The Sun’s outer atmosphere, known as the corona, is seen expanding into space in this image taken from the Metis Solar Orbiter instrument. Metis is a multi-wavelength device, operating at both visible and ultraviolet wavelengths. This is a coronagraph, which means it blocks bright sunlight from the Sun’s surface, thereby making visible the faint light that scatters particles within the corona. In this image, the red fuzzy disk represents the corona while the white disk is a mask to compress the image size to reduce the amount of unnecessary data related to the downlink. Image credits: ESA and NASA/Solar Orbiter/METIS team; Dr.. Tiloni et al (2023)
A cosmic alignment and several spacecraft exercises have yielded groundbreaking measurements that help solve the 65-year-old cosmic mystery of why the sun’s atmosphere is heating up.
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 has always been a mystery because the surface temperature of the Sun is only around 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 colder the further away from the heat source. However, the temperature of the corona is 150 times hotter than the surface.
There must be another way to transfer energy to the plasma, but what?
Theories and challenges of inquiry
Turbulence in the solar atmosphere has long been thought to cause significant heating of the plasma in the corona. But when 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, a spacecraft is positioned at a certain distance and cameras are used to view the sun and its atmosphere at different wavelengths. For in situ measurements, the spacecraft flies over the area it wants to explore and takes particle and magnetic field measurements in that part of space.
Both approaches have their respective advantages. Remote sensing shows large-scale results but does not reveal the details of the processes occurring in the plasma. Meanwhile, in-situ measurements provide very specific information about small-scale processes in plasma, but do not show how these affect large scales.
Multiple investigations on spacecraft
To get a complete picture, two spacecraft are needed. And that’s what heliophysicists currently have in the form of the European Space Agency’s Solar Orbiter spacecraft 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 on-site measurements. The Parker Solar Probe largely forgoes remote sensing of the Sun itself in order to get closer to making on-site measurements.
But to take full advantage of the complementary approach, Parker Solar Probe must be within the field of view of one of Solar Orbiter’s instruments. In this way, Solar Orbiter is able to record the large-scale consequences of what the Parker Solar Probe measures at the site.
ESA’s Solar Orbiter is one of two integrated spacecraft studying the Sun up close: it joins NASA’s Parker Solar Probe, which is already involved in the mission. Source: Solar Orbiter: medialab ESA/ATG; 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 Metis Solar Orbiter instrument. Metis is a coronagraph that blocks light from the surface of the Sun and takes images of the corona. This was the perfect tool to use for large-scale measurements, so Daniele started looking for a time when the Parker Solar Probe would be installed.
It is known that on June 1, 2022, both spacecraft will be in the correct orbital configuration – approx. Basically, Solar Orbiter will be observing the Sun, and Parker Solar Probe will be off to the side, very close but outside the field of view of the METS instruments.
When Daniele saw the problem, he realized that all it took to shine the Parker Solar Orbiter was a little practice with the Solar Orbiter: rotating it by 45 degrees and then pointing it slightly away from the Sun.
Yet when every space mission maneuver is carefully planned in advance, and when the spacecraft itself is designed to point only in a very specific direction, especially when faced with the frightening heat of the sun, it is not clear that the spacecraft operations team will allow such maneuvers. . deviation. However, once everyone understood the potential scientific benefits, 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 continues; 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,” said Daniele, who led the analysis of the data set. Through collaboration, they were able to make the first combined observations and estimate coronal heating rates in-situ.
“The ability to use Solar Orbiter and the Parker Solar Probe has opened up a new dimension to this research,” said Gary Zank, from the University of Alabama in Huntsville, US, and one of the authors of the resulting paper.
By comparing the new measured rates with theoretical predictions made by solar physicists over the years, Daniel shows that solar physicists were roughly correct in identifying 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 this disorder occurs is not unlike what happens when you stir your morning cup of coffee. By stimulating the random movement of a fluid, whether gas or liquid, energy is transferred to a smaller scale, culminating in the conversion of energy to heat. In the case of the solar corona, the fluid is also magnetized so that the stored magnetic energy is also available to be converted into heat.
The transfer of magnetic energy and kinetic energy from larger scales to smaller scales is the essence of turbulence. At the smallest scales, this allows fluctuations to eventually interact with individual particles, mostly protons, and heat them.
Conclusion and implications
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 measurements of this process.
“This is the first scientific research. This work is an important step forward in solving the problem of coronal heating,” said Daniel Müller, project scientist.
Solar Orbiter is a space mission resulting from international collaboration between ESA and NASA, managed by the European Space Agency.
2023-09-22 04:07:05
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