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We perform three-dimensional numerical simulations of stellar winds ofearly-M dwarf stars. Our simulations incorporate observationally reconstructedlarge-scale surface magnetic maps, suggesting that the complexity of themagnetic field can play an important role in the angular momentum evolution ofthe star, possibly explaining the large distribution of periods in field dMstars, as reported in recent works. In spite of the diversity of the magneticfield topologies among the stars in our sample, we find that stellar windflowing near the (rotational) equatorial plane carries most of the stellarangular momentum, but there is no preferred colatitude contributing to massloss, as the mass flux is maximum at different colatitudes for different stars.We find that more non-axisymmetric magnetic fields result in more asymmetricmass fluxes and wind total pressures $p_{\rm tot}$ (defined as the sum ofthermal, magnetic and ram pressures). Because planetary magnetospheric sizesare set by pressure equilibrium between the planet's magnetic field and $p_{\rmtot}$, variations of up to a factor of $3$ in $p_{\rm tot}$ (as found in thecase of a planet orbiting at several stellar radii away from the star) lead tovariations in magnetospheric radii of about 20 percent along the planetaryorbital path. In analogy to the flux of cosmic rays that impact the Earth,which is inversely modulated with the non-axisymmetric component of the totalopen solar magnetic flux, we conclude that planets orbiting M dwarf stars likeDT~Vir, DS~Leo and GJ~182, which have significant non-axisymmetric fieldcomponents, should be the more efficiently shielded from galactic cosmic rays,even if the planets lack a protective thick atmosphere/large magnetosphere oftheir own.

M-dwarf stellar winds: the effects of realistic magnetic geometry on rotational evolution and planets