Probing the Solar Wind Up Close
An artist's concept of the Parker Solar Probe spacecraft approaching the sun. Launched in 2018, its first pass in this past year has provided new data on solar wind activity and made critical contributions to our ability to forecast major space-weather events on Earth.
The sun glows with a surface temperature of about 5500 degrees Celsius but its hot outer layer, the corona, has a temperature of over a million degrees. The corona ejects a wind of charged particles, and in 1957 Eugene Parker realized that this wind, already known to be responsible for the direction of comets' tails, could move faster than the speed of sound and could easily bombard the Earth. He developed a theory for the solar wind, and today that wind is known for producing auroral glows and even disrupting global communications.
There are two important, longstanding, and related questions about the wind that astronomers have been working to answer: How does the corona become heated to temperatures so much hotter than the surface, and how does the corona generate and then shape the wind as it expels particles into space? The approximate answer to the first question involves the ionized material in the hot corona. The moving gas generates powerful magnetic field loops that, when they twist and break, can accelerate charged particles. The answer to the second question has been even more difficult to ascertain because the solar wind has only been sampled so far by spacecraft whose closest approach to the sun has been about thirty million miles, about the same distance from the sun as the orbit of Mercury. By this distance, however, scientists think that the wind has already undergone changes that obscure key details of its driving sources in the corona.
In August, 2018, NASA launched the Parker Solar Probe to approach within 3.8 million miles of the sun's surface in a series of progressively closer approaches to answer this and other pressing questions. In November 2018 and April 2019 the probe completed its first pass, coming about twice as close to the sun as any previous probe, and the results have just been published in a series of articles in Nature. CfA astronomers Justin Kasper, Anthony Case, Leon Golub, Kelly Korreck, and Michael Stevens, play leadership roles on the Parker team, including development of one of Parker's instruments, SWEAP (the Solar Wind Electrons Alphas and Protons Investigation). In the new papers, the team demonstrates for the first time that the solar wind near the sun is much more structured and dynamic than it is at Earth. The direction of the magnetic field, for example, undergoes rapid reversals that last for only minutes. Although some similar magnetic field behaviors had been spotted before, the large amplitude and the high occurrence rates of the reversals seen by Parker were surprising. The precise nature of these reverse structures is unknown, but astronomers suspect that plasma instabilities like these play a much larger role in the dynamics and energetics of the solar wind than had previously been expected.
In related discoveries, the spacecraft found that as the wind streams out into space, parts of it race ahead in high-velocity "rogue" waves with nearly double the speed of the solar wind. Parker flew through more than 1,000 of these spikes; they too are still mysterious. In fact, some particles are apparently being accelerated to speeds nearing the speed of light in events that can be impulsive as well as gradual. A third surprising result was how quickly the solar wind in general rotates around the sun. Models had suggested that the wind flows in this direction at a speed of a few kilometers per second, but the Parker Solar Probe measured it moving much faster, about 35 to 50 kilometers a second. The reason for this is also not known. Parker has several more years to orbit the sun in even closer approaches, and during this time the sun enter a more active phase. These first results have already demonstrated the success of the mission and signal that many more discoveries - and a new understanding of the solar wind - lies ahead.
"Alfvénic Velocity Spikes and Rotational Flows in the Near-Sun Solar Wind," J. C. Kasper et al., Nature, 4 Dec 2019.
"Probing the Energetic Particle Environment Near the Sun," D. J. McComas et al. Nature 576, 223, 2019.
"Highly Structured Slow Solar Wind Emerging from an Equatorial Coronal Hole," S. D. Bale et al. Nature, 4 Dec 2019.