Generation of Type U Bursts in Solar Radio Emission

Ledenev, V. G.

doi:10.1007/s11207-008-9192-x

Cited by 3 publications

(2 citation statements)

References 14 publications

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“…U-bursts and J-bursts, first reported by Maxwell & Swarup (1958), are believed to be the signatures of electron beams travelling along closed magnetic loops, although other alternatives exist (e.g. Haddock 1959;Takakura 1966;Ledenev 2008). The bursts form either an inverted 'U' or 'J' shape in the dynamic spectra.…”

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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“…The drift rate of bursts is around 10 MHz s −1 and the type III bursts occur every several seconds. The abrupt stop of the radio bursts at frequencies below 50 MHz strongly indicates the existence of J-bursts, where the exciting electron beam reaches the highest altitude of a closed magnetic structure and stops producing radio emission (Haddock 1959;Takakura & Kai 1966;Ledenev 2008;Reid & Kontar 2017b). At late times of the selected spectral box, meter bursts occur less frequently, and J-type bursts with frequencies down to 50−60 MHz are observed.…”

Section: Observationsmentioning

confidence: 99%

Temporal and spatial association between microwaves and type III bursts in the upper corona

Altyntsev

Reid²,

Meshalkina

et al. 2023

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One of the most important tasks in solar physics is the study of particles and energy transfer from the lower corona to the outer layers of the solar atmosphere. The most sensitive methods for detecting fluxes of non-thermal electrons in the solar atmosphere is observing their radio emission using modern large radioheliographs. We analyzed joint observations from the 13 April 2019 event observed by LOw-Frequency ARray (LOFAR) at meter wavelengths, and the Siberian Radio Heliograph (SRH) and the Badary Broadband Microwave Spectropolarimeter (BBMS) spectropolarimeter in microwaves performed at the time of the second PSP perihelion. During a period without signatures of non-thermal energy release in X-ray emission, numerous type III and/or type J bursts were observed. During the same two hours we observed soft X-ray brightenings and the appearance of weak microwave emission in an abnormally narrow band around 6 GHz. At these frequencies the increasing flux is well above the noise level, reaching 9 sfu. In the LOFAR dynamic spectrum of 53−80 MHz a region is found that lasts about an hour whose emission is highly correlated with 6 GHz temporal profile. The flux peaks in the meter waves are well correlated with extreme UV (EUV) emission variations caused by repeated surges from the bright X-point. We argue that there is a common source of non-thermal electrons located in the tail of the active region, where two loop systems of very different sizes interacted. The frequencies of type III and/or type J bursts are in accordance with large loop heights around 400 Mm, obtained by the magnetic field reconstruction. The microwave coherent emission was generated in the low loops identified as bright X-ray points seen in soft X-ray and EUV images, produced by electrons with energies several tens of keV at about twice the plasma frequency.

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