  A flare kernel associated with a C4 class flare was observed in a spectral
window including the He I triplet 10830 A and Si I 10827 A with a
spectropolarimeter on the Domeless Solar Telescope at Hida Observatory on
August 9th, 2015. Observed Stokes profiles of the He I triplet in the flare
kernel in its post-maximum phase are well reproduced through inversions
considering the Zeeman and the Paschen-Back effects with a three slab model of
the flare kernel, in which two slabs having up and downward velocities produce
emissions and one slab produces an absorption. The magnetic field strength
inferred from the emission components of the He I line is 1400 G, which is
significantly stronger than 690 G that is observed at the same location in the
same line 6.5 hours before the flare. In addition, photospheric magnetic field
vector derived from the Si I10827 A is similar to that of the flare kernel. To
explain this results, we suggest that the emission in the He I triplet during
the flare is produced in the deep layer, around which bombardment of
non-thermal electrons leads to the formation of a coronal temperature plasma.
Assuming a hydrogen column density at the location where the He I emissions are
formed, and a power-law index of non-thermal electron energy distribution, we
derived the low-energy cutoff of the non-thermal electron as 20 - 30 keV, which
is consistent with that inferred from hard X-ray data obtained by RHESSI.
