The four instruments of NASA’s Parker Solar Probe, a robotic spacecraft designed to explore the Sun’s atmosphere, have returned unprecedented science data from near our star, culminating in new discoveries published this week in a series of four papers in the journal Nature.
Illustration of NASA’s Parker Solar Probe approaching the Sun. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Steve Gribben.
Parker Solar Probe, launched on August 12, 2018, is part of NASA’s Living with a Star program to explore aspects of the Sun-Earth system that directly affect life and society.
The mission is named for Dr. Eugene Parker, famed solar physicist who in 1958 first predicted the existence of the solar wind, the stream of charged particles and magnetic fields that flow continuously from the Sun, bathing Earth. It’s the first NASA mission to be named for a living researcher.
“This first data from Parker reveals our star, the Sun, in new and surprising ways,” said Dr. Thomas Zurbuchen, associate administrator for science at NASA Headquarters in Washington.
“Observing the Sun up close rather than from a much greater distance is giving us an unprecedented view into important solar phenomena and how they affect us on Earth, and gives us new insights relevant to the understanding of active stars across galaxies. It’s just the beginning of an incredibly exciting time for heliophysics with Parker at the vanguard of new discoveries.”
Though it may seem placid to us here on Earth, the Sun is anything but quiet.
Our star is magnetically active, unleashing powerful bursts of light, deluges of particles moving near the speed of light and billion-ton clouds of magnetized material.
All this activity affects our planet, injecting damaging particles into the space where our satellites and astronauts fly, disrupting communications and navigation signals, and even triggering power outages.
It’s been happening for the Sun’s entire 5-billion-year lifetime, and will continue to shape the destinies of Earth and the other planets in our Solar System into the future.
“The Sun has fascinated humanity for our entire existence,” said Dr. Nour E. Raouafi, project scientist for Parker Solar Probe at the Johns Hopkins Applied Physics Laboratory.
“We’ve learned a great deal about our star in the past several decades, but we really needed a mission like Parker Solar Probe to go into the Sun’s atmosphere. It’s only there that we can really learn the details of these complex solar processes. And what we’ve learned in just these three solar orbits alone has changed a lot of what we know about the Sun.”
The dynamic solar wind
Observed near Earth, the solar wind is a relatively uniform flow of plasma, with occasional turbulent tumbles. But by that point it’s traveled over ninety million miles — and the signatures of the Sun’s exact mechanisms for heating and accelerating the solar wind are wiped out. Closer to the solar wind’s source, Parker Solar Probe saw a much different picture: a complicated, active system.
“The complexity was mind-blowing when we first started looking at the data,” said University of California, Berkeley’s Dr. Stuart Bale, lead for Parker Solar Probe’s FIELDS instrument suite, which studies the scale and shape of electric and magnetic fields.
“Now, I’ve gotten used to it. But when I show colleagues for the first time, they’re just blown away.”
“From Parker’s vantage point 15 million miles from the Sun, the solar wind is much more impulsive and unstable than what we see near Earth.”
Like the Sun itself, the solar wind is made up of plasma, where negatively charged electrons have separated from positively charged ions, creating a sea of free-floating particles with individual electric charge. These free-floating particles mean plasma carries electric and magnetic fields, and changes in the plasma often make marks on those fields.
The FIELDS instruments surveyed the state of the solar wind by measuring and carefully analyzing how the electric and magnetic fields around the spacecraft changed over time, along with measuring waves in the nearby plasma.
These measurements showed quick reversals in the magnetic field and sudden, faster-moving jets of material — all characteristics that make the solar wind more turbulent. These details are key to understanding how the wind disperses energy as it flows away from the Sun and throughout the solar system.
One type of event in particular drew the eye of the science teams: flips in the direction of the magnetic field, which flows out from the Sun, embedded in the solar wind.
These reversals — dubbed ‘switchbacks’ — last anywhere from a few seconds to several minutes as they flow over Parker Solar Probe.
During a switchback, the magnetic field whips back on itself until it is pointed almost directly back at the Sun.
Together, FIELDS and SWEAP (Solar Wind Electrons Alphas and Protons) measured clusters of switchbacks throughout Parker’s first two flybys.
“Waves have been seen in the solar wind from the start of the space age, and we assumed that closer to the Sun the waves would get stronger, but we were not expecting to see them organize into these coherent structured velocity spikes,” said Dr. Justin Kasper, principal investigator for SWEAP at the University of Michigan.
“We are detecting remnants of structures from the Sun being hurled into space and violently changing the organization of the flows and magnetic field. This will dramatically change our theories for how the corona and solar wind are being heated.”
The exact source of the switchbacks isn’t yet understood, but Parker Solar Probe’s measurements have allowed scientists to narrow down the possibilities.
Among the many particles that perpetually stream from the Sun are a constant beam of fast-moving electrons, which ride along the Sun’s magnetic field lines out into the Solar System.
