Universe Today
There’s lots of things you can do with balloons. Children’s birthday parties come to mind, as do retirement parties. But you can also observe the Sun in unprecedented detail with one - or at least with a suite of equipment attached to a gondola floating beneath one. That’s what the SUNRISE III mission did back in 2024, and scientists have had time to compile a series of three scientific papers on the results from that mission, which have been published in the Astrophysical Journal Letters.
So why use a balloon? The Earth’s atmosphere is turbulent, which can smear fine details of solar observation, such as the smallest magnetic features on the Sun. Lifting instruments above most of Earth’s atmosphere significantly sharpens the picture, and a balloon provides that benefit without all the mechanical constraints of a true rocket launch.
SUNRISE III included a series of instruments, such as a 1m solar telescope, hanging from a gondola underneath a helium balloon cruising at roughly 35 km. This gave it an almost unprecedentedly clear view of the solar atmosphere, which the mission took advantage of by using the Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) to measure the full Stokes polarimetry of our local star, allowing astronomers to reconstruct magnetic field directions and strengths between multiple atmospheric layers.
Fraser talks about how hot our Sun actually is.Some of those atmospheric layers were part of the “Quiet Sun” - the parts that get much less attention than the high energy, highly volatile “sunspots” that typically only take up a fraction of the Sun’s surface. Scientists have long thought of this chromosphere as being dominated by a “canopy” of vertical magnetic flux tubes. But data from SCIP shows that the canopy is fully of unexpected magnetic threads. These threads are relatively narrow at 725 km across (which is small compared to the rest of the Sun), and their magnetic strength fluctuates between 10 and 20 Gauss, alternating in a pattern that is similar to structures typically seen around sunspot penumbrae. Interestingly, they occasionally flipped their polarity entirely compared to the rest of the canopy around them in what is called an “opposite-polarity intrusion” (OPI).
“These findings suggest that the canopy field is not simply an expanding structure originating from network regions, but instead has a complex three-dimensional configuration containing numerous localized sub-structures,” the researchers said. They also explain that the canopy field seems an ideal place for small-scale magnetic reconnections that could release energy to heat the surrounding plasma.
The question of what causes these OPIs is the focus of another companion paper, but instead of just analyzing SUNRISE III data, this one turns to simulations - specifically a 3D radiative-magnetohydrodynamic simulation algorithm called MURaM-ChE. When the researchers put in the SUNRISE III data as an input to the simulation, they matched the OPI structures extremely well, and it became obvious what they actually were - twist magnetic flux ropes.
Fraser talks about our first glimpse at the Sun's poles from Solar Orbiter.Pretty much any time we do a story on the outer layer of the Sun, or on solar storms more generally, we mention these twisted flux ropes. They are around 2,000 to 7,000 km in length, and their “reconnection” is what seems to set up solar flares with the massive amount of energy that process releases. Now it appears they have threaded their way even into the relatively calm surface of the Sun, though their breaking and disconnecting doesn’t seem to happen as often there.
Another long-standing feature of our reporting on Solar physics is the plasma jets that shoot out from the poles for the Sun. They’ve been implicated in providing energy to the Sun’s corona, but we’ve never been able to successfully measure their true magnetic strength. Watching them in action with SCIP allowed scientists to make a pretty accurate estimation of those values though.
According to a third paper, the strength of the magnetic field is between 10 and 20 Gauss in the lower canopy - actually lower than previous estimates would have predicted. However, higher up above the “solar limb” (the apparent edge of the Sun), the magnetic field strength jumps up to around 40 Gauss.
Fraser discusses the possibility that we're due for a massive solar flare.That apparent jump in the value might actually be an instrumentation trick, though. At lower altitudes, the thicket of “spicules” as they are called is so dense that telescopes can’t see through them and can only calculate values for the outermost spicules. Further up, this canopy thins out, allowing the instruments to more directly measure their values.
These three papers combine to show how complex even the “quiet” face of the Sun actually is. And how useful balloons can be at getting instruments just far enough above the atmosphere to make a huge difference in the accuracy of their measurements. Future missions, such as SOLAR-C, will hopefully provide even more insight into how exactly all of these complex fields and ropes tie together to create our Sun’s corona - but at least for now it’s clear we’re only just scratching the surface of our home star.
Learn More:
NAOJ - Stratospheric Observatory Sunrise-III Reveals Intricate Solar Magnetic Structures
P. A. Ondratschek et al - Cause of Chromospheric Opposite Polarity Intrusions Discovered in Sunrise III/SCIP Data: MURaM-ChE Simulations Point to Twisted Flux Ropes
Y. Naito et al - Magnetic Field Distribution along Spicules Revealed with SUNRISE III/SCIP
M. Kubo et al - Three-dimensional Magnetic Field Structure of a Quiet-Sun Region Revealed by SUNRISE III/SCIP
UT - Solar Orbiter Catches the Sun’s Missing High-Speed Vibrations
UT - Previously Unseen Super-Hot Plasma Jets Heat the Sun's Corona
