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

Abstract

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.

LOFAR Observations of Fine Spectral Structure Dynamics in Type IIIb Radio Bursts

“…Instead of a classical interferometric approach, the spatially-resolved spectroscopic observations were performed using the LOFAR beam-formed mode (Stappers et al, 2011;van Haarlem et al, 2013), when the data from the LOFAR core stations are combined to form a number of "tied-array beams" covering an area of the sky; the beam size is of about λ/D ∼ 10 ′ at 32 MHz, where λ is the wavelength and D is the maximum baseline. The advantage of using the tiedarray beams is that it allows producing images with very high time resolution, which is not possible in the LOFAR interferometric mode (see Stappers et al, 2011;Morosan et al, 2014Morosan et al, , 2015Reid and Kontar, 2017;Chen et al, 2018, for details).…”

Radio observatories and instrumentation used in space weather science and operations

“…When these multiple beams are arranged in a grid-pattern in a single direction (at a single source) on the sky, the technique is known as "tied-array imaging", see Figure 2 for an example applied to solar observations using LOFAR. In recent years such a technique has been successfully employed to perform spatially resolved observations of solar radio bursts (e.g., Morosan et al, 2014Morosan et al, , 2015Reid & Kontar, 2017). Compared to interferometric imaging, the advantage of such a technique in the case of LOFAR is its much higher time-sampling, e.g.…”

A Review of Recent Solar Type III Imaging Spectroscopy

“…The bulk of magnetic flux is closed in the corona and so we might expect U-bursts and Jbursts to be observed more often than type III bursts when in fact the converse is true. Using the derived magnetic loop and electron beam parameters from LOFAR imaging spectroscopy, Reid and Kontar (2017a) analyzed the electron beam instability criteria, shown in Figure 5. For radio emission to be generated on closed magnetic fields, the loop needs to be long enough for a power-law accelerated electron beam to become Langmuir-wave unstable through time-of-flight.…”

“…Right: A schematic showing how electron beams require long flux tubes and/or high densities to produce J/U-bursts. Adapted from Reid and Kontar (2017a).…”

“…Type U and J bursts allow diagnosis of coronal densities within magnetic flux tubes confined to the corona. Using LOFAR, Reid and Kontar (2017a) found the coronal density profile from two J-bursts and one U-burst that occurred in quick succession. Estimating second harmonic emission, the density profile roughly fit enhanced standard density profiles, the 3.0× Baumbach-Allen Model (Allen, 1947), 3.5× Sittler model (Sittler and Guhathakurta, 1999) and 4.…”