January 10, 2023
Scientists with NASA’s Parker Solar Probe mission think they have discovered the processes that give birth to streams of charged particles — called the solar wind — released from the Sun’s corona, or upper atmosphere.
A team led by Nour E. Raouafi of the Johns Hopkins Applied Physics Laboratory (APL) found evidence that small-scale jetting in the Sun’s corona, driven by a process called magnetic reconnection, is responsible for the heating and acceleration of the solar wind. These mechanisms allow the solar wind to rise through the solar atmosphere, escape the Sun’s gravity and permeate the solar system. Through this process, the Sun loses both mass and energy to the solar wind. This study is accepted for publication in the Astrophysical Journal.
Scientists primarily used observations from Parker Solar Probe along with images from the Solar Dynamics Observatory (SDO) and the Geostationary Operational Environmental Satellite-R Series’ Solar Ultraviolet Imager (GOES-R/SUVI) instrument as well as high-resolution magnetic field data from the Big Bear Solar Observatory in California to arrive at these conclusions.
“We are excited about the use of the Parker data in concert with ground-based and other space-based observations to further our understanding of the solar wind,” said Peg Luce, deputy director for the Heliophysics Division at NASA Headquarters.
“This new data shows us how the solar wind gets going at its source,” said Raouafi, the study lead and the Parker Solar Probe project scientist at APL in Laurel, Maryland. “You can see the flow of the solar wind rising from tiny jets of million-degree plasma all over the base of the corona. These findings will have a huge impact on our understanding of the heating and acceleration of the coronal and solar wind plasma.”
Understanding the solar wind is fundamental to our understanding of our solar system and others throughout the universe — and is the primary science goal of the Parker Solar Probe mission. Made of electrons, protons and heavier ions, the solar wind courses through the solar system at roughly 1 million mph (1.6 million kph), extending the Sun’s magnetic field outward. When the solar wind interacts with Earth’s magnetic field, it can create stunning auroras, as well as disruptions in GPS and communications systems. The solar wind, and other stellar winds, can also shape planetary systems by affecting the composition and evolution of planetary atmospheres and influencing planets’ habitability.
The Sun is famous for spectacular displays of magnetic activity — coronal mass ejections, flares and sunspots — but smaller events are even more prevalent. Small jets (or “jetlets”) and bright spots about a few hundred miles wide occur on the Sun continuously, regardless of solar activity phase, just like the perpetual solar wind.
Observations from Parker previously showed ubiquitous folds in the solar wind magnetic field, called switchbacks. Scientists knew that learning how and where these structures form would offer insights into the genesis and heating of the solar wind itself.
“We had observations of the Sun for years and years,” said Guillermo Stenborg, a senior scientist at APL and a co-author on the paper. “Trying to understand what Parker Solar Probe is observing, notably the switchbacks, led us to have a fresh look at these data and discover the omnipresence of the tiny jets of hot coronal plasma.”
Magnetic reconnection drives these small-scale jetting phenomena. Reconnection is a common process among charged gases, called plasmas, that fuel stars and fill the near-vacuum of space.
These observations showed that magnetic reconnection is present in the lower solar atmosphere across the entire Sun, and like the solar wind, it is omnipresent throughout the solar cycle. Therefore, this ongoing process is a plausible driver for the constant solar wind, as opposed to other phenomena that wax and wane with the 11-year solar cycle.
“The tiny reconnection events we observed are, in a way, what Eugene Parker — namesake of the Parker Solar Probe mission — hypothesized years ago,” Raouafi added. “We are observing events that are among a variety of phenomena on a spectrum of different sizes. That spectrum ultimately concludes in the nanoflares that Eugene Parker predicted.”
The ubiquitous reconnection at small scales serves two purposes: heating the coronal plasma and producing the impulsive jetlets propelling this plasma into the corona and the solar wind.
In magnetic reconnection, a stressed magnetic field stores energy like a twisted rubber rope. During the reconnection, which is a reconfiguration of the magnetic field, the excess energy is released to the plasma in the form of heat, speed and waves. In the case of jets and jetlets, the plasma is released in the form of beams along the open magnetic field lines.
“Connecting the solar wind in situ measurements by Parker Solar Probe to the source of the solar wind was a very challenging task,” said Daniel Seaton, a scientist with the Southwest Research Institute and a co-author on the paper. “But the availability of new images and image processing techniques that better reveal the connections between the inner and outer corona made a big difference.”
“The magnetic field is the primary ingredient for the reconnection,” added Haimin Wang, distinguished professor at the New Jersey Institute of Technology and director of the Institute of Space Weather Sciences. “To access the magnetic fields driving the magnetic reconnection at the origin of the coronal jetlets, we needed very-high-resolution magnetograms such as those from the Big Bear Solar Observatory (BBSO) Goode Solar Telescope. We expect the 4-meter Daniel K. Inouye Solar Telescope to reveal even higher resolution magnetic fields, potentially resulting in observations of much more jetting at smaller scales.”
Analysis of the rate, mass and energy fluxes of small-scale jetting in the Sun’s atmosphere supports the proposal that the ubiquitous, small-scale jetting activity driven by magnetic reconnection can account for essentially all of the mass and energy lost by the Sun to the solar wind. Essentially, the mass and energy escaping the Sun through the estimated number of jetlets per day is equal to the mass and energy the Sun loses daily to the solar wind.
“Like farmers might light tiny fires to keep entire fields of crops warm during the harsh winter, the Sun uses tiny reconnection events to keep the entire corona hot and accelerate the solar wind,” Raouafi said.
This breakthrough could have implications for scientists’ knowledge of the Sun and our solar system, as well as other solar systems, based on similar characteristics in their stellar wind. With this achievement, the Parker Solar Probe team is closing in on the mission’s primary science objective.
Parker Solar Probe was developed as part of NASA’s Living With a Star program to explore aspects of the Sun-Earth system that directly affect life and society. The Living With a Star program is managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland, for NASA’s Science Mission Directorate in Washington. APL designed, built, manages and operates the spacecraft.
The Applied Physics Laboratory, a not-for-profit division of The Johns Hopkins University, meets critical national challenges through the innovative application of science and technology. For more information, visit www.jhuapl.edu.