NASA’s Parker Solar Probe Plunges Into Fast Solar Wind and Discovers Its Mysterious Source

NASA’s Parker Solar Probe (PSP) has made a breakthrough in understanding the structure and source of the solar wind, thanks to its proximity to the sun’s surface. It has detected high-energy particles that align with the flows in coronal holes, hinting at magnetic reconnection occurring within these regions. This discovery enhances our comprehension and prediction of solar storms that could potentially disrupt Earth. Credit: NASA GSFC/CIL/Brian Monroe

The PSP managed to approach the sun’s surface closely enough to unveil hidden granular details.

The PSP has flown close enough to the sun to reveal the intricate structure of the solar wind near its birthplace on the sun’s surface, offering insights that are usually obscured by the uniform surge of charged particles as the wind leaves the corona.

It is akin to perceiving water jets from a showerhead while being hit in the face with the water blast.

A research paper published on June 7 in Nature, led by a team of scientists including Stuart D. Bale from the University of California, Berkeley, and James Drake from the University of Maryland-College Park, reveals that the PSP has identified streams of high-energy particles that coincide with supergranulation flows within coronal holes. This suggests that these are the birthplaces of the high-speed solar wind.

Coronal holes are regions where magnetic field lines extend from the surface without looping back inward, thus forming open field lines that occupy the majority of the space around the sun. These holes typically occur at the poles during the sun’s quiet phases, and the fast solar wind they produce does not hit Earth. However, during the sun’s active phase, which occurs every 11 years when its magnetic field flips, these holes appear all over the surface, causing bursts of solar wind directed towards Earth.

Understanding the genesis and path of the solar wind will improve our predictions of solar storms, which, while creating stunning auroras on Earth, can disrupt satellites and the electrical grid.

According to Drake, understanding the mechanism behind the sun’s wind is essential for practical reasons on Earth. This knowledge affects our ability to understand how the sun releases energy and causes geomagnetic storms, which pose a threat to our communication networks.

Based on their analysis, the researchers liken the coronal holes to showerheads, with nearly evenly spaced jets emerging from bright spots where magnetic field lines are funneled into and out of the sun’s surface. The scientists propose that when oppositely directed magnetic fields pass each other in these funnels, which can be 18,000 miles wide, the fields often break and reconnect, propelling charged particles out of the sun.

By analyzing some extraordinarily high-energy particles that the PSP has detected—particles traveling 10 to 100 times faster than the average solar wind—the researchers infer that the wind could only have been produced by this process, known as magnetic reconnection. The PSP, launched in 2018, primarily aimed to resolve the two conflicting explanations for the origin of the high-energy particles that form the solar wind: magnetic reconnection or acceleration by plasma or Alfvén waves.

Bale concludes that the fast solar wind’s energy source comes from magnetic reconnection within these funnel structures within the coronal holes. This energy doesn’t originate uniformly within a coronal hole, but is structured within these supergranulation cells. This discovery is strong evidence that magnetic reconnection is the primary source.

The funnel structures likely align with the bright jetlets that can be observed from Earth within coronal holes, as recently reported by Nour Raouafi, a co-author of the study and the PSP project scientist at the Applied Physics Laboratory at Johns Hopkins University, which designed, built, manages, and operates the spacecraft.

Investigating the Sun

When the solar wind reaches Earth, 93 million miles from the sun, it has morphed into a uniform, turbulent flow of intertwining magnetic fields and charged particles that interact with Earth’s magnetic field and deposit electrical energy into the upper atmosphere. This interaction excites atoms, resulting in beautiful auroras at the poles, but also causes effects that permeate Earth’s atmosphere. Predicting the most intense winds, or solar storms, and their effects near Earth is a primary mission of NASA’s Living With a Star program, which funded the PSP.

The probe’s design allows it to examine the solar wind near its birthplace on the sun’s surface, or photosphere, and to understand how the wind’s charged particles are accelerated to escape the sun’s gravitational pull.

For this task, the PSP needed to approach closer than 25 to 30 solar radii, or approximately 13 million miles.

As the probe approached about 12 solar radii from the sun’s surface—5.2 million miles—the data clearly showed that the probe was traversing jets of material, rather than merely turbulence. Bale, Drake, and their colleagues traced these jets back to the supergranulation cells in the photosphere, where magnetic fields bunch up and funnel into the sun.

The debate was whether these funnels accelerated the charged particles through magnetic reconnection, slingshotting particles outward, or via waves of hot plasma—ionized particles and magnetic field—emanating from the sun, similar to surfing a wave.

However, the detection of extremely high-energy particles in these jets by PSP—which were tens to hundreds of kiloelectron volts (keV), as opposed to a few keV for most solar wind particles—confirmed to Bale that magnetic reconnection accelerates the particles and generates the Alfvén waves, which likely provide the particles an additional boost.

The probe will not be able to get any closer to the sun than about 8.8 solar radii above the surface—around 4 million miles—without damaging its instruments. Despite this, Bale anticipates that data from this altitude will confirm the team’s conclusions, though the sun is now entering its maximum activity phase, making the processes they are studying potentially harder to discern.

Bale reflects that had the PSP been launched at the sun’s maximum activity phase, understanding these processes would have been more challenging due to the increased solar chaos. He considers their launch timing during the solar minimum to be fortuitous.

Reference: “Interchange reconnection as the source of the fast solar wind within coronal holes” by S. D. Bale, J. F. Drake, M. D. McManus, M. I. Desai, S. T. Badman, D. E. Larson, M. Swisdak, T. S. Horbury, N. E. Raouafi, T. Phan, M. Velli, D. J. McComas, C. M. S. Cohen, D. Mitchell, O. Panasenco and J. C. Kasper, 7 June 2023, Nature.
DOI: 10.1038/s41586-023-05955-3

Frequently Asked Questions (FAQs) about Parker Solar Probe’s Discovery of Solar Wind Origins

More about Parker Solar Probe’s Discovery of Solar Wind Origins

• Parker Solar Probe’s Official NASA Website
• NASA’s explanation on Solar Wind
• Understanding Magnetic Reconnection
• More about Space Weather
• Living With a Star Program