scholarly journals LOFAR observations of radio burst source sizes and scattering in the solar corona

2020 ◽  
Vol 645 ◽  
pp. A11
Author(s):  
Pearse C. Murphy ◽  
Eoin P. Carley ◽  
Aoife Maria Ryan ◽  
Pietro Zucca ◽  
Peter T. Gallagher
Keyword(s):  
Solar Corona ◽  
Radio Source ◽  
Radio Wave ◽  
Radio Burst ◽  
Wave Scattering ◽  
Low Frequency ◽  
Density Fluctuations ◽  
Source Size ◽  
Burst Source ◽  
Low Frequencies

Low frequency radio wave scattering and refraction can have a dramatic effect on the observed size and position of radio sources in the solar corona. The scattering and refraction is thought to be due to fluctuations in electron density caused by turbulence. Hence, determining the true radio source size can provide information on the turbulence in coronal plasma. However, the lack of high spatial resolution radio interferometric observations at low frequencies, such as with the LOw Frequency ARray (LOFAR), has made it difficult to determine the true radio source size and level of radio wave scattering. Here we directly fit the visibilities of a LOFAR observation of a Type IIIb radio burst with an elliptical Gaussian to determine its source size and position. This circumvents the need to image the source and then de-convolve LOFAR’s point spread function, which can introduce spurious effects to the source size and shape. For a burst at 34.76 MHz, we find full width at half maximum (FWHM) heights along the major and minor axes to be 18.8′ ± 0.1′ and 10.2′ ± 0.1′, respectively, at a plane of sky heliocentric distance of 1.75 R⊙. Our results suggest that the level of density fluctuations in the solar corona is the main cause of the scattering of radio waves, resulting in large source sizes. However, the magnitude of ε may be smaller than what has been previously derived in observations of radio wave scattering in tied-array images.

scholarly journals LOFAR Imaging of the Solar Corona during the 2015 March 20 Solar Eclipse

2021 ◽  
Author(s):  
Aoife Maria Ryan ◽  
Peter T. Gallagher ◽  
Eoin P. Carley ◽  
Michiel A. Brentjens ◽  
Pearse C. Murphy ◽  
...  
Keyword(s):  
Solar Corona ◽  
Spatial Resolution ◽  
Solar Eclipse ◽  
Electromagnetic Waves ◽  
Imaging Techniques ◽  
Active Regions ◽  
Density Fluctuations ◽  
Source Size ◽  
Interferometric Imaging ◽  
Low Frequencies

<p>The solar corona is a highly-structured plasma which can reach temperatures of more than 2 MK. At low frequencies (decimetric and metric wavelengths), scattering and refraction of electromagnetic waves are thought to considerably increase the imaged radio source sizes (up to a few arcminutes). However, exactly how source size relates to scattering due to turbulence is still subject to investigation. The theoretical predictions relating source broadening to propagation effects have not been fully confirmed by observations, due to the rarity of high spatial resolution observations of the solar corona at low frequencies. Here, the LOw Frequency ARray (LOFAR) was used to observe the solar corona at 120–180 MHz using baselines of up to 3.5 km (corresponding to a resolution of 1–2’) during the partial solar eclipse of 2015 March 20. A lunar de-occultation technique was used to achieve higher spatial resolution (0.6’) than that attainable via standard interferometric imaging (2.4’). This provides a means of studying the contribution of scattering to apparent source size broadening. This study shows that the de-occultation technique can reveal a more structured quiet corona that is not resolved from standard imaging, implying scattering may be overestimated in this region when using standard imaging techniques. However, an active region source was measured to be 4’ using both de-occultation and standard imaging. This may be explained by increased scattering of radio waves by turbulent density fluctuations in active regions, which is more severe than in the quiet Sun.</p><p><br><br></p>

scholarly journals Radio Source Surveys

2002 ◽  
Vol 199 ◽  
pp. 3-10
Author(s):  
J. J. Condon
Keyword(s):  
Radio Source ◽  
Source Parameters ◽  
Low Frequency ◽  
Low Frequencies ◽  
Cluster Emission ◽  
New Class ◽  
Basic Source ◽  
Nearby Galaxies ◽  
Source Populations ◽  
Compact Sources

