  We present constraints on the structure of the chromosphere from observations
of the Ca II H line profile near and off the solar limb. We obtained a data set
of the Ca II H line in a field of view extending 20" across the limb. We
analyzed the spectra for the properties of off-limb spectra. We used tracers of
the Doppler shifts, such as the location of the absorption core, the ratio of
the two emission peaks H2V and H2R, and intensity images at a fixed wavelength.
The average off-limb profiles show a smooth variation with increasing limb
distance. The line width increases up to a height of about 2 Mm above the limb.
The profile shape is fairly symmetric with nearly identical H2V and H2R
intensities; at a height of 5 Mm, it changes into a single Gaussian without
emission peaks. We find that all off-limb spectra show large Doppler shifts
that fluctuate on the smallest resolved spatial scales. The variation is more
prominent in cuts parallel to the solar limb than on those perpendicular to it.
As far as individual structures can be unequivocally identified at our spatial
resolution, we find a specific relation between intensity enhancements and
Doppler shifts: elongated brightenings are often flanked all along their
extension by velocities in opposite directions. The average off-limb spectra of
Ca II H present a good opportunity to test static chromospheric atmosphere
models because they lack the photospheric contribution that is present in
disk-center spectra. We suggest that the observed relation between intensity
enhancements and Doppler shifts could be caused by waves propagating along the
surfaces of flux tubes: an intrinsic twist of the flux tubes or a wave
propagation inclined to the tube axis would cause a helical shape of the
Doppler excursion, visible as opposite velocity at the sides of the flux tube.
