Average Brightness Temperature Research Articles

Simulated and observed 37 GHz emission over Africa

A model for simulating polarized 37-GHz microwave emissions from vegetated surfaces is proposed. An equation expressing the radiative transfer is solved using a two-point Gauss quadrature method. Surface temperature, reflectance, normalized difference, and polarized brightness temperatures are obtained from processed multispectral satellite data. The model simulations are evaluated against satellite data collected between January 1982 and December 1983 along a transect going from rain forest to hot desert over Africa (0 deg to 20 deg N, 11 deg E). It is found that the temporal variation of the average brightness temperature matches closely the surface temperature. The seasonal variation of the polarization difference is confined within the latitude band of 8 deg to 18 deg N, but the normalized difference continues to show seasonal variation south of 8 deg N. The relations between the polarization difference and the normal difference and between the polarization difference and the reflectance are considered to be nonlinear and almost linear, respectively.

Experimental data for separation of vegetation and soil and estimation of soil moisture using passive microwaves

Abstract Microwave radiometer measurements of soil moisture content were made over bare and vegetated fields with dual polarized microwave radiometers at 1·55GHz (L-band) and 19·1 GHz (K.-band). Two typical Indian crops Bazra and Gawar have been studied. The bare field measurements were used to investigate the effect of soil texture on sensitivity of a radiometer to soil moisture and for soil moisture sampling depth. It is found that expression of soil moisture as available moisture content in the soil can minimize the texture effect. The estimated soil moisture sampling depth for L-band is 2-5 cm, while for K-band it is less than 2 cm. The vegetation cover affects the sensitivity of the radiometer to soil moisture. This effect is more pronounced the denser the vegetation and higher the frequency of observation. The measured polarization factor over a vegetated field at L-band was found to be appreciably reduced compared to that over a bare field. The difference between normalized brightness temperature f...

Determining microwave brightness temperatures from precipitating horizontally finite and vertically structured clouds

Microwave radiances that may be measured from satellite‐borne radiometers operating at 37 GHz have been computed as a function of rainfall rates from horizontally finite precipitating clouds that contain both ice and liquid hydrometeors. It is found that precipitating ice at the top of the cloud depresses brightness temperatures significantly. Detailed comparison between finite clouds and the equivalent sections of plane‐parallel clouds are made. Footprint averaged brightness temperatures from finite clouds are found to deviate considerably from the sectioned plane‐parallel approximation, especially for large rain rates. Better agreement is achieved if a plane‐parallel source function is used in the finite cloud model. The effect of shape and orientation of the precipitating cells with respect to the satellite has also been considered. It is found that the largest errors introduced by plane‐parallel theory besides the footprint‐filling errors are encountered when precipitating cells have large fractions of their surface below the freezing level or contain large amounts of ice.

Microwave remote sensing of plant water stress

The sensitivity of microwave (MW) emission to physical conditions of vegetation has been assessed by means of ground-based microwave and infrared radiometers. Measurements on corn and wheat have shown an inverse correlation between the normalized brightness temperature ( T N) from the Ka band (36 GHz) and the atmospheric water vapor pressure (VP) at the top of vegetation. From this observation, we show that a crop water stress index can be calculated by means of down-looking MW sensors, provided air temperature is known. A polarization index (PI) dependent only on microwave measurements was shown to be related to crop water stress.

Far-infrared brightness temperature of Saturn's disk and rings

We present far-infrared observations of Saturn in the wavelength band 76–116 μm, using a balloon-borne 75-cm telescope launched on 10 December 1980 from Hyderabad, India, when B′, the Saturnicentric latitude of the Sun, was 4°.3. Normalizing with respect to Jupiter, we find the average brightness temperature of the disk-ring system to be 90 ± 3° K. Correcting for the contribution from rings using experimental information on the brightness temperature of rings at 20 μm, we find T D, the brightness temperature of the disk, to be 96.9 ± 3.5° K. The systematic errors and the correction for the ring contribution are small for our observations. We, therefore, make use of our estimate of T D and earlier observations of Saturn when contribution from the rings was large and find that for wavelengths greater than 50 μm, there is a small reduction in the ring brightness temperature as compared to that at 20 μm.

Estimates of brightness temperatures from scanning radiometer data

In the analysis of antenna temperature maps of the earth obtained by satellite-borne microwave radiometers, estimates of brightness temperatures or averages of brightness temperatures over areas considerably smaller than the region sensed at a given position of the radiometer antenna are often needed. An application of the Backus-Gilbert methodology is made to obtain an objective criterion of the best resolution (in a least squares sense) obtainable from a given system and to investigate the trade-off between resolution and noise in the derived average brightness temperatures. The mathematically related problem of simultaneously analyzing antenna temperature measurements made at different frequencies by antennas with noncoincident antenna patterns is also considered.

