The Structure of NGC 1976 in the Radio Range, v2
  High angular resolution radio continuum images of NGC 1976 (M42, Orion A) at frequency=330 MHz (wavelength=91 cm), 1.5 GHz (20 cm) and 10.6 GHz (2.8 cm), have been aligned, placed on a common grid, smoothed to common resolutions of 80" (=0.16 pc at 420 pc) and 90" (=0.18 pc) and compared on a position-by-position basis. The results are not consistent with a single value of Te. The best fit to the continuum data is a multi-layer model based on radio recombination line (RRL) data with a monotonic variation, from Te=8500K in the higher intensity, more compact region at the rear of NGC 1976 to Te =6000K in the low intensity, extended region in the foreground. An estimate of temperature fluctuations toward the peak from this model yields t2=0.003. This is a factor of 10 lower than fluctuation values from optical collisionally excited line data. 
T. L. Wilson, S. Casassus, Katie M. Keating
astro-ph.GA