The initial PSP returns, which are reported in four papers published online today (Dec. 4) in the journal Nature, begin to lift the veil on Earth’s star, which has remained surprisingly mysterious despite forever being the brightest light in our sky.
“These four papers show that, by going into an unexplored region of the solar system, the PSP has already made great discoveries,” Daniel Verscharen, a researcher at the Mullard Space Science Laboratory at University College London, wrote in an accompanying “News and Views” piece in the same issue of Nature. Verscharen was not involved in any of the new studies.
Kissing the sun
The PSP launched in August 2018, on a $1.5 billion mission to help researchers better understand the inner workings of the sun.
Mission scientists are particularly interested in solving two long-standing puzzles: how the stream of particles flowing continuously from the sun, known as the solar wind, is accelerated to its tremendous velocities; and why the sun’s outer atmosphere, or corona, is so much hotter than its surface. (Corona temperatures can top 2 million degrees Fahrenheit, or 1.1 million degrees Celsius. The solar surface is downright temperate by comparison, at 11,000 F, or 6,000 C.)
The PSP is tackling these questions by brazenly barreling into the corona itself. Once every five months or so, the probe zooms through the sun’s sizzling atmosphere, getting unprecedented up-close looks at our star.
These closest approaches, or perihelion passages, have taken the PSP within 15 million miles (24 million kilometers) of the solar surface to date. Before this mission, the closest a probe had ever gotten to the sun was 26.55 million miles (42.73 million km)—a feat pulled off in 1976 by the Helios 2 craft, a joint effort of the United States and the former West Germany.
Helios 2 also set the record for the fastest speed relative to the sun, at 153,454 mph (246,960 km/h). This mark was broken by NASA’s Juno Jupiter orbiter, which reached 165,000 mph (265,000 km/h) during its arrival at the gas giant in July 2016. But the PSP is now the speed king: During the spacecraft’s first perihelion pass, on Nov. 6, 2018, the sun’s powerful gravity accelerated the PSP to a top speed of 213,243 mph (343,181 km/h).
Conditions in the corona are extreme, of course, so the PSP is equipped with some heavy-duty armor: a 4.5-inch-thick (11.4 centimeters) carbon-composite shield, which protects the craft and its four science instruments from intense heat and radiation.
Those instruments are the Fields Experiment (Fields), which is measuring electric and magnetic fields and waves, among other things; the Integrated Science Investigation of the Sun (ISoIS), which is characterizing the electrons, protons and heavy ions that are accelerated to high speeds in the sun’s atmosphere and beyond; the Wide-field Imager for Solar Probe (WISPR), a set of telescopes imaging the corona and environs; and the Solar Wind Electrons Alphas and Protons (SWEAP) investigation, which is studying the most abundant constituents in the solar wind (electrons, protons and helium ions).
The four new papers report what these instruments observed during the PSP’s first two perihelion passes, which occurred in November 2018 and April 2019.
Tracing the “slow” solar wind to its source
One of the studies, for example, found that Fields is starting to get the goods on the “slow” solar wind, a component of the stream that never exceeds about 1.1 million mph (1.8 million km/h). “Slow” is a relative term here; the “fast” solar wind zips along at about twice that velocity.
Scientists already knew that the fast solar wind originates in large coronal “holes”—patches where the outer atmosphere is considerably cooler and thinner than normal—near the sun’s poles. And the data from Fields suggest that the slow wind is coming from coronal holes as well, but from smaller ones near the solar equator.
Fields also observed surprising reversals in the solar magnetic field flowing past the spacecraft: The field sometimes flipped its orientation 180 degrees and then, within seconds or minutes, flipped back again.
“These switchbacks are probably associated with some kind of plasma jets,” study lead author and Fields principal investigator Stuart Bale, a physics professor at the University of California, Berkeley, said in a statement. “My own feeling is that these switchbacks, or jets, are central to the solar wind heating problem.”
Data from SWEAP, meanwhile, indicate that such reversals are “travelling S-shaped bends in the field lines coming from the sun,” as Verscharen put it, and that the flips are boosting the speed of the solar wind.
Findings from ISoIS help flesh out this emerging picture. The instrument’s data show that it takes energetic solar particles longer than previously expected to reach the PSP, perhaps because they’re traveling along the surprisingly S-shaped field lines.
And WISPR is giving scientists a clearer picture of the sun, the corona and the complex, roiling region immediately surrounding our star. WISPR’s images help put the information collected by the other instruments into proper context and also yield insights in their own right.
For example, the fourth new study reports, WISPR photos provide some evidence for a dust-free zone near the sun, which has been postulated but not yet directly detected. “The detailed images from the PSP also show spatial variations in the solar wind that are consistent with variations in the sun’s magnetic field on its surface, and reveal small blobs of plasma that are ejected from the sun and form part of the young solar wind,” Verscharen wrote.
The best is yet to come
The solar-wind answer is a partial one at the moment, and it’s still unclear how exactly the corona is heated so dramatically. But the PSP team has plenty of time to fill in the blanks, for these newly published results are just the beginning. The spacecraft is designed to continue studying the sun through 2025, and its perihelion passages will keep getting closer and closer, thanks to trajectory-sculpting flybys of Venus.
The PSP’s final science orbit, for example, will take it within just 3.83 million miles (6.16 million km) of the solar surface and feature top speeds of about 430,000 mph (690,000 km/h).
And these future close approaches will become more frequent, because the PSP’s path around the sun will shrink. The probe’s orbital period is currently about 150 Earth days but will be 88 days by mission’s end.
The length of that mission will allow the PSP team to study the sun at diverse stages of its 11-year activity cycle. So, the sun-kissing spacecraft should gather reams of interesting data that keep researchers busy for a long time to come.
“It is expected that PSP data will guide our understanding of the sun and the solar wind for many years,” Verscharen wrote. “New models and theories will be motivated by the spacecraft’s discoveries, and this knowledge will be transferable to other stars and astrophysical plasmas throughout the universe.”
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