Observations of the intensity distribution near the solar limb at 2.43 and 22.5/t, show the absence of limb brightening to within 1 or 2 arc sec of the limb. Observations at 1.2 mm indicate limb brightening at this wavelength. These results are compared with the Utrecht Reference Photosphere and with existing data on the solar flux in the millimeter range, and suggest that the temperature minimum is broad and extends above rs000 --2 β’ 10-L A sharp rise of temperature is required above zs000 = 10 -5.
In this paper we shall discuss a series of investigations that we have carried out during the past year to study the solar intensity distribution in the far infrared and millimeter regions. Part of the work has been reported previously (NoYzs, GINGERICH, and GOLDBERG, 1966;NOYES, 1966; NOYES et al., 1966), but some of the results are new.
In February and March of 1966 we made preliminary measurements of the solar limb intensity at 24.3/t in an attempt to see the brightening predicted by several current models of the solar atmosphere (e.g., NoYEs, GINGER~CH and GOLDBERG, 1966). The results were negative to within the 10-arcsec resolution limit of our measurements. This observation is described in Section 1.
Next we measured the infrared limb profile with higher spatial resolution during the November 12, 1966, total solar eclipse. This experiment has indicated that limb darkening at 22.5 # persists to the limit of our resolution, about 1 or 2 arcsec from the limb. We discuss the experiment and preliminary results in Section 2.
At the same time, we have analyzed several drift curves of the sun made at a wavelength of 1.2 mm, and found definite evidence for limb brightening at this wavelength. We describe these data and their analysis in Section 3.
Finally, in Section 4 we discuss the implications of these various results for the temperature structure of the low chromosphere.
1. Measurements of the Solar Limit Profile at 24.3/z
In February and March of 1966 we made several measurements of the solar limb intensity distribution, using the 5-foot f/14 telescope of the Lunar and Planetary