NASA's Parker Solar Probe surprised scientists with its groundbreaking views of the Sun as the first results from the mission were released on Wednesday, December 4, 2019. Scientists observed bursts of energetic particles never seen before on a small scale, and switchback-like reversals in the out-flowing solar magnetic field apparently whipping up the solar wind. The mission's goal is to shed light on the mysteries about the Sun.
"They’re striking and it’s hard to not think that they’re somehow important in the whole problem," said astrophysicist Stuart Bale and member of the mission team. Nicola Fox, director of NASA's heliophysics division, compared this unprecedented switchback phenomenon to the cracking of a whip.
Researchers said they also finally gathered evidence dust-free zone rotating around the Sun. Farther out, there is much more dust from vaporizing comets and asteroids that one of the 80 small viewfinders on a solar probe instrument was pierced by a grain.
"I can’t say that we don’t worry about the spacecraft. I mean, the spacecraft is going through an environment that we’ve never been before," Fox said.
When observed near Earth, the solar wind is a relatively uniform flow of plasma with occasional turbulent tumbles. However, closer to the solar wind's source, Parker Solar Probe caught a different picture– a complicated, active system.
"The complexity was mind-blowing when we first started looking at the data," said Stuart Bale, the University of California, Berkeley, lead for Parker Solar Probe’s FIELDS instrument suite. "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 24 million km (15 million miles) from the Sun, the solar wind is much more impulsive and unstable than what people see near Earth.
The solar wind is made up of plasma, like the Sun itself, where negatively charged electrons have separated from positively charged ions. This creates a sea of free-floating particles with an individual electric charge.
The particles indicate that plasma carries electric and magnetic fields, and changes in the plasma often produce marks on the fields.
“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 University of Michigan' Justin Kasper, principal investigator for Solar Wind Electrons Alphas and Protons (SWEAP).
Another question that the probe addressed is how 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," NASA wrote.
"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."
Parker Solar Probe was able to observe the solar wind while it was still rotating. Its solar wind instrument detected rotation starting more than 32 million km (20 million miles) from the Sun.
The speed of the rotation increased as Parker approached its perihelion point– stronger than many scientists had anticipated. It also transitioned more quickly than predicted to an outward flow, which is what helps mask these effects from where we usually sit, about 150 million km (93 million miles) from the Sun.
"The large rotational flow of the solar wind seen during the first encounters has been a real surprise," said Kasper.
"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."
In addition, another question nearing to be answered is the elusive dust-free zone. For the first time, Parker imagers observed the cosmic dust that is starting to thin out.
These images show dust starting to thin a little over 11 million km (7 million miles) from the Sun, and this decrease in dust continues steadily to the current limits of Wide-field Imager for Solar Probe's (WISPR) measurements at a little over 4 million miles from the Sun.
Parker Solar Probe's measurements have also provided a new perspective on two types of space weather events– energetic particle storms and coronal mass ejections.
Parker Solar Probe's Integrated Science Investigation of the Sun (ISʘIS) instruments have measured several never-before-seen energetic particle events– events so small that all trace of them is lost before they reach Earth or any of our near-Earth satellites.
"Even at solar minimum conditions, the Sun produces many more tiny energetic particle events than we ever thought," said David McComas, principal investigator for the ISOIS suite at Princeton University.
"These measurements will help us unravel the sources, acceleration, and transport of solar energetic particles and ultimately better protect satellites and astronauts in the future."
Furthermore, data from the WISPR instruments gave detail on structures in the corona and solar wind, including coronal mass ejections, billion-ton clouds of solar material that the Sun sends hurtling out into the solar system.
"Since Parker Solar Probe was matching the Sun's rotation, we could watch the outflow of material for days and see the evolution of structures," said Russ Howard, principal investigator for the WISPR suite.
"Observations near Earth have made us think that fine structures in the corona segue into a smooth flow, and we're finding out that's not true. This will help us do better modeling of how events travel between the Sun and Earth."
Parker is on course to sweep past Venus on December 26 for the second gravity-assist and make its fourth near solar encounter in January. The results were discussed in multiple studies in the Nature journal and were made during a quiet phase of solar activity.
"We’re just starting to scratch the surface of this fascinating physics," said Princeton University space plasma physicist David McComas, the chief scientist of one of the spacecraft’s instruments.
The Sun will go through an active phase as Parker comes nearer its target. "We can expect even more exciting results soon," said Daniel Verscharen of the University College London, who also studies the Sun but was not part of this particular mission.
Fox shared the results with astrophysicist Eugene Parker, professor emeritus at the University of Chicago for whom the spacecraft is named after.
"Alfvenic velocity spikes and rotational flows in the near-Sun solar wind" – Kasper, J. C. et al – Nature – DOI: 10.1038/s41586-019-1813-z
The prediction of a supersonic solar wind was first confirmed by spacecraft near Earth and later by spacecraft at heliocentric distances as small as 62 solar radii. These missions showed that plasma accelerates as it emerges from the corona, aided by unidentified processes that transport energy outwards from the Sun before depositing it in the wind. Alfvénic fluctuations are a promising candidate for such a process because they are seen in the corona and solar wind and contain considerable energy. Magnetic tension forces the corona to co-rotate with the Sun, but any residual rotation far from the Sun reported until now has been much smaller than the amplitude of waves and deflections from interacting wind streams. Here we report observations of solar-wind plasma at heliocentric distances of about 35 solar radii, well within the distance at which stream interactions become important. We find that Alfvén waves organize into structured velocity spikes with duration of up to minutes, which are associated with propagating S-like bends in the magnetic-field lines. We detect an increasing rotational component to the flow velocity of the solar wind around the Sun, peaking at 35 to 50 kilometres per second—considerably above the amplitude of the waves. These flows exceed classical velocity predictions of a few kilometres per second, challenging models of circulation in the corona and calling into question our understanding of how stars lose angular momentum and spin down as they age.
"Highly structured slow solar wind emerging from an equatorial coronal hole" – Bale, S. D. et al – Nature – DOI: 10.1038/s41586-019-1818-7
During the solar minimum, when the Sun is at its least active, the solar wind is observed at high latitudes as a predominantly fast (more than 500 kilometres per second), highly Alfvénic rarefied stream of plasma originating from deep within coronal holes. Closer to the ecliptic plane, the solar wind is interspersed with a more variable slow wind of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain, theories and observations suggest that they may originate at the tips of helmet streamer, from interchange reconnection near coronal hole boundaries, or within coronal holes with highly diverging magnetic fields. The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfvén-wave turbulence, heating by reconnection in nanoflares, ion cyclotron wave heating, and acceleration by thermal gradients. At a distance of one astronomical unit, the wind is mixed and evolved, and therefore much of the diagnostic structure of these sources and processes has been lost. Here we present observations from the Parker Solar Probe at 36 to 54 solar radii that show evidence of slow Alfvénic solar wind emerging from a small equatorial coronal hole. The measured magnetic field exhibits patches of large, intermittent reversals that are associated with jets of plasma and enhanced Poynting flux and that are interspersed in a smoother and less turbulent flow with a near-radial magnetic field. Furthermore, plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities that are associated with plasma heating and thermalization processes. Our measurements suggest that there is an impulsive mechanism associated with solar-wind energization and that micro-instabilities play a part in heating, and we provide evidence that low-latitude coronal holes are a key source of the slow solar wind.
Featured image credit: NASA/Johns Hopkins APL
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