Prospects of Solar Microwave Observations at the Ventspils Radio-Astronomy Center
The paper concerns the prospects, technical issues and unique possibilities of solar microwave observations using the RT-32 radio telescope of the Ventspils International Radio-Astronomy Center. The solar physics tasks are discussed that are to be solved based on the observations of the large-scale coronal structures and radio flux fluctuations of as well as the magnetic structure of solar active regions.
The reflection and diffraction of external communication and navigational transmitters from tall constructions and moving blades of wind turbines produce some short-pulse additional electromagnetic interference strong enough to fully disturb radio astronomical observations. The problem of short-pulse electromagnetic interference is distinctive to all radio telescopes surrounded by wind turbines. This problem became significant for Ventspils International Radio Astronomy Centre (VIRAC) after new wind park “Platene” of Winergy Ltd. was built in 2012 and radio telescopes RT-16 and RT-32 renovated and equipped with cryogenic high sensitive receivers. The paper deals with the analysis and evaluation of intensities and probabilities of short-pulse interferences produced by wind park “Platene” and its possible impact on radio astronomical observations at VIRAC radio telescopes.
The authors perform a comparative analysis of the efficiency of two types of low-power wind energy conversion systems with horizontal and vertical axis in the meteorological conditions of Latvia. The analysis is based on long-term wind speed measurements over the period of two years conducted by a network of 22 observation stations at the height of 10 m above the ground. The study shows that in the conditions of Latvia wind turbines with a horizontal axis are expected to work with greater efficiency than similar installations with a vertical axis. The paper presents the models of the spatial distribution of average wind speed, Weibull wind speed frequency distribution parameters and the values of the expected operational efficiency for small wind turbine generators. The modelling results are presented in the form of colour contour maps. Overall, the results of the study can serve as a tool for forecasting annual energy production and for estimating the feasibility of commercial use of wind energy at the height of 10 m in the territory of Latvia.
The paper studies spatial wind energy flow distribution in Latvia based on wind speed measurements carried out at an altitude of 10 m over a period of two years, from 2015 to 2016. The measurements, with 1 min increments, were carried out using certified measuring instruments installed at 22 observation stations of the Latvian National Hydrometeorological and Climatological Service of the Latvian Environment, Geology and Meteorology Centre (LEGMC). The models of the spatial distribution of averaged wind speed and wind energy density were developed using the method of spatial interpolation based on the historical measurement results and presented in the form of colour contour maps with a 1×1 km resolution. The paper also provides the results of wind speed spatial distribution modelling using a climatological reanalysis ERA5 at the altitudes of 10, 54, 100 and 136 m with a 31×31 km resolution. The analysis includes the comparison of actual wind speed measurement results with the outcomes of ERA5 modelling for meteorological observation stations in Ainazi, Daugavpils, Priekuli, Saldus and Ventspils.
Application of Recovering Procedures to RT-32 Radio Maps of the Sun
The generalized maximum entropy method (GMEM) is proven to recover both positive and negative local microwave sources on the 2.7 cm maps of the Sun taken with the radiotelescope RT-32 of the Ventspils International Radio Astronomy Center (VIRAC). The maps of the Sun need to be recovered due to high intensity of the side lobes of the RT-32 diagram pattern - up to 30% at 2.7 cm. The presence of negative local sources associated with Hα filaments is inferred from the 2.7 cm maps of the Sun (RT-32) compared with the 1.76 cm maps of the Sun (Nobeyama Radio Heliograph).
Sharp Changes in the Ordinary Mode Microwave Emission from a Stable Sunspot: Model Analysis
In the work, interpretation is given for a microwave source associated with a stable isolated sunspot as a sample to study the sharp changes in ordinary mode microwave emission. The appropriate microwave source of the solar active region NOAA 10325 was observed with the radio telescope RATAN-600 and the Nobeyama Radio Heliograph during March 30 - April 4, 2003. The radio emission brightness in ordinary mode is shown to be depressed below the brightness of the quiet Sun when the sunspot was near the solar disk centre (April 2-3) but sharply increased in the wavelength range 1.76-2.67 cm at the longitudes |θ| > 50°.
Microwave radiation by gyroresonance mechanism is analysed in relation to the particular features of the images taken in He I 10830 Å chromospheric line (Chromospheric Helium-I Imaging Photometer) and EUV emission lines (Coronal Diagnostic Spectrometer). We have constructed a new model according to which above the sunspot's surroundings at the levels of gyroresonance emission the atmosphere is as cold as a "sunspot plume" (T ~ 105-106 K, N ~ 109 cm-3). As could be supposed from the He I line intensity, the central part of the sunspot atmosphere contains open magnetic field and rarefied plasma (N ~ 108 cm-3, T ~ 105.5-106 K).
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.
The present paper deals with the calibration method of interferometers with antennas having a small effective area, on the quasinoise signals of GLONASS and GPS navigation satellites. Algorithms for calculation of antenna coordinates and instrumental delay from the analysis of correlation interferometer response to signals of satellites in the near field of the instrument were reviewed. The method was tested in VLBI experiments on interferometers with medium and large baselines that included radio telescopes of NIRFI and VIRAC. The values of the antenna coordinates and instrumental delay with an error within the limits of one discrete were obtained. The sources of measurement errors and ways to improve the accuracy of results were analysed.
The microwave regions with low brightness temperature are found to overlap the regions of the depressed coronal emission and open field lines at the periphery of two solar active regions (ARs). The imaging microwave observations of the Sun with the Nobeyama Radio heliograph at 1.76 cm, the MRO-14 radio telescope of Metsähovi Radio Observatory at 0.8 cm, and the RT-32 of Ventspils International Radio Astronomy Centre in the range 3.2-4.7 cm are used. To reduce the noise in the intensity distribution of the RT-32 maps of the Sun, one wavelet plane of “à trous” wavelet space decomposition is subtracted from each map. To locate the open-field regions, the full-Sun coronal magnetic fields with the potential field source surface (PFSS) model for RSS = 1.8 Rʘ are simulated. We conclude that the revealed LTRs present narrow coronal hole-like regions near two ARs and imply an extra investigation on the plasma outflow.