The brightness distribution over the moon's disk at 6 cm wavelength

On November 27, 1974, a map of the Moon was obtained at 6 cm wavelength with the 100-m-telescope in Effelsberg. The high angular and favourable temperature resolution allowed an interpretation of the observed brightness distribution. The dominant feature of the brightness distribution is the centre-to-limb variation, particularly noticeable in the direction of the poles. The exponent of the commonly adopted cos ()-law, describing the temperature variation across the lunar disk, is determined as 0.4. The North-South variation of the lunar surface temperature is estimated to be 30%; the depth of penetration (L _e) of electromagnetic waves of 6 cm wavelength is found to beL _e 17 m.     

The brightness distribution of the sun at 2.8cm wavelength

Maps of the Sun at 2.8 cm wavelength, observed with the 100-m telescope in Effelsberg, on 1972, October 31 and 1973, February 10, are discussed. The brightness distribution over the Sun is, with the exception of active regions, approximately constant and nearly sharp-edged. Regions of small enhancements in the radiation can be identified with zones of weak activity in the solar magnetograms. Optical filaments could also be seen in absorption at 2.8 cm wavelength, the optical depth being of the order of 0.2. Several active regions have been analysed with respect to their position relative to their optical counterparts and to their brightness temperature: The latter was found to reach almost 10⁶K in one case. No noticeable limb brightening could be observed; if any exists, it should be smaller than 5%. Likewise no noticeable elliptical deformation of the Sun's disk could be found. The geometrical thickness of the coronal layer, contributing to the undisturbed radiation at 2.8 cm was estimated to be a maximum of 4000 km.     

Coronal bright points at 6 cm wavelength

We report the results of the first observations of solar coronal bright points at 6 cm wavelength using the Very Large Array (VLA), with a spatial resolution of 1.2. The maximum brightness temperature of the sources observed is 3 × 10⁴ K with a mean value of 1 × 10⁴ K (above the quiet Sun value). The lifetime of most sources is between 5 and 20 min. The average diameter of the sources is about 5–15 arc. The sources are gaussian-like near the footpoint of miniature loops and they appear in groups. The observations indicate that significant fluctuations in the brightness temperature (sometimes quasi-periodic) and in the spatial extents of these sources can occur over periods of a few minutes.On leave from Beijing Observatory, Beijing, Peoples Republic of China.     

High-resolution observations of solar radio bursts at 2, 6, and 20 cm wavelength

The Very Large Array and the Westerbork Synthesis Radio Telescope have been used to observe eight solar bursts at 2, 6, or 20 cm wavelength with second-of-arc angular resolution. The regions of burst energy were all resolved with angular sizes between 5″ and 30″, brightness temperatures between 2 × 10⁷ K and 2 x 10⁸ K, and degrees of circular polarization between 10 and 90%. A series of 10 s snapshot maps are presented for the more intense bursts, and superimposed on photospheric magnetograms or Hα photographs. The impulsive phase of the radio bursts is located near the magnetic neutral line of the active regions, and between the flaring Hα kernels which mark the footpoints of magnetic loops. The impulsive phase of one 6 cm burst was smaller and spatially separated from both the preburst radio emission and the gradual decay phase of the burst. Another 6 cm burst exhibited preburst heating of the coronal loop in which the burst occurred. The plasma was probably heated at a lower level in the loop, while the burst energy was released several minutes later at a higher level. A multiple-spike 20 cm burst exhibited polarity inversions with degrees of circular polarization of 90%. The rapid changes in circular polarization are attributed to either a magnetically complex region or the emersion of new magnetic flux at coronal heights where magnetic field strengths H ≈ 300 to 400 G.     

