Centimetre Wavelengths Research Articles

Correlation processing and its use to mitigate the effects of multipath and other interfering signals in antenna ranges

Correlation processing has previously been used in radio communication systems to measure and characterise multipath environments, but it can equally well be applied to the elimination of multipath effects. The authors review the historical development of the technique in the context of its earlier use, and the background to its implementation as a method of eliminating or at least substantially reducing, the effects of multipath in antenna ranges. Correlation processing, which can be implemented either as a one-step, real-time process or used as a post-measurement option, also suppresses the effects of coherent and non-coherent interfering signals, so that the accuracy of antenna pattern measurement is greatly improved. At centimetre and millimetre wavelengths, it is possible to make accurate measurements in a normal laboratory environment without the need for an anechoic chamber. A simple proof-of-concept practical system is described, and example results are presented to demonstrate the validity of the technique and to compare it with time gating.

Solar radio proxies for improved satellite orbit prediction

Specification and forecasting of solar drivers to thermosphere density models is critical for satellite orbit prediction and debris avoidance. Satellite operators routinely forecast orbits up to 30 days into the future. This requires forecasts of the drivers to these orbit prediction models such as the solar Extreme-UV (EUV) flux and geomagnetic activity. Most density models use the 10.7 cm radio flux (F10.7 index) as a proxy for solar EUV. However, daily measurements at other centimetric wavelengths have also been performed by the Nobeyama Radio Observatory (Japan) since the 1950's, thereby offering prospects for improving orbit modeling. Here we present a pre-operational service at the Collecte Localisation Satellites company that collects these different observations in one single homogeneous dataset and provides a 30 days forecast on a daily basis. Interpolation and preprocessing algorithms were developed to fill in missing data and remove anomalous values. We compared various empirical time series prediction techniques and selected a multi-wavelength non-recursive analogue neural network. The prediction of the 30 cm flux, and to a lesser extent that of the 10.7 cm flux, performs better than NOAA's present prediction of the 10.7 cm flux, especially during periods of high solar activity. In addition, we find that the DTM-2013 density model (Drag Temperature Model) performs better with (past and predicted) values of the 30 cm radio flux than with the 10.7 flux.

The 30 cm radio flux as a solar proxy for thermosphere density modelling

The 10.7 cm radio flux (F10.7) is widely used as a proxy for solar UV forcing of the upper atmosphere. However, radio emissions at other centimetric wavelengths have been routinely monitored since the 1950 s, thereby offering prospects for building proxies that may be better tailored to space weather needs. Here we advocate the 30 cm flux (F30) as a proxy that is more sensitive than F10.7 to longer wavelengths in the UV and show that it improves the response of the thermospheric density to solar forcing, as modelled with DTM (Drag Temperature Model). In particular, the model bias drops on average by 0–20% when replacing F10.7 by F30; it is also more stable (the standard deviation of the bias is 15–40% smaller) and the density variation at the the solar rotation period is reproduced with a 35–50% smaller error. We compare F30 to other solar proxies and discuss its assets and limitations.

Perspective nickel-oxide cathodes technologies for centimetric range microwave devices

Three technologies of nickel-oxide cathodes based on nickel powder and on barium-calciumstrontium triple carbonate powder for microwave ovens of the centimetric wave length range devices are suggested. The optimum fraction of nickel powder for the production of cathodes is 45-71 microns. The optimum content of the triple carbonate is 10%. The optimum pressure of pressing is 7 t/cm2. The optimum temperature of agglomeration is 1100°C for 10 min. The quality improvement and yield of suitable cathodes are provided by the use of agglomerates of nickel and triple carbonates. The agglomerates contain nickel particles of the 10-25 microns fraction coated with the triple carbonate containing 50% of the carbonate and baked in dried hydrogen at 1000°C for 10 min. The formation of the cathode is carried out by mixing and agglomerating the 45-63 microns fraction and the nickel 45-71 microns fraction with the carbonates concentration 10%. In order to improve the cathodes quality it is expedient to anneal the agglomerates after their production in carbon dioxide at 200-300°C with a final purge by nitrogen at 1000°C for 10 minutes. The use of agglomerates based on nickel and the triple carbonate for the production of oxide-nickel cathodes provides more uniform distribution of the emission-active agent in the volume and on the surface of the cathodes.

