Sun, the atmosphere is hotter than the surface: Solar Orbiter reveals a new piece

The ESA spacecraft performs a ‘cosmic alignment’ with the NASA Parker Solar Probe spacecraft, thanks to this ‘space gymnastics’ scientists are one step away from solving the mystery

It comes from the European probe Solar Orbiter of theHex an important new piece to solving a 65-year-old mystery about Sun: the atmosphere of our largest star is hotter than its surface, it is a million degrees hot, 150 times hotter than the surface of the Sun. The new information – new measurements of the properties of the Sun’s atmosphere – came thanks to an alignment cosmic space between ESA’s Solar Orbiter spacecraft and the Parker Solar Probe NASA. A bit of ‘space gymnastics’ which gave the European ESA and NASA spacecraft the possibility of carrying out an “innovative measurement” which has now brought scientists one step away from solving the mystery. The European Space Agency announced the new milestone. The Sun’s atmosphere, ESA scientists recall, is called the corona, is made up of an electrically charged gas known as plasma and has a temperature of about one million degrees Celsius. And its temperature is a still unsolved mystery because the surface of the Sun is ‘only’ at around 6000 degrees. The delicate ‘space gymnastics’ maneuvers that made the alignment of Solar Orbiter with Parker Solar Probe possible were also carried out thanks to Italian technology, the attitude sensors of Leonardo on board both missions which, like space compasses, help probes and satellites to orient themselves and maintain the correct attitude. Also Italian is Solar Orbiter’s sophisticated Metis instrument created for the Italian Space Agency by Thales Alenia Space and OHB Italia. Italian is the scientist Daniele Telloni, researcher at the National Institute of Astrophysics (Inaf) at the Astrophysical Observatory of Turin, who led the study published in the journal The Astrophysical Journal Letters and who is part of the team working on Solar’s Metis instrument Orbiter. Metis is a coronagraph that blocks light from the Sun’s surface and takes photos of the corona – it is the perfect instrument to use for large-scale measurements.

Scientists remember that the Sun’s corona should be cooler than the surface because the Sun’s energy comes from the nuclear furnace in its core, and things naturally get colder the further they are from a heat source. Yet the corona is more than 150 times hotter than the surface. There must be another method at work to transfer energy into the plasma, but which one? It has long been suspected that turbulence in the solar atmosphere could cause significant heating of the plasma in the corona, but when it comes to studying this phenomenon, solar physicists run into a practical problem: it is impossible to collect all the data they need with a single spacecraft. There are two ways to study the Sun: remote sensing and in situ measurements. ESA explains that in remote sensing the spacecraft is positioned at a certain distance and uses cameras to observe the Sun and its atmosphere at different wavelengths. For in situ measurements, the spacecraft flies through the region it wants to investigate and takes measurements of particles and magnetic fields in that part of space. And both approaches have their advantages. Remote sensing shows large-scale results but not the details of the processes occurring in the plasma. Meanwhile, in situ measurements provide highly specific information about small-scale processes in the plasma, but do not show how this affects large scales. In short, to get the complete picture, two spacecraft are needed. This is exactly what solar physicists have currently been able to do thanks to the Solar Orbiter spacecraft led by ESA and NASA’s Parker Solar Probe. Solar Orbiter is designed to get as close to the Sun as possible and still perform remote sensing operations, along with in situ measurements. Parker Solar Probe largely forgoes remote sensing of the Sun itself to get even closer for its in situ measurements.

But to take full advantage of their complementary approaches, the Parker Solar Probe solar spacecraft would have to be within the field of view of one of Solar Orbiter’s instruments, so Solar Orbiter could record the large-scale consequences of what Parker Solar Probe was measuring in situ. Daniele Telloni, a researcher at the National Institute of Astrophysics (INAF) at the Turin Astrophysical Observatory, is part of the team behind Solar Orbiter’s Metis instrument. Metis is a coronagraph that blocks light from the Sun’s surface and takes photos of the corona. It is the perfect instrument to use for large-scale measurements and so Daniele started looking for the times when Parker Solar Probe would align with the ESA probe. Telloni discovered that on June 1, 2022 the two spacecraft would be in the correct orbital configuration, or almost so. Essentially, Solar Orbiter would be looking at the Sun and Parker Solar Probe would be just off to the side, incredibly close but just outside the field of view of the Metis instrument. As Daniele looked at the problem, he realized that all it would take to get the Parker Solar Probe into a favorable position: a bit of space gymnastics with Solar Orbiter, a 45-degree rotation and then pointing it slightly away from the Sun. But every maneuver of a space mission is carefully planned in advance, spacecraft are designed to point only in very specific directions, especially when faced with the Sun’s fearsome heat. So there was great doubt whether the spacecraft operations team would have authorized this maneuver. A real detour. But the potential scientific return immediately became clear to all the research teams and the decision was a clear ‘Yes’ to the maneuver. The roll and offset progressed, the Parker Solar Probe entered the field of view and together with 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” clarified Daniele Telloni who led the analysis of the data sets. Working together, the ESA and NASA research teams were able to make the first combined observational and in situ estimate of the coronal warming rate. The possibility of using both Solar Orbiter and Parker Solar Probe really opened up a completely new dimension in this research” commented Gary Zank of the University of Alabama, in Huntsville (USA), co-author of the article. Comparing the speed just measured with theoretical predictions made by solar physicists over the years, Daniele Telloni has shown that solar physicists were almost certainly right in identifying turbulence as a way to transfer energy. (by Andreana d’Aquino)