  We assemble a sample of disk star-forming galaxies from the Sloan Digital Sky
Survey Data Release 7, studying the structure of H$\alpha$ emission lines,
finding a large fraction of this sample contains boxy H$\alpha$ line profiles.
This fraction depends on galaxy physical and geometric parameters in the
following way: (1) it increases monotonically with star formation rate per unit
area ($\Sigma_{\rm SFR}$), and stellar mass ($M_*$), with the trend being much
stronger with $M_*$, from $\sim$0% at $M_*=10^{10}M_{\odot}$ to about 50% at
$M_*=10^{11}M_\odot$; (2) the fraction is much smaller in face-on systems than
in edge-on systems. It increases with galaxy inclination ($i$) while $i <
60\,^{\circ}$ and is roughly a constant of 25% beyond this range; (3) for the
sources which can be modeled well with two velocity components, blueshifted and
redshifted from the systemic velocity, these is a positive correlation between
the velocity difference of these two components and the stellar mass, with a
slope similar to the Tully-Fisher relation; (4) the two components are very
symmetric in the mean, both in velocity and in amplitude. The four findings
listed above can be understood as a natural result of a rotating galaxy disk
with a kpc-scale ring-like H$\alpha$ emission region.
