  Matter accreting onto the magnetic poles of a neutron star spreads under
gravity towards the magnetic equator, burying the polar magnetic field and
compressing it into a narrow equatorial belt. Steady-state, Grad-Shafranov
calculations with a self-consistent mass-flux distribution (and a
semi-quantitative treatment of Ohmic diffusion) show that, for $\Ma \gtrsim
10^{-5}\Msun$, the maximum field strength and latitudinal half-width of the
equatorial magnetic belt are $B_{\rm max} = 5.6\times 10^{15}
(\Ma/10^{-4}\Msun)^{0.32}$ G and $\Delta\theta = \max[3^{\circ}
(\Ma/10^{-4}\Msun)^{-1.5},3^{\circ} (\Ma/10^{-4}\Msun)^{0.5}(\dot{M}_{\rm
a}/10^{-8}\Msun {\rm yr}^{-1})^{-0.5}]$ respectively, where $\Ma$ is the total
accreted mass and $\dot{M}_{\rm a}$ is the accretion rate. It is shown that the
belt prevents north-south heat transport by conduction, convection, radiation,
and ageostrophic shear. This may explain why millisecond oscillations observed
in the tails of thermonuclear (type I) X-ray bursts in low-mass X-ray binaries
are highly sinusoidal: the thermonuclear flame is sequestered in the magnetic
hemisphere which ignites first. The model is also consistent with the
occasional occurrence of closely spaced pairs of bursts. Time-dependent,
ideal-magnetohydrodynamic simulations confirm that the equatorial belt is not
disrupted by Parker and interchange instabilities.
