NASA: A press conference held at the fall meeting of the American Geophysical Union (AGU), held on Tuesday (14) in New Orleans, USA, released the results of a historic event for world heliophysics: the first time a spacecraft terrestrial directly touched the surface of the Sun.
Part of the study “Parker solar probe enters the magnetically dominated solar corona”, published on Tuesday (14) in the scientific journal Physical Review Letters, the event took place on April 28, 2021, at 5:33 am GMT. At that moment, the unmanned spacecraft reached the outermost layer of the Sun’s atmosphere, known as the corona, and spent five hours there.
Unlike Earth, the Sun does not have a solid surface. However, it has a superheated atmosphere, made up of solar material that binds to the star through gravity and magnetic forces. As heat and increasing pressure push this material away from the core, it reaches a point where both gravity and magnetic fields become too weak to contain it.
Entering the solar atmosphere
The point of intersection between the end of the solar atmosphere and the beginning of the solar wind is called the Alfvén’s critical surface, which marks the beginning of the sun’s outer edge. The solar wind is made of solar material with enough energy to cross this critical boundary. It is so strong that it drags the Sun’s magnetic field with it as it crosses the solar system, towards Earth and beyond. In this movement, the internal wind waves do not have the speed to return.
However, the exact location of Alfvén’s critical surface was not known to scientists. Estimates made through remote images of the luminous envelope indicated for a region between ten and 20 solar rays from the surface of the Sun, between -7 to 14 million kilometers. “We were hoping that sooner or later we would find the corona for at least a short period of time,” said Justin Kasper, lead author of the article.
It was only during its eighth solar overflight, on April 28, that the Parker Solar Probe located the specific magnetic and particle conditions at 18.8 solar rays (or 13 million kilometers) above the solar surface, which indicated the scientists that the spacecraft had first crossed Alfvén and entered the solar atmosphere. At this point, Parker had crossed in and out of the crown about three times.