The primary goal of radio source surveys is to generate flux-limited samples. Sources selected at very low frequencies are dominated by unbeamed emission and give the only unbiased view of the parent populations used by “unification” models to account for the diversity of sources seen at high frequencies. Low-frequency surveys favor sources with exceptionally steep spectra. They include radio galaxies at high redshifts, radio halos of nearby galaxies, relic radio sources, diffuse cluster emission, pulsars that may be missed by traditional pulse searches, and a new class of unidentified compact sources. Flux densities from low-frequency surveys extend the spectra of known source populations to frequencies at which free-free and synchrotron absorption become significant and constrain basic source parameters. Finally, telescope fields-of-view scale ∝ λ2, so gridded surveys can be more efficient than directed observations of individual targets. This review covers recent and proposed low-frequency source surveys and their astronomical uses.

Imaging the Solar Corona during the 2015 March 20 Eclipse using LOFAR

2020 ◽  
Author(s):  
Aoife Maria Ryan ◽  
Peter T. Gallagher ◽  
Eoin P. Carley ◽  
Diana E. Morosan ◽  
Michiel A. Brentjens ◽  
...  
Keyword(s):  
Solar Corona ◽  
Spatial Resolution ◽  
Electromagnetic Waves ◽  
High Spatial Resolution ◽  
Low Frequency ◽  
Angular Width ◽  
Interferometric Imaging ◽  
Low Frequencies ◽  
Point Spread ◽  
Spread Function

<p>The solar corona is a highly-structured plasma which reaches temperatures of more than ~2MK. At low radio frequencies (≤ 400 MHz), scattering and refraction of electromagnetic waves are thought to broaden sources to several arcminutes. However, exactly how source size relates to scattering due to turbulence is still subject to investigation. This is mainly due to the lack of high spatial resolution observations of the solar corona at low frequencies. Here, we use the LOw Frequency ARray (LOFAR) to observe the solar corona at 120-180 MHz using baselines of up to ~3.5 km (~1--2’) during a partial solar eclipse of 2015 March 20. We use a lunar de-occultation technique to achieve higher spatial resolution than that attainable via traditional interferometric imaging. This provides a means of studying source sizes in the corona that are smaller than the angular width of the interferometric point spread function. </p>

scholarly journals Anisotropic radio-wave scattering from englacial water regimes, Mýrdalsjökull, Iceland

2007 ◽  
Vol 53 (182) ◽  
pp. 473-478 ◽  
Author(s):  
Kenichi Matsuoka ◽  
Throstur Thorsteinsson ◽  
Helgi Björnsson ◽  
Edwin D. Waddington
Keyword(s):  
Radio Wave ◽  
Wave Scattering ◽  
Flow Direction ◽  
Low Frequency ◽  
Radio Waves ◽  
Glacier Surface ◽  
Ice Flow ◽  
Scattering Model ◽  
Water Regimes ◽  
Strong Scattering

AbstractColinear-polarized 5 MHz radar profiling data were obtained on Mýrdalsjökull, a temperate glacier in Iceland. Radar transects, and therefore polarization planes, were aligned approximately parallel, transverse and oblique to the ice flow direction. Echoes from the shallower half to two-thirds of the ice were 10–20dB stronger on the oblique and longitudinal transects than those on the transverse transects. Anisotropy as a function of depth is clearly seen at the sites where the transects cross. Strong scattering on longitudinal transects apparently caused extinction of a radar-reflecting layer that was continuously profiled on the transverse transects. A radio-wave scattering model shows that scattering from a longitudinal water-filled conduit parallel to the glacier surface can explain the observed azimuthal variations of the echo. We conclude that low-frequency (~MHz) radio waves can help to characterize englacial water regimes.

scholarly journals LOFAR observations of a jet-driven piston shock in the low solar corona

2021 ◽  
Author(s):  
Ciara Maguire ◽  
Eoin Carley ◽  
Pietro Zucca ◽  
Nicole Vilmer ◽  
Peter Gallagher
Keyword(s):  
Radio Burst ◽  
Extreme Ultraviolet ◽  
Large Angle ◽  
Low Frequency ◽  
Density Fluctuations ◽  
Plasma Emission ◽  
Type Ii ◽  
Radio Waves ◽  
Type Ii Radio Bursts ◽  
Electron Density Fluctuations

