  We have systematically investigated ultraviolet (UV) emission from molecular
hydrogen (H$_{2}$) using the Interface Region Imaging Spectrometer (IRIS),
during three X-ray flares of C5.1, C9.7 and X1.0 classes on Oct. 25, 2014.
Significant emission from five H$_{2}$ spectral lines appeared in the flare
ribbons, interpreted as photo-excitation (fluorescence) due to the absorption
of UV radiation from two Si IV spectral lines. The H$_{2}$ profiles were broad
and consisted of two non-stationary components in red and in the blue wings of
the line in addition to the stationary component. The red (blue) wing
components showed small redshifts (blue shifts) of ~5-15 km s$^{-1}$ (~5-10 km
s$^{-1}$). The nonthermal velocities were found to be ~5-15 km s$^{-1}$. The
interrelation between intensities of H$_{2}$ lines and their branching ratios
confirmed that H$_{2}$ emission formed under optically thin plasma conditions.
There is a strong spatial and temporal correlation between Si IV and H$_{2}$
emission, but the H$_{2}$ emission is more extended and diffuse, further
suggesting H$_{2}$ fluorescence, and - by analogy with flare ''back-warming''
providing a means to estimate the depth from which the H$_{2}$ emission
originates. We find that this is 1871$\pm$157 km and 1207$\pm$112 km below the
source of the Si IV emission, in two different ribbon locations.

Solar and Stellar Astrophysics