Two spacecraft have provided a groundbreaking measurement that helps solve a 65-year-old cosmic mystery - why the sun's atmosphere is so hot. The sun's atmosphere is called the corona. It consists of electrically charged gas called plasma, with a temperature of about one million degrees Celsius. Its temperature is an eternal mystery, as 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 the farther away you are from the heat source, the cooler it naturally becomes.However, the temperature of the corona is more than 150 times higher than the surface. There must be another method of transferring energy to the plasma at work, what is it?
In this image captured by Solar Orbiter's Metis instrument, the Sun's outer atmosphere, known as the corona, can be seen extending into space. Metis is a multi-wavelength device that operates in both visible and ultraviolet wavelengths. It's a coronagraph, which means it blocks bright sunlight from the Sun's surface, making visible the dimmer light scattered by particles in the corona. In this image, the blurred red disk represents the coronagraph, while the white disk is a mask used to compress the image size to reduce the amount of unnecessary downlink data. Image credit: ESA and NASA/Solar Orbiter/Metis Group; D. Telloni et al. (2023)
Theoretical and investigative challenges
Turbulence in the solar atmosphere has long been suspected to cause substantial heating of plasma in the corona. But when studying this phenomenon, solar physicists encountered a practical problem: It was impossible to collect all the data needed with just one spacecraft.
There are two methods of studying the Sun: remote sensing and in-situ measurements. In remote sensing measurements, a spacecraft is placed at a distance and uses cameras to observe the sun and its atmosphere at different wavelengths. In an in-situ measurement, the spacecraft flies over the area it wants to study, taking measurements of particles and magnetic fields in that part of space.
Both methods have their advantages. Remote sensing can show large-scale results but cannot show the details of the processes occurring in the plasma. At the same time, in situ measurements can provide highly specific information about small-scale processes in the plasma, but cannot show how these processes affect large-scale processes.
Dual spacecraft survey
To get the full picture, two spacecraft would be needed. That's exactly what heliophysicists currently have, with the ESA-led Solar Orbiter spacecraft and NASA's Parker Solar Probe. Solar Orbiter is designed to get as close as possible to the Sun while performing remote sensing operations and in-situ measurements. The Parker Solar Probe has largely given up on remote sensing of the sun itself, instead moving closer to the sun to make on-the-spot measurements.
But to take full advantage of their complementarity, Parker Solar Probe must be within the field of view of one of Solar Orbiter's instruments. In this way, Solar Orbiter can record the vast amount of data generated by Parker Solar Probe's in-situ measurements.
ESA's Solar Orbiter is one of two complementary spacecraft studying the sun at close range: it joins NASA's Parker Solar Probe in its mission. Image credits: Solar Orbiter: ESA/ATGmedialab; Parker Solar Probe: NASA/Johns Hopkins APL
astrophysics coordination
Daniele Telloni, a researcher at the Italian National Institute for Astrophysics (INAF) at the Turin Astrophysical Observatory, is a member of the team behind Solar Orbiter's Metis instrument. Metis is a coronagraph that blocks light from the sun's surface and takes pictures of the corona. It was the perfect instrument for large-scale measurements, so Danielle set out to find when the Parker Solar Probe would line up.
He found that on June 1, 2022, the two spacecraft will be in the correct orbital configuration -- almost. Essentially, Solar Orbiter will be looking at the sun, while Parker Solar Probe is off to the side, very close but just outside the field of view of the Métis instrument.
When Daniel saw the problem, he realized that bringing the Parker Solar Probe into view would require just a small maneuver on Solar Orbiter: rolling it 45 degrees and pointing it slightly away from the sun.
But every movement of a space mission is carefully planned in advance, and the spacecraft itself is designed to point only in very specific directions, especially when dealing with the terrifying heat of the sun. It's unclear whether the spacecraft operations team would approve such a deviation. However, once the potential scientific payoff became clear to everyone, the decision was an unequivocal "yes."
ESA's Solar Orbiter mission will face the Sun from within the orbit of Mercury during its closest approach to the Sun. Image source: ESA/ATGmedialab
Breakthrough observations
The roll and deflection continued; Parker Solar Probe came into view, and for the first time the two spacecraft simultaneously measured the large-scale structure of the corona and the microphysical properties of the plasma.
"This work is the result of contributions from many, many people," said Daniel, who led the analysis of the dataset. Working together, they produced the first comprehensive observations and in-situ estimates of coronal heating rates.
"The ability to use Solar Orbiter and Parker Solar Probe simultaneously really opens up a whole new dimension to this research," said Gary Zank of the University of Alabama in Huntsville, a co-author of the paper.
By comparing the newly measured rates to years of theoretical predictions by solar physicists, Daniel showed that solar physicists were almost certainly correct in identifying turbulence as a mode of energy transfer.
Artist's concept of the Parker Solar Probe approaching the sun. Image source: NASA/JohnsHopkinsAPL/SteveGribben
The exact way turbulence creates this effect is not unlike what happens when you stir your coffee in the morning. By stimulating the random movement of a fluid (gas or liquid), energy is transferred to smaller scales, ultimately converting the energy into heat. In the corona, the fluid is also magnetized, so the stored magnetic energy can also be converted into thermal energy.
This transfer of magnetic and kinetic energy from larger to smaller scales is the essence of turbulence. At the smallest scales, it causes the waves to eventually interact with individual particles (mainly protons) and heat them.
Conclusion and Enlightenment
More work needs to be done before the problem of solar heating is solved, but now, thanks to Daniele's work, solar physicists have measured this process for the first time.
"This is a scientific first," said project scientist Daniel Müller.