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In this paper we present an analysis of the first high-resolution full Stokesvector spectropolarimetric observations of the active M dwarf AD Leo. Based onobservations collected in 2016 with the ESPaDOnS instrument at CFHT, we derivedthe least-squares deconvolved Stokes profiles and detected linear polarisationsignatures in spectral lines. At the same time, we discovered that the circularpolarisation profiles corresponding to our data set are significantly weakercompared to all archival spectra of AD Leo, which exhibited approximatelyconstant profiles over the timescale of at least 6 years until 2012. Magneticmaps obtained using Zeeman Doppler imaging confirm the sudden change in thesurface magnetic field. Although the total magnetic field energy decreased byabout 20% between 2012 and 2016, the field component responsible for theobserved circular polarisation signatures corresponds to a stronger fieldoccupying a smaller fraction of the stellar surface in the more recent map.These results represent the first evidence that active M dwarfs withdipole-dominated axisymmetric field topologies can undergo a long-term globalmagnetic variation.

A sudden change of the global magnetic field of the active M dwarf AD Leo revealed by full Stokes spectropolarimetric observations