In the Sun, something mystical is happening. Defying all logic, its atmosphere turns considerably hotter and hotter with the increase in its distance from the blazing surface of the Sun.
Temperatures at the corona - which is the slim, outermost layer of the Sun’s atmosphere - is more than 2 million degrees Fahrenheit; however, just 1000 miles below, the temperature of the underlying surface is 10,000 °F. The way Sun manages this action has remained obscure till date in astrophysics; researchers call it the coronal heating problem. NASA’s Parker Solar Probe - a new, landmark mission scheduled to launch not before August 11, 2018 - will fly through the corona itself, looking for hints to its behavior and providing the chance for researchers to solve this puzzle.
Observed from Earth, as we view it in visible light, the appearance of the Sun—which is silent, unvarying—is contradictory to the life and spectacle of our nearest star. Eruptions and intense bursts of radiation rock its turbulent surface, thereby hurling solar material at mind-blowing speeds to every corner of the solar system. This solar activity can set off space weather events that have the ability to harm astronauts and satellites, interrupt radio communications, and at their most severe, interfere with power grids.
The corona extends for millions of miles above the surface and roils with plasma, gases superheated to such an extent that they separate into an electric flow of ions and free electrons. Ultimately, it continues to flow outward as the solar wind, a supersonic stream of plasma that permeates the whole of the solar system. Therefore, the fact is that humans live well within the Sun’s extended atmosphere. In order to gain complete insights into the corona and all its mysteries, it is necessary to understand not only the star powering life on Earth but also the very space around Earth.
A 150-Year-Old Mystery
Much of the existing knowledge about the corona is deeply rooted in the history of total solar eclipses. Before the advent of spacecraft and advanced instruments, the only method for studying the corona from Earth was at the time of a total eclipse, when the bright face of the Sun is blocked by the Moon, exposing the surrounding, dimmer corona.
The story of the coronal heating problem started with the observation of a green spectral line at the time of total eclipse in 1869. Since distinctive elements emit light at characteristic wavelengths, researchers can use spectrometers to investigate light from the Sun and recognize its composition. However, the green line detected in 1869 did not correspond to any recognized elements on Earth. Researchers considered perhaps a new element had been discovered and named it coronium.
Seven decades later, a Swedish physicist found out that the element responsible for the emission was iron, superheated to the point where it was ionized 13 times and had only half the electrons of a normal atom of iron. Here lies the problem: Researchers calculated that such high levels of ionization would necessitate coronal temperatures of about 2 million degrees Fahrenheit—almost 200 times hotter than the surface.
For several decades, this deceivingly simple green line has been the Mona Lisa of solar science, puzzling researchers who cannot explain its existence. From the time its source was identified, it has been understood that the puzzle is only more complex than it was first found to be.
“I think of the coronal heating problem as an umbrella that covers a couple of related confusing problems,” stated Justin Kasper, a space scientist at the University of Michigan in Ann Arbor. Kasper is also principal investigator for SWEAP, short for the Solar Wind Electrons Alphas and Protons Investigation, an instrument suite aboard Parker Solar Probe. “First, how does the corona get that hot that quickly? But the second part of the problem is that it doesn’t just start, it keeps going. And not only does heating continue, but different elements are heated at different rates.” It is a fascinating hint in relation to heating in the Sun.
After discovering the hot corona, researchers and engineers have carried out considerable studies to understand its behavior. They have created robust instruments and models and launched spacecraft that observe the Sun around the clock. However, even the high-resolution observations and the most complex models can only partly describe coronal heating, and certain theories contradict each other. There is also the challenge of studying the corona from a distant position.
Although humans live within the expansive atmosphere of the Sun, the solar plasma and corona in near-Earth space vary drastically. The slow solar wind takes nearly four days to travel 93 million miles and reach Earth or the spacecraft that study it—a lot of time for it to mingle with other particles whizzing through space and lose its attributive features.
