WATCH: NASA Missions Make Unprecedented Map of Sun’s Magnetic Field

More than a hundred years later, the chromosphere remains the most mysterious of the Sun’s atmospheric layers

The chromosphere, photographed during the 1999 total solar eclipse. The red and pink hues – light emitted by hydrogen – earned it the name chromosphere, from the Greek “chrôma” meaning color. ( image)

( — 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 ringed the Moon’s silhouette.

More than a hundred years later, the chromosphere remains the most mysterious of the Sun’s atmospheric layers. Sandwiched 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 begin to dominate the Sun’s behavior.

Now, for the first time, a triad of NASA missions have peered into the chromosphere to return multi-height measurements of its magnetic field. The observations – captured by two satellites and the Chromospheric Layer Spectropolarimeter 2, or CLASP2 mission, aboard a small suborbital rocket – help reveal how magnetic fields on the Sun’s surface give rise to the brilliant eruptions in its outer atmosphere. The paper was published today in Science Advances.

A major goal of heliophysics – the science of the Sun’s influence on space, including planetary atmospheres – is to predict space weather, which often begins on the Sun but can rapidly spread through space to cause disruptions near Earth.

The chromosphere lies between the photosphere, or bright surface of the Sun that emits visible light, and the super-heated corona, or outer atmosphere of the Sun at the source of solar eruptions. The chromosphere is a key link between these two regions and a missing variable determining the Sun’s magnetic structure. ( image)

Driving these solar eruptions is the Sun’s magnetic field, the invisible lines of force stretching from the solar surface to space well past Earth. This magnetic field is difficult to see – it can only be observed indirectly, by light from the plasma, or super-heated gas, that traces out its lines like car headlights traveling a distant highway. Yet how those magnetic lines arrange themselves – whether slack and straight or tight and tangled – makes all the difference between a quiet Sun and a solar eruption.

“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 out the magnetic field lines in the corona, where solar eruptions take place, but the plasma is way too sparse for accurate readings. (The corona is far less than a billionth as dense as air at sea level.)

ABOVE VIDEO: NASA GSFC solar scientist Holly Gilbert explains a computer model of the sun’s magnetic field.

Instead, scientists measure the more densely packed photosphere – the Sun’s visible surface – two layers below. They then use mathematical models to propagate that field upwards into the corona.  This approach skips measuring the chromosphere, which lies between the two, instead, hoping to simulate its behavior.

Unfortunately the chromosphere has turned out to be a wildcard, where magnetic field lines rearrange in ways that are hard to anticipate. The models struggle to capture this complexity.

“The chromosphere is a hot, hot mess,” said Laurel Rachmeler, former NASA project scientist for CLASP2, now at the National Oceanic and Atmospheric Administration, or NOAA. “We make simplifying assumptions of the physics in the photosphere, and separate assumptions in the corona. But in the chromosphere, most of those assumptions break down.”

Institutions in the U.S., Japan, Spain and France worked together to develop a novel approach to measure the chromosphere’s magnetic field despite its messiness. Modifying an instrument that flew in 2015, they mounted their solar observatory on a sounding rocket, so named for the nautical term “to sound” meaning to measure. Sounding rockets launch into space for brief, few-minute observations before falling back to Earth. More affordable and quicker to build and fly than larger satellite missions, they’re also an ideal stage to test out new ideas and innovative techniques.
The Zeeman effect. This animated image shows a spectrum with several absorption lines – spectral lines produced when atoms at specific temperatures absorb a specific wavelength of light. When a magnetic field is introduced (shown here as blue magnetic field lines emanating from a bar magnetic), absorption lines split into two or more. The number of splits and the distance between them reveals the strength of the magnetic field. Note that not all spectral lines split in this way, and that the CLASP2 experiment measured spectral lines in the ultraviolet range, whereas this demo shows lines in the visible range. ( video)

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