Absolute brightness temperature measurements at 2.1-mm wavelength

Absolute measurements of the brightness temperatures of the Sun, new Moon, Venus, Mars, Jupiter, Saturn, and Uranus, and of the flux density of DR21 at 2.1-mm wavelength are reported. Relative measurements at 3.5-mm wavelength are also presented which resolve the absolute calibration discrepancy between The University of Texas 16-ft radio telescope and the Aerospace Corporation 15-ft antenna. The use of the bright planets and DR21 as absolute calibration sources at millimeter wavelengths is discussed in the light of recent observations.     

Planetary brightness temperature measurements at 1.4-mm wavelength

Precise relative measurements of the disk brightness temperatures of Venus, Mars, Jupiter, and Saturn have been made at a mean wavelength of 1.4 mm. The rings of Saturn contribute significantly to the observed total emission. Other results include a better understanding of the properties of the NRAO 11-m antenna near its high frequency limit and of atmospheric degradation of observations in this wavelength range.     

The 3.3-mm brightness distribution of the quiet sun

Center-to-limb brightness distribution measurements of the quiet Sun at a wavelength of 3.3 mm show that there is a slight limb brightening at this wavelength. Within the measurement accuracy of the system used, the limb brightening function is only radially dependent. At 3.3 mm, the measurements are consistent with a solar brightness curve that is flat to about r = 0.8 with a rapid increase to a peak value of about 1.3 at the limb. The results show that most of the central disk 3.3-mm emission comes from a thin layer of relatively constant temperature about 1500–3500 km above the photosphere. This work was supported by the U.S. Air Force under Contract No. F04701-69-C-0066.     

Mars: Subsurface properties from observed longitudinal variation of the 3.5-mm brightness temperature

Measurements at 3.5 mm of the disk-average brightness temperature of Mars during the 1978 opposition can be represented by

$\text{T}{}_{\mathrm{B}}\text{(Mars, 3 5 mm, Jan/Feb 1978)}\text{=}\textcolor{red}{}$

(The errors cited are from the internal scatter; the estimated absolute calibration uncertainty is 3%.) This longitudinal variation must be taken into account if Mars is to be used as a calibration source at millimeter wavelengths. The total range of the 3.5-mm variation is three to four times larger than both the 2.8-cm and 20-μm variations. This unexpected result can possibly be explained by subsurface scattering from rocks ?1.5-cm radius.     

The solar brightness temperature at millimeter wavelengths

The authors have previously discussed an improved method for obtaining the absolute solar brightness temperature using the new Moon as a calibration source. New measurements of the Sun-to-new Moon ratio at three frequencies near 36 GHz ( = 8 mm) and also at two frequencies near 93 GHz ( = 3 mm) are reported. The slopes of the solar brightness temperature spectrum based on these ratios are then discussed. The absolute solar brightness spectrum derived from all current available measurements is also presented and discussed.     

The brightness temperature of Saturn at decimeter wavelengths

Observations of the planet Saturn at wavelengths of 49.5 and 94.3 em are reported. The equivalent disk brightness temperatures were found to be 400 ± 65°K and 540 ± 110°K, respectively. It is suggested that the enhanced portion of the spectrum of the disk brightness temperature favours the idea that the observed long wavelength radiation comes from the planet's atmosphere.However, the possibility of a magnetic field associated with Saturn is not rejected by the observations. Part of the excess temperature could be attributed to weak synchrotron emission coming from a region outside the ring system.     

The quiet Sun brightness temperature at 127 MHz

A correction and analysis of routine total flux measurements made in Toru at 127 MHz allowed for the determination of the peak brightness temperature of the Sun (above 9 × 10⁵ K) in three periods grouped around the last minimum, and for estimation of the brightness temperature of the coronal holes: (7.3 ± 1.4) × 10⁵ K.     

