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A sunspot cools, and above it a dark filament, a prominence, hangs for weeks. Inside it, like a compressed spring, hides the magnetic field. It can be laid out in neat arches, or it can be twisted into a tight rope. This difference determines when and how it will blow: a rope is more unstable and ready to fly at the slightest nudge. For a long time, astronomers debated what exactly sits in the prominence channel hours before a flare, because ordinary images of the Sun's lower layer, the photosphere, did not give a clear answer.

A group of physicists from Sweden and other countries added a second floor. They took data from the chromosphere — the layer that lies just above the photosphere and glows pinkish during eclipses. Measuring chromospheric magnetic fields is difficult: the signal is weak, and the spectral lines (the rainbow bands used to infer the field) are blurred and sit at different heights. The new method automatically selects the correct height for each point and resolves the 180-degree ambiguity — when the field could point one way or the exact opposite, and it's unclear which.

They tested it on computer simulations where the true magnetic structure is known in advance. It turned out that using only the photosphere, you can make a mistake and take a rope for a simple arch. But when chromospheric measurements were included, the three-dimensional portrait of the field was reconstructed almost without distortion. Then they applied the method to real observations of an active region with a prominence, obtained at the Swedish Solar Telescope — an instrument with a one-meter mirror on the island of Palma. The reconstructed field clearly showed: inside the channel sits a twisted magnetic rope. The spring was cocked long before the jerk.

Now, with a reliable way to peek under the "lid" of a prominence, physicists can spot earlier where energy for a flare is building up. And that means better predicting those solar storms that sometimes hit satellites and power grids on Earth. The spring inside the dark filament is no longer a mystery.

Original abstract

as written by the authors arXiv:2606.10411

Solar eruptions are powered by the release of magnetic energy stored in the lower solar atmosphere, but the pre-eruptive magnetic configuration of filament channels remains difficult to determine. A central question is whether this energy is stored in a pre-existing magnetic flux rope or in a sheared arcade that forms a flux rope only during eruption. Resolving this ambiguity is critical for identifying instability thresholds and eruption triggers, yet photosphere-based extrapolations often provide insufficient constraints on the three-dimensional coronal field. Here, we introduce a data-driven magnetic field extrapolation framework that combines photospheric and chromospheric vector magnetograms in a unified multi-height optimization, while accounting for variable chromospheric formation heights and the 180{\deg} azimuthal ambiguity. Tests with radiative magnetohydrodynamic simulations show that photosphere-only extrapolations can misidentify the pre-eruptive magnetic configuration, whereas chromospheric vector constraints recover the three-dimensional structure substantially more accurately. Applied to multi-line spectropolarimetric observations of an active region filament obtained with the Swedish Solar Telescope, the method reveals a reconstructed magnetic field consistent with a pre-eruptive flux-rope configuration. These results show that chromospheric vector magnetic measurements can provide decisive constraints on filament magnetic configuration and open a path toward diagnosing magnetic-energy storage and instability in eruptive solar active regions.