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Parker Solar Probe and the weird case of the hot corona

Most of what we know about the corona is deeply rooted in the history of total solar eclipses. Parker Solar Probe will fly through this region looking for clues to the sun's behavior. This photo was taken in Madras, Oregon, during the Total Solar Eclipse on August 21
, 2017. Credit: NASA's Goddard Space Flight Center / Gopalswamy

There is something mysterious about the sun. Despite all logic, its atmosphere gets hotter as it moves away from the glowing surface of the sun.

Temperatures in the corona – the thin, outermost layer of the solar atmosphere – spray over 2 million degrees Fahrenheit, while just 1,000 miles below, the underlying surface simmers at a mild 10,000F. As the sun manages, this achievement remains the greatest unanswered questions in astrophysics; Scientists call it the problem of coronary warming. A new, groundbreaking mission, NASA's Parker Solar Probe, due to launch no earlier than August 11, 2018, will fly through the corona itself, seeking clues to its behavior and providing scientists with the opportunity to unravel this mystery.

From The Earth, as we see it in visible light, the appearance of the sun – calm, unchanging – deceives the life and drama of our next star. Its turbulent surface is shattered by eruptions and intense bursts of radiation that hurl the sun's material at incredible speed into every corner of the solar system. This solar activity can trigger space weather events that interfere with wireless communications, can damage satellites and astronauts, and, when they are heaviest, intervene in power grids.

Above the surface, the corona stretches for millions of miles and bubbles with plasma, gases overheating so much that they separate into an electric flux of ions and free electrons. Finally, it goes outward like the solar wind, a supersonic flow of plasma that permeates the entire solar system. And so people live well in the extended atmosphere of our sun. To fully understand the corona and its secrets, one must not only understand the star that drives life on Earth, but also the space around us.

A 150-Year-Old Mystery

Most of what we know about the corona is deeply rooted in the history of total solar eclipses. Before sophisticated instruments and spacecraft, the only way to study the corona from Earth during a total solar eclipse, when the moon blocks the bright face of the sun and the surrounding, weaker corona, was to visualize the green spectral line that was created during a total eclipse of Was observed in 1869. Because different elements emit light at characteristic wavelengths, scientists can use spectrometers to analyze the light of the sun and determine its composition. But the green line observed in 1869 did not correspond to any known elements on earth. Scientists thought they might have discovered a new element, and they called it Coronium.

It was not until 70 years later that a Swedish physicist discovered that the element responsible for the emission is iron, overheated until it was ionized 13 times with only half of the electrons of a normal iron atom. And that's the problem: Scientists calculated that such high levels of ionization would require coronal temperatures around 2 million degrees Fahrenheit – nearly 200 times hotter than the surface.

The problem of coronal warming remains one of the biggest unanswered questions in astrophysics. Learn how astronomers first found evidence of this mystery during a solar eclipse in the 1800s, and what scientists think today might explain it. Image: NASA Goddard Space Flight Center

For decades, this deceptively simple green line was the Mona Lisa of the solar sciences, confusing scientists who can not explain their existence. Since we identified his source, we understand that the puzzle is even more complex than it first appeared.

"I'm thinking of the coronary warming problem as an umbrella covering a few related confusing issues," said Justin Kasper, a space scientist at the University of Michigan at Ann Arbor. Kasper is also Principal Investigator for SWEAP, short for Solar Wind Electrons Alphas and Protons Investigation, a range of instruments onboard the Parker Solar Probe. "First, how does the corona get so hot so quickly, but the second part of the problem is that it not only starts, it continues, and not just heating, but different elements are heated at different rates." It is a fascinating indication of what happens to the warming in the sun.

Since the discovery of the hot corona, scientists and engineers have done a great deal of work to understand their behavior. They have developed powerful models and instruments and launched satellites that observe the sun around the clock. But even the most complex models and high-resolution observations can only partially explain coronal warming, and some theories contradict each other. There is also the problem of studying the corona from a distance.

