NASA’s Parker Solar Probe will help solve a long-running mystery (literally) surrounding our sun: Why is its outer atmosphere hotter than its surface?

In roughly two years, the probe will be the first human-made craft to enter the zone surrounding the sun where heating looks fundamentally different that what has previously been seen in space. This will allow researchers to test their theory that the heating is due to small magnetic waves traveling back and forth within the zone.

Solving the riddle would allow scientists to better understand and predict solar weather, which can pose serious threats to Earth’s power grid. And step one is determining where the heating of the sun’s outer atmosphere begins and ends—a puzzle with no shortage of theories.

A 500-year-old puzzle

“Whatever the physics is behind this superheating, it’s a puzzle that has been staring us in the eye for 500 years,” says Justin Kasper, professor of climate and space sciences at the University of Michigan and a principal investigator for the Parker mission. “In just two more years, Parker Solar Probe will finally reveal the answer.”

“When you’re below the Alfvén point, you’re in this soup of waves.”

The theory, and how the team will use Parker to test it, appear in The Astrophysical Journal Letters.

In this “zone of preferential heating” above the sun’s surface, temperatures rise overall. More bizarre still, individual elements are heated to different temperatures, or preferentially. Some heavier ions are superheated until they’re 10 times hotter than the hydrogen that is everywhere in this area—hotter than the core of the sun.

Such high temperatures cause the solar atmosphere to swell to many times the diameter of the sun and they’re the reason we see the extended corona during solar eclipses. In that sense, Kasper says, the coronal heating mystery has been visible to astronomers for more than a half millenium, even if the high temperatures were only appreciated within the last century.

Alfvén waves

This same zone features hydromagnetic “Alfvén waves” moving back and forth between its outermost edge and the sun’s surface. At the outermost edge, called the Alfvén point, the solar wind moves faster than the Alfvén speed, and the waves can no longer travel back to the sun.

“When you’re below the Alfvén point, you’re in this soup of waves,” Kasper says. “Charged particles are deflected and accelerated by waves coming from all directions.”

In trying to estimate how far from the sun’s surface this preferential heating stops, the University of Michigan team examined decades of observations of the solar wind by NASA’s Wind spacecraft.

They looked at how much of helium’s increased temperature close to the sun was washed out by collisions between ions in the solar wind as they traveled out to Earth. Watching the helium temperature decay allowed them to measure the distance to the outer edge of the zone.

“We take all of the data and treat it as a stopwatch to figure out how much time had elapsed since the wind was superheated,” Kasper says. “Since I know how fast that wind is moving, I can convert the information to a distance.”

Those calculations put the outer edge of the superheating zone roughly 10 to 50 solar radii from the surface. It was impossible to be more definitive since some values could only be guessed at.

Initially, Kasper didn’t think to compare his estimate of the zone’s location with the Alfvén point, but he wanted to know if there was a physically meaningful location in space that produced the outer boundary.

After reading that the Alfvén point and other surfaces have been observed to expand and contract with solar activity, Kasper and coauthor Kristopher Klein, a former University of Michigan postdoc and new faculty at University of Arizona, reworked their analysis looking at year-to-year changes rather than considering the entire Wind Mission.

“To my shock, the outer boundary of the zone of preferential heating and the Alfvén point moved in lockstep in a totally predictable fashion despite being completely independent calculations,” Kasper says. “You overplot them, and they’re doing the exact same thing over time.”

Parker Solar Probe on the move

So does the Alfvén point mark the outer edge of the heating zone? And what exactly is changing under the Alfvén point that superheats heavy ions? We should know in the next couple of years. The Parker Solar Probe lifted off in August 2018 and had its first rendezvous with the sun in November 2018—already getting closer to the sun than any other human-made object.

In the coming years, Parker will get even closer with each pass until the probe falls below the Alfvén point. In their paper, Kasper and Klein predict it should enter the zone of preferential heating in 2021 as the boundary expands with increasing solar activity. Then NASA will have information direct from the source to answer all manner of long-standing questions.

“With Parker Solar Probe we will be able to definitively determine through local measurements what processes lead to the acceleration of the solar wind and the preferential heating of certain elements,” Klein says.

