Radio Astronomical Measurements Research Articles

Simulation and Testing of a Linear Array of Modified Four-Square Feed Antennas for the Tianlai Cylindrical Radio Telescope

A wide bandwidth, dual polarized, modified four-square antenna is presented as a feed antenna for radio astronomical measurements. A linear array of these antennas is used as a line-feed for cylindrical reflectors for Tianlai, a radio interferometer designed for 21[Formula: see text]cm intensity mapping. Simulations of the feed antenna beam patterns and scattering parameters are compared to experimental results at multiple frequencies across the 650–1420[Formula: see text]MHz range. Simulations of the beam patterns of the combined feed array/reflector are presented as well.

STRONG THEORETICAL SUPPORT FOR THE ASSIGNMENT OF B11244 TOl-C3H+

Highly accurate quantum chemical calculations beyond CCSD(T) have been used to study the molecular cation l-C3H+ which is the carrier of harmonically related radio lines observed in the Horsehead photodissociation region and toward Sgr B2(N). Excellent agreement with spectroscopic and radioastronomical measurements is obtained for the rotational constant, with the calculated value of B 0 = 11246.4 MHz only 1.5 MHz or 0.01% above the experimental value. The unusually large ratio of centrifugal distortion constants D 0(exp.)/De(theor.) = 1.80 is attributed to the shallow CCC bending potential of l-C3H+ and is quantitatively reproduced by variational calculations within a pseudo-triatomic model. A comparative study of centrifugal distortion constants in a series of four linear interstellar molecules (C3N–, C3O, l-C3H+, and C3) is made and some general conclusions are drawn.

Measurements of the total electron content of the ionosphere by the radioastronomy Faraday method

In 2008 and 2009, when a deep solar-activity minimum was observed, the total electron content of the ionosphere was measured by the radioastronomy method using the ionospheric Faraday rotation of the polarization plane of a linear-polarized component of the diffuse radio-frequency (RF) radiation of the galaxy. In the measurements performed at a frequency of 290 MHz at the Radioastronomy observatory “Staraya Pustyn” of the Radiophysical Research Institute of Nizhny Novgorod, the previously studied region of the Celestial North Pole was used. The brightness temperature was calibrated by the Cassiopeia A radio source. The time dependence of the Cassiopeia The RF radiation stream density, which was obtained in the period of 1978–2011 from the measurement results at a frequency of 290 MHz, is shown. A new method of accounting for a spurious polarized signal allowing for its time dependence was developed and used, which makes it possible to improve the measurement frequency of the time variations of total electron content of the ionosphere. This method allowed us to more accurately measure the brightness temperature of the linearly polarized component of diffuse galaxy RF radiation in the direction of the Celestial North Pole, which amounted to 0.83 ± 0.07 K at a frequency of 290 MHz. For this region of the horizon, we demonstrate the spectra of the brightness temperature and the position angle of the polarization plane of the linearly polarized component of the galaxy diffuse RF radiation ranging from meter to centimeter wavelengths, which allows us to choose the required frequency for the measurements by the radioastronomy Faraday method in the future. The values of the total electron content of the ionosphere are compared with the corresponding values from the global ionospheric maps obtained from observations of the GPS satellite signals. The difference between the average values of total electron content of the ionosphere is 1.9∙1012 cm-2 according to all radioastronomy measurements and the corresponding means from the global ionospheric maps.

Farside explorer: unique science from a mission to the farside of the moon

Farside Explorer is a proposed Cosmic Vision medium-size mission to the farside of the Moon consisting of two landers and an instrumented relay satellite. The farside of the Moon is a unique scientific platform in that it is shielded from terrestrial radio-frequency interference, it recorded the primary differentiation and evolution of the Moon, it can be continuously monitored from the Earth–Moon L2 Lagrange point, and there is a complete lack of reflected solar illumination from the Earth. Farside Explorer will exploit these properties and make the first radio-astronomy measurements from the most radio-quiet region of near-Earth space, determine the internal structure and thermal evolution of the Moon, from crust to core, and quantify impact hazards in near-Earth space by the measurement of flashes generated by impact events. The Farside Explorer flight system includes two identical solar-powered landers and a science/telecommunications relay satellite to be placed in a halo orbit about the Earth–Moon L2 Lagrange point. One lander would explore the largest and oldest recognized impact basin in the Solar System— the South Pole–Aitken basin—and the other would investigate the primordial highlands crust. Radio astronomy, geophysical, and geochemical instruments would be deployed on the surface, and the relay satellite would continuously monitor the surface for impact events.

