Decameter U-burst Harmonic Pair from a High Loop

Dorovskyy, V. V.; Melnik, V. N.; Konovalenko, A. A.; Bubnov, I. N.; Gridin, A. A.; Shevchuk, N. V.; Rücker, H. O.; Poedts, Stefaan; Panchenko, M.

doi:10.1007/s11207-014-0615-6

Cited by 15 publications

(21 citation statements)

References 18 publications

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“…Using a series of 29 U-bursts observed below 160 MHz Labrum & Stewart (1970) used the time associated with the width of the 'U' in the dynamic spectra at twice the turnover frequency and, together with a density model, estimated average velocities of 0.25 c. If the exciter velocity of U-bursts and J-bursts remains roughly constant then the curvature in the dynamic spectrum must correspond to a substantial decrease in the density gradient within the coronal loop, but this assumption has yet to be explored. Recently Dorovskyy et al (2015) used the time delay between the fundamental and harmonic components of a U-burst to deduce the temperature within a magnetic loop, dependent upon the velocity of the exciting electron beam.…”

Section: Introductionmentioning

confidence: 99%

Imaging spectroscopy of type U and J solar radio bursts with LOFAR

Reid

Kontar

2017

A&A

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Context. Radio U-bursts and J-bursts are signatures of electron beams propagating along magnetic loops confined to the corona. The more commonly observed type III radio bursts are signatures of electron beams propagating along magnetic loops that extend into interplanetary space. Given the prevalence of solar magnetic flux to be closed in the corona, it is an outstanding question why type III bursts are more frequently observed than U-bursts or J-bursts. Aims. We use LOFAR imaging spectroscopy between 30-80 MHz of low-frequency U-bursts and J-bursts, for the first time, to understand why electron beams travelling along coronal loops produce radio emission less often. Radio burst observations provide information not only about the exciting electron beams but also about the structure of large coronal loops with densities too low for standard EUV or X-ray analysis. Methods. We analysed LOFAR images of a sequence of two J-bursts and one U-burst. The different radio source positions were used to model the spatial structure of the guiding magnetic flux tube and then deduce the energy range of the exciting electron beams without the assumption of a standard density model. We also estimated the electron density along the magnetic flux rope and compared it to coronal models. Results. The radio sources infer a magnetic loop 1 solar radius in altitude, with the highest frequency sources starting around 0.6 solar radii. Electron velocities were found between 0.13 c and 0.24 c, with the front of the electron beam travelling faster than the back of the electron beam. The velocities correspond to energy ranges within the beam from 0.7-11 keV to 0.7-43 keV. The density along the loop is higher than typical coronal density models and the density gradient is smaller. Conclusions. We found that a more restrictive range of accelerated beam and background plasma parameters can result in U-bursts or J-bursts, causing type III bursts to be more frequently observed. The large instability distances required before Langmuir waves are produced by some electron beams, and the small magnitude of the background density gradients make closed loops less facilitating for radio emission than loops that extend into interplanetary space.

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Section: Introductionmentioning

confidence: 99%

Imaging spectroscopy of type U and J solar radio bursts with LOFAR

Reid

Kontar

2017

A&A

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“…Due to the fact that the U-and J-bursts turnover points were unambiguously determined on the timefrequency plane it became possible to determine the real frequency ratio and the time delay between the components of a harmonic pair. Thus, on basis of joint observations with the radio telescopes UTR-2 and GURT (Ukraine) held on 8 August 2012 it was found that the turnover frequencies ratio in the harmonic pair was close to 2 [22] (Fig. 2(a)).…”

Section: U-and J-burstsmentioning

confidence: 82%

Exploration of the Solar Decameter Radio Emission With the Utr-2 Radio Telescope

Melnik

Konovalenko

Dorovskyy

et al. 2021

Radio phys. radio astron.

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Purpose: The overview of the scientifi c papers devoted to the study of the solar decameter radio emission with the world’s largest UTR-2 radio telescope (Ukraine) published for the last 50 years. Design/methodology/approach: The study and analysis of the scientifi c papers on both sporadic and quiet (thermal) radiation of the Sun recorded with the UTR-2 radio telescope at the decameter wavelength range. Findings: The most signifi cant observational and theoretical results of the solar radio emission studies obtained at the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine for the last 50 years are given. Conclusions: For the fi rst time, at frequencies below 30 MHz, the Type II bursts, Type IV bursts, S-bursts, drift pairs and spikes have been recorded. The dependences of these bursts parameters on frequency within the frequency band of 9 to 30 MHz were obtained. The models of their generation and propagation were suggested. Moreover, for the fi rst time the fi ne time-frequency structures of the Type III bursts, Type II bursts, Type IV bursts, U- and J-bursts, S-bursts, and drift pairs have been observed due to the high sensitivity and high time-frequency resolutions of the UTR-2 radio telescope. The super-fi ne structure of Type II bursts with a “herringbone” structure was identifi ed, which has never been observed before. New types of bursts were discovered: “caterpillar” bursts, “dog-leg” bursts, Type III bursts with decay, Type III bursts with changing drift rate sign, Type III-like bursts, Jb- and Ub-bursts, etc. An interpretation of the unusually high drift rates and drift rates with alternating signs of the Type III-like bursts was suggested. Based on the dependence of spike durations on frequency, the coronal plasma temperature profi le at the heliocentric heights of 1.5–3RS was determined. Usage of the heliographic and interferometric methods gave the possibility to start studies of the spatial characteristics – sizes and locations of the bursts emission sources. Thus, it was shown that at the decameter band, the Type III burst durations were defi ned by the emission source linear sizes, whereas the spike durations were governed by the collision times in the source plasma. It was experimentally proved that the effective brightness temperatures of the sources of solar sporadic radio emission at the decameter band may reach values of 1014–1015 K. In addition, it was found that the radii of the quiet Sun at frequencies 20 and 25 MHz are close to the distances from the Sun at which the local plasma frequency is equal to the corresponding observed frequency of radio emission in the Baumbach–Allen model. Key words: UTR-2; Sun; decameter radio emission; radio bursts; corona

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“…There are three possible factors that can lead to the delay between lower and higher harmonics (e.g. Dorovskyy et al, 2015;Kong et al, 2016). Firstly, the group speed of the lower harmonics is smaller than that of higher ones, so it takes a longer time for the lower harmonics to propagate to the Earth from the spike source as shown in Figure 6b.…”

Section: Causes Of Temporal Delay Of Lower Harmonicsmentioning

confidence: 99%

Harmonics of Solar Radio Spikes at Metric Wavelengths

Feng

Chen

et al. 2018

Sol Phys

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This paper presents the latest observations from the newly-built solar radio spectrograph at the Chashan Solar Observatory. On July 18 2016, the spectrograph records a solar spike burst event, which has several episodes showing harmonic structures, with the second, third, and fourth harmonics. The lower harmonic radio spike emissions are observed later than the higher harmonic bands, and the temporal delay of the second (third) harmonic relative to the fourth harmonic is about 30 -40 (10) ms. Based on the electron cyclotron maser emission mechanism, we analyze possible causes of the temporal delay and further infer relevant coronal parameters, such as the magnetic field strength and the electron density at the radio source.

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Decameter U-burst Harmonic Pair from a High Loop

Cited by 15 publications

References 18 publications

Imaging spectroscopy of type U and J solar radio bursts with LOFAR

Imaging spectroscopy of type U and J solar radio bursts with LOFAR

Exploration of the Solar Decameter Radio Emission With the Utr-2 Radio Telescope

Harmonics of Solar Radio Spikes at Metric Wavelengths

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