Time variability and structure of quiet Sun sources at 6 cm wavelength

Using the Westerbork Synthesis Radio Telescope (WSRT) we produced a synthesized map of a quiet Sun region on June 15, 1976, and studied the structure and time variability of the quiet emitting regions at 6 cm wavelength with a spatial resolution of 6 arc sec. Comparison of the 12hr synthesis map with Ca⁺ K filtergram shows that bright and dark features on the 6 cm quiet Sun synthesized map correspond to the chromospheric networks and cells observed in Ca⁺ K. All 6 cm bright features lie over bright Ca⁺ K network elements. The reverse correlation is not true, that is, not all bright Ca⁺ K network features have their 6 cm counterparts. Comparison with the photospheric magnetogram shows that about 72% of the photospheric magnetic field enhancements (¦B¦ 5 G) are coincident with 6 cm emissive regions. Only one 6 cm feature could be positively identified with a bipolar magnetic structure. This implies that no more than 20–25% of the 6 cm emitting features could be associated with X-ray bright points. Intercomparison of our 12hr two-dimensional synthesis map, a 4hr two-dimensional synthesis map (around meridian) and the one-dimensional fan beam scans of the quiet Sun region at 6 cm, along with the Ca⁺ K filtergram and photospheric magnetogram shows that: (1) All of the 15 time-varying elements at 6 cm were located on Ca⁺ K networks; (2) about 40% of the 15 time varying elements at 6 cm are coincident with enhancements of the photospheric magnetogram; (3) individual time-varying sources have minimum source size (FWHM) of 15 arc sec and maximum brightness temperature of 10⁵ K; (4) the life time of the time varying sources varies from a few minutes to several tens of minutes; (5) the intensity of the sources varies by factors of 2 to 7 over time periods of 1 min to tens of minutes; and (6) the sources tend to disappear for periods of up to tens of minutes and to reappear at the same locations.     

Fine structure of the Sun at 1.3 cm wavelength

The two-element interferometer at Hat Creek Observatory was used at 1.3 cm wavelength to study the fine structure of the radio emissive regions on the Sun. Observations of the quiet Sun at 1.3 cm show sudden changes in the fringe amplitude and phase, lasting for typically about 5–8 min. Assuming that these events are identical in nature, a plot of peak amplitude vs the projected baseline at the time of the event suggests emission from a region of angular size of about 10″. The corresponding brightness temperature is 50000 K. It is possible that these events may be related to the appearance and disappearance of groups of spicules or mottles.     

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.     

Radar map of Venus at 3.8 cm wavelength

Radar echoes from Venus have been mapped at a wavelength of 3.8 cm from approximately 270° to 10° in longitude and ?50° to +50° in latitude during the inferior conjunctions of 1969 and 1972. Observations made in April 1969 and again in June 1972 both show the same regions of high reflectivity as well as several large regions of low reflectivity.     

Solar brightness distribution and its variability at 3 millimeter wavelength

Solar mapping at 3.4 millimeter wavelength has shown the existence of various spatial structures in brightness distribution. The most prominent structure is presented by the local sources of the slowly varying component of the solar radio emission. Usually they coincide with active regions. Some sources have no corresponding optical counterparts. Using synoptic radio maps, latitudinal belts of enhanced brightness were detected at the north and south hemispheres. These belts seem to coincide with sunspot zones, but the enhanced emission exists independently of sunspot group appearance. Comparison of our maps with XUV images shows a noticeable resemblance. Sources above active regions and latitudinal belts of enhanced brightness are seen in both ranges as well as coronal holes.     

Observations of Saturn at a wavelength of 49.5 cm

Radio emission from the planet Saturn was detected and measured by an unusually efficient observing technique at a wavelength of 49.5cm. The corresponding equivalent disk brightness temperature was hence determined to be 390 ± 65°K, providing further evidence for a mild enhancement in the emission at long wavelengths. It is pointed out that the currently available measurements of the disk brightness temperature in the wavelength range 1mm-1m are, as a whole, inadequate for estimating with confidence the detailed shape of the spectrum and that the exiguous, long wavelength observations should be supplemented with more and accurate measurements.     

