Physical conditions in the partially ionized zone of the Orion nebula

We report the first reliable measurement of the nebular [O I] lambda 5577 e mission line in the Orion nebula. Diagnostic diagrams based on the [O I] la mbda lambda 5577, 6300, and [N I] XX3467, 5198, 5200 emission lines indicat e that the bulk of the nebular emission in the partially ionized zone of th e Orion nebula is produced in regions with low to moderate electron densiti es (2 x 10(3) - 4 x 10(4) cm(-3)) and with electron temperatures in the 890 0 to 12400 K range. This result implies that models based on emission origi nating in high-density partially ionized zones are not applicable to explai n the observed [Fe II] spectrum in the optical region.