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Attenuation of radiation at wavelengths of 1.25 and 2.0 mm
ATTENUATION
OF
RADIATION
WAVELENGTHS
OF
1.25 AND
V. F. ZABOLOTNIY’,
I.
AT 2.0mm
A. ISKHAKOV’, A. V. SOKOLOV~ and E. V. SUKHONIN~
‘State Sternberg Astronomical Institute, University Avenue 13, Moscow. U.S.S.R., and ‘Institute of Radioengineering and Electronics of U.S.S.R. Academy of Sciences, Marx Avenue Moscow, 103907, U.S.S.R.
18,
Abstract -The results of mcasurcments of radiation attenuation at wavelengths of I.25 and 2.0 mm in clouds. carried out in the Crimea. using the RT-22 radiotelcscopc arc prcscnted. This telescope has an antenna diameter of 22 m and is coupled to a Fabry Perot lntcrfcromctcr and a liquid-helium cooled n-type InSb detector.
During recent years, the absorption properties of the atmosphere at millimetre (mm) and submillimetre (sub-mm) wavelengths both in the presence and absence of clouds has come to be of great interest. Theoretically this problem is very difficult to solve, because there are not yet available models of clouds which correspond to the properties of real clouds. Therefore, experiments play an important role in the investigations of absorption properties of the atmosphere. At mm and sub-mm, however, data on zenith attenuation in clouds and in the pure atmosphere are few in number. This work is devoted to the experimental investigations of zenith attenuation in the real atmosphere at wavelengths of 1.2 and 2 mm in different areas of the U.S.S.R. The results of these investigations are compared with those obtained earlier at 0.45, 0.73 and 1.2 mm.‘,3 EXPERIMENTAL
EQUIPMENT
Some data at 0.45, 0.73. 1.2 mm have been obtained using a I m radiotelescope. As a receiving element, an n-InSb detector working at liquid helium temperatures has been used. Resolution was provided using a grating spectrometer and was about 1 cm-’ at 1 mm. The sensitivity of the radiometer was 0.3 K in the range 0.4-1.6 mm using a time constant of 1 sec. The width of the antenna diagram of the telescope at half power was 4 min of arc.(‘.4) Experimental equipment at 1.2 and 2mm consisted of the RT-22 radiotelescope (Crimea Astrophysical Observatory, Simiez) and receiving device (radiometer with n-InSb detector cooled to liquid helium temperature and Fabry-Perot interferometer) with 0.1 K sensitivity using a time constant of 1 sec. The 12 m parabolical antenna of this telescope had a field of view of 1.8 x 1.3 min of arc. METHODS The attenuation of mm ing of solar radiation and proved to be more exact, on the Sun. In this case, as follows :
OF
MEASUREMENTS
and sub-mm waves has been determined both with the receivwith the receiving of atmospheric radiation. The first method although its use requires the necessity of a precise guidance atmospheric absorption due to water vapor can be defined In PI - p,* pz - pq2
/-Ii20 = ~2
set
vz
-
p1
set
vl
(1)
where P, and P, are the intensities of received solar radiation at zenith angles ‘pr and ‘pz respectively; P,, and P,,,-intensities of sky radiation at the same values of zenith angles; p1 and p,--absolute humidity values near the Earth’s surface at the time when the values of P, and P2 were measured. During these measurements, the values of temperature, humidity and pressure have been recorded. Besides that, for theoretical analysis of measured results, radio-sonde measurements have been used. 815