Interferometer observations of the solar brightness distribution at 8.6 mm wavelength

The brightness distribution of the quiet Sun at 8.6 mm wavelength is synthesized from off-meridian observations using an eight element east-west interferometer with a maximum base line of 16.38 m (1913). The observed brightness distribution is practically flat from the disk center to the optical limb. The effective radius of the nearly uniform component is 1.01 R . If the limb brightening is present, the brightening located between 0.95 R and 1.01 R , and the total flux density of the limb brightening is less than 1% of the total flux density of the Sun. In addition to the nearly uniform component there exists a coronal component just outside the optical limb.     

Comments on the quiet Sun brightness distribution at 1.2 mm wavelength

A re-analysis of the brightness distribution of the quiet Sun at 1.2 mm wavelength is made. It is concluded that there is no evidence for uniform or limb-brightened distribution at this wave-length.     

Observations of the OH radical in comets at 18 cm wavelength

We present recent observations of the OH radical at λ 18 cm with the Nançay radio telescope in comets Meier (1978 XXI), Bradfield (1979 X), Meier (1980q), P/Encke (1980), and Bradfield (1980t). The analysis of the OH radio line shape is a powerful tool to study the kinematics of the coma. The expansion velocity of the OH molecules is found to be ≈1.5 km sec^?1 at r_h = 1 AU, and decreases with increasing heliocentric distance. The line profile is generally asymmetric, which demonstrates the Greenstein effect on the fluorescent excitation mechanism and/or anisotropic outgassing of the nucleus. In several cases, especially for comet Meier (1978 XXI), an asymmetry is also found in the east-west brightness distribution of the OH line, showing again the Greenstein effect and/or anisotropic outgassing. An excitation model by uv pumping and fluorescence of the OH radical, which agrees with the observations at least in the first order, and the application of Haser's model lead to the production rate of the parent molecule of OH. There is a close correlation between this gas production rate and the visual brightness of the comet. Our estimates of gas production rates are smaller than or equal to those obtained from uv measurements, but both radio and uv estimates depend heavily on the parameters used in Haser's models.     

Spike emission at 21 cm wavelength in post-flare loops

Electron-cyclotron maser instability produced by accelerated energetic electrons is today's most favoured process for spike emission on ultra-short time scales at dm wavelengths. The process of electron acceleration occurs primarily in pre-impulsive and impulsive phases. We present observations of spike bursts at 21 cm in a limb event that occurred in AR 5629 on 1989 August 17. Optical observations of the event from Beijing Astronomical Observatory show coronal loops that interact just before the spike groups appeared. This implies that additional particle acceleration might be produced by the interaction between emerging loops and original large loops, and become the source of the energetic electrons responsible for the spike emission appearing in the descending phase. Some important parameters are deduced.Paper presented at the 4th CESRA workshop in Ouranopolis (Greece) 1991.     

The variation of solar brightness at the extreme solar limb at centimetre radio waves

In this note some properties of the nonconvolved brightness profile close to the extreme solar limb are described. In particular, at 6 cm and 11 cm wavelength relatively strong, narrow spikes should occur, noticeably affected by inhomogeneities (for example spicules) in the solar atmosphere.     

A study of coronal bright points at 20 cm wavelength

We present the results of a study of coronal bright points observed at 20 cm with the VLA on a day when the Sun was exceptionally quiet. Microwave maps of bright points were obtained using data for the entire observing period of 5 hours, as well as for shorter periods of a few minutes. Most bright points, especially those appearing in the full-period maps appear to be associated with small bipolar structures on the photospheric magnetogram. Overlays of bright point (B.P.) maps on the Ca⁺K picture, show that the brightest part of a B.P. tends to lie on the boundary of a supergranulation network. In general, the bright points exhibit rapid variations in intensity. There is no systematic correlation between the size of a B.P. and its intensity; the apparently slow variation of B.P. emission may have rapid fluctuations superimposed on it.     

Upper limit to the 2 cm brightness temperature of asteroid 433 Eros

The brightness temperature of 433 Eros was less than 460°K during the close opposition in January 1975.