  We present high-resolution long-slit spectroscopy of a Balmer-dominated shock
in the northeastern limb of the Cygnus Loop with the Subaru high dispersion
spectrograph. By setting the slit angle along the shock normal, we investigate
variations of the flux and profile of the H-alpha line from preshock to
postshock regions with a spatial resolution of about 4 times 10^{15} cm. The
H-alpha line profile can be represented by a narrow (28.9+/-0.7 km/s) Gaussian
in a diffuse region ahead of the shock, i.e., a photoionization precursor, and
narrow (33.1+/-0.2 km/s) plus broad (130-230 km/s) Gaussians at the shock
itself. We find that the width of the narrow component abruptly increases up to
33.1+/-0.2 km/s, or 38.8+/-0.4 km/s if we eliminate projected emission
originating from the photoionization precursor, in an unresolved thin layer (<
4 times 10^{15} cm at a distance of 540 pc) at the shock. We show that the
sudden broadening can be best explained by heating via damping of Alfven waves
in a thin cosmic-ray precursor, although other possibilities are not fully
ruled out. The thickness of the cosmic-ray precursor in the Cygnus Loop (a soft
gamma-ray emitter) is an order of magnitude thinner than that in Tycho's Knot g
(a hard gamma-ray emitter), which may be caused by different energy
distribution of accelerated particles between the two sources. In this context,
systematic studies might reveal a positive correlation between the thickness of
the cosmic-ray precursor and the hardness of the cosmic-ray energy
distribution.
