Anomalous refraction at radio wavelengths.
Abstract
Anomalous refraction is often seen at millimeter wavelengths with the IRAM 30-m telescope and at short centimeter wavelengths with the Effelsberg 100-m telescope. The events are characterized by apparent displacements of radio sources from their true positions, by typically 3″ in both azimuth and elevation, for a few seconds of time, with some rarer events up to 40″for 30 seconds of time. This anomalous refraction is caused by variations in the "wet" component of the refractive index, and corresponds to typical changes of 0.5 mm in the electrical pathlength over baselines of 30 to 100 meters. The effect is seen at all elevations, suggesting that the gradients in moist air are occurring in the lower troposphere, relatively close to the telescope. The events can occur in clear weather, more typically in the afternoon, and generally less frequently in winter, when the air temperature is below freezing. On a mountain site, the effect may be enhanced when the level of an inversion layer rises above the altitude of the telescope. The two-dimensional image motion has also been measured with the 30-m telescope. For integrations longer than one minute, the effective radio "seeing disk" may be 3″to 6″ when anomalous refraction is present, which may broaden the telescope beam, thereby changing the calibration parameters, and giving a false impression of the diameter of the source being observed. The effect is clearly related to the phase variations observed with interferometers.
- Publication:
-
URSI/IAU Symposium on Radio Astronomical Seeing
- Pub Date:
- 1990
- Bibcode:
- 1990ursi.symp...31D
- Keywords:
-
- Atmospheric Refraction;
- Radio Astronomy;
- Radio Wave Refraction;
- Seeing (Astronomy);
- Tropospheric Scattering;
- Millimeter Waves;
- Radio Telescopes;
- Troposphere;
- Astronomy;
- Earth Atmosphere: Refraction;
- Earth Atmosphere: MM Radiation;
- Earth Troposphere: Radio Telescopes