Absolute measurements of brightness temperature of distributed cosmic radio radiation in the meter-wavelength range

Abstract

UDC 532.164.4 Results of measurements of absolute brightness temperatures of distributed cosmic radio radiation in the wavelength range 0.8-1.5 m from regions with coordinates = ~.5 ~ and ~ = 17h-20 h are presented. Measurements were performed by the "null" method involving substitution of a "black" disk, which produced uncertainties from 9 to 12% in the brightness temperature measurements, significantly superior to uncertainties provided by existing radio-isophote mapsor those recalculated to the frequencies studied herein (z15-30%). The standardized "null" regions can be used as references in performing relative measurements. One of the major problems in the study of distributed cosmic radio radiation is the comparison of radio-isophotes, i.e., the spatial distribution of brightness temperature of such radiation. The greatest difficulties occur in establishing an absolute temperature scale, i.e., absolute values of brightness temperatures in reference regions, relative to which brightness temperatures can be calculated in relative measurements. The uncertainties of absolute measurements presented in available radio-isophote maps range from 15 to 30%. These uncertainties are produced mainly by the contribution of radiation received by side and back lobes of the radiotelescope antenna, which is difficult to calculate, as well as uncertainties in the antenna parameter values and the thermal calibration of the receiving apparatus. Relative temperature values indicated on the isophotes are known tomuch greater accuracy, since in relative measurements it is possible to create conditions under which the contribution to antenna temperature of interfering radio radiation entering the side and rear lobes remains constant during the measurement process and can be eliminated. The brightness temperatures of "reference" regions T~ ef should then be close to minimal so that uncertainties in their absolute measurement will have less effect on uncertainties in brightness temperature values measured relative to T~ of. In the meter wavelength range minimum brightness temperatures are about 300~ The possibility then arises (see [i]) of measuring such temperatures by the null method of substituting a "black" disk, the brightness temperature of which is close to that of the surrounding medium, i.e., 300~ The "black" disk method [2] is widely used in radio astronomy and antenna technology. The essence of the method may be described as follows. A disk coated with an "absolute" absorbing material at temperature T o is used as a radio radiation brightness temperature reference. The radiation from the disk is first measured (plane of the disk perpendicular to the antenna pattern), and then the disk is removed from the pattern in a manner such that no construction elements are added or disappear. The antenna remains fixed during the measurements, only the disk appearing and disappearing in the direction of the'main lobe. The temperature increment of the antenna is then [2]