On the comparison of radio-astronomical measurements of the height structure of magnetic field with results of model approximations

The results of microwave observations of the polarized emission of active regionsmade with the RATAN-600 radio telescope are used to develop the method for determining the structure of the magnetic field of these regions at coronal heights. About 1000-G-strong magnetic fields are observed in the solar atmosphere at rather high altitudes (from 10 to 25 Mm). This result is confirmed fairly well by the ultraviolet observations of magnetic loops, it is consistent with earlier radio-astronomical observations of the magnetic field at the height of the transition region, and it corresponds as well, if interpreted in terms of the dipole magnetic field model, to the vertical gradients of the photospheric magnetic field.

Errata [to "Peacetime Radio Astronomical Measurements Using World War II Radar Equipment" (Jun 08 148-153)

Errata [to "Peacetime Radio Astronomical Measurements Using World War II Radar Equipment" (Jun 08 148-153)

Laboratory studies of the HNCO molecular spectrum for precise spectroscopy of dark clouds

Detailed studies of the internal motions of dark clouds using spectral lines of many molecules require a laboratory frequency accuracy of the order of a few m s−1. Based on our laboratory studies of the HNCO rotational spectrum in the ground vibrational state, we have increased significantly the accuracy of frequency calculation in a wide range of quantum numbers. We have achieved an (1σ) uncertainty for rotational transitions in the K a = 0, 1 states recalculated to the Doppler velocity scale ≤2 m s−1 for all frequencies <1.1 THz. This value allows radio-astronomical measurements with an accuracy comparable to that of the highest-precision observations based on spectral lines of other molecules.

Comparison of model prediction with measurements of galactic background noise at L-band

The spectral window at L-band (1.413 GHz) is important for passive remote sensing of surface parameters such as soil moisture and sea surface salinity that are needed to understand the hydrological cycle and ocean circulation. Radiation from celestial sources (mostly galactic) is strong in this window, and an accurate accounting of this background radiation is often needed for calibration. This paper presents a comparison of the background radiation predicted by a model developed from modern radio astronomy measurements with measurements made with several modern L-band remote sensing radiometers. The comparison validates the model and illustrates the magnitude of the correction necessary in remote sensing applications.

Spectroscopic Measurements of Interstellar Molecules in Astronomy

Spectroscopic measurements of interstellar molecules are described. Since 1963 interstellar molecules have been discovered primarily by means of radio astronomical measurements, and some molecules have been detected in the infrared, optical, and ultraviolet regions. More than 120 species were detected including simple hydride, complex organic molecules, ring molecules, molecular ions, and radicals. Because of high frequency resolution fine- and hyperfine structure lines are easily observed by radio astronomical measurements, which are sometimes difficult to be resolved in ground-based laboratory measurements. Measured molecular spectra provide us with not only the precise motions of molecular clouds but excitation temperatures and molecular abundances as well. The Atacama Large Millimeter- and submillimeter Array (ALMA) is under construction in Chile through international collaboration of Japan, the US and the EU, and will soon unveil various mysteries of the universe.

Noise and the Strong‐Signal Limit in Radio Astronomical Measurement

ABSTRACTThe random error of radio astronomical measurements is usually computed in the weak‐signal limit, which assumes that the system temperature is sensibly the same on and off source, or with and without a spectral line. This assumption is often very poor. We give examples of common situations in which it is important to distinguish the system noise in signal‐bearing and signal‐free regions.

