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SYFY WIRE Bad Astronomy

Solar Orbiter sees the Sun bursting with tiny explosions of light

By Phil Plait
Artwork depicting Solar Orbiter approaching the Sun. Credit: ESA/Medialab

The European Space Agency's Solar Orbiter — or SolO — spacecraft launched in February 2020, beginning a seven-year-long mission to intensively study the Sun. It's loaded with 10 different scientific instruments to investigate a multitude of solar phenomena, including things that happen on its surface and the inner corona, and will help astronomers understand the peculiar mechanisms that go on there.

It just finished its commissioning phase, where the instruments are turned on and checked out after launch. Other spacecraft have gotten closer to the Sun (notably NASA's Parker Solar Probe), but no other mission gets this close that is also equipped with sunward-facing imagers, capable of taking high-resolution shots of our nearest star.

SolO is not only performing well, it's already started taking data despite not being in its true operational orbit yet. And one thing it's found is already exciting solar astronomers, because it's a phenomenon never seen before: Tiny (well, tiny for the Sun, still terrifyingly powerful on a human scale) bursts of light they've nicknamed "campfires."

An image of the Sun from Solar Orbiter shows a “campfire” (arrowed), a small burst of energy that appears to be happening all over the Sun’s surface. This image is about 380,000 km wide (about the distance from the Earth to the Moon)

These small-scale explosions were seen all over the Sun's surface. They've never been spotted before, partly because they're too small to be seen clearly by other missions. SolO was about 77 million kilometers from the Sun (roughly halfway between the Earth and Sun) on 30 May 2020 when it took these shots. The images were taken with SolO's Extreme Ultraviolet Imager, which from that distance can see details as small as 400 km across on the Sun.

It's not clear what these campfires actually are. An exciting possibility is that they're nanoflares, like solar flares but much smaller in scale. Solar flares occur when the magnetic field lines of the Sun get tangled up, reconnect with each other (like a gigantic short circuit), and release the energy stored up in them. This is a titanic amount of energy; the big flares can explode with the energy of billions of nuclear weapons (note that just doing its usual business, the Sun releases about 100 billion megatons* of energy of second). Happily they don't happen often.

Nanoflares would work more or less the same way but would be a more continuous event, with lots of them popping off somewhere on the Sun all the time. They're theoretical, proposed to explain a big solar mystery: Why the Sun's corona is so ridiculously hot.

Images from Solar Orbiter reveal many small “campfires” on the Sun (arrowed), what may be miniature versions of much larger solar flares. Credit: Solar Orbiter/EUI Team (ESA & NASA); CSL, IAS, MPS, PMOD/WRC, ROB, UCL/MSSL

The Sun's surface temperature is about 5500° Celsius. However, above it is an extremely thin atmosphere of gas that's called the corona — it's the gorgeous wispy tenuous stuff you see surrounding the Sun during a total solar eclipse. The weird thing is the corona is very hot, reaching temperatures of a million degrees! Usually, stars get cooler as you move up from the core outward, but for some reason once you're off the surface things get superheated. How?

Lots of ideas have been debated, but nanoflares are a good one. If they behave like teensy solar flares, then having them explode all over the Sun would heat the plasma there and sent it away from the surface, heating the corona tremendously. The problem is they've never been actually seen … but that's why these campfires seen by SolO are so very interesting. If they turn out to be nanoflares then this could solve the corona mystery!

Right now it's not possible to say, but further observations, especially with other instruments on SolO, may be able to shed more light (haha) on this.

A visible-light image of the Sun from Solar Orbiter’s Polarimetric and Helioseismic Imager shows granules, the tops of giant convection cells, on the Sun’s surface. Credit: Solar Orbiter / PHI Team / ESA & NASA

Mind you, SolO isn't even in its operational orbit yet. Over the next few years it will pass by Venus twice and the Earth once, using their gravity to change its orbit, dropping it even closer to the Sun. At closest approach it'll be just 43 million kilometers from the Sun, and will provide even clearer images. The tilt of the orbit will also be changed so that the spacecraft will be able to see the Sun's poles, which are very difficult to observe from Earth. That should be interesting; I'm reminded that when Juno flew over Jupiter's poles the view was completely different than expected, and all sorts of weird stuff was seen.

One of SolO's big goals is to help us better understand how the solar wind gets its start, too. This is the stream of subatomic particles blowing away from the Sun to the tune of 1.5 million tons per second, screaming away at speeds of 400 kilometers per second and sometimes far more. Like the coronal heating, the exact mechanism that launches the solar wind isn't well understood, so hopefully SolO (especially combined with observations from other solar observatories) will help clear that up.

Mind you, this isn't just some academic exercise. The Sun provides all the light and heat for us here on Earth, which I approve of, and would be reason enough to try to understand it better. But the Sun also blows out all sorts of scary storms, which can and have directly affected us on Earth as well; a big storm can destroy satellites, harm astronauts in orbit, and create widespread blackouts that can be extraordinarily damaging — just read about a storm that very happily missed us in 2012 to get a taste of just how bad this can be.

SolO is just getting started. The nominal mission is for it to keep looking at the Sun for seven years. That's a long time, and I hope the data it gathers continues to surprise us.

*A megaton is the energy equivalent of detonating a million tons of TNT, which is defined as 4 quadrillion Joules. If you're not familiar with that energy unit it hardly matters, because that's a lot of them and in the end it just means that the Sun is brain-crushingly powerful.