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Fine structure of type III solar radio bursts from Langmuir wave motion in turbulent plasma

Nature Astronomy volume 5, pages 796–804 (2021)Cite this article

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Abstract

The Sun frequently accelerates near-relativistic electron beams that travel out through the solar corona and interplanetary space. Interacting with their plasma environment, these beams produce type III radio bursts—the brightest astrophysical radio sources seen from Earth. The formation and motion of type III fine frequency structures is a puzzle, but is commonly believed to be related to plasma turbulence in the solar corona and solar wind. Combining a theoretical framework with kinetic simulations and high-resolution radio type III observations using the Low-Frequency Array, we quantitatively show that the fine structures are caused by the moving intense clumps of Langmuir waves in a turbulent medium. Our results show how type III fine structure can be used to remotely analyse the intensity and spectrum of compressive density fluctuations, and can infer ambient temperatures in astrophysical plasma, substantially expanding the current diagnostic potential of solar radio emission.

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Fig. 1: Dynamic spectra and associated radio contours of solar type III radio bursts.
Fig. 2: Simulated beam-generated Langmuir wave energy densities and their associated power density spectra.
Fig. 3: Simulated type III dynamic spectra.
Fig. 4: Magnification of a simulated and observed solar type III stria.
Fig. 5: Relation between the level of background density fluctuations and type III radio burst intensity fluctuations.
Fig. 6: Power density spectra from observed and simulated type III burst dynamic spectra.

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Data availability

The simulation datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request. LOFAR data used during the current study are publicly available on the LOFAR Long Term Archive at https://lta.lofar.eu/ under the project codes LC3_012 and LC4_016. The data used to make the plots in the paper are available on the UCL Research Data Repository using https://doi.org/10.5522/04/14140007.

Code availability

The code used to make the plots in the paper is available on the UCL Research Data Repository using https://doi.org/10.5522/04/14140679. The code used to generate the datasets in our study is currently in preparation to be made publicly available. In the interim period, the code can be made available from the corresponding author on reasonable request.

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Acknowledgements

We acknowledge support by the Science and Technology Facilities Council (STFC) Consolidated Grant ST/L000533/1. This work benefited from the Royal Society grant RG130642. This paper is based (in part) on data obtained with the International LOFAR Telescope (ILT) under project codes LC3_012 and LC4_016. LOFAR17 is the Low-Frequency Array designed and constructed by ASTRON. It has observing, data processing and data storage facilities in several countries that are owned by various parties (each with their own funding sources), and that are collectively operated by the ILT foundation under a joint scientific policy. The ILT resources have benefitted from the following recent major funding sources: CNRS-INSU, Observatoire de Paris and Université d’Orléans, France; BMBF, MIWF-NRW, MPG, Germany; Science Foundation Ireland (SFI), Department of Business, Enterprise and Innovation (DBEI), Ireland; NWO, The Netherlands; The Science and Technology Facilities Council, UK

Author information

Authors and Affiliations

  1. School of Physics and Astronomy, University of Glasgow, Glasgow, UK

    Hamish A. S. Reid & Eduard P. Kontar

  2. Department of Space and Climate Physics, University College London, London, UK

    Hamish A. S. Reid

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Contributions

Both H.A.S.R. and E.P.K. contributed towards the theoretical, numerical and data analysis required for this study, and for the writing of the text. Figures were created by H.A.S.R.

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Correspondence to Hamish A. S. Reid.

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Peer review information Nature Astronomy thanks Peter Yoon and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Illustration of the observed type III radio intensity fluctuations.

The peak flux for the fundamental emission of the three type III bursts observed by LOFAR, shown in Fig. 1, on a, 16th April, b, 24th June and c, 16th September. Errors calculated from the square root of the radio flux are indicated in red. The smoothed functions, used to calculate the values of ΔI/I are shown in green.

Extended Data Fig. 2 Illustration of the simulated type III radio intensity fluctuations.

a, Peak brightness temperature as a function of frequency for the simulated type III dynamic spectra propagating through plasma where Δn/n = 2.5 × 10−3. The smoothed function used to calculate ΔI/I is overplotted in green. b, same as panel a but Δn/n = 2.5 × 10−4.

Extended Data Fig. 3 Simulated beam-drive Langmuir wave energy density and associated power density spectra.

a, Beam-generated Langmuir wave energy density ULW as a function of distance. The background plasma has a sinusoidal perturbation with amplitude A = 0.001 and wavelength λ = 50 Mm in blue and λ = 10 Mm in red. The lower panel highlights the modulation in the background plasma gradient length scale \(L=0.5{n}_{e}^{-1}d{n}_{e}/dx\). b, Power density spectrum of the Langmuir wave energy density minus the unperturbed case from the simulations shown in panel a. The peaks occur at the wavenumber k1 = 2π/λ Mm−1 and the harmonics. The simulation with a lower wavelength density fluctuations clearly modulates the Langmuir waves at higher wavenumbers.

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Reid, H.A.S., Kontar, E.P. Fine structure of type III solar radio bursts from Langmuir wave motion in turbulent plasma. Nat Astron 5, 796–804 (2021). https://doi.org/10.1038/s41550-021-01370-8

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