Solar wind is a stream of protons (nuclei of hydrogen) and a little helium from the Sun. The Parker probe, diving closer to the Sun than ever before, found that in the young solar wind, fast clumps fly out — bunches that overtake the general flow, though never reaching light speed. Such a bunch slams into the surrounding plasma and, like a hammer on an anvil, strikes up magnetic waves.

These waves immediately bounce back: they siphon off kinetic energy from the bunch and turn it into heat. Computer simulations revealed something unexpected: the faster the bunch, the more strongly it brakes itself. Its speed drops by more than half, and the protons heat up and spread sideways — on the velocity plot, instead of a sharp peak you get a broad, flat 'hammer head', with the remnants of the fast core forming a handle.

Wave frequency analysis (similar to spectroscopy) matched theory exactly. This mechanism — fast streams create waves that slow them down — is universal: it works near black holes, in the Big Bang era, and shows up in the cosmic microwave background as a rise in entropy. Understanding it helps predict space weather.