  The effects of a vertical static magnetic field on the flow structure and
global transport properties of momentum and heat in liquid metal
Rayleigh-B\'enard convection are investigated. Experiments are conducted in a
cylindrical convection cell of unity aspect ratio, filled with the alloy GaInSn
at a low Prandtl number of $\mathit{Pr}=0.029$. Changes of the large-scale
velocity structure with increasing magnetic field strength are probed
systematically using multiple ultrasound Doppler velocimetry sensors and
thermocouples for a parameter range that is spanned by Rayleigh numbers of
$10^6 \le \mathit{Ra} \le 6\times 10^7$ and Hartmann numbers of $\mathit{Ha}
\le 1000$. Our simultaneous multi-probe temperature and velocity measurements
demonstrate how the large-scale circulation is affected by an increasing
magnetic field strength (or Hartmann number). Lorentz forces induced in the
liquid metal first suppress the oscillations of the large-scale circulation at
low $\mathit{Ha}$, then transform the one-roll structure into a cellular
large-scale pattern consisting of multiple up- and downwellings for
intermediate $\mathit{Ha}$, before finally expelling any fluid motion out of
the bulk at the highest accessible $\mathit{Ha}$ leaving only a near-wall
convective flow that persists even below Chandrasekhar's linear instability
threshold. Our study thus proves experimentally the existence of wall modes in
confined magnetoconvection. The magnitude of the transferred heat remains
nearly unaffected by the steady decrease of the fluid momentum over a large
range of Hartmann numbers. We extend the experimental global transport analysis
to momentum transfer and include the dependence of the Reynolds number on the
Hartmann number.
