NASA's Parker Solar Probe reached the sun's expanding atmosphere, known as the corona, and stayed there for five hours. This spacecraft is the first to enter the outer boundary of the sun.
"This marks the realization of the primary goal of the Parker mission and marks a new era in understanding the physics of the corona," said Justin C. Kasper, deputy chief technology officer at BWX Technologies and a professor at the University of Michigan. The mission is led by the Johns Hopkins University Applied Physics Laboratory (JHU/APL).
The probe made the first direct observations of the sun's atmosphere, measuring phenomena previously estimated.
The sun's outer edge begins at the Alfven critical surface: the lowest point where the sun and its gravitational and magnetic forces directly control the solar wind. Many scientists believe the sudden reversal of the sun's magnetic field, known as a "line of change," is generated from this region.
"The concept of sending a spacecraft into the sun's magnetized atmosphere — close enough to the Sun that the magnetic energy is greater than the kinetic and thermal energy of the ions and electrons — predates NASA," Kasper said.
In 2018, NASA launched the Parker Solar Probe, to eventually reach the corona and make humanity's first visit to a star.
In April, the probe spent five hours under Alfven's critical surface, making direct contact with the solar plasma. Beneath the surface, the pressure and energy of the sun's magnetic field are stronger than those of the particles. The spacecraft passed up and down the surface three times during the encounter. This is the first time a spacecraft has entered the corona and touched the sun's atmosphere.
Surprisingly, the researchers found that the critical surface of Alfven was wrinkled. It's not clear why the pseudo-streamer would push Alfven's critical surface away from the sun.
The researchers noted that there was much less variation below Alfven's critical surface than above it. This finding could mean that no broken lines form inside the corona. In addition, a lower magnetic reconnection rate on the sun's surface may inject less mass into the observed flow, resulting in less reversion.
The probe also recorded some evidence of a potential energy boost in the corona's interior, which could point to unknown physical phenomena affecting heating and dissipation.
"We've been watching the sun and its corona for decades, and we know there's interesting physics there to heat and accelerate solar wind plasma. Still, we can't say exactly what the physics is, "said Nour E. Raouafi, JHU/APL Parker Solar Exploration Project scientist."With the Parker Solar Probe now flying into the magnetically dominant corona, we will gain a long-awaited insight into the inner workings of this mysterious region."
The observations took place during the Parker Solar Probe's eighth encounter with the Sun. All data is publicly available in the NASA PSP archive. Several previous studies have predicted that the probe will cross the sun's boundary for the first time in 2021.
The Packard Solar Probe is the fastest known object ever built and has made many discoveries since its launch, including the dangers of explosions that cause space weather and ultra-fast dust.
The new findings suggest that direct observations from spacecraft have gone a long way toward illuminating the physics of coronal heating and solar wind formation. Having reached its goal of making contact with the Sun, the Parker Solar Probe will now go deeper into the sun's atmosphere and stay longer.
According to Gary Zank, a member of the National Academy of Sciences and co-investigator of the probe's Solar Wind Electron Alpha and Proton (SWEAP) instrument, "The importance of this event and the observations made by parker Solar Probe cannot be overstated. For more than 50 years since the beginning of the space age, the coronal boundary has been addressing the unanswered question of how the corona is heated to more than a million degrees to drive the solar wind. The first measurements of the sub-Alfvenic solar wind may represent the most important step in understanding the physics behind solar wind acceleration since Parker's formation model."
"This event is something that many solar physicists have been dreaming about during their careers!"Zank added.
The results were published in Physical Review Letters and announced at a press conference at the American Geophysical Union Fall 2021 meeting on December 14.