

This study reports decade-long spectropolarimetric measurements of magnetic fields in the chromospheres of M-dwarfs, revealing complex, variable magnetic structures that influence stellar activity and potential exoplanet habitability.

Contribution

First direct measurements of chromospheric magnetic fields in M-dwarfs, providing new insights into stellar magnetic complexity and activity.

Findings

Chromospheric magnetic fields reach hundreds of Gauss.

Magnetic field polarity often opposes the photospheric field.

Magnetic field strength and sign vary with height and time.

Abstract

Magnetic fields in the upper atmospheres of solar-like stars are believed to provide an enormous amount of energy to power the hot coronae and drive large-scale eruptions that could impact the habitability of planetary systems around these stars. However, these magnetic fields have never been routinely measured on stars beyond the solar system. Through decade-long spectropolarimetric observations, we have now achieved the measurements of magnetic fields in the lower and middle chromospheres of three M-dwarfs. Our results indicate that the line-of-sight component of the chromospheric magnetic fields can reach up to hundreds of Gauss, whose sign frequently opposes that of the photospheric field. The measurements highlight the magnetic field complexity and the variation with height close to the surface of these M-dwarfs. They provide critical constraints on the energy budget responsible for heating and eruptions of stellar upper atmospheres, and enable assessments of how stellar magnetic activity may affect exoplanet environments.