V. F. ZAEIOLOTNIY rt al.
816
In some cases. when attenuation is measured in the clear atmosphere and clouds. one can use the method of vertical scanning of the antenna. The total zenith attenuation may be defined from the formula: T0 - T, In ____ T, - T2 (ptt9 + r““),, =
(2)
set q1
-
set q2
where Tl, T, and To are the brightness temperatures if the zenith angles of antenna are ‘pr. v)~ and if it looks at same region which has brightness temperature T&temperature of the air near the earth surface. In the case Tct = 0, Eqn 2 gives us the attenuation of radiation in pure atmosphere. In the case of broken clouds the brightness temperatures T1 and T2 have been measured in an atmosphere with and without cloud and for the same zenith angle cp. Values of cloud attenuation will be determined from: 1 q-b = ___
set
ln
(I,
To - Tt 7-0 -
(3)
T2
where To is the brightness temperature if the antenna is looking at the region which has the same temperature as air near the Earth’s surface. If solar radiation propagates through the cloud, the zenith attenuation in it can be found. from : r;;
1 = lnc set (p P2
(4)
where PI and P2 are the intensities of the received signal in cloud and at these zenith angles cp. The error of attenuation measurements in clouds 15:j;. RESULT
OF
without it was about
MEASUREMENT
The analysis of attenuation data in water vapour showed that dependence of total attenuation from Earth’s surface on humidity is linear. Such regularity was observed in transparency windows at wavelengths of 0.45, 0.73 and 1.2 mm (see Fig. 1). It is interesting that the measured values of total zenith attenuation in water vapour are in a good agreement with calculation based on data for height profiles of temperature, pressure and humidity obtained from radio-sonde measurements. From the measurements one can conclude that total zenith attenuation in water vapor has a continuous variation depending on the humidity. For example, if humidity near the Earth’s surface changed from 10.6 to 15.4 g/m3 zenith attenuation had the following
o/o;.25 / O/O
2-
96 /
P.
Fig. I. Dependence
1 Ii 34566
2
4 me3
of zenith attenuation on thickness of Earth dity near the Earth’s surface.
atmosphere
from absolute
humi-
Attenuation
of radiation
Table Type of clouds
Zenith 0.45
AC Cu cu tong
1.5 17.5 51
attenuation 0.73
817
I. (dB) at wavelengths I.2
0.6 10 20
0.36 I.6 19
(mm) 2.0 0.16 9.0
values: 4.9-7.8 dB at 1.2 mm and 3.4-4.4 dB at 2 mm. This result is confirmed by other data obtained in this work.“.‘,” Cloud attenuation changes very strongly depending on wavelength, type of cloud and season. It is established by the result of attenuation investigations in different seasons that zenith attenuation in winter is 2-3 times less than in summer. Besides, in winter powerful Cu and Cu tong clouds do not occur, which in summer cause strong attenuation of radiation. Mean values of attenuation measured during the summer at 0.45, 0.73, 1.2 and 2 mm and for an air temperature near the Earth’s surface, 25”C, are given in Table 1. The above-mentioned results of measurements are of quantitative parameters which determine only the order of zenith attenuation values in clouds, including the effects of microstructure in time and space and the site distribution function, and so data about the physical state of particles in clouds will require in future the development of statistical methods of attenuation description in clouds. REFERENCES i Elektrouika. I. VARDANYAN. A. S.. I. A. ISKHAKOV, A. V. SOKOLOV & E. V. SUKHONIN. Radiotrkhnika 18. 217 (1973). ? WRIXON. G. T. Conf. Proc. 4th Eur. Microwave Conf. Montreux, Surbitan, p. 222 (1974). ; : SOKOLOV, A. V.. E. V. SUKHONIN & 1. A. ISKHAKOV. Proc. 4th Colloquium on Microwave Communication, Budapest. 2430 June (1974). 4. VARDANYAN. A. S.. A. N. VISTAVKIN. I. A. ISKHAKOV, V. N. LISTVIN. N. A. SAVICH & A. V. SOKOLOV. Asrrm. J. 49, 986 (1972). 5. AFONCHENKOV it al.. Astron. J. 53, 178 (1973). 6. ULABY. F. T. & A. W. STRAITON IEEE Tram. Am. Propag. AP-17. 337 (1969). 7. KISLYAKOV. A. G. & K. C. STANKEV~CH I~e.sti~a Vc’z’oc, Radiophysica. IO, 1244 (1967).
