NASA Missions Make Unprecedented Map of Sun’s Magnetic Field


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.

Chromosphere during solar eclipse

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.Credits: Luc Viatour

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.

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.)

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.

Layers of the Sun

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.Credits: Credits: NASA’s Goddard Space Flight Center

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.

Launching from the White Sands Missile Range in New Mexico, the rocket shot to an altitude of 170 miles (274 kilometers) for a view of the Sun from above Earth’s atmosphere, which otherwise blocks certain wavelengths of light. They set their sights on a plage, the edge of an “active region” on the Sun where the magnetic field strength was strong, ideal for their sensors.

As CLASP2 peered at the Sun, NASA’s Interface Region Imaging Spectrograph or IRIS and the JAXA/NASA Hinode satellite, both watching the Sun from Earth orbit, adjusted their telescopes to look at the same location. In coordination, the three missions focused on the same part of the Sun, but peered to different depths.

Hinode focused on the photosphere, looking for spectral lines from neutral iron formed there. CLASP2 targeted three different heights within the chromosphere, locking onto spectral lines from ionized magnesium and manganese. Meanwhile, IRIS measured the magnesium lines in higher resolution, to calibrate the CLASP2 data. Together, the missions monitored four different layers within and surrounding the chromosphere.

Eventually the results were in: The first multi-height map 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 principal investigator at NASA’s Marshall Space Flight Center in Huntsville, Alabama. “I know it sounds esoteric – but you’ve just showed the magnetic field at four heights at the same time. Nobody does that!”

The most striking aspect of the data was just how varied the chromosphere turned out to be. Both along the portion of the Sun they studied and at different heights within it, the magnetic field varied significantly.

“At the Sun’s surface we see magnetic fields changing over short distances; higher up those variations are much more smeared out. In some places, the magnetic field didn’t reach all the way up to the highest point we measured whereas in other places, it was still at full strength.”

The team hopes to use this technique for multi-height magnetic measurements to map the entire chromosphere’s magnetic field. Not only would this help with our ability to predict space weather, it will tells us key information about the atmosphere around our star.

“I’m a coronal physicist – I’m really interested in the magnetic fields up there,” Rachmeler said. “Being able to raise our measurement boundary to the top of the chromosphere would help us understand so much more, help us predict so much more – it would be a huge step forward in solar physics.”


To measure magnetic field strength, the team took advantage of the Zeeman effect, a century-old technique. (The first application of the Zeeman effect to the Sun, by astronomer George Ellery Hale in 1908, is how we learned that the Sun was magnetic.) The Zeeman effect refers to the fact that spectral lines, in the presence of strong magnetic fields, splinter into multiples. The farther apart they split, the stronger the magnetic field.

Demonstration of Zeeman effect

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.Credits: NASA’s Goddard Space Flight Center/Scott Weissinger

The chaotic chromosphere, however, tends to “smear” spectral lines, making it difficult to tell just how far apart they split – that’s why previous missions had trouble measuring it. CLASP2’s novelty was in working around this limitation by measuring “circular polarization,” a subtle shift in the light’s orientation that happens as part of the Zeeman effect. By carefully measuring the degree of circular polarization, the CLASP2 team could discern how far apart those smeared lines must have split, and thereby how strong the magnetic field was.

They’ll have a chance to take that step forward soon: A re-flight of the mission was just greenlit by NASA. Though the launch date isn’t yet set, the team plans to use the same instrument but with a new technique to measure a much broader swath of the Sun.

“Instead of just measuring the magnetic fields along the very narrow strip, we want to scan it across the target and make a two-dimensional map,” McKenzie said.


CLASP2’s launch

More about NASA’s Sounding Rockets

By Miles Hatfield

NASA’s Goddard Space Flight Center, Greenbelt, Md.Last Updated: Feb 19, 2021Editor: Miles Hatfield

4.7 3 votes
Article Rating
Newest Most Voted
Inline Feedbacks
View all comments
February 21, 2021 3:08 am

so the models didnt work cos it was chaotic?
hmm like they also dont work here for the same reason?

Reply to  ozspeaksup
February 21, 2021 4:05 am

Yeah, but the difference is, on the sun, there are no climastrologists, so nobody to think the sun shines from his ass.
Oh, and don’t use words like chaotic, it really gets up nyolci’s ass.

Reply to  ozspeaksup
February 21, 2021 4:08 am

Well, electric currents are huge unknown for astrophysicists, magnetic field is not just bits of iron scattered on piece of paper. It might take long time before effects of the solar magnetic field on our environment are fully understood. In this case science will advance only with next generational change of the current crop of scientist.

Reply to  Vuk
February 21, 2021 4:28 am

Solar flares are short and powerful events, far more interesting than just counting sunspots and for us here on the Earth might cause undesirable effects, and yet electric currents are at the core of flares structure

Joel O'Bryan
Reply to  Vuk
February 21, 2021 12:51 pm

Your cartoon lacks the reconnection event between field lines carried on plasma filaments.
It’s the re-connection event between twisted field lines that allows the release of the stored magnetic energy in the plasma filaments to blast through the corona and thus superheating it to produce the X-ray flashes.

