  Photoheating of the gas in low-mass dark matter (DM) haloes prevents baryons
from cooling, leaving the haloes free of stars. Gas in these 'dark' haloes
remains exposed to the ultraviolet background (UVB), and so is expected to emit
via fluorescent recombination lines. We present a set of radiative transfer
simulations, which model dark haloes as spherical gas clouds in hydrostatic
equilibrium with a DM halo potential, and in thermal equilibrium with the UVB
at redshift z = 0. We use these simulations to predict surface brightnesses in
H-alpha, which we show to have a characteristic ring-shaped morphology for
haloes in a narrow mass range between 10^9.5 and 10^9.6 M_sun. We explore how
this emission depends on physical parameters such as the DM density profile and
the UVB spectrum. We predict the abundance of fluorescent haloes on the sky,
and discuss possible strategies for their detection. We demonstrate how
detailed observations of fluorescent rings can be used to infer the properties
of the haloes which host them, such as their density profiles and the
mass-concentration relation, as well as to directly measure the UVB amplitude.