A Sensitive Search for Predicted Methanol Maser Transitions with the Australia Telescope Compact Array

AbstractWe have used the Australia Telescope Compact Array to search for a number of centimetre wavelength methanol transitions which are predicted to show weak maser emission towards star formation regions. Sensitive, high spatial, and spectral resolution observations towards four high-mass star formation regions which show emission in a large number of class II methanol maser transitions did not result in any detections. From these observations, we are able to place an upper limit of ≲ 1300 K on the brightness temperature of any emission from the 31A+–31A−, 17−2–18−3 E (vt = 1), 124–133 A−, 124–133 A+, and 41A+–41A− transitions of methanol in these sources on angular scales of 2 arcsec. This upper limit is consistent with current models for class II methanol masers in high-mass star formation regions and better constraints than those provided here will likely require observations with next-generation radio telescopes.

Clear sky atmosphere at cm-wavelengths from climatology data

We utilise ground-based, balloon-borne and satellite climatology data to reconstruct site and season-dependent vertical profiles of precipitable water vapour (PWV). We use these profiles to solve radiative transfer through the atmosphere, and derive atmospheric brightness temperature ($T_{\rm atm}$) and optical depth ($\tau$) at centimetre wavelengths. We validate the reconstruction by comparing the model column PWV with photometric measurements of PWV, performed in clear sky conditions pointed towards the Sun. Based on the measurements, we devise a selection criteria to filter the climatology data to match the PWV levels to the expectations of the clear sky conditions. We apply the reconstruction to the location of a Polish 32-metre radio telescope, and characterise $T_{\rm atm}$ and $\tau$ year-round, at selected frequencies. We also derive the zenith distance dependence for these parameters, and discuss the shortcomings of using planar, single-layer, and optically thin atmospheric models in continuum radio-source flux-density measurement calibrations. We obtain PWV-$T_{\rm atm}$ and PWV-$\tau$ scaling relations in clear sky conditions, and constrain limits to which the actual $T_{\rm atm}$ and $\tau$ can deviate from those derived solely from the climatological data. Finally, we suggest a statistical method to detect clear sky that involves ground-level measurements of relative humidity. Accuracy is tested using local climatological data. The method may be useful to constrain cloud cover in cases when no other (and more robust) climatological data are available.

WEAK MASER EMISSION OF METHYL FORMATE TOWARD SAGITTARIUS B2(N) IN THE GREEN BANK TELESCOPE PRIMOS SURVEY

A non-LTE radiative transfer treatment of cis-methyl formate (HCOOCH3) rotational lines is presented for the first time using a set of theoretical collisional rate coefficients. These coefficients have been computed in the temperature range 5-30 K by combining coupled-channel scattering calculations with a high accuracy potential energy surface for HCOOCH3-He. The results are compared to observations toward the Sagittarius B2(N) molecular cloud using the publicly available PRIMOS survey from the Green Bank Telescope. A total of 49 low-lying transitions of methyl formate, with upper levels below 25 K, are identified. These lines are found to probe a presumably cold (~30 K), moderately dense (~1e4 cm-3) and extended region surrounding Sgr B2(N). The derived column density of ~4e14 cm-2 is only a factor of ~10 larger than the column density of the trans conformer in the same source. Provided that the two conformers have the same spatial distribution, this result suggests that strongly non-equilibrium processes must be involved in their synthesis. Finally, our calculations show that all detected emission lines with a frequency below 30 GHz are (collisionally pumped) weak masers amplifying the continuum of Sgr B2(N). This result demonstrates the importance and generality of non-LTE effects in the rotational spectra of complex organic molecules at centimetre wavelengths.

Heavily loaded ferrite-polymer composites to produce high refractive index materials at centimetre wavelengths

A cold-pressing technique has been developed for fabricating composites composed of a polytetrafluoroethylene-polymer matrix and a wide range of volume-fractions of MnZn-ferrite filler (0%–80%). The electromagnetic properties at centimetre wavelengths of all prepared composites exhibited good reproducibility, with the most heavily loaded composites possessing simultaneously high permittivity (180 ± 10) and permeability (23 ± 2). The natural logarithm of both the relative complex permittivity and permeability shows an approximately linear dependence with the volume fraction of ferrite. Thus, this simple method allows for the manufacture of bespoke materials required in the design and construction of devices based on the principles of transformation optics.

Weathering history of an exposed bedrock fault surface interpreted from its topography

Morphologic features of bedrock fault scarps are underutilized in studying faulting and weathering history, partly because of a lack of accurate quantitative parameters for topography. The study employs ground-based LiDAR to measure five patches at different levels on the same fault surface and then calculates roughness in the form of power spectral density in directions parallel and perpendicular to the slip. The power spectral density and spatial frequency typically follow a power law for each fault patch, showing approximately linear relationships in a log–log plot. However, due to additional power introduced by weathering, all spectral curves, especially those parallel to the slip, can be divided into two segments, lower-frequency (wavelengths of several centimeters – several meters) and higher-frequency (wavelengths of several centimeters and below) domains. This shows that the topographic features at different spatial scales are dominated by different mechanical processes: faulting abrasion in the lower-frequency domain and the weathering process in the higher-frequency domain. Moreover, we develop two parameters to quantify the degree of weathering of a fault outcrop, which is significant to describe the evolution of the fault-scarp and infer the date of faulting under calibration.

