Overview

As part of the Small Solar Observation Program (approved under the ISAS Small Mission Program in FY2016 and successfully concluded in FY2024), a team led by the National Astronomical Observatory of Japan (NAOJ) with ISAS involvement developed the SUNRISE Chromospheric Infrared spectroPolarimeter (SCIP) for the international balloon-borne solar observatory Sunrise-III. During its 2024 flight, SCIP successfully obtained high-resolution and high-sensitivity spectro-polarimetric observations of the Sun, measuring how the Sun's light varies with wavelength (spectrum) and the orientation of the light waves (polarization).

Analysis of these observations has revealed that the chromosphere above the quiet Sun, which covers the majority of the solar surface, contains remarkably complex magnetic structures that had previously remained undetected. These findings provide important new insights into the long-standing mystery of how the solar chromosphere and corona are heated.

Key Results

The solar surface (photosphere) consists of magnetically active regions, such as sunspots, and the much more extensive quiet-Sun regions, where magnetic fields are relatively weak. Although numerous small-scale magnetic structures are known to exist in the quiet Sun and extend into the chromosphere (the Sun's atmosphere that sits above the photosphere), direct measurements of these weak chromospheric magnetic fields have been extremely difficult from ground-based observatories because atmospheric turbulence severely limits observational precision.

Operating at an altitude of approximately 35 km in the stratosphere, SCIP enables continuous measurements of magnetic fields from the photosphere to the chromosphere with minimal influence from atmospheric disturbances. These observations have provided an unprecedented view of the magnetic environment in the quiet Sun.

Magnetic fields concentrated in the photosphere expand outward with height and form umbrella-like structures known as magnetic canopies in the chromosphere. Detailed analysis of the SCIP data revealed the presence of numerous fine-scale, filamentary magnetic structures within these canopies. Some of these structures even exhibit magnetic polarity opposite to that of their surrounding environment (Figure 1).

Figure 1. Magnetic field distributions in the photosphere (left) and chromosphere (right) of a quiet-Sun region observed by SCIP. The color scale is given in gauss, with red indicating positive polarity and blue indicating negative polarity. In regions where magnetic fields are concentrated in the photosphere, the chromospheric magnetic fields expand over a much broader area. Within these canopy-like structures, numerous filamentary magnetic features can be seen, including structures with polarity opposite to that of their surroundings, as highlighted by the green circles.
Credit: NAOJ / Sunrise-III / SCIP Team

Numerical simulations suggest that these filamentary structures are produced when magnetic field lines become twisted by convective motions on the solar surface (Figure 2). Regions where oppositely directed magnetic fields are brought into close proximity are favorable sites for magnetic reconnection, a process that converts magnetic energy into heat. The discovery therefore provides an important clue to understanding the mechanisms responsible for heating the solar chromosphere and corona.

Figure 2. Chromospheric magnetic field distribution reproduced by numerical simulations (left). Filamentary structures with magnetic polarity opposite to their surroundings are clearly visible. The corresponding magnetic field lines are shown from a top-down view (upper right) and a side view (lower right), revealing a complex network of twisted and intertwined magnetic field lines.
Credit: MPS / Sunrise-III

The observations also yielded the first detailed measurements of magnetic fields within spicules, jet-like structures that extend from the solar surface into the corona. Spicules are believed to play a key role in transporting energy into the Sun's outer atmosphere. The data reveal how magnetic field strength and configuration vary with height along these structures (Figure 3), opening a new avenue for quantitatively investigating energy transport through the solar atmosphere.

Figure 3. Chromospheric intensity image of the solar limb observed by SCIP (top) and the corresponding line-of-sight magnetic field map (bottom). The dark region in the upper left corresponds to the background space beyond the solar limb. These observations reveal the vertical distribution of magnetic fields within spicules, jet-like structures extending upward from the solar surface. The black line marks the position of the solar limb. In the line-of-sight magnetic field map, the color scale is given in gauss, with red indicating magnetic fields directed away from the observer and blue indicating magnetic fields directed toward the observer.
Credit: NAOJ / Sunrise-III / SCIP Team

Comment from a collaborator at ISAS/JAXA

This achievement was made possible through a matching-fund scheme combining support from the Japan Society for the Promotion of Science (JSPS) and the ISAS Small Mission Program. SCIP is capable of measuring solar polarization signals with an accuracy of 0.03-0.04% in a one-second exposure.

Two key technologies used by SCIP, the Polarization Modulation Unit (PMU), which enables high-precision polarization measurements, and the scan mirror mechanism used to construct two-dimensional maps of the Sun, were originally developed under the leadership of ISAS/JAXA for the SOLAR-C mission. Their successful application to a balloon-borne observatory has directly enabled these groundbreaking discoveries.

The on-going development of SOLAR-C, which is designed to observe higher layers of the solar atmosphere with even greater capability, is expected to make significant contributions toward resolving the coronal heating problem.

Publications

The early science results from Sunrise-III have been published in a special Focus Issue of The Astrophysical Journal Letters.

THE ASTROPHYSICALJOURNAL LETTERS　Sunrise-III Early Science
https://iopscience.iop.org/collections/apjl-260303-220_Focus-on-Sunrise-III

• Three-dimensional Magnetic Field Structure of a Quiet-Sun Region Revealed by SUNRISE III/SCIP
  Masahito Kubo et al.
  The Astrophysical Journal Letters
  DOI: 10.3847/2041-8213/ae909b
• Magnetic Field Distribution along Spicules Revealed with SUNRISE III/SCIP
  Yoshihiro Naito et al.
  The Astrophysical Journal Letters
  DOI: 10.3847/2041-8213/ae8f38
• Cause of Chromospheric Opposite-Polarity Intrusions Discovered in Sunrise III/SCIP Data: MURaM-ChE Simulations Point to Twisted Flux Ropes
  Patrick A. Ondratshcek et al.
  The Astrophysical Journal Letters
  DOI: 10.3847/2041-8213/ae9607