<p>The Sun produces highly dynamic and eruptive events that can drive shocks through the corona. These shocks can accelerate electrons, which result in plasma emission in the form of a type II radio burst. Despite a large number of type II radio bursts observations, the precise origin of coronal shocks is still subject to investigation. Here we present a well-observed solar eruptive event that occurred on 16 October 2015, focusing on a jet observed in the extreme ultraviolet by the SDO Atmospheric Imaging Assembly, a streamer observed in white-light by the Large Angle and  Spectrometric Coronagraph, and a metric type II radio burst observed by the LOw-Frequency Array (LOFAR) radio telescope. For the first time, LOFAR has interferometrically imaged the fundamental and harmonic sources of a type II radio burst and revealed that the sources did not appear to be co-spatial, as would be expected from the plasma emission mechanism. We correct for the separation between the fundamental and harmonic using a model which accounts for the scattering of radio waves by electron density fluctuations in a turbulent plasma. This allows us to show the type II radio sources were located ∼0.5 R<sub>sun</sub> above the jet and propagated at a speed of ∼1000 km s<sup>−1</sup>, which was significantly faster than the jet speed of ∼200 km s<sup>−1</sup>. This suggests that the type II burst was generated by a piston shock driven by the jet in the low corona.</p>

scholarly journals Can a Strong Radio Burst Escape the Magnetosphere of a Magnetar?

2021 ◽  
Vol 922 (1) ◽  
pp. L7
Author(s):  
Andrei M. Beloborodov
Keyword(s):  
Radio Source ◽  
Radio Wave ◽  
Cross Section ◽  
Radio Burst ◽  
Scattered Radiation ◽  
Low Density ◽  
Radio Bursts ◽  
Outer Magnetosphere ◽  
Scattered Spectrum

Abstract We examine the possibility that fast radio bursts (FRBs) are emitted inside the magnetosphere of a magnetar. On its way out, the radio wave must interact with a low-density e ± plasma in the outer magnetosphere at radii R = 109–1010 cm. In this region, the magnetospheric particles have a huge cross section for scattering the wave. As a result, the wave strongly interacts with the magnetosphere and compresses it, depositing the FRB energy into the compressed field and the scattered radiation. The scattered spectrum extends to the γ-ray band and triggers e ± avalanche, further boosting the opacity. These processes choke FRBs, disfavoring scenarios with a radio source confined at R ≪ 1010 cm. Observed FRBs can be emitted by magnetospheric flare ejecta transporting energy to large radii.

scholarly journals Speckles in Interstellar Radio-Wave Scattering

1991 ◽  
Vol 131 ◽  
pp. 238-241
Author(s):  
K. M. Desai ◽  
C. R. Gwinn ◽  
J. Reynolds ◽  
E. A. King ◽  
D. Jauncey ◽  
...  
Keyword(s):  
Radio Source ◽  
Radio Wave ◽  
Wave Scattering ◽  
Speckle Pattern ◽  
Complete Information ◽  
Radio Waves ◽  
Interstellar Plasma ◽  
Interstellar Scattering ◽  
Vela Pulsar ◽  
Scattering Material

AbstractObservations of speckles in the scattering disk of the Vela pulsar are presented and speckle techniques for studying and circumventing scattering of radio waves by the turbulent interstellar plasma are discussed. The speckle pattern contains, in a hologrammatic fashion, complete information on the structure of the radio source as well as the distribution of the scattering material. Speckle observations of interstellar scattering of radio waves are difficult because of their characteristically short timescales (≈seconds) and narrow bandwidths (≈kHz). Here, we present first observations, taken at 13 cm wavelength with elements of the SHEVE VLBI network, of speckles in interstellar scattering.

scholarly journals Radio Source Counts at Ultra Low Frequencies: Results, Significance and Implication for Observational Cosmology

1995 ◽  
Vol 164 ◽  
pp. 455-455
Author(s):  
K.P. Sokolov
Keyword(s):  
Radio Source ◽  
Large Scale ◽  
Low Frequency ◽  
Space Distribution ◽  
Observational Cosmology ◽  
Radio Sources ◽  
Extragalactic Radio Source ◽  
Low Frequencies ◽  
Extended Radio ◽  
Frequency Source

Analysis of cosmological evolution effects in the low-frequency source counts at 25 MHz obtained with the UTR-2 radio telescope and conclusions on the large-scale extragalactic radio source space distribution are presented. The data require the existence of a decrease in the space distribution of the most distant extended radio sources.

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