Is AD Leo entering a polarity reversal? Long-term monitoring of the large-scale magnetic field with ESPaDOnS, NARVAL and SPIRou

Zeeman-Doppler Imaging has been applied on numerous stars to reconstruct their surface magnetic field topology and provide observational feedback on their internal structure. Several types of magnetic field topologies in low-mass fully convective M dwarfs have been reported and their explanation requires either two coexisting and stable dynamo branches (known as dynamo bistability) or long-term magnetic cycles with polarity reversals, but there is no definite conclusion on the matter. Shedding more light on activity cycles for M dwarfs is important to optimise radial velocity surveys of exoplanets as well as inform transmission spectroscopy studies about the modulation of stellar winds in which close-in planets are embedded. We analysed near-infrared spectropolarimetric observations of the active M dwarf AD Leo taken with SPIRou at CFHT between 2019 and 2020. We examined the long-term behaviour of the longitudinal magnetic field and we recovered the magnetic field geometry via both Zeeman-Doppler Imaging and a Principal Component Analysis. Including archival optical data sets, we found evidence of a secular evolution of the magnetic field in the form of a varying intensity and reduced axisymmetry (from 90% to 50%). Throughout the entire time series, the topology remained simple, i.e. predominantly poloidal and dipolar. This suggests that low-mass M dwarfs with a dipole dominated magnetic field can undergo magnetic cycles.