For the first time in history, a spacecraft has touched the sun.
NASA's Parker Solar Probe, which carries instruments built at UC Berkeley, flew through the sun's upper atmosphere - the corona - for a few hours on April 28, 2021, sampling particles and magnetic fields for the first time from one of the hottest places in the solar system.
Getting this close to the sun was one of the main objectives of the mission, which was launched in 2018. For the past three years, the probe has been circling the sun and observing the solar corona, as well as the planet Venus.
The feat marks a major milestone for NASA, which has launched numerous satellites to monitor the sun, the solar wind and its effect on Earth, but has never gotten inside the corona to study the processes responsible for superheating the corona and pushing the solar wind to supersonic speeds. Such measurements from inside the corona will be critical for understanding and forecasting extreme space weather events that can disrupt telecommunications and damage satellites around Earth.
The results were announced today (Dec. 14) during a press conference at the 2021 fall meeting of the American Geophysical Union in New Orleans. Papers describing the results have been accepted for publication in the journals Physical Review Letters and the Astrophysical Journal.
The sun doesn't have a solid surface, but Parker Solar Probe passed into a portion of the sun's atmosphere that contains material that is bound to the sun by gravity and magnetic forces. Farther out, the gravitational forces and magnetic fields are too weak to contain this energetic material, which streams from the sun as the solar wind. When the solar wind hits Earth, it compresses our planet's magnetic field, generates auroras and can disrupt orbiting satellites and telecommunications on the surface.
As Parker Solar Probe passed through the corona on its ninth encounter with the sun, the spacecraft flew by structures called coronal streamers. These structures can be seen as bright features moving upward in the upper images and angled downward in the lower row. Such a view is only possible because the spacecraft flew above and below the streamers inside the corona. Until now, streamers have only been seen from afar. They are visible from Earth during total solar eclipses. (Images courtesy of NASA/Johns Hopkins APL/Naval Research Laboratory)
That solar touch point, known as the Alfvén critical surface, marks the end of the solar atmosphere and the beginning of the solar wind.
The probe's Solar Wind Electrons Alphas and Protons (SWEAP) instrument, much of which was designed and built at Berkeley's Space Sciences Laboratory (SSL), made key observations that showed that the charged particles around the probe were moving slowly enough to remain captured by the sun, proving that it was within the Alfvén critical surface and, thus, the solar atmosphere.
"This is an important measurement because it provides the first opportunity to determine the plasma properties in the region in which the solar wind makes the super-Alfvenic transition," said Davin Larson, a project scientist at SSL who was part of the team that built the Solar Probe Analyzer for ions (SPAN-Ion) instrument on SWEAP.
Until now, researchers were unsure exactly where the Alfvén critical surface lay. Estimates, based on remote images of the corona, had put it somewhere between 10 and 20 solar radii from the center of the sun - 4.3 to 8.6 million miles. Parker Solar Probe's spiral trajectory brings it slowly closer to the sun, and during the last few passes, it was consistently below 20 solar radii (9% of the Earth's distance from the sun), putting it in the position to cross - if the estimates were correct.
On April 28, during its eighth flyby of the sun, Parker Solar Probe encountered the specific magnetic and particle conditions that told scientists it had crossed the Alfvén critical surface and finally entered the solar atmosphere.
Switchbacks and solar wind origins
In addition to entering the solar corona, the probe on a recent flyby pinpointed the origin of distinctive, zigzag-shaped structures called switchbacks in the solar wind. In 2019, Berkeley physicist Stuart Bale and his colleagues discovered that these switchbacks are plentiful close to the sun, not oddities confined to the sun's polar regions. From a much closer vantage - half the distance of that in 2019 - the probe has now revealed that the switchbacks originate near the visible surface of the sun, the so-called photosphere.
Artist's rendition of the Parker Solar Probe in the sun's atmosphere, or corona. (Image courtesy of NASA)
The clues came as Parker Solar Probe orbited closer to the sun on its sixth flyby, less than 25 solar radii out. Data showed switchbacks occur in patches and, relative to other elements, have a higher percentage of helium, which is known to come from the photosphere. The switchbacks' origins were further narrowed when the scientists found the patches aligned with magnetic funnels that emerge from the photosphere between convection cell structures called supergranules.
"The structure of the regions with switchbacks matches up with a small magnetic funnel structure at the base of the corona," said Bale, Berkeley professor of physics and lead author of the new switchbacks paper now in the Astrophysical Journal. "This is what we expect from some theories, and this pinpoints a source for the solar wind itself."
The scientists think that, in addition to being the birthplace of switchbacks, the magnetic funnels might be where one component of the solar wind originates. The solar wind comes in two different varieties, fast and slow, and the funnels could be where some particles in the fast solar wind come from.
Understanding where and how the components of the fast solar wind emerge, and if they're linked to switchbacks, could help scientists answer a long-standing solar mystery - how the corona is heated to millions of degrees.
While the new findings indicate where switchbacks are made, the scientists can't yet confirm how they're formed. One theory suggests they might be created by waves of plasma that roll through the region like ocean surf. Another contends they're made by an explosive process known as magnetic reconnection, which is thought to occur at the boundaries where the magnetic funnels come together.
"My instinct is, as we go deeper into the mission and lower and closer to the sun, we're going to learn more about how magnetic funnels are connected to the switchbacks," Bale said, "and hopefully resolve the question of what process makes them."
The first passage through the solar corona, which lasted only a few hours, is one of many planned for the mission. Parker Solar Probe will continue to spiral closer to the sun, eventually reaching a location as close as 10 solar radii from the center. Upcoming flybys, the next of which is happening January 2022, will likely bring Parker Solar Probe through the corona again.
Because the size of the corona is driven by solar activity, as the sun's 11-year activity cycle - the solar cycle - ramps up, the outer edge of the corona will expand, giving Parker Solar Probe a greater chance of being inside the corona for longer periods of time.
