The chromosphere, built during the total solar eclipse in 1999. The colors red and pink – light emitted by hydrogen – earned the name chromosphere, from the Greek “chroma” meaning color. Credit: Luc Viatour
For decades after its discovery, observers could only see the solar chromosphere for a few minutes: during a complete solar eclipse, when a bright red glow circled the lunar silhouette.
More than a century later, the chromosphere remains the most mysterious of the atmospheric layers of the sun. Surrounded between the clear surface and the ethereal solar corona, the sun’s outer atmosphere, the chromosphere is a place of rapid change, where temperatures rise and magnetic fields begin to take control of the sun’s behavior. .
Now, for the first time, a third of NASA missions have moved into the chromosphere to return multiple measurements of its magnetic field. The observations – captured by two satellites and a Spectromolarimeter 2 mission of Chromospheric Layer Spectropolarimeter 2, or CLASP2, aboard a small suborbital rocket – will help reveal how magnetic fields on the sun’s surface cause a spectacular explosion. in the open air. The paper was published on February 19, 2021, in Advances in science.
The main goal of heliophysics – the science of the sun’s influence on space, including planetary atmospheres – is to predict space weather, which often starts with the Sun but can spread rapidly through space. space to create a riot near the Earth.
Driving these solar explosions is the sun’s magnetic field, the invisible lines of force that stretch from the surface of the sun to a place far beyond Earth. This magnetic field is difficult to see – it can only be seen indirectly, with light from plasma, or fully-heated gas, which follows its lines like car lights traveling on a distant highway. But despite how these magnetic lines align themselves – whether slow or straight or tight and connected – make the difference between a silent Sun and a solar eclipse.
“The sun is both beautiful and mysterious, with sustained activity stimulated by its magnetic fields,” said Ryohko Ishikawa, a solar physics expert at the Japan National Astronomical Observatory in Tokyo and lead author of the paper.
The chromosphere lies between the photosphere, or bright surface of the sun that emits visible light, and the fully-heated corona, or outer atmosphere of the sun at the source of a solar explosion. The chromosphere is a key link between these two regions and a variable is required determining the magnetic structure of the sun. Credit: Credits: NASA Goddard Space Flight Center
It would be great if researchers could read out the magnetic field lines in the corona, where solar explosion occurs, but the plasma is far too scattered for accurate readings. (The corona is far less than a billion thick as air at sea level.)
Instead, scientists measure the denser photosphere – the visible surface of the sun – two layers below. They then use mathematical models to extend that range upwards into the corona. This approach jumps out measuring the chromosphere, which lies between the two, instead, hoping to resemble behavior.
Unfortunately, the chromosphere has turned out to be a wild card, where magnetic field lines rearrange in ways that are hard to imagine. The models are struggling to capture this complexity.
“The chromosphere is a hot, hot message,” said Laurel Rachmeler, a former NASA project scientist for CLASP2, now at the National Oceanic and Atmospheric Administration, or NOAA. “We simplify assumptions about physics in the photosphere, and take individual assumptions into the corona. But in the chromosphere, most of those assumptions break down. ”
Institutions in the US, Japan, Spain and France worked together to develop a new way to measure the magnetic field of the chromosphere despite its complexity. Changing its flying instrument in 2015, they put their solar observatory on an acoustic rocket, so named for the marine term “to sound” meaning measurement. Acoustic rockets launch into space for brief observations, just minutes before they fall back to Earth. Build more accessible and faster and fly the larger satellite missions, they are also a good place to try out new ideas and innovative ways.
Launching from the White Sands Missile Range in New Mexico, the rocket fired to a height of 170 miles (274 kilometers) for a view of the Sun from above the Earth’s atmosphere, which otherwise blocks certain waves of light. They set their sights on a plage, as an “active area” on the Sun where the strength of the magnetic field was strong, perfectly suited to their senses.
As shown by CLASP2 at the Sun, NASA or IRIS Interface Division Image Spectrograph and the JAXA/ NASA Hinode Satellites, both looking at the sun from Earth’s orbit, modified their telescope to look at the same place. In coordination, the three missions focused on the same part of the Sun, but went to different depths.
