arXiv subjects

A. A. Konovalenko

Publications and source records attributed to A. A. Konovalenko.

18 recordsLinked to original sources

arXiv2023

Spatial properties of the complex decameter type II burst observed on 31 May 2013

We present the results of observations of complex powerful type II burst associated with narrow Earth-directed CME, which was ejected at around 11 UT on 31 May 2013. The observations were performed by radio telescope UTR-2, which operated as local interferometer, providing the possibility of detection of the spatial parameters of the radio emission source. There are also polarization data from URAN-2 radio telescope. The CME was detected by two space-born coronagraphs SOHO/LASCO/C2 and STEREO/COR1-BEHIND, and was absolutely invisible for STEREO-AHEAD spacecraft. The associated type II burst consisted of two successive parts of quite different appearance on the dynamic spectrum. The first burst was narrow in frequency, had cloudy structure and was completely unpolarized while the second one represented rich herring-bone structure and exposed high degree of circular polarization. Both parts of the whole event reveal band splitting and well distinguished harmonic structure. The positions and sizes of the sources of the type II burst were found using cross-correlation functions of interferometer bases. The sources of the type II bursts elements were found to be of about 15 arcmin in size in average, with the smallest ones reaching as low as 10 arcmin. Corresponding brightness temperatures were estimated. In most cases these temperatures were between $10^{11}$ and $10^{12}$ K with maximum value as high as $10^{14}$ K. The spatial displacement of the source was measured and model independent velocities of the type II burst sources were determined.

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arXiv2020

Solar Type U Burst Associated with a High Coronal Loop

An inverted U burst with equally developed ascending and descending branches observed by Giant Ukrainian Radio Telescope (GURT) on 18 April 2017 in meter wavelengths band is discussed. This U burst was attributed to the high coronal loop above the limb active region NOAA 12651. Under the assumption that, associated with the burst, the coronal loop confines isothermal plasma stratified according to a Boltzmann density relation, the geometrical and physical parameters of the loop were estimated. According to our model coronal loops may contain plasma which is up to 20 times denser than the surrounding coronal plasma. In general the proposed model gives the relation between the plasma temperature and the height of the loop in such a way that under the given parameters of the associated U burst, higher loops contain cooler plasma and vice versa. An alternative method of coronal loop height determination was suggested. Assuming that the observed U burst and the preceding type III burst were generated by the same exciter we define the height of the loop from the delay of the former with respect to the latter at certain frequency. We show that defining of coronal loops heights by another independent method, e.g. interferometric or tied-array imaging may solve the uncertainty of the inside-the-loop plasma temperature determination.

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arXiv2020

Properties of Type III and Type IIIb Bursts in the Frequency Band of 8-80 MHz during PSP Perihelion at the Beginning of April 2019

Properties of type III and type IIIb bursts in the frequency band of 8-80 MHz observed by the radio telescopes Ukrainian Radio Interferometer of NASU-2 (URAN-2) (Poltava) and Giant Ukrainian Radio Telescope (GURT) (Kharkiv) during the Parker Solar Probe (PSP) perihelion in April 2019 are discussed. These correspond of those that were observed by PSP at frequencies <19 MHz. We analyze dependences of drift rates and durations on frequency for these bursts. We show that drift rate dependences on frequency agree well with those derived from the Newkirk corona if source velocities are between 0.17 and 0.2 c for both type IIIb bursts and type III bursts under the assumption that the first ones are fundamentals and the second ones are their harmonics. However, all observational dependences are flatter in comparison with the dependences for a Newkirk corona. We assume that this can be related with coronal temperature decreasing at heliocentric distances from 1.35 to 6.5 solar radii. Duration dependencies of type IIIb and type III bursts on frequency in the range of 10-70 MHz are also obtained. We note that the durations and drift rates of type III bursts as well as their dependences on frequency differ essentially from those for type IIIb bursts.