“The predictions in this paper suggest that these processes are operating below the Alfvén surface, a region close to the sun that no spacecraft has visited, meaning that these preferential heating processes have never before been directly measured.”

Kasper is the principal investigator of the Solar Wind Electrons Alphas and Protons Investigation on the Parker Solar Probe. SWEAP’s sensors scoop up the solar wind and coronal particles during each encounter to measure velocity, temperature, and density, and shed light on the heating mystery.

NASA’s Wind Mission funds the work.

Source: University of Michigan

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NASA is inviting people around the world to submit their names online to go on a microchip aboard NASA’s Parker Solar Probe mission launching in summer 2018. The mission will travel through the sun’s atmosphere, facing brutal heat and radiation conditions—and your name will go along for the ride.

“This probe will journey to a region humanity has never explored before,” says Thomas Zurbuchen, the associate administrator for the Science Mission Directorate at NASA Headquarters in Washington. “This mission will answer questions scientists have sought to uncover for more than six decades.”

“Parker Solar Probe is, quite literally, the fastest, hottest—and, to me, coolest—mission under the sun.”

Understanding the sun has always been a top priority for space scientists. Studying how the sun affects space and the space environment of planets is the field known as heliophysics. The field is not only vital to understanding Earth’s most important and life-sustaining star, it supports exploration in the solar system and beyond.

Submissions will be accepted until April 27. Learn more and add your name to the mission here: http://go.nasa.gov/HotTicket

Fast and hot

The spacecraft, about the size of a small car, will travel directly into the sun’s atmosphere about 4 million miles from the star’s surface. The primary science goals for the mission are to trace how energy and heat move through the solar corona and to explore what accelerates the solar wind as well as solar energetic particles. The mission will revolutionize our understanding of the sun, where changing conditions can spread out into the solar system, affecting Earth and other worlds.

To perform these unprecedented investigations, the spacecraft and instruments will be protected from the sun’s heat by a 4.5-inch-thick carbon-composite shield, which will need to withstand temperatures outside the spacecraft that reach nearly 2,500 F. This state-of-the-art heat shield will keep the four instrument suites designed to study magnetic fields, plasma and energetic particles, and image the solar wind at room temperature.

The spacecraft speed is so fast, at its closest approach it will be going at approximately 430,000 mph. That’s fast enough to get from Washington, DC, to Tokyo in under a minute.

“Parker Solar Probe is, quite literally, the fastest, hottest—and, to me, coolest—mission under the sun,” says project scientist Nicola Fox of the Johns Hopkins Applied Physics Laboratory. “This incredible spacecraft is going to reveal so much about our star and how it works that we’ve not been able to understand.”

How the Parker Solar Probe got its name

In May 2017, NASA renamed the spacecraft from the Solar Probe Plus to the Parker Solar Probe in honor of astrophysicist Eugene Parker. The announcement took place at a ceremony at the University of Chicago, where Parker is professor emeritus in the astronomy and astrophysics department.

Our sun likely had a twin called ‘Nemesis’

This was the first time NASA named a spacecraft for a living person.

NASA missions most often get new names after launch and certification. In this case, given Parker’s accomplishments within the field, and how closely aligned this mission is with his research, the decision was made to honor him prior to launch, in order to draw attention to his important contributions to heliophysics and space science.

In the 1950s, Parker proposed a number of concepts about how stars—including our sun—give off energy. He called this cascade of energy the solar wind, and he described an entire complex system of plasmas, magnetic fields, and energetic particles that make up this phenomenon. Parker also theorized an explanation for the superheated solar atmosphere, the corona, which is—contrary to what was expected by physics laws—hotter than the surface of the sun itself. Many NASA missions have continued to focus on this complex space environment defined by our star.

Parker Solar Probe is part of NASA’s Living with a Star Program. NASA’s Goddard Space Flight Center in Greenbelt, Maryland manages LWS for NASA’s Science Mission Directorate in Washington. The Johns Hopkins Applied Physics Laboratory, in Laurel, Maryland, manages the Parker Solar Probe mission for NASA. APL is designing and building the spacecraft and will also operate it.

Source: Johns Hopkins University

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