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Suffice to say that seeing things better produces major scientific breakthroughs.
In an article published July 18 in The Astrophysical Journal a team of scientists led by Craig DeForest-solar physicist at the branch of Southwest Research Institute in Boulder, Colorado-demonstrating that this historical trend still holds . Using advanced algorithms and data-cleaning techniques, the team discovered fine-grained structures never previously detected in the outer corona – the atmosphere at a million degrees of the Sun – by analyzing the images taken by NASA's STEREO satellite. The new results also provide a foreshadowing of what could be seen by NASA's Parker Solar Probe, which after its launch in summer 2018 will orbit directly across this region.
The outer crown is the source of the solar wind, the charge current of the particles that flow from the Sun in all directions. Measured near the Earth, magnetic fields embedded in the solar wind are interlaced and complex, but what causes this complexity remains unclear. "In deep space, the solar wind is turbulent and gusty," said DeForest. "But how did this happen, did he let the Sun smooth and become turbulent when he went through the solar system, or did the bursts speak to us about the Sun itself?"
. the source of the solar wind in every detail. If the sun itself causes turbulence in the solar wind, then we should be able to see complex structures from the beginning of the wind journey.
But existing data does not show such a fine structure, at least, until "The previous images of the crown showed the region as a smooth, laminar structure," said Nicki Viall, a solar physicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and co-author of the study. "It turns out that this apparent fluidity was just due to the limitations of our image resolution."
The Study
To understand the crown, DeForest and his colleagues began with coronagraphic images. by a special telescope that blocks the light from the surface (much brighter).
These images were generated by the COR2 coronagraph aboard the NASA Solar and Earth Observatory Observatory, or STEREO-A, which surrounds the Sun. and Venus
In April 2014, STEREO-A would soon be behind the Sun, and scientists wanted to get some interesting data before the communications were briefly interrupted.
So they organized a special data collection campaign for three days. COR2 has taken longer and more frequent exposures of the crown than usual. These long exposures allow more time for light from weak sources to hit the detector's instrument – allowing it to see the details it would miss.
But scientists did not want longer exposure pictures, they wanted a higher resolution. Options were limited. The instrument was already in the space; unlike Galileo, they could not tinker with the material itself. Instead, they adopted a software approach, avoiding data of the highest quality possible by improving the signal-to-noise ratio of COR2.
What is the signal-to-noise ratio?
-noise ratio is an important concept in all scientific disciplines. It measures how much you can distinguish the thing you want to measure – the signal – things you do not do – the noise.
For example, say you have a big hearing. You notice the slightest cry of mice late at night; you can spy on the murmurs of the schoolchildren huddled twenty meters away. Your hearing is impeccable – when the sound is weak.
But it's a completely different ball game when you're in the front row of a rock concert. Other sounds in the environment are just too powerful; no matter how attentively you listen, the mouse whines and whispers (the signal, in this case) can not cut through the music (noise).
The problem is not your hearing-it's the wrong signal-to-noise ratio.
Cor2 coronographs are like your hearing. The instrument is sensitive enough to image the crown in great detail, but in practice, its measurements are polluted by noise from the space environment and even the wiring of the instrument itself. The key innovation of DeForest and his colleagues was to identify and separate this noise, to increase the signal-to-noise ratio and to reveal the outer ring in unprecedented detail.
The Analysis
The first step to improve the signal-to-noise ratio had already been taken: images with longer exposure. Longer exposures allow more light in the detector and reduce noise levels – the team estimates noise reduction by a factor of 2.4 for each image and a factor of 10 by combining them over a period of time. 20 minutes.
were up to sophisticated algorithms, designed and tested to extract the true crown of noisy measurements.
They filtered light from background stars (which create bright spots in the picture that are not really part of the crown). They corrected the small differences (a few milliseconds) over the duration of the shutter opening of the camera. They removed the basic brightness of all images and standardized it so that the brighter regions do not wash the faintest.
But one of the most difficult obstacles is inherent to the crown: the blur due to the solar wind. To overcome this source of noise, DeForest and his colleagues performed a special algorithm to smooth their images over time.