Radio Observations of Supernova 1987A

AbstractSupernovae and their remnants are believed to be prodigious sources of Galactic cosmic rays and interstellar dust. Understanding the mechanisms behind their surprisingly high production rate is helped by the study of nearby young supernova remnants. There has been none better in modern times than SN1987A, for which radio observations have been made for over a quarter of a century. We review extensive observations made with the Australia Telescope Compact Array (ATCA) at centimetre wavelengths. Emission at frequencies from 1 to 100 GHz is dominated by synchrotron radiation from an outer shock front which has been growing exponentially in strength from day 3000, and is currently sweeping around the circumstellar ring at about 4000 km s−1. Three dimensional models of the propagation of the shock into the circumstellar medium are able to reproduce the main observational features of the remnant, and their evolution. We find that up to 4% of the electrons encountered by the shock are accelerated to relativistic energies. High-frequency ALMA observations will break new ground in the understanding of dust and molecule production.

Receiving And Data Acquisition Systems Of Rt-32 For Vlbi Observations / Rt-32 Uztveršanas Un Datu Reģistrācijas Sistēmas Vlbi Novērojumiem

Abstract Radiotelescope RT-32 is a fully steerable 32-m parabolic antenna located at Irbene and belonging to Ventspils International Radio Astronomy Centre (VIRAC). Currently, the work on upgrading and repair of its receiving hardware and data acquisition systems is of high priority for the VIRAC. One of the main scientific objectives for the VIRAC Radioastronomical observatory is VLBI (very long baseline interferometry) observations in centimetre wavelengths in collaboration with world VLBI networks, such as European VLBI network (EVN), Low Frequency VLBI network (LFVN), and others. During the last years the room in the secondary focus of telescope was reconstructed, and several new receivers were installed. Currently, RT-32 observations are carried out in four different bands: 92 cm, 18 cm, 6 cm, and 2.5 cm. First three of them are already successfully employed in diversified VLBI experiments. The receiver on 2.5 cm band has only one linear polarized chain and is used mainly for the methanol maser single dish observations. The apparatus system of RT-32 is equipped with two independent VLBI data acquisition systems: TN-16, and DBBC in combination with MK5b. Both systems are employed in interferometric observations depending on the purpose of experiment and the enabled radiotelescopes. The current status of RT-32, the availability of its receiving and data acquisition units for VLBI observations and the previous VLBI sessions are discussed.

Radio imaging-spectroscopy observations of the Sun in decimetric and centimetric wavelengths

AbstractTo address fundamental processes in the solar eruptive phenomena it is important to have imaging-spectroscopy over centimetric-decimetric wave range. The Chinese Spectral Radioheliograph (CSRH) in 0.4-15 GHz range with high time, space and frequency resolutions is being constructed to achieve this goal. The perspectives to open new observational windows on solar flares and CMEs will be achieved by mapping the radio emission from unstable electron populations during the basic processes of energy release. CSRH is located in a radio quiet region in Inner Mongolia of China. The array of CSRH-I in 0.4-2.0 GHz with 40 4.5m antennas has been established and starts test observations. The 60 2m antennas for array of CSRH-II in 2-15 GHz have been mounted and assembled. The progress and current status of CSRH are introduced.

A search for pre-substellar cores and proto-brown dwarf candidates in Taurus: multiwavelength analysis in the B213-L1495 clouds

In an attempt to study whether the formation of brown dwarfs (BDs) takes place as a scaled-down version of low-mass stars, we conducted IRAM30m/MAMBO-II observations at 1.2 mm in a sample of 12 proto-BD candidates selected from Spitzer/IRAC data in the B213-L1495 clouds in Taurus. Subsequent observations with the CSO at 350 micron, VLA at 3.6 and 6 cm, and IRAM30m/EMIR in the 12CO(1-0), 13CO(1-0), and N2H+(1-0) transitions were carried out toward the two most promising Spitzer/IRAC source(s), J042118 and J041757. J042118 is associated with a compact (<10 arcsec or <1400 AU) and faint source at 350 micron, while J041757 is associated with a partially resolved (~16 arcsec or ~2000 AU) and stronger source emitting at centimetre wavelengths with a flat spectral index. The corresponding masses of the dust condensations are ~1 and ~5 Mjup for J042118 and J041757, respectively. In addition, about 40 arcsec to the northeast of J041757 we detect a strong and extended submillimetre source, J041757-NE, which is not associated with NIR/FIR emission down to our detection limits, but is clearly detected in 13CO and N2H+ at ~7 km/s, and for which we estimated a total mass of ~100 Mjup, close to the mass required to be gravitationally bound. In summary, our observational strategy has allowed us to find in B213-L1495 two proto-BD candidates and one pre-substellar core candidate, whose properties seem to be consistent with a scaled-down version of low-mass stars.

