  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 the 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, both
significantly expanding the current diagnostic potential of solar radio
emission.
