Universe Today
The Sun’s magnetic fields are a twisty, curvy, ever changing mess. In particular, our star’s polar regions host regions called polar coronal holes that host invisible magnetic highways that stretch out into interplanetary space. But there’s a lot we don’t know about how those highways actually work, and in particular how they give the particles that form the fast solar wind an extra “kick” that sends them zooming at hundreds of kilometers per second. A new paper from a team led by Dr. Yuhang Gao and Prof. Hui Tian at Peking University, and published recently in the journal National Science Review, thinks they might have found an answer by using high-speed shots from Solar Orbiter to detect never-before seen rapid, high-frequency magnetic waves in those areas.
Key to this particular discovery is camera speed. Older instruments, such as NASA’s Solar Dynamics Observatory (SDO) took pictures every 12 seconds, and had a spatial resolution of around 1,100 km for every pixel. Upgraded equipment offered much faster, and higher resolution imagery. Solar Orbiter’s Extreme Ultraviolet Imager (EUI) captures an image every five seconds, and with details down to 420 km per pixel - more than double the precision and twice the speed of earlier instruments.
Using data from both instruments for a period in September 2021, the authors subjected both to an automated magnetic wave tracking program called the Northumbria University Wave Tracking (NUWT). Taking a close look at solar plumes - ray-like magnetic structures that stick out from the Sun’s north pole - the authors noted a distinct difference in the number of waves between the two observatories. With the SDO, the algorithm caught 560 waves events, whereas with the EUI it caught 2,318 - more than 4 times as many.
Fraser talks about Solar Orbiter.The difference seemed to come down to frequency. 38% of the waves seen by the EUI had wavelengths shorter than 100 seconds, whereas only 9% of the waves detected by SDO had the same wavelengths. Another determining factor was speed - i.e. how fast the plasma thread moved. SDO’s average was around 9.9 km/s, whereas EUI’s averaged 15.4 km/s. And since energy scales with velocity, these faster moving waves carry much more power. Based on the paper’s calculation, these faster moving waves carry 2.6 times greater power than previous estimates.
Theories abound about where these types of magnetic ripples come from. The researchers offer up three potential sources - first is when magnetic loops run into unconnected magnetic field lines, causing the lines to snap and reconfigure, and simultaneously launching high-frequency ripples up into space. A second explanation is that they form as part of a giant jet of plasma called a spicule, which can directly pump these fast transverse waves up into the plumes above them. A third explanation is that larger, slower moving waves might bounce off of each other, creating “turbulent cascades” of smaller, higher frequency waves.
While their creation mechanism is still somewhat unclear, these waves could help answer two interconnected solar physics mysteries - why is the outer layer of the Sun so hot, and how do the particles from the fast solar wind get accelerated to such incredible speeds. According to the paper, the fast waves that travel into the Sun’s upper atmosphere suffer from effects such as resonant absorption and phase mixing, transferring much of their energy into the surrounding environment.
Fraser answers the question of whether we can directly harvest energy from the Sun, instead of just through solar conversion.Some of that energy likely is translated directly into the heat that drives the absurd temperatures in the Sun’s outer layers, which can reach over 1,000,000℃. Yet more of that energy can potentially be transferred directly into the kinetic processes that shove the ions up to the speeds needed to form the fast solar wind.
To be clear, these mechanisms aren’t yet well defined, and the overall energy calculated as part of this single dataset is still around an order of magnitude lower than what other models suggest would be needed to fully launch the solar wind. However, it proves that there is a massive amount of energy hiding in plain sight behind the limitations of our technology.
That technology still has room for improvement, with missions like the planned Solar Polar-orbit Observatory (SPO) expected to launch in early 2029. With its unique viewing angle, it will offer unprecedented insights into the magnetic chaos happening at the Sun’s poles. And its instruments might even be fast enough to capture even more high-energy waves, and help solve those mysteries once and for all.
Learn More:
Science China Press / EurekAlert - Hidden high-frequency waves found in the Sun’s polar corona may help power the fast solar wind
Y. Gao et al - High-frequency magnetohydrodynamic waves with substantial energy in the solar polar corona
UT - The Solar Orbiter is Giving Us an Unprecedented Look at the Sun's Poles
UT - We're Finally Seeing the Sun's Mixed Up Magnetism at its Poles