We may live in the expansive atmosphere of the sun, but the corona and solar plasma in the near-Earth space are dramatically different. It takes about four days for the slow solar wind to travel 93 million miles and reach the Earth or spacecraft it studies – plenty of time to mingle with other particles that travel across space and lose their distinctive features.

Studying this Homogeneous Soup Plasma for evidence of coronal warming is like trying to study the geology of a mountain by scouring sediments thousands of miles downstream in a river delta. On the way to the corona, Parker Solar Probe will scan particles that have just warmed up, eliminate the uncertainties of a 93-million-mile journey, and return the most perfect measurements of the corona ever taken to Earth.

"All our work Over the years this point has come: we have realized that we can never fully resolve the coronary heating problem until we send a probe to measure in the corona itself," said Nour Raouafi, Deputy Project Scientist and Solar Physicist by Parker Solar Probe at Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland

Traveling to the Sun is an idea older than NASA itself, but it took decades to develop the technology that developed enables her journey. During this time, scientists have determined exactly what types of data – and corresponding instruments – they need to complete a picture of the corona and answer that ultimate burning question.

Above the surface, the corona pictured here spans millions of miles, bubbling with plasma. Finally, it goes outward like the solar wind, a supersonic flow of plasma that permeates the entire solar system. View animated GIF: https://www.nasa.gov/sites/default/files/thumbnails/image/sunloop1.gif Credit: NASA Goddard Space Flight Center / Lisa Poje / Genna Duberstein

Explain the Secrets of the Corona

Parker Solar Probe will test two major theories to explain coronal warming. The outer layers of the sun are constantly cooking and rusting with mechanical energy. As massive cells of charged plasma move through the sun – much like bubbles rise through a pot of boiling water – their fluid motion creates complex magnetic fields that extend far into the corona. Somehow, the tangled fields channel this wild energy into the corona as heat – as they do what every theory tries to explain.

One theory suggests that electromagnetic waves are the root of the corona's extreme heat. Perhaps this boiling motion brings magnetic waves of a particular frequency – called Alfvén waves – from deep into the sun into the corona, which drives charged particles into the atmosphere and heats them, a bit like ocean waves that drive surfers to shore and accelerate.

Another suggests that bomb explosions, so-called nanoflares, divert heat into the solar atmosphere via the solar surface. As with their larger counterparts, solar flares, Nanoflares, are expected by an explosive process called magnetic reconnection. Turbulent cooking on the sun twists and twists magnetic field lines, builds up tensions and stresses until they break explosively – like a rubber band breaking over – particles accelerate and heat up.

The two theories are not necessarily mutually exclusive. In fact, many scientists, to complicate matters, think that both could be involved in heating the corona. Sometimes, for example, the magnetic re-connection that triggers a nanoflare can also trigger Alfvén waves, which then further heat the surrounding plasma.

The other big question is how often do these processes occur – constant or in different outbursts? Answering this requires a level of detail that we do not have from 93 million miles away.

"We're getting closer to heating, and there are times when Parker Solar is spinning samples or simultaneously orbiting the sun's speed," said Eric Christian, a space scientist at NASA's Goddard Space Flight Center in Greenbelt , Maryland, and a member of the mission's scientific team. "This is an important part of science, and as we hover over the same place, we see the evolution of warming."

A close-up of the convection of the sun, or boiling, motion, with a small sunspot forming to the right of Hinode, a collaboration between NASA and Japan Aerospace Exploration Agency (JAXA). The outer layers of the sun are constantly cooking and rusting with mechanical energy. This flowing motion creates complex magnetic fields that extend far into the corona. Credit: NASA / JAXA / Hinode

Uncovering Evidence

Once the Parker solar probe reaches the corona, how will it help scientists distinguish whether waves or nanoflares power the heater? In particular, while the spacecraft carries four instrument clusters for a variety of research types, two will yield data useful for solving the coronal heating secrecy: the FIELDS Experiment and SWEAP

Invisible Forces Surveyor, FIELDS, led by the University of California, Berkeley It directly measures electrical and magnetic fields to understand the shocks, waves, and magnetic reconnections that heat the solar wind.

