The scorching gas streaming from the Sun is not a continuous flow but rather a soup of charged particles. In this soup, waves constantly boil up: electric fields flare and fade like ripples on water from a tossed stone. One such wave is the ion-ion acoustic wave. It arises when two kinds of ions (atoms that have lost electrons) move at different speeds. It's like two opposing streams on a highway: as they overtake each other, they create eddies.
Scientists want to learn how to catch these waves using data from a single probe. To do this, they applied the "field-particle correlation" method: it shows how energy flows from the electric field to particles and back. Imagine a swing that you push in rhythm, and it pushes you back: that's the picture. The method distinguishes such pushes from mere jitter. In a computer model reproducing solar wind conditions, the researchers found clear traces of these waves in the behavior of protons and electrons.
But here's the catch: these waves are very short-lived. They flare up and vanish faster than a spacecraft can take the next measurement. Current probes, such as Parker Solar Probe, measure too infrequently. So catching these waves in real space remains impossible — only in simulation. Perhaps future instruments with faster sampling will be able to hear this whisper of the solar wind.
The solar wind blows at speeds from 300 to 800 kilometers per second, but its particles are so sparse that in a cubic centimeter there are fewer than in the best vacuum on Earth.
In Arthur C. Clarke's story "The Solar Wind," races of sailboats driven by the pressure of sunlight remind us how finely we sense the streams from our star.