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arXiv2020

Radio signature of a distant behind-the-limb CME on 2017 September 6

We discuss properties of a Type IV burst, which was observed on 2017 September 6, as a result of the powerful flare X 9.3. At decameter wavelengths this burst was observed by the radio telescopes STEREO A, URAN-2, and NDA at frequencies 5 - 35 MHz. This moving Type IV burst was associated with a coronal mass ejection (CME) propagating in the southwest direction with a speed of 1570 km/s. The maximum radio flux of this burst was about 300 s.f.u. and the polarization was more than 40%. In the frequency range of 8-33 MHz it continued for more than 2 hr. For STEREO A the associated CME was behind the limb, its longitudinal angle was about 160 degrees. This moving Type IV burst was observed by STEREO A at frequencies of 5-15 MHz in spite of the low sensitivity of STEREO A. This means that the radio emission directivity of a Type IV burst is rather wide. Assuming the plasma mechanism of Type IV radio emission we derived the plasma density distribution in the CME core at distances of 5.6 Rs and 9.8 Rs and its mass to be about 10^16 g . It is planned that the minimum perihelion of the Parker Solar Probe (PSP) spacecraft will be at about 9 Rs. So we discuss in what conditions PSP will be in if it crosses a similar CME core.

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arXiv2018

Solar bursts as can be observed from the lunar farside with a single antenna at very low frequencies

Earth-based observations are complicated by the opacity of Earth's ionosphere at very low frequencies and strong man-made radio frequency interference. This explains long standing interest in building a low frequency radio telescope on the farside of the Moon. Experience from ground-based observations near the ionospheric cutoff in dealing with the interference, ionosphere, and wide-field imaging/dynamic range problems provides crucial information for future radioastronomic experiments on the Moon. In this purpose we observed non-intensive solar bursts on the example of solar drift pairs (DP) at decameter-meter wavelengths with large and small arrays as well as by a single crossed active dipole. We used the large Ukrainian radio telescope UTR-2, the URAN-2 array, a subarray of the Giant Ukrainian radio telescope (GURT) and a single crossed active dipole to get the spectral properties of radio bursts at the frequency range of 8-80 MHz during solar observations on July 12, 2017. Statistical analysis of upper and lower frequencies, at which DPs are recorded, shows that the occurrence of forward DPs is more preferable at lower frequencies of the decameter range of observations in comparison with reverse DPs generated more likely at meter wavelengths. We conclude that DPs can be detected not only by antenna arrays, but even by a single crossed active dipole. Thus the latter antenna has a good potential for future low-frequency radio telescopes on the Moon.

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arXiv2018

Revisiting the frequency drift rates of decameter type III solar bursts observed in July-August 2002

Estimating for the frequency drift rates of type III solar bursts is crucial for characterizing their source development in solar corona. According to Melnik et al. (Solar Phys. 269, 335, 2011), the analysis of powerful decameter type III solar bursts, observed in July-August 2002, found a linear approximation for the drift rate versus frequency. The conclusion contradicts to reliable results of many other well-known solar observations. In this paper we report on the reanalysis of the solar data, using a more advanced method. Our study has shown that decameter type III solar bursts of July-August 2002, as standard type III ones, follow a power law in frequency drift rates. We explain possible reasons for this discrepancy.

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arXiv2018

Decameter Type IV Burst Associated with a behind-the-limb CME Observed on 7 November 2013

We report on the results of observations of a type IV burst by URAN-2 (Ukrainian Radio interferometer of Academy Scienses) in the frequency range 22 - 33 MHz, which is associated with the CME (coronal mass ejection) initiated by a behind-the-limb active region (N05E151). This burst was observed also by the radio telescope NDA (Nancay Decameter Array) in the frequency band 30 - 60 MHz. The purpose of the article is the determination of the source of this type IV burst. After analysis of the observational data obtained with the URAN-2, NDA, STEREO (Solar-Terrestrial Relations Observatory) A and B spacecraft, and SOHO (Solar and Heliospheric Observatory)spacecraft we come to the conclusion that it is a core of a behind-the-limb CME. We conclude that the radio emission can escape the center of the CME core at a frequency of 60 MHz and originates from the periphery of the core at frequency 30 MHz due to occultation by the solar corona at corresponding frequencies. We find plasma densities in these regions supposing the plasma mechanism of radio emission. We show that the frequency drift of the start of the type IV burst is governed by an expansion of the CME core. Type III bursts, which were observed against this type IV burst, are shown to be generated by fast electrons propagating through the CME core plasma. A type II burst registered at frequencies 44 - 64 MHz and 3 - 16 MHz was radiated by a shock with a velocity of about 1000 km s^{-1} and 800 km s^{-1}, respectively.

