Above the Sun's limb hangs a cloud of tenuous, hot gas — a prominence. It can linger for weeks, then suddenly break loose and fly off into space. On April 15, 2023, the SPICE instrument on the Solar Orbiter probe got its first detailed look at such a cloud in ultraviolet light.

The SPICE spectrometer split the prominence's light into colors, much as a prism splits a white beam into a rainbow. In this spectrum, bright hydrogen lines, Lyman-beta and Lyman-gamma, stand out. Their brightness and width reveal what's going on inside the cloud: how dense, hot, and transparent it is.

It turns out that just before breaking away, the prominence begins to "breathe": density and temperature in different parts change unevenly. Some regions brighten, others dim. It's like the wind picking up and dying down before a storm, but on a scale of hundreds of thousands of kilometers.

Meanwhile, the ground-based GONG telescope was imaging the same cloud in the red light of hydrogen (the H-alpha line). By comparing two images taken at different times, scientists calculated that the prominence was rising at about 30 kilometers per second. That's like driving from Moscow to St. Petersburg in half a minute.

Such measurements matter not only to astronomers. When a prominence breaks off, it becomes a coronal mass ejection — a cloud of charged particles that heads toward Earth and can trigger magnetic storms. The earlier we spot the first signs of detachment, the more accurate our space weather forecasts.