The attempted polarity reversal and evolving magnetic environment of AD Leo

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
² School of Mathematics and Physics, University of Queensland, St Lucia, QLD, 4072 Australia
³ Centre for Planetary Habitability (PHAB), Department for Geosciences, University of Oslo, Oslo, Norway
⁴ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, IRAP/UMR 5277, 14 avenue Edouard Belin, F-31400 Toulouse, France
⁵ Laboratoire Univers et Particules de Montpellier, Université de Montpellier, CNRS, F-34095 Montpellier, France
⁶ School of Mathematical and Physical Sciences, Macquarie University, Macquarie Park, NSW, 2113 Australia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 29 April 2025
Accepted: 11 February 2026

Abstract

Context. In the past two decades, the observed large-scale magnetic field of the active M dwarf star AD Leo has evolved from being strongly negative to mildly negative, raising a suspicion that it might be at the imminence of switching polarity (i.e. becoming positive). Although magnetic field reversals are observed every 11 years in the solar magnetic field, in the context of M dwarfs, magnetic field reversals are still poorly understood and so far not predictable. Further, no reversals have yet been observed for fast-rotating M dwarfs. Moreover, it is known that the magnetic field of stars impacts their surrounding space weather environment. Studying how space weather evolves over time is thus crucial for examining planetary habitability.

Aims. We examine the properties of AD Leo’s large-scale magnetic field, which was recently found to be trending towards a polarity reversal. We also investigate how the space weather environment changes in response to the evolution of the large-scale magnetic field, by modelling the wind of AD Leo.

Methods. We analysed spectropolarimetric data collected by ESPaDOnS and SPIRou in late 2022 and early 2023. With the optical and near-infrared data we computed the longitudinal magnetic field, and with the near-infrared data we reconstructed the large-scale magnetic field using Zeeman-Doppler imaging. Using five magnetograms, from between 2019 and 2023, we simulated three-dimensional Alfvén wave-driven stellar winds using the state-of-the-art space weather code SWMF.

Results. Although we see an evolution of the large-scale magnetic field of AD Leo, we find no polarity reversal, but rather a restoration of the field to a simpler and axisymmetric configuration and consistently negative values for the longitudinal magnetic field strength. Previous work found the longitudinal field to get as weak as −46 G in 2020, and rather than continued weakening it now appears to be strengthening. Our new large-scale field reconstruction for AD Leo is characterised by a highly axisymmetric, poloidal-dipolar field with an increased mean large-scale field strength from 93 G to 145 G. However, the mean strength is still diminished compared to maps produced between 2007 and 2016. Our simulations of the space weather environment around AD Leo find the stellar mass loss rates to be on average ∼2.54 × 10¹⁰ kg/s – an order of magnitude greater than the solar mass loss rate. Additionally, we examine the space weather experienced by hypothetical planets orbiting at the bounds of the habitable zone around AD Leo. We find that the entirety of the habitable zone resides beyond the AlfvÉn surface. Further, magnetised habitable zone planets (with planetary field strengths greater than 0.34 G) would likely be shielded from the incident wind and atmospheric erosion would be negligible (excluding effects from coronal mass ejections and flares). Additionally, we find the complexity of the wind velocity and wind pressure structures to evolve with the changing axisymmetry of the stellar large-scale magnetic field, resulting in more variable conditions along the orbits at certain epochs.