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arXiv2018

Decameter Type III Bursts with Changing Frequency Drift-Rate Signs

We discuss properties of type III bursts which change sign of their drift rate from negative to positive and vice versa. Moreover such bursts may change sign of their drift rates more than once. These specific type III bursts were observed simultaneously by radio telescopes UTR-2, URAN-2 and NDA in frequency band 8-41 MHz. The negative drift rates of these bursts are close to those of usual decameter type III bursts and variate from -0.84 MHz/s to -5.56 MHz/s. The positive drift rates of specific type III bursts vary in the wider range from 0.44MHz/s to 12 MHz/s. Unlike inverted U-bursts these type III bursts still drift from the high frequencies to the low frequencies in spite of the change of the drift rates signs. Our basic explanation of the positive drift rate of these type III burst differs from the common assumption that positive drift rates of Type III bursts are connected with electron beam propagation towards the Sun. We propose that, even if electron beams move outward from the Sun, they can generate type III bursts with positive drift rates if in some regions of the solar corona the group velocities of type III radio emissions are lower than the velocities of the electron beams.

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arXiv2018

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.

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arXiv2018

Solar Type IV bursts at frequencies 10-30 MHz

The results of the first observations of Type IV bursts at frequencies 10-30 MHz are presented. These observations were carried out at radio telescopes UTR-2 (Kharkov, Ukraine) and URAN-2 (Poltava, Ukraine) during the period 2003-2006. Detection of Type IV bursts in wide band from 10 to 30MHz with high sensitivity and time resolution allowed to study their properties in details. These bursts have fluxes 10-2000s.f.u. at maximum phase. Their durations are about 1-2 hours and even more. Some of Type IV bursts drift from high to low frequencies with drift rates about 10kHz/s. All observed Type IV bursts have fine structures in the form of sub-bursts with durations from 2s to 20s and frequency drift rates in a majority of 1-2MHz/s. In most cases, sub-bursts with negative drift rates were registered. Sometimes sub-bursts in absorption with durations 10-200s against Type IV burst background have been observed. The Type IV burst observed on July 22, 2004 had zebra structure, in which single zebra stripes had positive, negative and infinite drift rates.

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arXiv2016

Separating Nightside Interplanetary and Ionospheric Scintillation with LOFAR

Observation of interplanetary scintillation (IPS) beyond Earth-orbit can be challenging due to the necessity to use low radio frequencies at which scintillation due to the ionosphere could confuse the interplanetary contribution. A recent paper by Kaplan {\it et al} (2015) presenting observations using the Murchison Widefield Array (MWA) reports evidence of night-side IPS on two radio sources within their field of view. However, the low time cadence of 2\,s used might be expected to average out the IPS signal, resulting in the reasonable assumption that the scintillation is more likely to be ionospheric in origin. To verify or otherwise this assumption, this letter uses observations of IPS taken at a high time cadence using the Low Frequency Array (LOFAR). Averaging these to the same as the MWA observations, we demonstrate that the MWA result is consistent with IPS, although some contribution from the ionosphere cannot be ruled out. These LOFAR observations represent the first of night-side IPS using LOFAR, with solar wind speeds consistent with a slow solar wind stream in one observation and a CME expecting to be observed in another.

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arXiv2015

Unusual Solar Radio Burst Observed at Decameter Wavelengths

An unusual solar burst was observed simultaneously by two decameter radio telescopes UTR-2 (Kharkov, Ukraine) and URAN-2 (Poltava, Ukraine) on 3 June 2011 in the frequency range 16-28 MHz. The observed radio burst has some unusual properties, which are not typical for the other types of solar radio bursts. The frequency drift rate of it was positive (about 500 kHz s$^{-1}$) at frequencies higher than 22 MHz and negative (100 kHz s$^{-1}$) at lower frequencies. The full duration of this event varies from 50 s up to 80 s, depending on the frequency. The maximum radio flux of the unusual burst reaches $\approx 10^3$ s.f.u and its polarization does not exceed 10%. This burst has a fine frequency-time structure of unusual appearance. It consists of stripes with the frequency bandwidth 300-400 kHz. We consider that several accompanied radio and optical events observed by SOHO and STEREO spacecraft are possibly associated with the reported radio burst. A model that may interpret the observed unusual solar radio burst is proposed.

