Two solar probes are getting closer to solving the mystery of the solar corona

2023-09-14 15:00:05

The outer atmosphere of the Sun is called corona. It is made up of an electrically charged gas known as plasma and has a temperature of around a million degrees centigrade. Its temperature is a mystery, since the Sun’s surface only reaches about 6,000 degrees.

The plasma of the solar corona is a million degrees, while the surface is only 6,000 degrees. This issue has been unresolved for 65 years.

How can it be? The corona should be colder than the surface because the Sun’s energy comes from the nuclear furnace at its core, and things naturally cool the further they get from the heat source. However, the corona is more than 150 times hotter than the surface.

There must be another method to transfer energy to plasma, but which one? It has long been suspected that the turbulence of the solar atmosphere can cause significant heating of the coronal plasma. But when it comes to investigating this phenomenon, solar physicists run into a practical problem: It’s impossible to gather all the data they need with a single spacecraft.

Remote sensing and measurements on site

There are two ways to investigate the Sun: remote sensing and measurements on site. In the remote sensing, the spacecraft sits at a certain distance and uses cameras to observe our star and its atmosphere at different wavelengths. For its part, in the measurements on sitethe spacecraft flies through the region it wants to investigate and makes measurements of the particles and magnetic fields in that part of space.

Both approaches have their advantages. Remote sensing shows the large scale results, but not the details of the processes taking place in the plasma. For their part, the measurements on site provide very specific information about the processes to be small scale in plasma but they do not show how they affect on a large scale.

To get a full picture, two spacecraft are needed. This is exactly what solar physicists currently have with the ESA-led Solar Orbiter and NASA’s Parker Solar Probe.

Solar Orbiter can perform remote sensing operations and Parker Solar Probe can get even closer to the Sun to take in situ measurements

Solar Orbiter (in which NASA also collaborates) is designed to get as close to the Sun as possible and carry out satellite operations. remote sensing and in situ measurements. Parker Solar Probe largely renounces remote sensing of the Sun to get even closer and carry out its measurements on site.

But to take full advantage of its complementary approaches, Parker Solar Probe would have to be within the field of view of one of Solar Orbiter’s instruments. That way, the second could record the large-scale consequences of what the first was measuring. on site.

Daniele Telloniresearcher at the Italian National Institute of Astrophysics (INAF) at the Turin Astrophysical Observatory, is part of the team responsible for the Metis instrument of the Solar Orbiter.

Metis is a coronagraph which blocks light from the Sun’s surface and takes images of the corona. It’s the perfect instrument for large-scale measurements, so Daniele began looking for moments when the Parker Solar Probe would align.

Solar Orbiter’s Metis instrument observes the Sun’s corona. / ESA & NASA/Solar Orbiter/Metis team; D. Telloni et al (2023)

Alignment of the two ships

He discovered that the June 1, 2022the two spaceships would be in the correct orbital configuration, almost. Essentially, Solar Orbiter would be facing the Sun and Parker Solar Probe would be right next to it, tantalizingly close but just outside the Metis instrument’s field of view.

When Daniele thought about the problem, he realized that all it took to get the Parker Solar Probe into view was a little ‘gymnastics’ with the Solar Orbiter: a 45 degree turn and then aim it slightly far from the Sun.

But when all the maneuvers of a space mission are carefully planned in advance, and the spacecraft themselves are designed to point only in very specific directions, especially when faced with the fearsome heat of the Sun, it was not clear that the space operations team the ship would authorize such a deviation. However, once everyone was clear about the potential scientific yield, the decision was a clear “yes.”

The two spacecraft have produced the first simultaneous measurements of the large-scale configuration of the solar corona and the microphysical properties of the plasma

The turning and pointing continued. The Parker Solar Probe entered the field of view and together the spacecraft produced the first simultaneous measurements of the large-scale configuration of the solar corona and the microphysical properties of the plasma.

“This work is the result of the contribution of many people,” says Daniele, who led the analysis of the data sets. Working together, they have been able to carry out the first combined observational and on site of the coronal heating rate. Now they publish the results in The Astrophysical Journal Letters.

“The ability to use both the Solar Orbiter and the Parker Solar Probe has really opened up a whole new dimension in this research,” he says. Gary Zankfrom the University of Alabama in Huntsville (USA) and co-author of the study.

The turbulent atmosphere of the Sun

By comparing the newly measured index with theoretical predictions made by solar physicists over the years, Daniele has shown that solar physicists were almost certainly correct in identifying the turbulence as a way to transfer energy.

The specific way in which turbulence does this is not very different from what happens when stir the coffee in the morning. By stimulating the random movements of a fluid, whether a gas or a liquid, the energy is transferred to smaller and smaller scaleswhich culminates in the transformation of energy into heat.

In the case of the solar corona, the fluid is also magnetizedso the stored magnetic energy is also available to be converted into heat.

The transfer of magnetic energy and motion from large to small scales generates turbulence and, at small scales, fluctuations interact with particles and heat them

This transfer of magnetic energy and motion from larger to smaller scales is the very essence of turbulence. On the smallest scales, it allows fluctuations to eventually interact with individual particles, mostly protons, and heat them.

We have to keep working before we could say that the problem of solar heating is solved, but now, thanks to Daniele’s work, solar physicists have their first measurement of this process.

“It is a scientific scoop. “This work represents an important step forward in solving the problem of coronal heating,” he concludes. Daniel Mülleranother of the project scientists.