A groundbreaking new measurement by the Solar Orbiter spacecraft and the Parker Solar probe brings scientists closer than ever to solving a long-standing mystery surrounding the sun. Oddly, our host star’s atmosphere, or corona, is incredibly hotter than the sun’s surface despite being further away from the obvious source of the sun’s heat – and this is a conundrum that has troubled physicists for about 65 years.
The collaboration between these two instruments was made possible when the Solar Orbiter, operated by the European Space Agency (ESA), performed some space gymnastics. These maneuvers allowed the spacecraft to simultaneously observe the sun and NASA’s Parker Solar Probe. Ultimately, this allowed for simultaneous solar observations between the two, which together indicated that the turbulence is likely heating the solar corona to incredible temperatures.
“The ability to use both Solar Orbiter and Parker Solar Probe really opened up a whole new dimension in this research,” said Gary Zank, co-author of a study on the findings and a researcher at the University of Alabama in Huntsville. said in a statement.
This collaboration could finally solve the so-called “coronal heating mystery,” which revolves around the heat discrepancy between the corona, made of thin, nebulous electrically charged gas called plasma, and the sun’s surface, or photosphere.
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What is the mystery of coronal heating?
The corona can reach temperatures of up to 1.8 million degrees Fahrenheit (1,000,000 degrees Celsius), while 1,600 miles below it, the photosphere only reaches temperatures of about 10,800 degrees Fahrenheit (6,000 degrees Celsius).
This is a worrying fact because the sun’s core, where the nuclear fusion of hydrogen into helium occurs, is where the vast majority of solar heat comes from. This is as if the air about a foot above a campfire is hotter than the air an inch away from the flames.
The heat mismatch also means that there must be another heating mechanism at play directly at the crown. Until now, this mechanism had eluded scientists, but turbulence in the sun’s atmosphere significantly heating the coronal plasma has long been considered a plausible explanation. However, this hypothesis was impossible to investigate with data from a spacecraft.
Satellites can study the sun in two ways: they can get closer and make in situ measurements like NASA’s Parker Solar Probe does, or they can make more remote investigations like the Solar Orbiter. The Solar Orbiter studies the corona from about 26 million miles (42 million kilometers) away from the sun, while the Parker Solar Probe challenges the sun’s blazing plasma as it passes by about 4 million miles (6.4 million km) from the solar surface.
But there is a trade-off between the two approaches.
Remote sensing can see extensive details about the sun, but suffers when it comes to making observations about what physics is at play in the coronal plasma. On the other hand, in situ observations can measure that plasma in more detail but tend to miss the bigger solar picture.
This means that by combining large-scale measurements of events on the Sun made by the Solar Orbiter with detailed observations of the same phenomenon made by the Parker Solar Probe we could be presented with the complete picture of the Sun with all the intricate details filled in – the best of both worlds.
However, this is not as simple as it seems. To facilitate this collaboration, the Parker Solar Probe would need to be in the field of view of one of the Solar Orbiter’s instruments as the two observe the sun from their relative positions.
How scientists got the “best of both worlds” to potentially solve a solar mystery
A team of astronomers, including researcher Daniele Telloni of the National Institute for Astrophysics (INAF), discovered that on June 1, 2022, the two solar observatories would be within a short distance of the desired orbital configuration to engage in such a team.
While the Solar Orbiter would be looking at the sun, the Parker Solar Probe would be just off to the side, only slightly out of sight of the ESA spacecraft’s Metis instrument, a device called a “coronagraph” that blocks light from the sun. photosphere for imaging the corona and is ideal for distant, large-scale observations.
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To perfectly align the two spacecraft and bring the Parker Solar Probe within sight of Metis, the Solar Orbiter performed a 45-degree rotation and was then pointed slightly away from the sun.
The data collected following this well-planned and authorized maneuver by the spacecraft’s operations team paid off, revealing turbulence that could actually transfer energy in the way solar physicists had theoretically predicted would cause coronal heating.
Turbulence drives coronal heating in a similar way to what happens when coffee is stirred here on Earth. Energy is transferred on a smaller scale through random movements in a fluid or gas – coffee and plasma – and this converts that energy into heat. In the case of the corona, the plasma is magnetized and this means that the stored magnetic energy can also be converted into heat.
The transfer of magnetic and motion or kinetic energy from larger scales to smaller scales is the very essence of this turbulence and, on smaller scales, allows the fluctuations to interact with individual particles, mostly positively charged protons, heating them.
This, however, does not mean that the mystery of coronal heating is “a closed case”. Solar scientists have yet to confirm the mechanism suggested by these findings and the collaboration between Parker Solar Probe and Solar Orbiter.
“This is a scientific first. This work represents a significant step forward in solving the coronal heating problem,” said Daniel Müller, Solar Orbiter project scientist.
The team’s research was published Thursday (September 14) in Astrophysical Journal Letters.