Response : Sea Straits and Glacial Periods in the Red Sea

Anomalous averaged infrared brightness temperatures of the Galilean satellites of Jupiter reported by Gross (1975) are rejected as falsely conceived and lacking physical reality. It is argued that the calculations of equilibrium temperatures should be corrected, whereupon predictions would be in satisfactory agreement with observations, in conformity with the radiometric method of determining the diameters of asteroids and satellites. The IR irradiance and the related disk-averaged brightness temperature for the spectral band are recommended as more relevant. Attention is drawn to some interesting discrepancies between calculated and observed temperatures of the Jovian satellites which merit further investigation.

Galilean Satellites: Anomalous Temperatures Disputed

Anomalous averaged infrared brightness temperatures of the Galilean satellites of Jupiter reported by Gross (1975) are rejected as falsely conceived and lacking physical reality. It is argued that the calculations of equilibrium temperatures should be corrected, whereupon predictions would be in satisfactory agreement with observations, in conformity with the radiometric method of determining the diameters of asteroids and satellites. The IR irradiance and the related disk-averaged brightness temperature for the spectral band are recommended as more relevant. Attention is drawn to some interesting discrepancies between calculated and observed temperatures of the Jovian satellites which merit further investigation.

A Theory of Type I Solar Radio Bursts

(Proc. Astron. Soc. Australia). A theory is developed to account for the observed properties of type I storm bursts in terms of plasma radiation – that is, electromagnetic radiation at the electron plasma frequency resulting from the non-linear scattering of electron plasma waves on plasma ions. Now the average brightness temperature of a type I source is greater than 109 K, or even higher if, because of coronal scattering, the apparent source size is larger than the true source size. For brightness temperatures as high as 109 K the non-linear scattering must be of the induced kind in which electromagnetic radiation below the frequency of the electron plasma waves is amplified. For such radiation to be strongly circularly polarized in the o-mode, as observed in type I bursts, requires that the amplification be more effective in the o-mode than in the x-mode (Figure 1). This is found to be so for plasma waves excited by electrons travelling parallel to the magnetic field. The electric field of the plasma waves is then also parallel to the magnetic field. The non-linear scattering is more efficient for that magnetoionic mode which has the greater component of electric field in the same direction. This mode is the o-mode.

Absolute measurements and computed values for Martian irradiance between 10.5 and 12.5 μm

The combination of seasonal and orbital changes in Martian insolation result in complex latitude dependent surface temperature variations that effect the total radiance of the planet as seen from the earth. These surface temperature variations have been calculated, based upon a computer simulation of the thermal environment of the planet. The temperature variations are then integrated to yield the total radiance of the planet as seen from the earth as a function of time. The absolute radiance of Mars was measured on April 4, 1971, with a balloon-borne radiometer system operating in the wavelength range between 10.5 and 12.5 μm. The average brightness temperature of the Mars disk determined from these measurements was 254°K with a 1 σ error of 4°K.

A recalibration of the quiet Sun millimeter spectrum based on the Moon as an absolute radiometric standard

The solar millimeter continuum between 1 and 20 mm is recalibrated using observations of the average lunar brightness temperature at the center of lunar disk and new Moon brightness temperatures. The solar data are placed on a common scale according to the average lunar brightness temperature distribution proposed by Linsky. A least-squares parabolic regression curve is proposed for the solar millimeter continuum. A small departure from this regression curve near 8 mm may indicate the existence of an absorption feature.

The Moon as a Proposed Radiometric Standard for Microwave and Infrared Observations of Extended Sources

view Abstract Citations (75) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Moon as a Proposed Radiometric Standard for Microwave and Infrared Observations of Extended Sources Linsky, Jeffrey L. Abstract Measured values of the average midnight and morning terminator infrared brightness temperatures of the central portion of the lunar disk can quite accurately determine the mean surface temperature despite likely horizontal and vertical inhomogeneities of the thermal properties of the lunar soil. These data together with laboratory measurements on lunar soil and in situ temperature measurements in and on the lunar surface lead to a mean surface temperature of 220.5 K + 2.5 percent and a mean temperature 35 K hotter at a depth of 1 m. The monthly average brightness temperature is then estimated as a function of wavelength between 10 m and 100 cm taking into account likely temperature dependencies of the thermal conductivity and loss tangent, and the variation of emissivity with wavelength. The accuracy with which the Moon can be used as an absolute radiometric standard for extended sources is estimated based on the likely range of lunar thermal and electromagnetic properties. Subject headings: infrared - Moon - radio radiation - radio radiation, planetary Publication: The Astrophysical Journal Supplement Series Pub Date: February 1973 DOI: 10.1086/190266 Bibcode: 1973ApJS...25..163L full text sources ADS |