OF
RADIATION
WAVELENGTHS
OF
1.25 AND
V. F. ZABOLOTNIY’,
I.
AT 2.0mm
A. ISKHAKOV’, A. V. SOKOLOV~ and E. V. SUKHONIN~
‘State Sternberg Astronomical Institute, University Avenue 13, Moscow. U.S.S.R., and ‘Institute of Radioengineering and Electronics of U.S.S.R. Academy of Sciences, Marx Avenue Moscow, 103907, U.S.S.R.
18,
Abstract -The results of mcasurcments of radiation attenuation at wavelengths of I.25 and 2.0 mm in clouds. carried out in the Crimea. using the RT-22 radiotelcscopc arc prcscnted. This telescope has an antenna diameter of 22 m and is coupled to a Fabry Perot lntcrfcromctcr and a liquid-helium cooled n-type InSb detector.
During recent years, the absorption properties of the atmosphere at millimetre (mm) and submillimetre (sub-mm) wavelengths both in the presence and absence of clouds has come to be of great interest. Theoretically this problem is very difficult to solve, because there are not yet available models of clouds which correspond to the properties of real clouds. Therefore, experiments play an important role in the investigations of absorption properties of the atmosphere. At mm and sub-mm, however, data on zenith attenuation in clouds and in the pure atmosphere are few in number. This work is devoted to the experimental investigations of zenith attenuation in the real atmosphere at wavelengths of 1.2 and 2 mm in different areas of the U.S.S.R. The results of these investigations are compared with those obtained earlier at 0.45, 0.73 and 1.2 mm.‘,3 EXPERIMENTAL
EQUIPMENT
Some data at 0.45, 0.73. 1.2 mm have been obtained using a I m radiotelescope. As a receiving element, an n-InSb detector working at liquid helium temperatures has been used. Resolution was provided using a grating spectrometer and was about 1 cm-’ at 1 mm. The sensitivity of the radiometer was 0.3 K in the range 0.4-1.6 mm using a time constant of 1 sec. The width of the antenna diagram of the telescope at half power was 4 min of arc.(‘.4) Experimental equipment at 1.2 and 2mm consisted of the RT-22 radiotelescope (Crimea Astrophysical Observatory, Simiez) and receiving device (radiometer with n-InSb detector cooled to liquid helium temperature and Fabry-Perot interferometer) with 0.1 K sensitivity using a time constant of 1 sec. The 12 m parabolical antenna of this telescope had a field of view of 1.8 x 1.3 min of arc. METHODS The attenuation of mm ing of solar radiation and proved to be more exact, on the Sun. In this case, as follows :
OF
MEASUREMENTS
and sub-mm waves has been determined both with the receivwith the receiving of atmospheric radiation. The first method although its use requires the necessity of a precise guidance atmospheric absorption due to water vapor can be defined In PI - p,* pz - pq2
/-Ii20 = ~2
set
vz
-
p1
set
vl
(1)
where P, and P, are the intensities of received solar radiation at zenith angles ‘pr and ‘pz respectively; P,, and P,,,-intensities of sky radiation at the same values of zenith angles; p1 and p,--absolute humidity values near the Earth’s surface at the time when the values of P, and P2 were measured. During these measurements, the values of temperature, humidity and pressure have been recorded. Besides that, for theoretical analysis of measured results, radio-sonde measurements have been used. 815
