Solve a 65-year-old cosmic mystery

The Sun’s outer atmosphere, known as the corona, can be seen extending into space in this image from Solar Orbiter’s Metis instrument. Metis is a multi-wavelength device, operating on visible and ultraviolet wavelengths. It is a coronagraph, which means it blocks bright sunlight from the Sun’s surface, leaving visible the faint light that scatters particles in the corona. In this image, the fuzzy red disk represents the corona while the white disk is a mask to compress the image size to reduce the amount of unnecessary data associated with the downlink. Image credit: ESA and NASA/Solar Orbiter/METIS team; Dr.. Tiloni et al. (2023)

A cosmic alignment and a few spacecraft exercises have provided a groundbreaking measurement that helps solve the 65-year-old cosmic mystery of why the sun’s atmosphere is warming.

The sun’s atmosphere is called the corona. It consists of an electrically charged gas known as plasma Its temperature is about one million degrees Celsius.

Its temperature is always a mystery because the surface temperature of the Sun is only about 6,000 degrees Celsius. The corona should be cooler than the surface because the Sun’s energy comes from the nuclear furnace at its core, and things naturally get cooler the farther away from the heat source. However, the corona is more than 150 times hotter than the surface.

There must be another way to transfer energy to the plasma, but what?

Theories and challenges of investigation

It has long been suspected that turbulence in the solar atmosphere could lead to significant heating of the plasma in the corona. But when it comes to investigating this phenomenon, solar physicists face a practical problem: It’s impossible to collect all the data they need with just one spacecraft.

There are two ways to explore the Sun: remote sensing and in situ measurements. In remote sensing, the spacecraft is positioned at a certain distance and cameras are used to look at the sun and its atmosphere at different wavelengths. For in situ measurements, the spacecraft flies through the area it wants to explore and takes measurements of particles and magnetic fields in that part of space.

Both approaches have their advantages. Remote sensing shows large-scale results but not details of the processes occurring in the plasma. Meanwhile, in situ measurements provide very specific information about small-scale processes in the plasma, but do not show how this affects the large scale.

Dual investigation in spacecraft

To get the full picture, two spacecraft are needed. And that’s exactly what heliophysicists currently have in the form of the Solar Orbiter spacecraft piloted by the European Space Agency and NASA’s Parker Solar Probe. Solar Orbiter is designed to get as close to the Sun as possible and still perform remote sensing, as well as in situ measurements. The Parker Solar Probe largely ignores remote sensing of the Sun itself to get closer to make its measurements in situ.

But to take full advantage of their complementary approach, the Parker Solar Probe must be within the field of view of one of the Solar Orbiter’s instruments. In this way, Solar Orbiter was able to record the large-scale consequences of what the Parker Solar Probe was measuring in situ.

ESA’s Solar Orbiter is one of two integrated spacecraft studying the Sun at close range: it joins NASA’s Parker Solar Probe, which was already involved in its mission. Source: Solar Orbiter: ESA/ATG medialab; Parker Solar Probe: NASA/Johns Hopkins APL

Astrophysical coordination

Daniele Telloni, a researcher at the Italian National Institute of Astrophysics (INAF) at the Astrophysical Observatory in Turin, is part of the team behind the Solar Orbiter’s Metis instrument. Metis is a coronagraph that blocks light from the Sun’s surface and takes pictures of the corona. It’s the perfect tool to use for large-scale measurements, so Daniele started looking for times when the Parker Solar Probe would line up.

It was found that on June 1, 2022, the two spacecraft will be in the correct orbital configuration – approximately. Essentially, the Solar Orbiter will be looking at the Sun, and the Parker Solar Probe will be just off to the side, dramatically close but outside the field of view of the METS instrument.

When Daniele looked at the problem, he realized that all it would take to shine light on the Parker Solar Orbiter was a little exercise with the Solar Orbiter: a 45-degree roll and then pointing it slightly away from the Sun.

But when every space mission maneuver is carefully planned in advance, and when the spacecraft themselves are designed to point only in very specific directions, especially when dealing with the Sun’s frightening heat, it was not obvious that the spacecraft operations team would authorize such a maneuver. deviation. However, once everyone was clear about the potential scientific return, the decision was a clear “yes.”

ESA’s Solar Orbiter mission will encounter the Sun from within Mercury’s orbit at its closest approach. Source: ESA/ATG medialab

Hack notes

Roll and offset steering proceeded; The Parker Solar Probe entered the field of view, and together the spacecraft produced the first-ever simultaneous measurements of the large-scale composition of the solar corona and the microphysical properties of the plasma.

“This work is the result of the contributions of many people,” says Daniele, who led the analysis of the datasets. Working together, they were able to make the first joint observational and in situ estimate of the coronal heating rate.

“Being able to use both the Solar Orbiter and the Parker Solar Probe has opened up a whole new dimension in this research,” says Gary Zank, of the University of Alabama in Huntsville, USA, and a co-author of the resulting paper.

By comparing the newly measured rate with theoretical predictions made by solar physicists over the years, Daniele showed that solar physicists were roughly correct in their identification of turbulence as a means of energy transfer.

Artist’s concept of the Parker Solar Probe spacecraft approaching the Sun. Image credit: NASA/Johns Hopkins APL/Steve Gribben

The specific way the disorder does this is no different than what happens when you stir your morning cup of coffee. By stimulating random movements of a fluid, whether gas or liquid, energy is transferred to ever smaller scales, which culminates in the conversion of energy into heat. In the case of the solar corona, the fluid is also magnetized and thus stored magnetic energy is also available to be converted into heat.

Such a transfer of magnetic energy and kinetic energy from larger to smaller scales is the essence of turbulence. On the smallest scales, it allows the fluctuations to finally interact with individual particles, mostly protons, and heat them.

Conclusions and implications

Much more work is needed before we can say that the problem of solar heating has been solved, but now, thanks to Daniel’s work, solar physicists have been able to make the first measurement of this process.

“This is a scientific first. This work represents an important step forward in solving the problem of coronal heating,” says Daniel Müller, project scientist.

Solar Orbiter is a space mission of international cooperation between ESA and ESA NASAmanaged by the European Space Agency.