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arXiv2015

Fine and superfine structure of Decameter-Hectometer type II burst on 2011 June 7

The characteristics of the type II bursts with herringbone structure observed both by ground based radio telescopes (UTR-2, URAN-2) and spaceborn spectrometers (STEREO A-B) are discussed. The burst was recorded on 7 June, 2011 in the frequency band 3--33~MHz. It was characterized by extremely rich fine structure. The statistical analysis of more than 300 herringbone sub-bursts constituting the burst was performed separately for the positively (reverse) and negatively (forward) drifting sub-bursts. The sense and the degree of circular polarization of the herringbone sub-bursts were measured in the respectively wide frequency band (16--32~MHz). A second order fine frequency structure of the herringbone sub-bursts was firstly observed and processed. Using STEREO COR1 (A,B) and SOHO LASCO C2 images the direction and radial speed of the CME responsible for the studied type II burst were determined. The possible location of the type II burst source on the flank of the shock was found.

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arXiv2015

LOFAR tied-array imaging and spectroscopy of solar S bursts

Context. The Sun is an active source of radio emission that is often associated with energetic phenomena ranging from nanoflares to coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), numerous millisecond duration radio bursts have been reported, such as radio spikes or solar S bursts (where S stands for short). To date, these have neither been studied extensively nor imaged because of the instrumental limitations of previous radio telescopes. Aims. Here, Low Frequency Array (LOFAR) observations were used to study the spectral and spatial characteristics of a multitude of S bursts, as well as their origin and possible emission mechanisms. Methods. We used 170 simultaneous tied-array beams for spectroscopy and imaging of S bursts. Since S bursts have short timescales and fine frequency structures, high cadence (~50 ms) tied-array images were used instead of standard interferometric imaging, that is currently limited to one image per second. Results. On 9 July 2013, over 3000 S bursts were observed over a time period of ~8 hours. S bursts were found to appear as groups of short-lived (<1 s) and narrow-bandwidth (~2.5 MHz) features, the majority drifting at ~3.5 MHz/s and a wide range of circular polarisation degrees (2-8 times more polarised than the accompanying Type III bursts). Extrapolation of the photospheric magnetic field using the potential field source surface (PFSS) model suggests that S bursts are associated with a trans-equatorial loop system that connects an active region in the southern hemisphere to a bipolar region of plage in the northern hemisphere. Conclusions. We have identified polarised, short-lived solar radio bursts that have never been imaged before. They are observed at a height and frequency range where plasma emission is the dominant emission mechanism, however they possess some of the characteristics of electron-cyclotron maser emission.

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arXiv2014

LOFAR tied-array imaging of Type III solar radio bursts

The Sun is an active source of radio emission which is often associated with energetic phenomena such as solar flares and coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), the Sun has not been imaged extensively because of the instrumental limitations of previous radio telescopes. Here, the combined high spatial, spectral and temporal resolution of the Low Frequency Array (LOFAR) was used to study solar Type III radio bursts at 30-90 MHz and their association with CMEs. The Sun was imaged with 126 simultaneous tied-array beams within 5 solar radii of the solar centre. This method offers benefits over standard interferometric imaging since each beam produces high temporal (83 ms) and spectral resolution (12.5 kHz) dynamic spectra at an array of spatial locations centred on the Sun. LOFAR's standard interferometric output is currently limited to one image per second. Over a period of 30 minutes, multiple Type III radio bursts were observed, a number of which were found to be located at high altitudes (4 solar radii from the solar center at 30 MHz) and to have non-radial trajectories. These bursts occurred at altitudes in excess of values predicted by 1D radial electron density models. The non-radial high altitude Type III bursts were found to be associated with the expanding flank of a CME. The CME may have compressed neighbouring streamer plasma producing larger electron densities at high altitudes, while the non-radial burst trajectories can be explained by the deflection of radial magnetic fields as the CME expanded in the low corona.

