Magnetic topology of low-mass stars : changes in coronal structure and emission measures across the Fully Convective Boundary
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M Dwarfs are becoming increasingly important in the investigation of magnetic fields. These low mass stars span a range of masses that coincide with a change in the internal structure from partly convective to fully convective. Despite this, these stars remain extremely magnetically active and exhibit strong, stable magnetic fields. Reconstructed maps of the radial magnetic field component at the stellar surface can be produced through Zeeman-Doppler Imaging (ZDI). In this thesis I extrapolate the 3D coronal magnetic field from the reconstructed magnetic maps and investigate the change in the coronal structure across the fully-convective boundary, for a sample of early-to late-M dwarfs. ZDI has the ability to map surface magnetic fields but only the net large-scale field is detected. I create synthesised maps to model the hidden magnetic field not detected by ZDI and examine the effect this small-scale field has on coronal properties. I find that the dipole component of the magnetic field is prominent on each star and influences the large-scale structure but that the high multipoles cannot be ignored when modelling the open flux bearing the stellar wind. This open flux is responsible for angular momentum loss and the spin down time of the star and I find that the magnitude of the predicted open flux increases with decreasing mass. I examine the X-ray emission measure and reproduce the observed trend of a rise, a saturation and a further decline of the X-ray luminosity with Rossby number. I find that when the synthesised small-scale field is scaled with respect to the large-scale field the X-ray activity-rotation relation is unaffected by the increased surface flux. I also find that the magnetic field structure is not significantly affected by the addition of a carpet of small-scale field. As such the spin-down times and angular momentum loss due to the open flux are also unaffected. For a small sub-section of the stellar sample, I investigate the impact of forcing the magnetic field geometries towards symmetric and anti-symmetric configurations about the equator. I find that these assumptions have only a small effect on coronal properties. At spectral type M7, a deviation from the well-established L[sub]X-L[sub]R relation occurs in the radio band. I adopt the theory behind Earth’s auroral kilometric radiation to demonstrate that the electron cyclotron maser instability, a plausible explanation for radio emission, is able to form within the corona, close to the stellar surface. I also find that the light curve of this radio emission is highly dependent on the magnetic field structure and stellar inclination.
Thesis, PhD Doctor of Philosophy
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