Debris discs at centimetre wavelengths: planetesimal populations in young extrasolar Kuiper belts

ABSTRACT Two luminous debris discs around 100 Myr old have been searched for 1 cm dust emission, to a depth three times greater than in any such previous study. Very low limits were set towards both HD 377 and HD 104860 (noise levels of 12–14 Jy), extending the spectral range from 70 m to 3 mm where cool dust has previously been sought. The almost-identical fluxes of the two systems were merged into an average spectral energy distribution, which was then fitted using a distribution of grain sizes. The canonical infinite collisional cascade, with numbers of particles n(D) ∝ D−3.5 for diameter D, overpredicts the 1-cm flux, which should have been detected at over 3σ for the merged system. Preferred solutions have truncated grain populations with largest particles around 1.5–4 mm in diameter, and slightly flatter distributions, up to n(D) ∝ D−3.1. The lack of cm-sized and larger particles is reminiscent of the similar deficit inferred around comets from spacecraft encounters. Such departures from simple power-law distributions of particles have been predicted in recent models where some size regimes are more readily broken up than others. A deficit of cm-scale particles can explain the fits we obtain, and reduces the total masses inferred for the comet belts of these stars.

Water-temperature effect on the spectral density of wind gravity waves and on sea-surface roughness

The results of hourly measurements of sea roughness and hydrometeorological parameters, which were automatically taken from special buoys over a long period of time, were used. These buoys were located in the open regions of both the Atlantic and Pacific oceans in different climatic zones; the mean water-surface temperature around the buoys varies from 1–3°C to 26–28°C. In addition to measurement results, the tables contain data on the spectral density of sea roughness for a wide range of frequencies. An analysis of these data, which was made for a short-wave region of the wind-wave spectrum, for the first time revealed a noticeable watertemperature dependence of the spectral density of wind waves within the frequency range 0.30–0.40 Hz, which corresponds to wave lengths of 9-4 m. The presence of such dependence is explained by a rapid temperature change in kinematic sea-water viscosity. Earlier, we indicated the temperature dependence of only very short spectral components that relate to a centimetric wavelength range. The statistical significance of the watertemperature effect on the spectral density of waves of the indicated frequency is supported by the results of a variance analysis. Temperature variations in the parameter of sea-surface roughness, which is determined, first of all, by the energy of the spectral shortwave region, are estimated. Altimetry is the basic method which is used in remotely determining the velocity of near-water wind. This method allows one to obtain records of deviations of the sea surface from the geoid surface and to calculate (on the basis of these records) the spectral density of wave components of almost any frequency. It is known that the wave-spectrum components in the region of low frequencies are almost always affected by ripple. Consequently, the energy of these components is determined not only by wind forcing, and only the components in the range of frequencies exceeding approximately 0.3 Hz are purely windy. Therefore, using the results of sea-surface altimetry in order to determine the velocity of near-water wind, one should use the spectral densities of wave components in this frequency region. The water-temperature dependence of the spectral density of short wind waves is manifested only in a certain frequency interval, which supports this recommendation.

Radio continuum observations of Class I protostellar discs in Taurus: constraining the greybody tail at centimetre wavelengths★

We present deep 1.8 cm (16 GHz) radio continuum imaging of seven young stellar objects in the Taurus molecular cloud. These objects have previously been extensively studied in the sub-mm to NIR range and their SEDs modelled to provide reliable physical and geometrical parametres.We use this new data to constrain the properties of the long-wavelength tail of the greybody spectrum, which is expected to be dominated by emission from large dust grains in the protostellar disk. We find spectra consistent with the opacity indices expected for such a population, with an average opacity index of beta = 0.26+/-0.22 indicating grain growth within the disks. We use spectra fitted jointly to radio and sub-mm data to separate the contributions from thermal dust and radio emission at 1.8 cm and derive disk masses directly from the cm-wave dust contribution. We find that disk masses derived from these flux densities under assumptions consistent with the literature are systematically higher than those calculated from sub-mm data, and meet the criteria for giant planet formation in a number of cases.