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arXiv2013

Radio seismology of the outer solar corona

Observed oscillations of coronal loops in EUV lines have been successfully used to estimate plasma parameters in the inner corona (< 0.2 R_0, where R_0 is the solar radius). However, coronal seismology in EUV lines fails for higher altitudes because of rapid decrease in line intensity. We aim to use radio observations to estimate the plasma parameters of the outer solar corona (> 0.2 R_0). We use the large Ukrainian radio telescope URAN-2 to observe type IV radio burst at the frequency range of 8-32 MHz during the time interval of 09:50-12:30 UT in April 14, 2011. The burst was connected to C2.3 flare, which occurred in AR 11190 during 09:38-09:49 UT. The dynamic spectrum of radio emission shows clear quasi-periodic variations in the emission intensity at almost all frequencies. Wavelet analysis at four different frequencies (29 MHz, 25 MHz, 22 MHz and 14 MHz) shows the quasi-periodic variation of emission intensity with periods of 34 min and 23 min. The periodic variations can be explained by the first and second harmonics of vertical kink oscillation of transequatorial coronal loops, which were excited by the same flare. The apex of transequatorial loops may reach up to 1.2 R_0 altitude. We derive and solve the dispersion relation of trapped MHD oscillations in a longitudinally inhomogeneous magnetic slab. The analysis shows that a thin (with width to length ratio of 0.1), dense (with the ratio of internal and external densities of > 20) magnetic slab with weak longitudinal inhomogeneity may trap the observed oscillations. Seismologically estimated Alfvén speed inside the loop at the height of 1 R_0 is 1000 km/s. Then the magnetic field strength at this height is estimated as 0.9 G. Extrapolation of magnetic field strength to the inner corona gives 10 G at the height of 0.1 R_0.

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arXiv2007

Decameter Type III-Like Bursts

Starting from 1960s Type III-like bursts (Type III bursts with high drift rates) in a wide frequency range from 300 to 950MHz have been observed. These new bursts observed at certain frequency being compared to the usual Type III bursts at the same frequency show similar behaviour but feature frequency drift 2-6 times higher than the normal bursts. In this paper we report the first observations of Type III-like bursts in decameter range, carried out during summer campaigns 2002 - 2004 at UTR-2 radio telescope. The circular polarization of the bursts was measured by the radio telescope URAN-2 in 2004. The observed bursts are analyzed and compared with usual Type III bursts in the decameter range. From the analysis of over 1100 Type III-like bursts, their main parameters have been found. Characteristic feature of the observed bursts is similar to Type III-like bursts at other frequencies, i.e. measured drift rates (5-10 MHz/s) of this bursts are few times larger than that for usual Type III bursts, and their durations (1-2 s) are few times smaller than that for usual Type III bursts in this frequency band.

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arXiv2006

Radio recombination lines from the largest bound atoms in space

In this paper, we report the detection of a series of radio recombination lines (RRLs) in absorption near 26 MHz arising from the largest bound carbon atoms detected in space. These atoms, which are more than a million times larger than the ground state atoms are undergoing delta transitions (n~1009, Delta n=4) in the cool tenuous medium located in the Perseus arm in front of the supernova remnant, Cassiopeia A. Theoretical estimates had shown that atoms which recombined in tenuous media are stable up to quantum levels n~1500. Our data indicates that we have detected radiation from atoms in states very close to this theoretical limit. We also report high signal-to-noise detections of alpha, beta and gamma transitions in carbon atoms arising in the same clouds. In these data, we find that the increase in line widths with quantum number (proportional to n^5) due to pressure and radiation broadening of lines is much gentler than expected from existing models which assume a power law background radiation field. This discrepancy had also been noted earlier. The model line widths had been overestimated since the turnover in radiation field of Cassiopeia A at low frequencies had been ignored. In this paper, we show that, once the spectral turnover is included in the modeling, the slower increase in line width with quantum number is naturally explained.

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