Reply to  Joel O'Bryan
February 21, 2021 2:21 pm

It is image taken from an AGU paper I had stored on my computer. Solar flare magnetic reconnections are far more complex affairs, but if anyone is interested there is collaborative paper
written under guidance of Eric Priest (acquaintance of mine), solar scientist and  professor of mathematics at St Andrews University, UK, recipient of numerous awards including Hale Prize from the AAS in 2002, the.lead author of numerous papers on solar physics matters, including (for me too technical to adequately comprehend) book “Magnetohydrodynamics of the Sun”
with whole chapter on solar flares
p.s add .pdf to the link

Reply to  Vuk
February 21, 2021 2:44 pm

Note to Charles the Mod
I posted a link for JO’B above, for a book which has been in print for number of years now not meant for wide distribution. It would be greatly appreciated if you could delete it sometime later today. Thanks.

Reply to  Vuk
February 21, 2021 4:50 am

I’m not sure how predictable the Sun’s magnetic field will ever be. When a moving plasma is subjected to a magnetic field, it is deflected. The changed direction of the plasma movement changes the magnetic field, and so on, and so forth.

The simplest example I can think of is with electric arc welding. link Magnetic fields can really mess up a weld by deflecting the arc plasma away from where you want it to go. Just multiply that simple problem by a gazillion and you get something like the problem of predicting the behavior of the Sun’s magnetic field.

It’s like turbulent convection on steroids.

Joel O'Bryan
Reply to  commieBob
February 21, 2021 1:07 pm

“It’s like turbulent convection on steroids magnetics.”

In aerodynamics, CFD with unstructured finite element (differential) methods are used to model and predict turbulent and laminar air flows. Of course it means nothing until the underlying mathematics and outputs in the analyses are validated in wind tunnel testing.

But to added to the complexity of CFD, with solar phenomenon the magnetics have to integrated, so Magneto-HydroDynamics (MHD) is a field unto itself that takes CFD complexity to a whole other level, and usually requiring time on a supercomputer to solve even basic problems. And those outputs don’t mean much more than a 3rd tier journal paper until physicists are able to compare them to some kind of observation product like the CLASP2 data discussed here.

Alan Robertson
February 21, 2021 11:23 am

I just want to be around to see the first photos of a black dwarf star.

Reply to  Alan Robertson
February 21, 2021 1:01 pm

Hi Alan
Not at the moment, but there is no need to worry you will eventually catch-up with it.  
Your  black dwarf  star has lost all its energy (its temperature is the same
as space background radiation), i.e it doesn’t contain any information.
In the theory of the holographic universe all of its  information is preserved in the universe’s boundary well beyond the universe’s observable event horizon, hence no photons can reach you photographic plate, but you are also going to end up there too, so do not despair, just be a bit more patient. Once you are there your time will slow down to zero so you can observe it for the absolute eternity.  

Rod Evans
February 21, 2021 12:19 pm

What’s with all this interest in looking at the Sun? Don’t these stellar scientist know, it plays no part in our Earth’s wellbeing?
Maybe they will find some CO2 lurking about? That seems to be the complete answer to everything these days. So much for 42, what did that Douglas Adams guy know anyway….?

Joel O'Bryan
February 21, 2021 12:46 pm

“Not only would this help with our ability to predict space weather, it will tells us key information about the atmosphere around our star.”

Sadly this was originally GISS’s charter in NASA in the 1960’s and 1970’s. James Hansen’s environmental activism took GISS away from that principle mission to studying Earth Climate with computer models. GISS instead now still chases his desire to stop coal extraction and burning and using in silico simulation produced Climate Change doomerism as a weapon to that end. As an environmentalist that required his “getting in bed” with the Socialists come Marxists, a bargain with the devil.

James F. Evans
February 21, 2021 1:44 pm

If there are magnetic fields, there are also electric fields and electric currents (flows of plasma [ionized matter]).

Joel O'Bryan
Reply to  James F. Evans
February 21, 2021 3:09 pm

In a capacitor, the energy is stored in the E-field.
In an induction coil, energy is stored in the B field. It is best to think of the physical system here as windings of inductor coils where the energy that is released when a looped filament (winding) is snipped is magnetic energy.

James F. Evans
Reply to  Joel O'Bryan
February 24, 2021 6:42 pm

What’s to say one is stronger than the other, is the energy stored in the magnetic field (“B field”) or the electric field?

No electric field… no magnetic field… full stop.

February 21, 2021 4:30 pm

Instead of models and assumptions “breaking down” can they just say…


February 21, 2021 10:49 pm

“Did you find something?”

“Yeah, I found a Corona!”

“A Corona?”

“Yeah! It was a Corona-Corona, but I only found halfa it! Nyuk nyuk nyuk!”

%d bloggers like this:
Verified by MonsterInsights