The quiet sun brightness temperature at 408 MHz

The flux of the radio quiet Sun and the brightness temperature at 408 MHz (73 cm) are derived from measurements with the E-W Nançay interferometer and the E-W arm of the Medicina North Cross. It is shown that the lowest envelopes, which defined the radio quiet Sun, correspond to transits of extended coronal holes across the disk of the Sun.     

Distribution of centimeter-wave brightness temperature of solar polar region

The brightness temperature distributions of the solar atmosphere in the polar region at the distances from one to two solar radii during the solar activity minimum are reported. Observations of the maximum phase of the solar eclipse of March 29, 2006 were carried out simultaneously on two sectors of the RATAN-600 radio telescope over a wide range of centimeter waves, 1–31 cm. This study is based on a comparison of models and observations carried out on the northeastern sector of the RATAN-600.     

Synchrotron self-Compton models of high brightness temperature radio sources

We re-examine the maximum brightness temperature that a synchrotron source can sustain by adapting standard synchrotron theory to an electron distribution that exhibits a deficit at low energy. The absence of low energy electrons reduces the absorption of synchrotron photons, allowing the source to reach a higher brightness temperature without the onset of catastrophic cooling. We find that a temperature of ∼10¹⁴ K is possible at GHz frequencies. In addition, a high degree of intrinsic circular polarisation is produced. We compute the stationary, synchrotron and self-Compton spectrum arising from the continuous injection of such a distribution (modelled as a double power-law) balanced by radiative losses and escape, and compare it with the simultaneously observed multi-wavelength spectrum of the BL Lac object S5 0716+714. This framework may provide an explanation of other high brightness-temperature sources without the need for mechanisms such as coherent emission or proton synchrotron radiation.     

Multifrequency spectra of solar brightness temperature derived from eclipse observations

Changes in solar radio-brightness temperature were derived at 2.8,19.3 and 22.2 GHz from the observations of radio flux during the total eclipse of 1980 February 16. High-resolution MEM spectra of the brightness temperature fluctuations at the three frequencies showed periodicities ranging from 3.5 min to 128 min. Between 3.5 min and 14.6 min there are several periodicities of comparable significance common to the three operating frequencies. If the corresponding variations in brightness temperature are assumed to result from spatial variations in the solar radio emission, the observed periodicities imply scale sizes in the range 76000 km to 320000 km.     

On the limit of brightness temperature in nonstationary radio sources

Observations imply that extragalactic radio sources must be in nonstationary states. One possible way to interpret a observed high brightness temperature which exceeds the inverse Compton limit is by means of a nonstationary condition (Kellerman and Pauliny-Toth 1969, Slysh 1992). This paper investigates the maximum brightness temperature of nonstationary radio sources by incorporating the continuity equation for the electron number density and the radiative transfer equation. In radio sources with a high enough magnetic field, synchrotron losses should be considered and the maximum brightness temperature will not achieve the maximum value suggested by Slysh (1992). Strong acceleration canonly shorten the time to achieve the brightness temperature limit and doesnot violate the KPT limit.     

Ultraviolet ion chamber measurements of the solar minimum brightness temperature

The solar ultraviolet flux in the wavelength bands 1580–1640 Å and 1430–1470 Å (FWHM) has been measured using photon ion chambers carried on the satellite WRESAT I (1967-118A). These observations of the integrated ultraviolet flux from the entire disk indicate a value of (4570 ± 50) K for the solar temperature minimum. The results are compared with other estimates of the minimum value of the solar brightness temperature.Died August, 1971.     

