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.