Solar Wind: Waves Give Birth to Radio Bursts express
In the stream of charged particles that constantly blows from the Sun, radio bursts sometimes occur — short, powerful signals at frequencies you can pick up on an ordinary radio. Until now, it was unclear how exactly these signals are born in that stream. Now, instruments on the Solar Orbiter spacecraft have witnessed the entire process firsthand.
The Sun ejects beams of electrons — particles with a negative charge. These beams, like speedboats, cut through the solar wind. As they do, they excite waves of a special type in the plasma (a hot gas of charged particles): Langmuir waves. Previously, it was thought that these waves simply scatter off random irregularities. But new data show that the waves can split into two parts, and one of them is precisely a radio wave.
The scientists verified that the waves really do split, rather than just coincidentally lining up. To do this, they measured the frequencies and directions of all the waves, compared their phases (oscillation shifts), and timed their appearance. Everything matched theoretical predictions. Moreover, computer simulations showed that the splitting occurs inside a smooth "dip" in density — a region where there are slightly fewer particles than around it. Such a dip traps the waves, preventing them from running away, and the splitting has time to complete.
This discovery is an important step toward understanding how the Sun "talks" to us in the language of radio waves. Now we know one of the mechanisms that converts the energy of electron beams into radio signals that reach Earth.
Original abstract
as written by the authors arXiv:2601.18495
The nonlinear decay of Langmuir/$$\mathcal{Z}$$-mode waves into electromagnetic $$\mathcal{Z}$$-mode wave radiation at the plasma frequency is observed for the first time in the solar wind. This finding was enabled by the unprecedented high-resolution electric and magnetic field measurements provided by the Radio Plasma Waves (RPW) instrument aboard the Solar Orbiter spacecraft, which encountered an electron beam associated with a Type III radio burst. The decay process is definitively identified through multiple lines of evidence: satisfaction of frequency and wavevector resonance conditions, strong phase coherence and temporal coincidence between the interacting waves, exclusion of competing mechanisms, and full agreement with theoretical predictions. Particle-in-cell simulations, conducted under close beam-plasma conditions, successfully reproduce the key features of the observations. Notably, they suggest that the wave packet observed by Solar Orbiter may be trapped within an extended, nearly flat-bottomed density well, where the decay process is not overcome by wave scattering on random density fluctuations and subsequent mode conversion effects.
Type III radio bursts from the Sun can be heard on an ordinary radio: they sound like short whistles, similar to birdsong.
In Stanislaw Lem's novel "Solaris," the planet's ocean communicates with humans using radio waves — possibly employing similar plasma processes.