The spectrum of titan in the far-infrared and microwave regions

The pressure-induced absorptions of gaseous nitrogen (N₂) and methane (CH₄) are computed on the basis of the collisional lineshape theory of G. Birnhaum and E.R. Cohen [Canad. J. Phys.54, 593–602 (1976)]. Laboratory data at 300 and 124°K for N₂ and at 296 and 195°K for CH₄ are used to determine the collisional time constant and their temperature dependence. The spectrum of Titan from the microwave to the far-infrared region (0.1–600 cm^?1) is then modeled using these opacities and a temperature profile of Titan's atmosphere derived from the Voyager 1 radio occultation experiment. The model atmosphere is composed of N₂ and CH₄, their relative proportions being determined by the vapor pressure law of CH₄. A model with gaseous opacity alone is ruled out by the far-infrared observations. An additional opacity, thought to be associated with a methane cloud, is confirmed. The effective temperature of Titan is estimated at T_e = 83.2 ± 1.4°K.     

On the radio optical depth of the layer where the temperature equals the brightness temperature

It is usually believed that the radio optical depth of the layer h* where the temperature equals the brightness temperature is independent of the frequency. This assumption is criticized from the theoretical point of view and the behaviour of (h*) as a function of the frequency v is computed for two different solar models.     

The radio brightness of Titan

Observations of Titan with the NRAO interferometer yield a brightness temperature of 115 ± 40°K at 8085MHz, giving an estimate for the mean surface temperature of 135 ± 45°K.     

Long-term variations in the microwave brightness temperature of the Uranus atmosphere

We present a self-consistent, 36-year record of the disk-averaged radio brightness of Uranus at wavelengths near 3.5 cm. It covers nearly half a uranian year, and includes both equatorial and polar viewing geometries (corresponding to equinox and solstice, respectively). We find large (greater than 30 K) changes over this time span. In agreement with analyses made of more limited microwave data sets, our observations suggest the changes are not caused by geometric effects alone, and that temporal variations may exist in the deep uranian troposphere down to pressures of tens of bars. Our data also support an earlier suggestion that a rapid, planetary-scale change may have occurred in late 1993 and early 1994. The seasonal record presented here will be useful for constraining dynamical models of the deep atmosphere, and for interpreting observations made during Uranus' 2007 equinox passage. As part of a multi-wavelength observing campaign for this event, the Goldstone-Apple Valley Radio Telescope (GAVRT) project will continue to make frequent, single-dish observations near 3.5 cm.     

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.     

Greenhouse models of the atmosphere of titan

The greenhouse effect is calculated for a series of model atmospheres of Titan containing varying proportions of methane, hydrogen, helium, and ammonia. The pressure induced transitions of hydrogen and methane are the major sources of infrared opacity. For each model atmosphere we first computed its temperature structure with a radiative-convective equilibrium computer program and then generated its brightness temperature spectrum to compare with observed values. This comparison indicates that the methane-to-hydrogen ratio is 1_?.67⁺², the surface pressure is at least 0.4atm, and the surface temperature at least 150°K. In addition, except possibly close to the surface, the amount of ammonia is far less than the saturation vapor value. Large amounts of helium may also be present. Many of the successful model atmospheres have methane condensation clouds in the upper troposphere, which help reconcile spectroscopic gas abundances and the observed ultraviolet albedo of Titan with the gas amounts required for the greenhouse effect. The occurrence of large amounts of hydrogen may be a prerequisite for the occurrence of large amounts of methane in the atmosphere and vice versa. This hypothesis may help explain why Titan is the only satellite in our solar system known to have an atmosphere.     

A chromospheric temperature inversion and its implications on millimeter brightness distribution

The brightness temperature curve of the quiet Sun at millimeter wavelengths suggests a possible inversion in the mid-millimeter range. Interpreting this as a result of an actual inversion in the chromospheric temperature structure, and example of a model chromoshere is presented whose calculated temperature curve exhibits such an inversion. This model is then tested for radial brightness distribution at millimeter wavelengths. Comparing the calculated distributions at 3.2 mm and 6 mm with eclipse measurements made with parabolic cylinders at 3.2 mm and 8.6 mm shows qualitative agreement, allowing for instrumental smoothing. It is conluded that a chromospheric temperature inversion, either actual or effective, could account for the inversion suggested by millimeter data and also the complex brightness distributions measured during eclipses with parabolic cylinders.