Pearse Murphy
Low frequency radio wave scattering and refraction can have a dramatic effect on the observed size and position of radio sources in the solar atmosphere (corona).
The scattering and refraction is thought to be due to fluctuations of electron density caused by turbulent motion. Hence, determining the true radio source size can provide information on the turbulence in the plasma of the sun's atmosphere.
However, the lack of high spatial resolution radio interferometric observations at low frequencies with radio telescopes such as the LOw Frequency ARray (LOFAR) have made it difficult to determine the true radio source size.
Here we directly fit a LOFAR interferometric observation of a solar radio burst to determine its source size and position. This circumvents the need for using imaging algorithms, which can introduce spurious effects on source size and shape.
A burst at 34.76 MHz is found to have a size of 18.8 arcmin ± 0.1 arcmin and 10.2 arcmin ± 0.1 arcmin across the longest and shortest directions of the source respectively. The radio burst is determined to be at a distance of 1.95 solar radii from the centre of the sun.
From this, the relative level of root mean squared density fluctuations in the solar corona was found to be < 0.16. Our results suggest that the level of density fluctuations in the solar corona is the major cause of the scattering of radio waves, resulting in large source sizes. However, the level of scattering may be smaller than previously derived through observations of radio wave scattering.
Title: Observations of Radio Burst Source Sizes and Scattering in the Solar Corona
The text box outlines the relevant background and context for the poster.
Figure 1: The figure contains two panels. The top panel (a) shows the solar X-ray light curves measured by the Geostationary Operational Environmental Satellite from 12:00UT-14:00UT on 2015-10-17. Two vertical lines indicate the time period 13:21UT-13:23UT. Two lines connect from this time range to the bottom panel (b). Panel b shows a dynamic spectrum observed by the LOw Frequency ARray containing a solar radio burst. The time range in panel b is 13:21UT-13:23UT and the frequency range is 25MHz-85MHz. There is an inset in panel b which zooms in to a portion of the radio burst from 13:21:42UT-13:21:52UT and 34.2MHz-35.4MHz. The inset shows fine scale frequency structure in the radio burst which is marked by a cross.
Slide 2 contains two text boxes to be read left then right and one figure along the bottom.
Title: "Methods: Fitting in Fourier Space to Learn About a Solar Radio Burst"
The left text box describes how a radio image is created from an interferometric observation. The right text box explains why our method differs from "standard" imaging procedures.
Figure 2: The figure consists of three panels (a,b,c). Panel a shows the amplitude of the radio burst measured in Fourier space. Each point represents a pair of antennae in Fourier space, also known as a baseline, and their colour shows the amplitude received by that baseline. The background colour map is the amplitude of a fitted model. An ellipse denotes the 50 percent level of the fitted amplitude. Panel b is almost identical to panel a except that it shows the phase of the radio burst and model. Panel c shows a plot of amplitude versus angular scale for each baseline. Two curves in panel c show where the fitted model would lie on this plot.
Slide 3 contains two text boxes to be read top then bottom and one figure on the right hand side.
Title: "Results: Using a Solar Radio Burst to Learn About Radio Wave Scattering"
The top text box describes how the size of the radio burst in the model fit is bigger than the size theoretically predicted from fine frequency structure in Figure 1b.
The bottom text box describes how radio wave scattering is the cause of increased source size and gives the level of scattering estimated from the observation.
Figure 3: The figure shows contours from 50 percent to the maximum of the radio intensity for the reconstructed radio image. The contours are overlaid on a 171 angstrom image of the Sun observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory.
Slide 4 contains two text boxes on the top half of the slide to be read left then right and a summary text box on the bottom half of the slide.
Title: "Conclusions: It’s More Complicated Than We Thought!"
The top left text box states that the calculated value for the level of radio wave scattering depends on the characteristic length over which scattering occurs.
The top right text box explains how the level of scattering determined by this work is an upper limit only, as may be the case with other studies in the literature.
The summary text box recounts the main points given in each slide.