Solar Threads Beyond the Disk express
The Sun's edge looks like a fuzzy fringe: thin jets of hot gas, spicules, shoot upward from it. They live only a few minutes and are constantly changing. In 2024, a balloon-borne telescope managed to spot not only the jets themselves but also their faint glow in hydrogen lines that had never been captured there before.
These are three Paschen lines — Pa 15, Pa 16, and Pa 17. This is light emitted by a hydrogen atom when its electron jumps from distant orbits down to the third level. The higher the line number, the fainter the emission, but the more transparent the gas is to that light. That's why the new lines proved useful: they let us peer into the most rarefied regions where conventional observations go blind.
Measurements showed that gas velocities derived from the new lines are lower than those from the older calcium line. This means the hydrogen light comes from more transparent regions where many jets with different speeds mix along the line of sight. Averaging smears the picture, but it's exactly what reveals the gas's tenuousness.
Comparing the brightness of the three lines with each other didn't match calculations. That hints that processes we haven't accounted for are involved in forming these lines — possibly additional heating or microscopic plasma motions. Now solar physicists have a new tool to figure out what the Sun's tenuous outer layer, the chromosphere, is made of and how it behaves.
Original abstract
as written by the authors arXiv:2608.14034v1
We report the detection of high-order hydrogen Paschen emission lines (Pa~15, Pa~16, and Pa~17) in the quiet-Sun chromosphere off the solar limb using the Chromospheric Infrared SpectroPolarimeter (SCIP) on board the {\sc Sunrise~iii} balloon telescope. These lines reveal thread-like structures resembling spicules and exhibit systematically smaller Doppler velocities than Ca~II~854.2~nm, suggesting that they are optically thinner and more affected by line-of-sight averaging, especially near the limb. Non-LTE radiative transfer synthesis using the spherically symmetric one-dimensional code \texttt{rhsphere} reproduces the overall spectral properties. The observed ratios among three Paschen lines show systematic deviations from synthetic and theoretical results, suggesting that additional physical effects may influence the formation of high-order Paschen lines. The study demonstrates the potential of high-order Paschen lines as a new diagnostic of optically thin plasma in the off-limb chromosphere.
Spicules on the Sun look like grass in the wind: about a million are visible at once, and each one lives less than ten minutes.