  We present results from an Halpha emission-line survey in a one square degree
area centered on the Orion Nebula Cluster, obtained with the Wide Field Grism
Spectrograph-2 on the 2.2-meter telescope of the University of Hawaii. We
identified 587 stars with Halpha emission, 99 of which, located mainly in the
outer regions of the observed area, have not appeared in previous Halpha
surveys. We determined the equivalent width (EW) of the line, and based on it
classified 372 stars as classical T Tauri stars (CTTS) and 187 as weak line T
Tauri stars (WTTS). Simultaneous r', i' photometry indicates a limiting
magnitude of r' ~ 20 mag, but the sample is incomplete at r' > 17 mag. The
surface distribution of the Halpha emission stars reveals a clustered and a
dispersed population, the former consisting of younger and more massive young
stars than the latter. Comparison of the derived EWs with those found in the
literature indicates variability of the Halpha line. We found that the typical
amplitudes of the variability are not greater than a factor 2-3 in the most
cases. We identified a subgroup of low-EW stars with infrared signatures
indicative of optically thick accretion disks. We studied the correlations
between the equivalent width and other properties of the stars. Based on
literature data we examined several properties of our CTTS and WTTS subsamples
and found significant differences in mid-infrared color indices, average
rotational periods, and spectral energy distribution characteristics of the
subsamples.