These electrons always flow strictly along the shape of the field lines moving out from the Sun, regardless of whether the north pole of the magnetic field in that particular region is pointing towards or away from the Sun.
But Parker Solar Probe measured this flow of electrons going in the opposite direction, flipping back towards the Sun — showing that the magnetic field itself must be bending back towards the Sun, rather than Parker Solar Probe merely encountering a different magnetic field line from the Sun that points in the opposite direction.
This suggests that the switchbacks are kinks in the magnetic field — localized disturbances traveling away from the Sun, rather than a change in the magnetic field as it emerges from the Sun.
Parker Solar Probe’s observations of the switchbacks suggest that these events will grow even more common as the spacecraft gets closer to the Sun.
The mission’s next solar encounter on January 29, 2020, will carry the spacecraft nearer to the Sun than ever before, and may shed new light on this process. Not only does such information help change our understanding of what causes the solar wind and space weather around us, it also helps us understand a fundamental process of how stars work and how they release energy into their environment.
The rotating solar wind
Some of Parker’s measurements are bringing scientists closer to answers to decades-old questions. One such question is about how, exactly, the solar wind flows out from the Sun.
Near Earth, we see the solar wind flowing almost radially — meaning it’s streaming directly from the Sun, straight out in all directions. But the Sun rotates as it releases the solar wind; before it breaks free, the solar wind was spinning along with it.
This is a bit like children riding on a playground park carousel — the atmosphere rotates with the Sun much like the outer part of the carousel rotates, but the farther you go from the center, the faster you are moving in space. A child on the edge might jump off and would, at that point, move in a straight line outward, rather than continue rotating.
In a similar way, there’s some point between the Sun and Earth, the solar wind transitions from rotating along with the Sun to flowing directly outwards, or radially, like we see from Earth.
Exactly where the solar wind transitions from a rotational flow to a perfectly radial flow has implications for how the Sun sheds energy. Finding that point may help us better understand the lifecycle of other stars or the formation of protoplanetary disks, the dense disks of gas and dust around young stars that eventually coalesce into planets.
Now, for the first time — rather than just seeing that straight flow that we see near Earth — Parker Solar Probe was able to observe the solar wind while it was still rotating.
It’s as if Parker Solar Probe got a view of the whirling carousel directly for the first time, not just the children jumping off it. Parker’s solar wind instrument detected rotation starting more than 20 million miles (32.2 million km) from the Sun, and as the spacecraft approached its perihelion point, the speed of the rotation increased.
The strength of the circulation was stronger than many scientists had predicted, but it also transitioned more quickly than predicted to an outward flow, which is what helps mask these effects from where we usually sit, about 93 million miles (150 million km) from the Sun.
“The large rotational flow of the solar wind seen during the first encounters has been a real surprise,” Dr. Kasper said.
“While we hoped to eventually see rotational motion closer to the Sun, the high speeds we are seeing in these first encounters is nearly ten times larger than predicted by the standard models.”
NASA’s Parker Solar Probe saw cosmic dust (illustrated here) — scattered throughout our Solar System — begin to thin out close to the Sun, supporting the idea of a long-theorized dust-free zone near the Sun. Image credit: NASA’s Goddard Space Flight Center / Scott Wiessinger.
Dust near the Sun
Another question approaching an answer is the elusive dust-free zone.
Our Solar System is awash in dust — the cosmic crumbs of collisions that formed planets, asteroids, comets and other celestial bodies billions of years ago.
Scientists have long suspected that, close to the Sun, this dust would be heated to high temperatures by powerful sunlight, turning it into a gas and creating a dust-free region around the Sun. But no one had ever observed it.
For the first time, Parker Solar Probe’s imagers saw the cosmic dust begin to thin out.
Because WISPR — Parker Solar Probe’s imaging instrument — looks out the side of the spacecraft, it can see wide swaths of the corona and solar wind, including regions closer to the Sun.
These images show dust starting to thin a little over 7 million miles (11.2 million km) from the Sun, and this decrease in dust continues steadily to the current limits of WISPR’s measurements at a little over 4 million miles (6.4 million km) from the Sun.
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S.D. Bale et al. Highly structured slow solar wind emerging from an equatorial coronal hole. Nature, published online December 4, 2019; doi: 10.1038/s41586-019-1818-7
R.A. Howard et al. Near-Sun observations of an F-corona decrease and K-corona fine structure. Nature, published online December 4, 2019; doi: 10.1038/s41586-019-1807-x
J.C. Kasper et al. Alfvénic velocity spikes and rotational flows in the near-Sun solar wind. Nature, published online December 4, 2019; doi: 10.1038/s41586-019-1813-z
D.J. McComas et al. Probing the energetic particle environment near the Sun. Nature, published online December 4, 2019; doi: 10.1038/s41586-019-1811-1
