The Effect of Combined Magnetic Topologies on Thermally Driven Winds
Date Submitted
2017-04-21 09:15:26
SolWind
Adam Finley
University of Exeter
Poster
Sean Matt
Stellar winds dominate the space environment surrounding cool stars, which has implications for planetary interactions along with controlling the angular momentum loss from the stars themselves. The rotational evolution of these stars is known to depend on a multitude of stellar parameters, including magnetism. Cool stars are observed to host a variety of magnetic field topologies, typically globally dominated by a combination of the lowest order field modes; dipole, quadrupole and octupole. These mixed fields can be seen during the solar magnetic cycle, where an oscillation from dipolar to quadrupolar geometry is observed with a continuous range of intermediate mixed field states in between. The changing magnetic topology modifies the effectiveness of magnetic braking for a given stellar wind. The Alfvén radius in ideal MHD is seen as a lever arm for the torque, as such we us it to characterise the angular momentum loss from magnetised stellar winds. We quantify the impact of mixed dipolar and quadrupolar fields on the Alfvén radius using MHD wind simulations containing mixed field geometries, all of which range in magnetic field strengths and reside in the slow-rotator regime. We argue that the mixed dipole and quadrupole fields of thermally driven stellar winds are approximately either dipole dominated or quadrupole dominated, and produce a broken power law scaling to describe this behaviour. More generally, we find the lowest order topology to be the most significant in determining the field morphology and spin-down torque, we observe this to continue for higher order mixed field combinations.