For decades after its discovery, observers could only see the solar chromosphere for a few fleeting moments: during a total solar eclipse, when a bright red glow circled the moon’s silhouette.
The chromosphere, photographed during the total solar eclipse in 1999. The red and pink tones – light emitted by hydrogen – earned it the name chromosphere, from the Greek “Chrôma”, which means color. Credits: Luc Viatour
More than a hundred years later, the chromosphere remains the most mysterious of the Sun’s atmospheric layers. Located between the bright surface and the ethereal solar corona, the sun’s outer atmosphere, the chromosphere is a place of rapid change where temperature rises and magnetic fields dominate the sun’s behavior.
Now, for the first time, a triad of NASA missions has looked into the chromosphere to return measurements of their magnetic field at multiple heights. The observations recorded by two satellites and the Chromospheric Layer Spectropolarimeter 2 or the CLASP2 mission on board a small suborbital rocket show how magnetic fields on the solar surface lead to brilliant eruptions in the outer atmosphere. The paper was published today in Science Advances.
A primary goal of heliophysics – the science of the sun’s influence on space, including the planet’s atmosphere – is to predict space weather, which often starts on the sun but can spread rapidly in space and cause disturbances near the earth.
These solar flares are powered by the sun’s magnetic field, the invisible lines of force that extend from the sun’s surface into space far behind the earth. This magnetic field is difficult to see – it can only be seen indirectly through light from the plasma or superheated gas tracing its lines like car headlights on a distant highway. But how these magnetic lines arrange themselves – whether loose and straight or tight and tangled – makes the difference between a calm sun and a sunburst.
“The sun is both beautiful and mysterious, with constant activity triggered by its magnetic fields,” said Ryohko Ishikawa, solar physicist at the National Astronomical Observatory of Japan in Tokyo and lead author of the paper.
Ideally, researchers could read the magnetic field lines in the corona where solar flares take place, but the plasma is far too sparse for accurate readings. (The corona is far less than a billionth as dense as air at sea level.)
Instead, scientists measure the more densely packed photosphere – the sun’s visible surface – two layers below. They then use mathematical models to spread this field up into the corona. This approach instead skips measuring the chromosphere that lies between the two in hopes of simulating their behavior.
The chromosphere lies between the photosphere, or the bright surface of the sun, which emits visible light, and the overheated corona, or the outer atmosphere of the sun, at the source of solar flares. The chromosphere is an important link between these two regions and a missing variable that determines the magnetic structure of the sun. Credits: Credits: NASA’s Goddard Space Flight Center
Unfortunately, the chromosphere has turned out to be a wildcard where the magnetic field lines rearrange in ways that are difficult to predict. The models try to capture this complexity.
“The chromosphere is a hot, hot mess,” said Laurel Rachmeler, former NASA project scientist for CLASP2, now with the National Oceanic and Atmospheric Administration (NOAA). “We make simplifying assumptions of physics in the photosphere and separate assumptions in the corona. But most of these assumptions break down in the chromosphere. “
Institutions in the US, Japan, Spain, and France have worked together to develop a novel approach to measuring the magnetic field of the chromosphere despite its disorder. When they modified an instrument that flew in 2015, they mounted their solar observatory on a rocket named after the nautical term “to sound”, which means “to measure”. Sound rockets launch into space for brief observations lasting a few minutes before falling back to Earth. They are cheaper and faster to build and fly than larger satellite missions. They are also an ideal stage to test new ideas and innovative techniques.
The rocket launched from the White Sands Missile Range in New Mexico and shot to an altitude of 172 miles to get a glimpse of the sun over the Earth’s atmosphere, which otherwise blocks certain wavelengths of light. They have a plague in their sights, the edge of an “active region” on the sun where the magnetic field strength was strong, ideal for their sensors.
While CLASP2 was looking at the sun, NASA’s Interface Region Imaging Spectrograph (IRIS) and the Hinode satellite JAXA / NASA, both of which observed the sun from orbit, adjusted their telescopes to look at the same location. In coordination, the three missions focused on the same part of the sun but looked at different depths.
Hinode concentrated on the photosphere and looked for spectral lines from neutral iron formed there. CLASP2 aimed at three different heights within the chromosphere and fixed spectral lines made of ionized magnesium and manganese. In the meantime, IRIS has measured the magnesium lines in higher resolution in order to calibrate the CLASP2 data. Together, the missions monitored four different layers within and around the chromosphere.
Finally, the results were in: The first map with multiple heights of the chromosphere’s magnetic field.
“When Ryohko first showed me these results, I just couldn’t stay in my seat,” said David McKenzie, CLASP2 investigator at NASA’s Marshall Space Flight Center in Huntsville, Alabama. “I know it sounds esoteric – but you just showed the magnetic field at four heights at the same time. Nobody does that! “
The most striking aspect of the data was how different the chromosphere was. The magnetic field varied considerably both along the part of the sun they were studying and at different heights.
“We see magnetic fields on the sun’s surface that change over short distances. further up these variations are much more smeared. In some places the magnetic field did not reach the highest point that we measured, while in other places it was still at full strength. “
The team hopes to use this technique to make magnetic measurements at multiple altitudes to map the magnetic field of the entire chromosphere. Not only would this help predict space weather, but it would also provide vital information about the atmosphere around our star.
“I’m a coronal physicist – I’m really interested in the magnetic fields up there,” said Rachmeler. “To be able to push our limit of measurement to the top of the chromosphere would help us understand so much more, so much more to predict – it would be a great advance in solar physics.”
MEASURING MAGNETIC FIELDS
To measure the magnetic field strength, the team used the Zeeman effect, a centuries-old technique. (The first application of the Zeeman effect to the sun by astronomer George Ellery Hale in 1908 revealed that the sun was magnetic.) The Zeeman effect refers to the fact that spectral lines splinter in multiples in the presence of strong magnetic fields. The further they divide, the stronger the magnetic field.
The Zeeman Effect. This animated image shows a spectrum with multiple absorption lines – spectral lines that are created when atoms absorb a certain wavelength of light at certain temperatures. When a magnetic field is applied (shown here as blue magnetic field lines emanating from a bar magnet), the absorption lines split into two or more. The number of divisions and the distance between them shows the strength of the magnetic field. Note that not all spectral lines were split this way and that the CLASP2 experiment measured spectral lines in the ultraviolet range, while this demo shows lines in the visible range. Credits: Goddard Space Flight Center of NASA / Scott Weissinger
The chaotic chromosphere, however, has a tendency to “smear” spectral lines, making it difficult to tell how far apart they are – so previous missions have had trouble measuring them. The novelty of CLASP2 was to circumvent this limitation by measuring “circular polarization”, a subtle shift in light orientation that occurs as part of the Zeeman effect. By carefully measuring the degree of circular polarization, the CLASP2 team was able to determine how far apart these smeared lines must be and how strong the magnetic field was.
You will have the chance to take this step forward soon: a return flight of the mission was only illuminated in green by NASA. Although the launch date has not yet been set, the team plans to use the same instrument, but with a new technique, to measure a much wider solar strip.
“Instead of just measuring the magnetic fields along the very narrow strip, we want to scan it over the target and create a two-dimensional map,” said McKenzie.
Start of CLASP2
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By Miles Hatfield
NASA’s Goddard Space Flight Center, Greenbelt, Md.Last updated: February 19, 2021 Publisher: Miles Hatfield