Langmuir wave motion observed in the most intense radio sources in the sky

Langmuir wave motion observed in the most intense radio sources in the sky
Figure 1. Dynamic spectra (left) and associated radio contours (right) of a solar type III radio burst observed by LOFAR on the 24 June 2015 at 12:18:20 UT. The LOFAR contours are at 75% of the peak flux of the type III bursts going from 40 MHz to 30 MHz in the colour sequence white-blue-green-yellow-red. The LOFAR beam contour at 75% for 30 MHz is shown in the top left corner in white. The background is the Sun in EUV at 171 Angstroms observed by AIA. Credit: Image from Reid & Kontar, Nature Astronomy, 2021.

The sun routinely produces energetic electrons in its outer atmosphere that subsequently travel through interplanetary space. These electron beams generate Langmuir waves in the background plasma, producing type III radio bursts that are the brightest radio sources in the sky (Suzuki and Dulk, 1985). These solar radio bursts also provide a unique opportunity to understand particle acceleration and transport, which is important for our prediction of extreme space weather events near the Earth. However, the formation and motion of type III fine frequency structures (see Figure 1) is a puzzle, but is commonly believed to be related to plasma turbulence in the solar corona and solar wind.

A recent work by Reid and Kontar combines a with kinetic simulations and high-resolution type III observations using the Low Frequency Array (LOFAR) and quantitatively demonstrates that the fine structures are caused by the moving intense clumps of Langmuir waves in a turbulent medium. These results show how type III fine can be used to remotely analyze the intensity and spectrum of compressive density fluctuations, and can infer in astrophysical plasma, both significantly expanding the current diagnostic potential of solar radio emission.

The radio fine structures (Figure 1) have a small drift in frequency caused by the motion of Langmuir wave clumps moving through space at their group velocity. Measuring this frequency drift (Figure 2) reveals the Langmuir wave group velocity, and subsequently the background thermal velocity. This new technique increases the scope of solar radio bursts to be used as a remote plasma temperature diagnostic. The observation infers a corresponding coronal plasma temperature around 1.1 MK. The radio fine structure also provides an additional way to estimate the electron beam bulk velocity, which is mostly controlled by the beam energy density.

Langmuir wave motion observed in the most intense radio sources in the sky
Figure 2. Magnification of one type III fine structure from the LOFAR data (left) and the simulations (right). The black dashed lines show a linear fit to the drift, estimating a constant velocity of 0.69 Mm/s for the observed type III burst and 0.6 Mm/s for the simulated type III burst. Credit: Image from Reid & Kontar, Nature Astronomy, 2021.

In summary, the results create a framework for exploiting the diagnostic potential of radio burst fine structure to estimate plasma temperatures and density turbulence. This new potential is especially relevant given the enhanced resolution of new-age ground-based radio telescopes that are resolving much more fine structure originating from the . Moreover, the closer proximity of Parker Solar Probe and Solar Orbiter to radio emission originating in the very high corona or , and hence higher sensitivity, allows fine structures to be detected in situ.


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Source position and duration of a solar type III radio burst observed by LOFAR

More information: Hamish A. S. Reid et al, Fine structure of type III solar radio bursts from Langmuir wave motion in turbulent plasma, Nature Astronomy (2021). DOI: 10.1038/s41550-021-01370-8
Journal information: Nature Astronomy

Citation: Langmuir wave motion observed in the most intense radio sources in the sky (2021, September 8) retrieved 20 September 2021 from https://phys.org/news/2021-09-langmuir-motion-intense-radio-sources.html
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