We investigate the change in stellar magnetic topology across the fully-convective boundary and its effects on coronal properties .
We consider both the magnitude of the open flux that influences angular momentum loss in the stellar wind and X-ray emission measure .
We use reconstructed maps of the radial magnetic field at the stellar surface and the potential-field source surface method to extrapolate a 3D coronal magnetic field for a sample of early-to-mid M dwarfs .
During the magnetic reconstruction process it is possible to force a solution towards field geometries that are symmetric or antisymmetric about the equator but we demonstrate that this has only a modest impact on the coronal tracers mentioned above .
We find that the dipole component of the field , which governs the large-scale structure , becomes increasingly strong as the stellar mass decreases , while the magnitude of the open ( wind-bearing ) magnetic flux is proportional to the magnitude of the reconstructed magnetic flux .
By assuming a hydrostatic and isothermal corona we calculate X-ray emission measures ( in magnitude and rotational modulation ) for each star and , using observed stellar densities as a constraint , we reproduce the observed X-ray saturation at Ro \leq 0.1 .
We find that X-ray rotational modulation is not a good indicator of magnetic structure as it shows no trend with Rossby number but can be useful in discriminating between different assumptions on the field geometry .