Waves too quick for older telescopes to see ripple through the Sun’s polar corona, and they carry a big share of the fast solar wind’s energy.
Solar physicists can work out how much energy it takes to push the Sun’s quickest wind out into space. For decades they haven’t been able to find that much energy within the Sun’s own atmosphere.
A camera on a spacecraft 55 million miles (88 million kilometers) away has now caught the waves they were missing, and the fastest of them carry the most energy.
Yuhang Gao and Hui Tian at Peking University led the work, which rests on eighteen minutes of pictures from one small patch of one pole.
Plumes carry waves off the solar pole
Each of the Sun’s poles is covered by a coronal hole. Inside it the magnetic field lines run straight out into space instead of arching back down to the surface. Anything traveling along one of those lines keeps going.
That’s where the fast solar wind starts, a stream of charged particles that leaves at hundreds of miles a second. It’s the reason that space weather forecasters watch the Sun at all, because that stream disturbs satellites and power grids when it reaches us.
Standing inside the hole are coronal plumes, narrow bright rays that reach far above the surface and keep a wave moving along their length instead of letting it spread out.
Push one of the fine threads inside a plume sideways and the whole thread sways, and the sway travels upward. Solar physicists call that a kink wave.
A faster camera caught more waves
Gao’s team worked with the Extreme Ultraviolet Imager on Solar Orbiter, a European Space Agency spacecraft. On September 14, 2021, it took 225 pictures of the north polar region in eighteen minutes, one every five seconds. Each pixel covers 130 miles (210 km).
The researchers laid 17 horizontal slices across the plumes, from roughly 4,700 up to 25,600 miles (7,500 to 41,200 kilometers) above the surface. The top slice was a little more than three Earth-widths out.
An automated program followed every thread that swayed and picked out 2,318 wave events.
For comparison, the team ran the same routine on pictures that NASA’s Solar Dynamics Observatory took at the same moment from near Earth. That camera takes a frame every 12 seconds, and each of its pixels covers twice as much ground. The count there came to 560.
Observations by SDO/AIA. (a) Original AIA 17.1 nm image, with the dashed box marks the ROI. (b) The ROI transformed to polar coordinates. (c) Image after denoising and unsharp masking, revealing fine plume structures. The dotted line indicate an example of slits (with a width of 5 pixels) for TD-map generation. (d) Example TD map showing kink waves detected by NUWT. (e) Heliographic configuration of Solar Orbiter and SDO (close to the Earth) on September 14, 2021, generated with Solar-MACH. Click image to enlarge.Quick waves carry the most energy
The two instruments disagreed most about the quickest waves. Of the Solar Orbiter events, 38 percent completed a full swing in under 100 seconds. In the Solar Dynamics Observatory data, only 9 percent did.
A quick wave doesn’t shift a thread very far. Sideways displacement was about the same in both data sets, roughly 200 miles (320 kilometers). Covering that distance in less time means moving faster, though, and speed is what carries energy.
Solar Orbiter clocked a typical sideways speed of 9.6 miles per second (15.4 kilometers per second), against 6.2 (10) from the older instrument. Sorted by how often each wave repeated, the quick ones carried more than twice the energy of the slow ones.
All told, the Solar Orbiter waves come in at 30 to 50 watts per square meter, about 2.6 times what the Solar Dynamics Observatory data gives. The same corona over the same eighteen minutes, and more than double the energy, purely because one camera blinks faster.
The solar wind requires more energy
Even the higher figure doesn’t close the gap. Other researchers estimate that heating the corona and accelerating the fast solar wind takes something near 500 watts per square meter, roughly ten times what these waves deliver.
The 30 to 50 figure also rests on a number nobody has measured directly: how much of a plume’s volume the swaying threads actually occupy. Gao’s team assumed half, the value earlier studies used.
Another team’s measurements point to a fraction as low as 2 percent. If the true value is between 5 and 10 percent, the energy drops by a factor of five to ten.
Waves don’t run continuously either. Each event lasts only part of the observing window, and accounting for the quiet stretches cuts both estimates by another 30 to 40 percent.
None of that touches the comparison, which is what the paper is really about. Earlier teams worked from the same assumptions, so whatever the absolute number turns out to be, the faster camera keeps finding more energy than the slower one.
Three processes could make the waves
Where the quick waves come from is still an open question. The team lists three candidates and endorses none.
Magnetic loops that arch back to the surface can snap and rejoin the open field lines, shaking the plume above.
Loops in quiet parts of the Sun run 3,100 to 25,000 miles (4,000 to 42,000 kilometers) long, and at the speeds these waves travel, that produces swings lasting 20 to 266 seconds. That covers the range Solar Orbiter recorded.
Spicules are the second candidate, jets of gas that shoot up from the surface right below a plume. Another team recently traced a kink wave in one spicule directly into the plume overhead, swinging at the same rate.
The third is turbulence. Slow waves reflecting back down the plume, or running into each other head-on, can break down into faster ones.
Polar missions could settle the question
Until recently every telescope aimed at the poles looked at them from the side. Everything along that line of sight lands in one image, which makes it hard to say where a wave begins or to follow it on the way up.
Solar Orbiter has started tilting out of that plane, and last year it sent back the first pictures of the Sun’s poles. Parker Solar Probe, on its closest approach yet, has been measuring where the Sun’s magnetic grip on the wind finally gives way.
Nobody has yet watched one of these waves from the moment it forms to the moment its energy turns into heat. That needs a camera looking down at a pole rather than across it.
“Future missions such as China’s planned Solar Polar-orbit Observatory will provide a more direct view of the Sun’s polar regions,” said Hui Tian. That mission is scheduled to launch in 2029.
The full study was published in the journal National Science Review.
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