Smoothing in time with a twist
If you have already done a "double-take", you know a thing or two about smoothing over time. A double glance, to check your first look, is just a simple way to combine two "measurements" taken at different times, to a degree that you can trust.
Smoothing time transforms this idea into an algorithm. The principle is simple: take two (or more) images, superimpose them and average their pixel values. The random differences between the images will eventually cancel out, leaving behind only what is consistent between them.
But when it comes to the crown, there is a problem: it is a dynamic, moving and changing structure. The solar material always moves away from the Sun to become the solar wind. Smoothing in time would create a motion blur – the same type of blur that you see in moving object photographs. It's a problem if your goal is to see the fine details.
To undo the blur of the solar wind, scientists used a new procedure: while they were doing their smoothing, they estimated the solar wind speed and moved the images
To understand how this approach works , think about taking snapshots of the highway when cars pass. If you simply cut back your images, the result would be a big, unclear mess – too much has changed between snapshots.
But if you could understand the speed of traffic and move your images to follow them, suddenly the details
For DeForest and his coauthors, the cars were the fine-scale structures of the crown, and the traffic on the highways were the solar wind
. Corona to tell you how fast things are moving. To determine exactly how many times images had to be shifted before averaging, they scanned the images pixel by pixel, correlating them to each other to calculate their similarity. Finally, they found the ideal point, where the nested parts of the images were as similar as possible. The amount of shift corresponded to an average solar wind speed of about 136 miles per second. By shifting each image by that amount, they aligned the images and smoothed them out, or averaged them.
"We smoothed, not only in space, not only in time, but in a mobile coordinate system," explains DeForest. This allowed us to create a motion blur that was not determined by the wind speed, but by the speed with which the characteristics changed in the wind.
DeForest and his collaborators had high quality images of the crown. The most surprising result was not a specific physical structure – it was the mere presence of the physical structure in itself.
Compared to the dynamic and turbulent inner crown, scientists had considered the outer crown as smooth and homogeneous. But this smoothing was only an artifact of poor signal-to-noise ratio:
"When we removed as much noise as possible, we realized that the crown is structured, until the optical resolution of the instrument ". DeForest says.
Like the individual grass strands that you only see when you are up close, the complex physical structure of the crown has been revealed in unprecedented detail. And among these physical details, three major discoveries emerged.
The Structure of Coronal Banners
Coronal streamers – also known as helmet streamers, because they look like a knight's pointed helmet – are bright structures develop on regions of the Sun with increased magnetic activity. Easily observed during solar eclipses, the magnetic loops on the surface of the Sun are stretched to sharp points pointed by the solar wind and can explode into coronal mass ejections, or large explosions of matter that eject parts of the Sun into the sun. surrounding area
. DeForest and the treatment of STEREO observations by his co-authors reveal that the streamers themselves are much more structured than previously thought.
"What we found is that there is not a single banner". "The flutes themselves are made up of a myriad of fine strands that together produce a brighter feature."
The Alfvén area
Where does the crown and the solar wind end? A definition points to the surface of Alfvén, a theoretical limit where the solar wind begins to move faster than waves can cross it. In this bordering region, disturbances occurring at a point farther away from the solar material can never retreat fast enough to reach the Sun.
"The material that flows beyond the surface of Alfvén is lost forever by the Sun," says DeForest.
Physicists have long believed that the Alfvén surface was only a surface layer or sheet where the solar wind suddenly reached a critical speed. But that's not what DeForest and his colleagues have found.
"What we conclude is that there is no clean Alfvén surface," said DeForest. "There is a wide" no man's land "or" Alfvén zone "where the solar wind is gradually disconnecting from the Sun, rather than a single clear boundary."
The observations reveal a fragmented frame where, at a given distance from the Sun, plasma moves fast enough to stop the communication in the opposite direction, and nearby flows are not. The water courses are close enough and thin enough to scramble the natural boundary of the Alfvén surface to create a vast, partially disconnected region between the crown and the solar wind.
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