Cool stars with outer convective envelopes are observed to have magnetic
fields with a variety of geometries, which on large scales are dominated by a
combination of the lowest order fields such as the dipole, quadrupole and
octupole modes. Magnetised stellar wind outflows are primarily responsible for
the loss of angular momentum from these objects during the main sequence.
Previous works have shown the reduced effectiveness of the stellar wind braking
mechanism with increasingly complex, but singular, magnetic field geometries.
In this paper, we quantify the impact of mixed dipolar and quadrupolar fields
on the spin-down torque using 50 MHD simulations with mixed field, along with
10 of each pure geometries. The simulated winds include a wide range of
magnetic field strength and reside in the slow-rotator regime. We find that the
stellar wind braking torque from our combined geometry cases are well described
by a broken power law behaviour, where the torque scaling with field strength
can be predicted by the dipole component alone or the quadrupolar scaling
utilising the total field strength. The simulation results can be scaled and
apply to all main-sequence cool stars. For Solar parameters, the lowest order
component of the field (dipole in this paper) is the most significant in
determining the angular momentum loss.Comment: 15 pages + 9 figures (main), 3 pages + 1 figure (appendix), accepted
for publication to Ap