Evolution of Polarization for Radiation Crossing a Plasma Layer of Quasi‐transverse Propagation and the Interpretation of Radioastronomical Measurements

First the complete change of polarization is obtained for radiation crossing a layer of quasi-transverse propagation in a plasma where the longitudinal component of the magnetic field changes signs along the propagation direction, while the transverse component is different from zero. The analysis is made for arbitrary input polarization. The results are then used for the interpretation of previously published measurements of radio emission from solar active regions. It is found that if a sufficiently narrow detection bandwidth is used, more information can be obtained from measurements than has been considered up to now. The present analysis is also of interest for other astrophysics problems.

Distribution of the longitudinal component of the magnetic field of prominences from radioastronomical measurements

We present the results of prominence observations at RT-22 CrAO in April, May, and September 1993 at 13.5 mm (intensity and circular polarization) and at 8.2 mm (intensity) wavelengths carried out by circular scanning using the combined scalar horn. On the assumption of thermal cyclotron radiation we have measured the values of the longitudinal component of the magnetic field for 18 prominences. The histogram of the distribution of the longitudinal component indicates the presence of maxima in the vicinity of 8, 15, and 25– 30 G. Our results are compared with the distributions obtained from optical measurements.

125?170 GHz SIS-receiver with closed cycle refrigeration system

A mechanically tunable SIS receiver covering the frequency range from 125 to 170 GHz is described. For cooling at 2.8 K, a closed cycle refrigeration system has been developed that has a cooling power of 350 mW at 2.8 K. The IF is centered at 1.4 GHz with a bandwidth of 600 MHz. For preamplification, a cryogenic 2-stage HEMT amplifier has been developed that has a noise temperature of about 7 K. The best narrow-band spot noise temperature of the receiver is 28 K (DSB) at 133 GHz. Typical broadband (600MHz) values are between 50 and 200 K depending on the frequency. The receiver is used for radioastronomical measurements at the Cologne 3-m radiotelescope.

Zur Interpretation der Messungen der Lichtablenkung am Sonnenrand

AbstractThe agreement between radiastronomical measurements of the gravitational deflection of light at the solar limb and the theory of relativity is compatible with the exces found by most optical observers. The optical excess is caused by stars so near to the Sun that radioastronomical observations are impossible. Optical and radioastronomical measurements both make it likely that there is an additional term inversely proportional to a power higher than the first of the distance from the Sun. Results found by BOUET indicate that this additional term is of solar origin.

Spatial Filtering Radio Astronomical Data: One-Dimensional Case

Radio astronomical measurements of radio brightness are made by pointing an antenna at a regular array of points in the sky and measuring the received noise power at each point. In the absence of receiver noise, the measured brightness is the convolution of the true brightness distribution with the antenna effective area (i.e., receiving power pattern), evaluated at the point of observation. Front-end noise in the radiometer receiver adds fluctuations inversely proportional to the observing time at each measured point. From such data, we calculate optimum mean-square estimates for two quantities: measured brightness between observations, and true brightness at and between observations. The first is interpolation; the second, called restoration, partially deconvolves the antenna pattern from the measured data. We determine the errors associated with each, as functions of: (1) receiving antenna pattern, (2) separation between observations, and (3) radiometer output signal-to-noise ratio. These results permit the construction of maps of measured and true brightness, with known mean-square errors. In this paper we study the one-dimensional version of this problem, assuming a large number of measured points. We find that measured points should be separated by about half the (full) 3-dB beamwidth for conventional antennas. Restoration is more costly than interpolation.

Planetary Radio Astronomy Observations from Voyager 2 Near Saturn

Planetary radio astronomy measurements obtained by Voyager 2 near Saturn have added further evidence that Saturnian kilometric radiation is emitted by a strong dayside source at auroral latitudes in the northern hemisphere and by a weaker source at complementary latitudes in the southern hemisphere. These emissions are variable because of Saturn's rotation and, on longer time scales, probably because of influences of the solar wind and Dione. The electrostatic discharge bursts first discovered by Voyager 1 and attributed to emissions from the B ring were again observed with the same broadband spectral properties and an episodic recurrence period of about 10 hours, but their occurrence frequency was only about 30 percent of that detected by Voyager 1. While crossing the ring plane at a distance of 2.88 Saturn radii, the spacecraft detected an intense noise event extending to above 1 megahertz and lasting about 150 seconds. The event is interpreted to be a consequence of the impact, vaporization, and ionization of charged, micrometer-size G ring particles distributed over a vertical thickness of about 1500 kilometers.

