Sunrise-III is an international collaborative project built around a one-meter solar telescope suspended beneath a high-altitude balloon. During its 2024 flight, the observatory floated through Earth’s stratosphere at an altitude of roughly 35 kilometers, far above the turbulent air that blurs images taken from the ground. From this vantage point the telescope can observe the Sun with very little atmospheric distortion, achieving a level of detail that ground-based instruments struggle to match even with the most sophisticated adaptive optics. The gondola carried three instruments developed by teams in different countries, each designed to probe a different layer of the solar atmosphere.

Among them, the Sunrise Chromospheric Infrared spectroPolarimeter, or SCIP, was developed under the leadership of the National Astronomical Observatory of Japan. SCIP is a near-infrared spectropolarimeter, an instrument that measures the polarization of sunlight at infrared wavelengths. Because magnetic fields alter the polarization states of spectral lines through physical processes such as the Zeeman effect, these polarization signatures encode the strength and direction of the magnetic field at the height where each spectral line forms. SCIP’s particular strength is its ability to continuously observe the magnetic field from the photosphere, the visible surface of the Sun, up through the chromosphere, the layer of atmosphere immediately above it.

The new observations targeted quiet-Sun regions, which cover most of the solar surface. Although these areas appear placid compared with active regions dominated by sunspots, they are permeated by magnetic fields that are simply weaker and more finely structured. That weakness is precisely what makes them difficult to measure. Ground-based telescopes must contend with blurring caused by Earth’s atmosphere, which smears out the small-scale magnetic features before their light can be analyzed. The high-precision measurements made possible by Sunrise-III’s stratospheric platform allowed the team to map the chromospheric magnetic field above quiet regions in fine detail for the first time.

What SCIP revealed was a surprise. Above a quiet-Sun region, the team identified thin, elongated, thread-like magnetic structures embedded within what solar physicists call the magnetic canopy. The canopy forms in the chromosphere as magnetic fields concentrated at the photosphere arc outward with increasing altitude, creating an overarching arch of magnetic structures above the quieter field below. The conventional picture treated this canopy as a relatively simple, smoothly expanding structure. The Sunrise-III data showed instead that it contains numerous thin, elongated magnetic substructures, more like a dense weave of fine threads than a broad, uniform sheet.

To understand what these threads represent, the team turned to numerical simulations. The simulations reproduced the thread-like structures seen by SCIP and indicated that they are associated with magnetic field lines twisted by motions at the solar surface. In this picture, the constant churning of convective plasma at the photosphere grabs the feet of magnetic field lines and winds them up, imparting a twist that propagates upward into the chromosphere. When viewed in observation, such twisted bundles of field lines appear as the fine, elongated threads that SCIP detected. The agreement between the observed structures and the simulated twisted field lines provides a physical interpretation for a phenomenon that had previously been resolved only in models.

In a separate set of observations, Sunrise-III turned its attention to a quiet-Sun region near the solar limb, the visible edge of the Sun. Viewing the Sun edge-on allows instruments to look at atmospheric structures from the side rather than from above, which is particularly valuable for studying features that extend vertically. There the observatory mapped the magnetic fields of spicules, jet-like structures that extend upward from the solar surface into the chromosphere and beyond. The measurements revealed how the distribution of the magnetic field varies with height above the limb, providing a vertical profile of magnetism through a region that is otherwise extremely difficult to characterize.

Spicules have long interested solar physicists because they are ubiquitous, appearing across the solar surface, and because they may play a role in transporting mass and energy into the upper solar atmosphere. By measuring their magnetic fields directly, Sunrise-III adds a crucial dimension to their study. Knowing how the field strength and geometry change with height in and around spicules constrains models of how these jets are driven and whether they can channel enough energy upward to matter for atmospheric heating. The new height-resolved magnetic maps from a quiet region near the limb represent exactly the kind of observational constraint that has been missing.

The broader significance of the results lies in one of the enduring puzzles of solar physics: why the Sun’s outer atmosphere is so much hotter than its visible surface. The photosphere sits at roughly 5,500 degrees Celsius, yet the chromosphere above it is hotter still, and the corona beyond it reaches temperatures of more than a million degrees. Some form of energy transport must carry heat upward against this gradient, and magnetic fields are the leading candidate, since they can channel energy generated by surface motions into the upper atmosphere. The discovery that quiet-Sun regions are filled with fine, twisted magnetic substructures suggests that even the calmest parts of the Sun may be quietly feeding energy into the atmosphere above them.

Because quiet regions cover most of the solar surface, the magnetic complexity found there could be globally important rather than a local curiosity. If twisted field lines throughout the quiet Sun routinely carry energy upward, the cumulative contribution to chromospheric and coronal heating could be substantial, complementing the energy released by active regions during flares and eruptions. The Sunrise-III results provide important clues to understanding how energy is transported from the solar surface into the atmosphere, and they demonstrate the value of stratospheric observation as a bridge between ground-based telescopes and space missions. As the team continues to analyze the 2024 flight data, the fine magnetic weaving above the quiet Sun may yield further insights into the hidden machinery that powers our nearest star.

Subject of Research: Chromospheric magnetic field structure in quiet-Sun regions observed by the balloon-borne Sunrise-III solar observatory

Article Title: Stratospheric observatory Sunrise-III reveals intricate solar magnetic structures

Article References: Stratospheric observatory Sunrise-III reveals intricate solar magnetic structures. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Sunrise-III, solar physics, magnetic fields, quiet Sun, chromosphere, spicules, magnetic canopy, spectropolarimetry, balloon observatory, NAOJ, solar atmosphere heating, The Astrophysical Journal Letters