V. F. ZAEIOLOTNIY rt al.
816
In some cases. when attenuation is measured in the clear atmosphere and clouds. one can use the method of vertical scanning of the antenna. The total zenith attenuation may be defined from the formula: T0 - T, In ____ T, - T2 (ptt9 + r““),, =
(2)
set q1
-
set q2
where Tl, T, and To are the brightness temperatures if the zenith angles of antenna are ‘pr. v)~ and if it looks at same region which has brightness temperature T&temperature of the air near the earth surface. In the case Tct = 0, Eqn 2 gives us the attenuation of radiation in pure atmosphere. In the case of broken clouds the brightness temperatures T1 and T2 have been measured in an atmosphere with and without cloud and for the same zenith angle cp. Values of cloud attenuation will be determined from: 1 q-b = ___
set
ln
(I,
To - Tt 7-0 -
(3)
T2
where To is the brightness temperature if the antenna is looking at the region which has the same temperature as air near the Earth’s surface. If solar radiation propagates through the cloud, the zenith attenuation in it can be found. from : r;;
1 = lnc set (p P2
(4)
where PI and P2 are the intensities of the received signal in cloud and at these zenith angles cp. The error of attenuation measurements in clouds 15:j;. RESULT
OF
without it was about
MEASUREMENT
The analysis of attenuation data in water vapour showed that dependence of total attenuation from Earth’s surface on humidity is linear. Such regularity was observed in transparency windows at wavelengths of 0.45, 0.73 and 1.2 mm (see Fig. 1). It is interesting that the measured values of total zenith attenuation in water vapour are in a good agreement with calculation based on data for height profiles of temperature, pressure and humidity obtained from radio-sonde measurements. From the measurements one can conclude that total zenith attenuation in water vapor has a continuous variation depending on the humidity. For example, if humidity near the Earth’s surface changed from 10.6 to 15.4 g/m3 zenith attenuation had the following
o/o;.25 / O/O
2-
96 /
P.
Fig. I. Dependence
1 Ii 34566
2
4 me3
of zenith attenuation on thickness of Earth dity near the Earth’s surface.
atmosphere
from absolute
humi-
Attenuation
of radiation
Table Type of clouds
Zenith 0.45
AC Cu cu tong
1.5 17.5 51
attenuation 0.73
817
I. (dB) at wavelengths I.2
0.6 10 20
0.36 I.6 19
(mm) 2.0 0.16 9.0
values: 4.9-7.8 dB at 1.2 mm and 3.4-4.4 dB at 2 mm. This result is confirmed by other data obtained in this work.“.‘,” Cloud attenuation changes very strongly depending on wavelength, type of cloud and season. It is established by the result of attenuation investigations in different seasons that zenith attenuation in winter is 2-3 times less than in summer. Besides, in winter powerful Cu and Cu tong clouds do not occur, which in summer cause strong attenuation of radiation. Mean values of attenuation measured during the summer at 0.45, 0.73, 1.2 and 2 mm and for an air temperature near the Earth’s surface, 25”C, are given in Table 1. The above-mentioned results of measurements are of quantitative parameters which determine only the order of zenith attenuation values in clouds, including the effects of microstructure in time and space and the site distribution function, and so data about the physical state of particles in clouds will require in future the development of statistical methods of attenuation description in clouds. REFERENCES i Elektrouika. I. VARDANYAN. A. S.. I. A. ISKHAKOV, A. V. SOKOLOV & E. V. SUKHONIN. Radiotrkhnika 18. 217 (1973). ? WRIXON. G. T. Conf. Proc. 4th Eur. Microwave Conf. Montreux, Surbitan, p. 222 (1974). ; : SOKOLOV, A. V.. E. V. SUKHONIN & 1. A. ISKHAKOV. Proc. 4th Colloquium on Microwave Communication, Budapest. 2430 June (1974). 4. VARDANYAN. A. S.. A. N. VISTAVKIN. I. A. ISKHAKOV, V. N. LISTVIN. N. A. SAVICH & A. V. SOKOLOV. Asrrm. J. 49, 986 (1972). 5. AFONCHENKOV it al.. Astron. J. 53, 178 (1973). 6. ULABY. F. T. & A. W. STRAITON IEEE Tram. Am. Propag. AP-17. 337 (1969). 7. KISLYAKOV. A. G. & K. C. STANKEV~CH I~e.sti~a Vc’z’oc, Radiophysica. IO, 1244 (1967).