On Rapid Interstellar Scintillation of Quasars: PKS 1257-326 Revisited

AbstractThe line of sight towards the compact, radio loud quasar PKS 1257–326 passes through a patch of scattering plasma in the local Galactic ISM that causes large and rapid, intra-hour variations in the received flux density at centimetre wavelengths. This rapid interstellar scintillation (SS) has been occurring for at least 15 years, implying that the scattering “screen” is at least 100 AU in physical extent. Through observations of the ISS we have measured microarcsecond-scale “core shifts” in PKS 1257-326, corresponding to changing opacity during an intrinsic outburst. Recent analysis of VLA data of a sample of 128 quasars found 6 sources scintillating with a characteristic time-scale of &lt; 2 hours, suggesting that nearby scattering screens in the ISM may have a covering fraction of a few percent. That is an important consideration for proposed surveys of the transient and variable radio sky.

The immediate environment of the Class 0 protostar VLA 1623, on scales of ∼50-100 au, observed at millimetre and centimetre wavelengths

We present high angular resolution observations, taken with the Very Large Array (VLA) and Multiple Element Radio Linked Interferometer Network (MERLIN) radio telescopes, at 7mm and 4.4cm respectively, of the prototype Class 0 protostar VLA1623. At 7mm we detect two sources (VLA1623A & B) coincident with the two previously detected components at the centre of this system. The separation between the two is 1.2arcsec, or ~170AU at an assumed distance of 139pc. The upper limit to the size of the source coincident with each component of VLA1623 is ~0.7arcsec, in agreement with previous findings. This corresponds to a diameter of ~100AU at an assumed distance of 139pc. Both components show the same general trend in their broadband continuum spectra, of a steeper dust continuum spectrum shortward of 7mm and a flatter spectrum longward of this. We estimate an upper limit to the VLA1623A disc mass of <0.13Msol and an upper limit to its radius of ~50AU. The longer wavelength data have a spectral index of \alpha~0.6+/-0.3. This is too steep to be explained by optically thin free-free emission. It is most likely due to optically thick free-free emission. Alternatively, we speculate that it might be due to the formation of larger grains or planetesimals in the circumstellar disc. We estimate the mass of VLA1623B to be <0.15M$sol. We can place a lower limit to its size of ~30x7 AU, and an upper limit to its diameter of ~100AU. The longer wavelength data of VLA1623B also have a spectral index of \alpha~0.6+/-0.3. The nature of VLA1623B remains a matter of debate. It could be a binary companion to the protostar, or a knot in the radio jet from VLA1623A.

William Ernest Burcham CBE. 1 August 1913 — 5 November 2008

William Ernest Burcham, known almost always as Bill, was an experimental nuclear physicist whose research was chiefly concerned with nuclear reactions and with radioactive β-decay. As an undergraduate, research student and postdoctoral student he worked in the Cavendish Laboratory in Cambridge during the last few years of Lord Rutherford’s life. During World War II he was one of those who pioneered the use of centimetre wavelengths in airborne radar. As a university teacher after the war, first in Cambridge and then in Birmingham, he helped for more than 30 years to re-establish nuclear studies as an academic discipline in the UK.

SEMIEMPIRICAL MODELS OF THE SOLAR ATMOSPHERE. III. SET OF NON-LTE MODELS FOR FAR-ULTRAVIOLET/EXTREME-ULTRAVIOLET IRRADIANCE COMPUTATION

Semiempirical atmospheric models of solar surface features as observed at moderate resolution are useful tools for understanding the observed solar spectral irradiance variations. Paper I described a set of models constructed to reproduce the observed radiance spectrum for solar surface features at ~2 arcsec resolution that constitute an average over small-scale features such as granulation. Paper II showed that a revision of previous models of low-chromospheric inter-network regions explains the observed infrared CO lines in addition to the UV and radio continuum from submillimeter to centimetric wavelengths. The present paper (1) shows that the Ca II H and K line wing observations are also explained by the new quiet-Sun-composite model, (2) introduces new low-chromospheric models of magnetic features that follow the ideas in Paper II, (3) introduces new upper chromospheric structures for all quiet-Sun and active-region models, and (4) shows how the new set of models explains EUV/FUV observations of spectral radiance and irradiance. This paper also discusses the chromospheric radiative-loss estimates in each of the magnetic features. The new set of models provides a basis for the spectral irradiance synthesis at EUV/FUV wavelengths based on the features observed on the solar surface.