Solar flares are a sudden release of magnetic energy stored in the corona. The standard model, developed back in the 1960s–70s, assumes that magnetic reconnection in the corona accelerates electrons, which then crash into the chromosphere and heat it. The heated material expands and rushes upward, a process known as chromospheric evaporation. However, high-resolution observations showed that upflows appear too early and too deep to be explained solely by electron impacts. This work joins a series of studies that, using new instruments such as Sunrise III, attempt to peer into the lower layers of the Sun's atmosphere and test how well the standard model matches reality.
Observations were conducted from the stratospheric balloon Sunrise III, which ascended to an altitude of about 37 km, where the atmosphere barely interferes. The SUSI instrument, an ultraviolet spectropolarimeter and imager, scanned an active region in the 310–400 nm range. Thanks to the diffraction limit, the resolution reached about 59 km on the solar surface, several times better than previous missions. The researchers analyzed spectral lines of iron Fe II at 325.59 nm and Fe II at 329.58 nm, which form in the deep chromosphere, at heights of 0.5–1 Mm. Doppler shifts of these lines allowed measuring plasma velocities. For comparison, hard X-ray data from the Fermi/GBM and ASO-S/HXI spacecraft were used, along with model calculations using the RADYN code.
In the pre-flare phase, 5–10 minutes before the onset of the impulsive rise in hard X-rays, distinct Doppler components with shifts of ±10 km/s were detected in the Fe II spectra. Blue wings of the lines indicate upflows, red wings downflows. The spatial scales of these flows reach 60–100 km, close to the instrument's resolution limit. Upflows are weaker in intensity than downflows and are arranged in narrow bands at the edges of brighter downflow regions. During the impulsive phase, the pattern persists: blue and red components coexist. The hard X-ray emission in the pre-flare phase is thermal in nature with a temperature of about 15 MK, while during the impulse a non-thermal component appears, but with a very soft spectrum (index 7–9) and a low cutoff energy (10–25 keV). Modeling with the RADYN code using such electron beam parameters does not reproduce the observed Doppler shifts: the synthesized lines show only a weak asymmetry of the wings without distinct components at ±10 km/s.
The results challenge the standard model of solar flares. If upflows in the deep chromosphere appear before the arrival of energetic electrons, then energy must be delivered by some other means. Possible candidates include thermal conduction from the corona, Alfvén waves, or local magnetic reconnection right in the lower layers. None of these mechanisms yet explains the full picture. This work differs from neighboring studies in that it is the first to detect evaporation at such low altitudes and in such an early phase, using Fe II lines that had not been applied to flares before. This opens a new window into diagnosing the lower solar atmosphere.
The topic will likely evolve toward more detailed modeling of energy transport in the chromosphere, taking into account the inhomogeneous structure of the magnetic field. New observations with even higher temporal and spatial resolution will be needed, especially in the ultraviolet range. It is also important to improve the sensitivity of hard X-ray detectors to capture weak signals in the pre-flare phase. Combined analysis of data from different heights in the solar atmosphere will help build a complete picture of mass circulation during a flare.
The results will impact solar flare physics, space weather models, and the interpretation of stellar flare observations on other stars.
Conduct similar observations for more powerful flares and in other active regions. Develop new numerical models that include local reconnection in the chromosphere.
This work is directly related to the unresolved problem of energy transport in solar flares: how energy from the corona is delivered to the lower atmospheric layers. Existing models cannot explain the early appearance of upflows. This points to the need to revise mechanisms of chromospheric heating and particle acceleration.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This study analyzes observations from Sunrise III in Fe II lines formed in the deep chromosphere and finds Doppler components of ±10 km/s, indicating upward and downward flows. It shows that upward flows appear 5–10 minutes before the impulsive rise of hard X-rays, and RADYN simulations using the observed electron beam parameters do not reproduce the observed shifts.
Where its strength is
The strength of the work lies in the direct observational evidence of early upflows in the deep chromosphere, which challenges the standard flare model and opens the way to exploring alternative energy transport mechanisms.
Where it could go next
Worth trying
Why this may matter
If early upflows are indeed unrelated to the electron beam, this would require a revision of energy release mechanisms in flares and could impact our understanding of coronal heating in general.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.60, against a usual 0.64. It looks like few have drilled nearby.
The stratospheric observatory Sunrise III ascended to an altitude of 37 km, where the sky is 100 times clearer for ultraviolet light than at sea level. This allowed achieving a resolution of 59 km on the Sun, comparable to the size of a small city.
In Arthur C. Clarke's novel 'A Solar Storm', solar flares threaten Earth, but the mechanisms of their occurrence are described schematically. This work shows how much more complex the real physics is: even in the lower layers of a star's atmosphere, processes occur that we are only beginning to understand.