Measurements of Microwave Emission from a Foam-Covered, Wind-Driven Sea

Abstract Measurements were made from aircraft of the 1.55-cm microwave emission from the North Sea and North Atlantic at surface wind speeds ranging from less than 5 to 25 m sec−1. Brightness temperatures in the nadir direction increased almost linearly with wind speed from 7 to 25 m sec−1 at a rate of about 1.2C (m sec−1)−1. At 70° from nadir the rate was 1.8C (m sec−1)−1. This increase was directly proportional to the occurrence of white water on the sea surface. At wind speeds <7 m sec−1, essentially no white water was observed and brightness temperatures in the nadir direction were ∼120K; at wind speeds of 25 m sec−1 white water cover was on the order of 30% and average brightness temperatures at nadir were −142K. Maximum brightness temperatures for foam patches large enough to fill the entire radiometer beam were 220K.

A Measurement of the Sky Brightness Temperature at 408 MHz

The absolute value of the background brightness temperature has been measured at a radiofrequency of 408 MHz. Observations were made with a large pyramidal "standard gain" horn aerial and the aerial temperatures were compared direct with the temperature observed with the receiver input connected to a matched resistive load of known temperature. For the region within 240 of the south celestial pole an average brightness temperature of 23� 9�K was obtained. Observations of northern regions indicate that values from previous determinations at northern latitudes are 4-5 degK too high.

Measurements of Venus and Jupiter Near 1-CM Wavelength

view Abstract Citations (24) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Measurements of Venus and Jupiter Near 1-CM Wavelength Law, Sara E. ; Staelin, David H. Abstract Observations of Venus and Jupiter were made from January to March, 1966. The wavelengths were 1.18, 1.28, 1.35, 1.43, and 1.58 cm. The average brightness temperatures of Venus were 400°, 418°, 436°, 451°, and 477° K, respectively, and those of Jupiter were 1200, 1200, 107°, 112°, and 136° K, respectively. These spectra are compared with various atmospheric models. The observations of Venus permit an upper limit to be placed on water-vapor abundance. The measured brightness temperatures of Jupiter are approximately 1.5 standard deviations less than theoretical spectra Publication: The Astrophysical Journal Pub Date: December 1968 DOI: 10.1086/149826 Bibcode: 1968ApJ...154.1077L full text sources ADS |

Spectrum measurements of the sun near 1 cm wavelength

The solar brightness temperature was measured at frequencies of 19.0, 21.0, 22.2, 23.5, and 25.5 GHz on 8 days in February and March, 1966. The observed average brightness temperatures were 10 800, 10 900, 11 000, 10 700, and 9 800°K, respectively. The daily values were close to these averages.

Mercury: Observations of the 3.4-Millimeter Radio Emission

Observations of the 3.4-millimeter radio emission from Mercury during 1965 and 1966 yielded the following relationship between average brightness temperature T(B) of the disk and the planetocentric phase angle i: T(B) = 277 (+/- 12) + 97 (+/- 17) cos [i + 29 deg (+/- 10 deg)] degrees K The errors are statistical standard; the phase shift corresponds to a phase lag-that is, the maximum and minimum of insolation lag the maximum and minimum of planetary radiation.

Solar Observations at 3.2 MM.

view Abstract Citations (27) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Solar Observations at 3.2 MM. Simon, Michal Abstract We discuss the extensive observations of the Sun at 3.2-mm wavelength carried out during the summer of 1964 with a radio telescope of 3' beam width. An average brightness temperature of 6402 + 215 K was measured at the center of the solar disk. No consistent limb brightening was observed. Brightnesstemperature maps were made and are correlated with Ha photographs and magnetograms. Regions of enhanced 3 2-mm emission are shown to correlate well with Ha-active regions and regions of strong magnetic fields. Particularly high enhancements were observed in regions that later flared. One flare was observed but not from its beginning. No bursts were detected. Publication: The Astrophysical Journal Pub Date: May 1965 DOI: 10.1086/148239 Bibcode: 1965ApJ...141.1513S full text sources ADS |

Calculations of a Slowly Varying Component at 4.3 MM.

view Abstract Citations (1) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Calculations of a Slowly Varying Component at 4.3 MM. Edelson, S. Abstract If the average electron density in a coronal condensation is 2.4 X 10 and the electron temperature is 3 X 106 K, then we may expect fluctuations of the order of 0.5-4 per cent in the total solar flux at 0.43 cm The average optical depth for these dense regions is found to be 0000425 between the heights of 18500-66000km above the photosphere, and their average brightness temperature the order of 1300 K. Publication: The Astrophysical Journal Pub Date: May 1962 DOI: 10.1086/147326 Bibcode: 1962ApJ...135..827E full text sources ADS |

Corrigendum

The average central brightness temperature given on page 565 is incorrect and should be 3�l06 �K. No other part of this paper is affected by this change.