The effect of temperature

The temperature of the atmosphere affects the flux levels in the near-IR, mid-IR and sub-mm quite differently. Table 1 gives a quantitative indication of these differences at four representative wavelengths by listing the value of the Planck function at four different temperatures. These are chosen as typical values for the atmosphere above mid-latitude sites such as Siding Spring (10C) and Mauna Kea (0C) and above the South Pole (C at the top of the inversion layer and C at the surface). Also listed are measured fluxes at the South Pole and the inferred emissivities for the atmosphere.

At these temperatures, the near-IR flux is on the Wien side of the Planck function, and hence shows the strongest dependence on temperature. Reductions of two orders of magnitude should be obtainable at the Pole at 2.4m. However, as Ashley et al. (1996) and Nguyen et al. (1996) report, the observed reduction is usually only by a factor of . The cause of this discrepancy appears to be airglow emission (Phillips et al., 1999). In fact, at wavelengths below 2.3m, airglow actually dominates over thermal emission from the atmosphere. On the other hand, at 3.8m the measured background levels are actually times less than at temperate sites, wheareas a simple estimate from the blackbody fluxes would suggest a drop of just a factor of 10. In the mid-IR the difference is even more marked, the measured reduction being a factor of 10 compared to an expectation of just 2-3 times based on the temperature drop alone. Clearly the drop in temperature from a mid-latitude site is not the only factor causing the reduction in background fluxes in the near- and mid-IR at the South Pole. In the sub-mm the temperature drop has only a small influence on the level of the background, and is not responsible for the 50% drop in flux at Pole compared to a mid-latitude site.

Table: The effect of temperature change on IR and sub-mm fluxes

Temperature	2.4m	3.8m	11m	350m
10C	4.3 x 10-2	2.6 x 101	7.0 x 103	1.4 x 102
0C	2.0 x 10-2	1.6 x 101	5.9 x 103	1.3 x 102
-40C	4.6 x 10-4	1.5 x 100	2.6 x 103	1.1 x 102
-70C	1.0 x 10-5	1.4 x 10-1	1.1 x 103	1.0 x 102
Typical Measured Flux at South Pole	1.0 x 10-4	1.0 x 10-1	2.0 x 101	6.0 x 101
Inferred Emissivity	20%	7%	1%	50%

Values of the Planck function in Jy/arcsecond² are listed for a range of temperatures and wavelengths, representative of temperate and Antarctic observing sites. Shown for comparison are measurements of the sky flux from the South Pole and the inferred emissivity for the atmosphere.

The sensitivity of the near-IR emission to temperature implies that even at an ideal site small variations in the temperature of the atmosphere will cause large changes in the background flux. Therefore near-IR astronomy requires a site that is not only cold, but also has as stable a temperature as possible. In contrast, sub-mm, and even mid-IR observations, are not so strongly affected by local variations in temperature.

© Copyright Astronomical Society of Australia 1997