FIGURE 1 - Total attenuation along vertical path through a clear atmosphere A: Average atmosphere with water vapour concentration of 7.5 g/mJ (1.5 cm equivalent depth) at the Earth’s surface B : Average atmosphere with water vapour concentration of 0.5 g/m3 (1 mm equivalent depth) at the Earth’s surface C: Dry atmosphere The most intense emission features are measured near the plane of the Milky Way resulting in brightness temperatures of about 100 K. In other regions of our Galaxy, brightness temperatures from normal gas range down to about 1 K. Abnormal gas, with high velocities, provides temperatures which range down to arbitrarily small values, and sensitivity requirements are severe. Special studies of normal gas, including for example, the measurement of Zeeman splitting to derive the magnetic field in the Galaxy, also place severe requirements on sensitivity; with modern equipment, integration times of 50 hours are sometimes necessary [Verschuur, 1971]. The zero level of the intensity scale is usually found by interpolation between the wings of the profiles which are considered to contain no hydrogen emission. This method of zero-level determination requires an addition of about 100 kHz, or more, which must be observed beyond the wings of the line itself. To eliminate the contribution of the continuum background emission, frequency-switching techniques are generally employed. Frequency switching over 1.5 or 2 MHz is typical, but observations of high-velocity gas may require switching over 3 to 5 MHz, usually on both the high and low frequency sides of the line. In addition to the emission arising from the hyperfine transition, absorption effects are also present and extensively studied. The frequency requirements for absorption studies are similar to those for emission studies. Absorption observations provide unique information on the spin temperature of hydrogen concentrations, and provide measures which are very sensitive to low temperature.
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