RECOMMENDING MAGNETIC FIELDS
To measure the strength of a magnetic field, the team took advantage of Zeeman’s influence, a century-old technique. (The first claim of Zeeman’s influence on the Sun, by astronomer George Ellery Hale in 1908, is how we learned that the sun was magnetic.) Zeeman’s influence refers to the presence of celestial lines , in the presence of strong magnetic fields, splinter into multiples. The further apart they differ, the stronger the magnetic field.
Zeeman victory. This animated image shows a spectrum with several lines of absence – celestial lines formed when atoms at a certain temperature receive a specific wavelength of light. When a magnetic field is introduced (shown here as blue lines of a magnetic field emanating from a magnetic bar), absorption lines are divided into two or more. The number of splits and the distance between them reflect the strength of the magnetic field. Note that not all spectral lines intersect in this way, and the CLASP2 test measured spherical lines in the ultraviolet range, but this demo shows lines in the visible range. Credit: NASA Goddard Space Flight Center / Scott Weissinger
The chaotic chromosphere, however, tends to “smear” celestial lines, making it difficult to tell exactly how far apart they separated – which is why previous missions had difficulty measuring. The CLASP2 novelty worked around this limitation by measuring “circular polarization,” a subtle movement in the direction of light that occurs as part of a Zeeman effect. By carefully measuring the degree of circular polarization, the CLASP2 team could determine how far apart these smear lines need to be separated, and thus how strong the magnetic field was.
Hinode focused on the photosphere, looking for celestial lines from neutral iron formed there. CLASP2 targeted three different heights within the chromosphere, locking on celestial lines from ionized magnesium and manganese. At the same time, IRIS measured the magnesium lines in higher resolution, to capitalize CLASP2 data. Together, the missions observed four different layers in and around the chromosphere.
Finally the results were: The first multipurpose map of the magnetic field of the chromosphere.
“When Ryohko first showed me these results, I couldn’t stay in my seat,” said David Mackenzie, chief CLASP2 observer at NASA’s Marshall Flight Center in Huntsville, Alabama. “I know it’s esoteric – but you’ve just shown the magnetic field at four heights at once. Nobody does that! ”
The most striking feature of the data was just how different the chromosphere was. Both on the part of the Sun they studied and at different altitudes within it, there was a great change in the magnetic field.
“At the surface of the sun we can see magnetic fields changing over short distances; higher up these changes are much more smeared. In some places, the magnetic field all the way to the highest point we measured did not reach it but in others it was still at full strength. ”
The team hopes to use this method for multi-height magnetic measurement to map the magnetic field of the entire chromosphere. Not only would this help our ability to predict space weather, it will tell us key information about the atmosphere around our star.
“I’m a physicist – I’m very interested in the magnetic fields up there,” said Rachmeler. “Raising our measurement limits to the top of the chromosphere would help us understand a lot more, it would help us predict a lot more – it would be a big step forward in physics the sun. ”
They will soon have the opportunity to take that step forward: A re-flight of the mission was made green by NASA. Although the start date has not yet been set, the team plans to use the same instrument but with a new way to measure a much wider swath of the Sun.
“Instead of just measuring the magnetic fields on the very narrow strip, we want to scan it over the target and make a two-dimensional map,” McKenzie said.
Read a unique map of the solar magnetic field created by CLASP2 Space Experiment for more information on this research.
Fact: “Mapping solar magnetic fields from the Photosphere to the base of the Corona” by Ryohko Ishikawa, Javier Trujillo Bueno, Tanausú del Pino Alemán, Takenori J. Okamoto, David E. McKenzie, Frédéric Auchère, Ryouhei Kano, Donguk Song, Masaki Yoshida, Laurel A. Rachmeler, Ken Kobayashi, Hirohisa Hara, Masahito Kubo, Noriyuki Narukage, Taro Sakao, Toshifumi Shimizu, Yoshinori Suematsu, Christian Bethge, Bart De Pontieu, Alberto Sainz Dalda, Genevieve D. Vigil, Amy Alinesinagerger . , Luca Belluzzi, Jiri Stepan, Andrés Asensio Ramos, Mats Carlsson and Jorrit Leenaarts, 19 February 2021, Advances in science.
DOI: 10.1126 / sciadv.abe8406