Alexander A. Konovalenko

Decameter Type III-Like Bursts

We report the first observations of Type III-like bursts at frequencies 10 30 MHz. More than 1000 such bursts during 2002 2004 have been analyzed. The frequency drift of these bursts is several times that of decameter Type III bursts. A typical duration of the Type III-like bursts is 1 2 s. These bursts are mainly observed when the source active region is located within a few days from the central meridian. The drift rate of the Type III-like bursts can take a large value by considering the velocity of Type III electrons and the group velocity of generated electromagnetic waves.

Fine Structure of Decametric Type II Radio Bursts

We have performed a comparative analysis of the fine structure of two decametric type II bursts observed on July 17 and August 16, 2002, with the 1024-channel spectrograph of the UTR-2 radio telescope in the frequency range 18.5–29.5 MHz and with the IZMIRAN spectrograph in the frequency range 25–270 MHz. The August 16 burst was weak, ∼2–5 s.f.u., but exhibited an unusual fine structure in the form of broadband fibers (Δfe > 250–500 kHz) that drifted at a rate characteristic of type II bursts and consisted of regular narrow-band fibers (Δfe > 50–90 kHz at 24 MHz) resembling a rope of fibers. The July 17 burst was three orders of magnitude more intense (up to 4500 s.f.u. at 20 MHz) and included a similar fiber structure. The narrow fibers were irregular and shorter in duration. They differed from an ordinary rope of fibers by the absence of absorption from the low-frequency edge and by slow frequency drift (slower than that of a type II burst). Both type II bursts were also observed in interplanetary space in the WIND/WAVES RAD2 spectra, but without any direct continuation. Analysis of the corresponding coronal mass ejections (CMEs) based on SOHO/LASCO C2 data has shown that the radio source of the type II burst detected on August 16 with UTR-2 was located between the narrow CME and the shock front trailing behind that was catching up with the CME. The July 17 type II fiber burst also occurred at the time when the shock front was catching up with the CME. Under such conditions, it would be natural to assume that the emission from large fibers is related to the passage of the shock front through narrow inhomogeneities in the CME tail. Resonant transition radiation may be the main radio emission mechanism. Both events are characterized by the possible generation of whistlers between the leading CME edge and the shock front. The whistlers excited at shock fronts manifest themselves only against the background of enhanced emission from large fibers (similar to the continuum modulation in type IV bursts). The reduction in whistler group velocity inside inhomogeneities to 760 km s−1 may be responsible for the unusually low drift rate of the narrow fibers. The magnetic field inside inhomogeneities determined from fiber parameters at 24 MHz is ∼0.9 G, while the density should be increased by at least a factor of 2.

Sensitivity of an Active Antenna Array Element for the Low-Frequency Radio Telescope GURT

The recently developed new generation, low-frequency Giant Ukrainian Radio Telescope (GURT) is built nearby the well-known Ukrainian T-shaped Radio Telescope. The new facility employs a phased antenna array composed of many subarrays of

State-of-the-art of low frequency radio astronomy, relevant antenna systems and international cooperation in Ukraine

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The decameter spikes as a tool for the coronal plasma parameters determination

active antenna elements. In this paper, the parameters of the active antenna used as array element are studied, with special attention paid to sensitivity. The electrical and noise parameters are calculated using computer simulation and wave techniques for noise modeling of two-port networks. The results of numerical calculations of the sensitivity are given in terms of the sky noise dominance (SND) and system equivalent flux density of the GURT element within 10–80 MHz. The calculated results are compared with in situ measurements.

SNRs of two active antenna designs: Inverted V vs horizontal dipole

The low frequency radio astronomy (decameter-meter range, frequencies of 10-300 MHz) currently demonstrates rapid progress all over the world. New generations of large antennas - LOFAR, LWA, MWA and others - have been created in many countries. At the same time Ukrainian radio astronomical systems UTR-2 and URAN still remain the largest and most informative ones at the lowest frequency range available for the ground-based radio astronomy (below 33 MHz), especially after their radical modernization during the most recent years. A great number of top priority results have been obtained on the basis of these radio telescopes. The results prove a high significance of the low frequency radio astronomy for astrophysics. Substantial part of these results have been obtained in the course of many year cooperation between Ukraine on one side and France, Austria, Germany and other countries on the other. Creation of new low frequency instruments GURT (Ukraine) and LSS/NenuFAR (France) for the wide frequency range of 10-80 MHz opens up new possibilities for research and fruitful cooperation.

Effect of the ground screen on sensitivity of low-frequency radio telescope array element

The paper is devoted to the analysis of the unusual event observed on 14 June 2012 in the frequency range 8–42 MHz. During this event the radiation flux changed stepwise two times. Assuming that these changes of radiation flux could be associated with the changes of the coronal plasma parameters (temperature, magnetic field) and using spikes as a tool for the determination of those parameters we traced how the temperature and magnetic field varied during the time of observations. According to the model proposed in the paper the magnetic field was about 1.9 G and the temperature varied in the range of 0.1–0.6 × 106 K at the heights 1.6–3.3 solar radii.

Variation of phasing system parameters of GURT active antenna subarray in a wide scan range

We present the results of numerical analysis of two design versions of active antenna which were considered for using in phased antenna array of GURT radio telescope at the stage of preliminary design. Both antenna versions are supplied with the same preamplifier but their dipole shapes differ. The first active antenna version has linear horizontal dipole, the second one — inverted V-dipole. The differences of input impedances, efficiencies, and mismatch coefficients of dipoles with preamplifier as well as the effect of these parameters on noise temperatures and SNR of antennas are analyzed in detail.

Observations of the Solar Continuum Radio Emission at Decameter Wavelengths

The Giant Ukrainian Radio Telescope (GURT) is a new generation radio telescope with active phased antenna array intended to operate within 10–80 MHz range. The radio telescope is being constructed now in Ukraine. The array elements are active antennas in form of flat wire dipoles integrated with low-noise preamplifiers. In the paper we present a comparative analysis of two GURT array element versions that differ in presence or absence of wire grid ground screen under the dipole. The analysis results are given and discussed.

30-element active antenna array as a prototype of a huge low-frequency radio telescope

In the abstract the results of transfer coefficient calculations of active antenna subarray of GURT radio telescope for all possible scan directions are presented. Also these results are supplied by measurements of signal changing with beam switching at subarray of GURT radio telescope which prove the correctness of the computations. The presented results could be useful for designing of new active antenna arrays and interpreting data obtained at present antenna arrays.