SWEAP – led by the Harvard-Smithsonian Astrophysical Observatory in Cambridge, Massachusetts – is the complement to half the study, collecting data on the hot plasma itself. It counts the most abundant particles in the solar wind – electrons, protons and helium ions – and Measures their temperature, how fast they move after being heated and in which direction.

Together, the two instrument suites paint a picture of the electromagnetic fields that are believed to be responsible for the warming, as well as the just-heated solar particles swirling through the corona. The key to their success are high-resolution measurements that are able to resolve interactions between waves and particles in fractions of a second.

Parker Solar Probe will appear within 3.9 million miles of the sun's surface – and that distance may seem large, the spacecraft is well positioned to detect signatures of coronal warming. "Although magnetic reconnections occur lower down near the solar surface, the spacecraft will see the plasma immediately after it occurs," said Goddard solar scientist Nicholeen Viall. "We have the ability to hold our thermometer directly in the corona and watch the temperature rise, compare that to the study of plasma that was heated from Earth four days ago, where there are many 3D structures and time-critical information

This part of the corona is a completely unexplored area, and scientists expect sights that are different from anything they have seen so far. Some think the plasma there is thin and thin like cirrus clouds. Or maybe it will look like massive pipe cleaner-like structures emanating from the sun.

"I'm pretty sure if we get that first round of data back, we'll see the solar wind at lower altitudes near the sun prickly and impulsive," said Stuart Bale of the University of California, Berkeley, Astrophysicist and FIELDS Principal Investigator , "I would spend my money on making the data much more exciting than what we see near Earth."

Artistic concept of the NASA's Parker Solar Probe. The spacecraft will fly through the solar corona to track how energy and heat move through the star's atmosphere. Credit: NASA / Johns Hopkins APL

The data is complicated enough – and comes from several instruments – that scientists need some time to put together an explanation for coronal warming. And because the sun's surface is not smooth and varies everywhere, Parker Solar Probe has to make several passes across the sun to tell the full story. But scientists are confident that they have the tools to answer their questions.

The basic idea is that every proposed mechanism for heating has its own unique signature. If Alfvén waves are the source of the corona's extreme heat, FIELDS will detect their activity. As heavier ions are heated at different rates, it appears that different classes of particles interact with these waves in a specific way; SWEAP will characterize their unique interactions.

When Nanoflares are responsible, scientists expect beams of accelerated particles to shoot in opposite directions – a telltale sign of explosive magnetic reconnection. Where magnetic reconnection occurs, they should also detect hot spots where magnetic fields change rapidly and the surrounding plasma heats up. Discoveries Are Present

Solar scientists are seething with zeal and excitement: Parker The mission of Solar Probe marks a turning point in the history of astrophysics and they have a real chance to unlock the secrets that have been their field since messed up for almost 150 years.

By combining the inner workings of the corona, scientists gain a deeper understanding of the dynamics that trigger space weather events that shape conditions in near-Earth space. But the applications of this science extend beyond the solar system. The sun opens a window to understand other stars – especially those that also have sun-like heater stars that could potentially enhance habitable environments, but are too far to ever study. And if you look at the basic physics of plasmas, you could probably teach scientists a lot about how plasmas behave elsewhere in the universe, like in clusters of galaxies or around black holes.

It is also quite possible that we have not even imagined the greatest discoveries to come. It's hard to predict how solving coronary warming will change our understanding of the space around us, but fundamental discoveries like these have the ability to change science and technology forever. Parker Solar Probe's voyage introduces human curiosity into an unprecedented region of the solar system, where every observation is a potential discovery.

"I'm almost certain we'll discover new phenomena that we do not know about now, and that's very exciting for us," Raouafi said. "Parker Solar Probe will make history by helping us to understand coronal warming – as well as solar wind acceleration and solar energy particles – but I think it also has the potential to steer the direction of the future of solar physics."

NASA is preparing for the launch of Parker Solar Probe, a mission to touch the sun

Provided by:
Goddard Space Flight Center of NASA

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