On March 30, 2022, two spacecraft — BepiColombo and STEREO-A — simultaneously felt a shockwave from the Sun. The first was at 0.6 times the Earth-Sun distance, the second at exactly one such distance. They were almost in a line, like two microphones in the path of sound, and this allowed us to hear how the solar wind changes its voice along the way.
At first, the particles flew in an orderly column: fast ones overtook slow ones, and the instruments recorded a sharp spike. But then, after the main wave passed, the signal stretched out like an echo in the mountains. On the farther spacecraft, this echo lasted longer. This meant the particles were no longer flying straight but were meandering, lingering, and bouncing off invisible obstacles — magnetic fields that rippled along the path.
For the first time, scientists not only noticed this loss of order but also measured it: they adjusted the scattering strength in a computer model to match the observations. It turned out that by the end of the event, the particle's mean free path — the distance it travels without hitting a magnetic fold — had shrunk to about 0.5–1.0 times the Earth-Sun distance. It's as if a long-distance runner gradually finds himself in a crowd where it's harder and harder to keep his pace.
So two probes, accidentally aligned, allowed us to peek into the invisible kitchen of the solar wind. Now we better understand why space weather forecasts sometimes fail: particles don't arrive on schedule but wander like travelers in a fog.
The particle's mean free path at the end of the event was about 0.5–1.0 astronomical units — roughly the distance from Earth to the Sun. For comparison, in room air, a molecule travels only about 70 nanometers before colliding.
In Arthur C. Clarke's novel "Solar Wind," racing ships with solar sails use particle streams from the Sun, but no one there accounts for how magnetic folds slow and scatter that stream.