Multiple Alfven wave reflections excited by Io: Origin of the Jovian decametric arcs

The recent Voyager radio astronomy measurements near Jupiter show that the Jovian decametric radiation consists of numerous discrete features called decametric arcs which are observed at all Jovian longitudes. It is generally believed that these arcs are produced by an interaction of Io with the Jovian magnetosphere. In this paper we propose that the large number of decametric arcs is caused by multiple reflections of a standing Alfven wave current system excited by Io. Estimates of the reflection coefficient at the ionosphere and other damping processes show that a large number of reflections can occur, with the Alfven wave current system possibly extending completely around Jupiter. This source geometry can account for a number of otherwise puzzling characteristics of the Jovian decametric radiation.

Far infrared laser magnetic resonance spectrum of CH

Laser magnetic resonance spectra between 0 and 17 kG have been recorded and analyzed for (J′ ← J″) = ( 7 2 ← 5 2 ) , ( 5 2 ← 3 2 ) , and ( 3 2 ← 1 2 ) transitions in the CH molecule, using the optically pumped far infrared lasers: 118.8 μm (CH 3OH), 180.7 μm (CD 3OH), 554.4 μm (CH 2CF 2), 561.3 μm (DCOOD), and 567.9 μm (CH 2CHCl). Other transitions in CH were detected with the 13CH 3OH laser at 115.8, 149.3, and 203.6 μm. The CH radical was generated in a low-pressure methane and atomic fluorine flame within the laser cavity. Analysis of the M′ J ← M″ J structure yields wavenumbers for the rotational transitions mentioned above of 84.3494, 55.3397, and 17.8376 cm −1, respectively. Combining results from the M J analysis with the J = 1 2 Λ- doubling interval derived from radioastronomy measurements yields Λ-doubling values for the J = 3 2 , 5 2 , and 7 2 states of 0.0237, 0.1620, and 0.3759 cm −1, respectively. Both the rotational intervals and the Λ-doublings are in good agreement with earlier less precise optical results. Analysis of the hyperfine structure yields values for the Frosch and Foley hyperfine parameters of a = +52, b = −74, c = +52, and d = +43.6 MHz, in good agreement with recent ab initio estimates and radioastronomy measurements.

Use of classical pertubation theories for the calculation of barycentric aberration and Doppler Effect

Modern techniques have considerably improved the accuracy of radioastronomical measurements. Examples are the high resolution in frequency, time and position for spectroscopic, pulsar, and VLBI observations. There is an increasing demand for accurate reduction of such measurements to an inertial frame of reference, preferably the barycenter of the solar system and the mean equator and equinox of a standard epoch. Reductions of this kind include the computation of general precession, nutation, aberration and Doppler effect. For general precession and nutation, there are internationally accepted algorithms which can be programmed in form of self-consistent subroutines. Aberration and Doppler shifts are determined by the barycentric velocity of the Earth and by diurnal effects. Until now, as far as I know, a self-consistent algorithm of sufficient accuracy for computation of the barycentric velocity components of the Earth has not been available. The need for such an algorithm is indicated by the recommendations of Commission 4 (Ephemerides) of the IAU (1976), that “stellar aberration shall be computed from the total velocity of the Earth …”, and that “reductions to apparent places should be computed rigorously …”. In practice, this can be done by reading and interpolating ephemerides obtained from numerical integrations.

Reduction of the Baseline Ripple on Spectra Recorded with the Parkes Radio Telescope

One of the factors determining system sensitivity in radio astronomy measurements of microwave emission and absorption spectra is the flatness of the spectrometer response in the absence of any spectral features. Ripple appears as a quasi-sinusoidal variation of this baseline and occurs whenever two or more components of the same signal reach the receiver by different paths and interfere.