Picture a guitar string stretched between two frets. When you pluck it, a standing wave forms: some points on the string oscillate strongly, while others, the nodes, remain almost still. The chromosphere, a layer of the solar atmosphere sandwiched between the photosphere and the corona, behaves in much the same way. Only instead of frets, it is the sharp temperature gradients that reflect the waves back.

In pores, which are small dark spots with strong magnetic fields, slow magnetoacoustic waves propagate along the field lines. If they are trapped in a resonant cavity, they should form standing waves with characteristic signatures: at the nodes, the oscillation amplitude is minimal, and the phase jumps by 180°. It is precisely such nodes that a team of astronomers has now caught for the first time.

They observed a pore in active region NOAA 12848 on July 21, 2021, with the Swedish Solar Telescope. The CRISP spectropolarimeter scanned the hydrogen Hα line at 17 wavelengths, allowing them to reconstruct temperature and velocity oscillations at different heights in the chromosphere. Then, using wavelet analysis, they isolated oscillations with a frequency of 6.1 mHz (a period of about 164 seconds) and measured the phase differences between signals from different heights.

The result was clear: at a height corresponding to a shift of about 0.30 Å from the Hα line center, the temperature phase jumped by 180°, and its power dropped sharply. That is the resonant node. The height of the node varied in space and time: in the center of the pore it rose to 0.2 Å, while at the edges it dropped to 0.4 Å. Outside the pore, nodes often could not be identified, indicating that traveling waves dominate there.

The observed mixture of standing and propagating waves is consistent with the model of a "leaky" resonator: part of the energy escapes into the corona. This discovery provides, for the first time, direct observational evidence for the existence of a resonant cavity in the chromosphere of a pore. Previously, one could only infer it from indirect signs, but now the nodes themselves have been caught, an unambiguous marker of standing waves.

There is also a practical benefit: the position of the node can be used to estimate the height of the reflecting layer and the sound speed in active regions. This is a new tool for chromospheric seismology, which previously existed only in theory. If nodes can be measured at several frequencies, it will be possible to construct a dispersion map and refine the temperature profile. Thus, a solar pore, like a guitar string, can reveal its internal structure through the character of its oscillations.