  Flare ribbons are bright manifestations of flare energy dissipation in the
lower solar atmosphere. For the first time, we report on high-resolution
imaging spectroscopy observations of flare ribbons situated off-limb in the
H$\beta$ and Ca II 8542 {\AA} lines and make a detailed comparison with
radiative hydrodynamic simulations. Observations of the X8.2-class solar flare
SOL2017-09-10T16:06 UT obtained with the Swedish Solar Telescope reveal bright
horizontal emission layers in H$\beta$ line wing images located near the
footpoints of the flare loops. The apparent separation between the ribbon
observed in the H$\beta$ wing and the nominal photospheric limb is about 300 -
500 km. The Ca II 8542 {\AA} line wing images show much fainter ribbon
emissions located right on the edge of the limb, without clear separation from
the limb. RADYN models are used to investigate synthetic spectral line profiles
for the flaring atmosphere, and good agreement is found with the observations.
The simulations show that, towards the limb, where the line of sight is
substantially oblique with respect to the vertical direction, the flaring
atmosphere model reproduces the high contrast of the off-limb H$\beta$ ribbons
and their significant elevation above the photosphere. The ribbons in the Ca II
8542 {\AA} line wing images are located deeper in the lower solar atmosphere
with a lower contrast. A comparison of the height deposition of electron beam
energy and the intensity contribution function shows that the H$\beta$ line
wing intensities can be an useful tracer of flare energy deposition in the
lower solar atmosphere

Solar and Stellar Astrophysics