Analyzing this homogenous soup of plasma for hints to coronal heating is similar to attempting to analyze the geology of a mountain by sifting through sediment in a river delta thousands of miles downstream. Parker Solar Probe will travel to the corona and sample just-heated particles, eliminating the uncertainties of a 93-million-mile journey and transmitting to Earth the most precise measurements of the corona ever recorded.
“All of our work over the years has culminated to this point: We realized we can never fully solve the coronal heating problem until we send a probe to make measurements in the corona itself,” stated Nour Raouafi, Parker Solar Probe deputy project scientist and solar physicist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
Although travel to the Sun is a concept older than NASA itself, it has taken decades to engineer the technology that renders this journey possible. In the meantime, researchers have determined precisely what types of data—and corresponding instruments—are required to complete a picture of the corona and answer these ultimate blazing questions.
Explaining the Corona’s Secrets
Parker Solar Probe will investigate two principal theories to explain coronal heating. Sun’s outer layers constantly boil and roil with mechanical energy. When massive cells of charged plasma roil through the Sun—quite similar to distinct bubbles rolling up through a pot of boiling water—complex magnetic fields extending deep into the corona are generated by their fluid motion. For some reason, the tangled fields route this fierce energy into the corona as heat—how they achieve this is what every theory attempts to explain.
One theory hypothesizes that the corona’s extreme heat is due to electromagnetic waves. Possibly that boiling motion initiates magnetic waves of a specific frequency—known as Alfvén waves—from deep within the Sun out into the corona, which make charged particles to spin and heat the atmosphere, somewhat similar to the way ocean waves push and accelerate surfers toward the shore.
Another theory proposes that heat into the solar atmosphere is dumped by bomb-like explosions, known as nanoflares, over the surface of the Sun. Similar to their larger equivalents, solar flares, nanoflares are considered to arise out of an explosive process known as magnetic reconnection. Magnetic field lines are twisted and contorted due to turbulent boiling on the Sun, increasing the stress and tension until they explosively snap—similar to breaking an over-wound rubber band—heating and accelerating particles in their wake.
It is not necessary that the two theories are mutually exclusive. As a matter of fact, to complicate matters, many researchers consider both could be involved in heating the corona. At times, for instance, the magnetic reconnection that triggers a nanoflare could also initiate Alfvén waves, which then heat the surrounding plasma more.
There is one more big question to be answered: What is the frequency of occurrence of these processes—constantly or in distinct bursts? A specific level of detail is needed to answer that question, and that detail cannot be achieved from 93 million miles afar.
“We’re going close to the heating, and there are times Parker Solar Probe will co-rotate, or orbit the Sun at the same speed the Sun itself rotates,” stated Eric Christian, a space scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and member of the mission’s science team. “That’s an important part of the science. By hovering over the same spot, we’ll see the evolution of heating.”
Uncovering the Evidence
Upon arriving at the corona, how will the Parker Solar Probe help researchers differentiate whether nanoflares or waves drive heating? Although the spacecraft carries four instrument suites for different types of research, specifically two will obtain data useful for solving the coronal heating puzzle: the FIELDS experiment and SWEAP.
Surveyor of invisible forces, FIELDS, headed by the University of California, Berkeley, directly measures magnetic and electric fields to gain insights into the shocks, waves and magnetic reconnection events that heat the solar wind.
SWEAP, which is headed by the Harvard-Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, is the complementary half of the study, collecting data related to hot plasma itself. It counts the most plentiful particles in the solar wind—protons, electrons, and helium ions—and measures their temperature, the speed with which they move after being heated, and their direction of movement.
Jointly, the two instrument suites offer an illustration of the electromagnetic fields considered to be responsible for heating, and also the just-heated solar particles spinning through the corona. High-resolution measurements are the key to their success, due to their potential resolve interactions between particles and waves at just fractions of a second.
Parker Solar Probe will swoop within 3.9 million miles of the Sun’s surface—and although this distance might appear great, the spacecraft is well-positioned to detect signs of coronal heating. “Even though magnetic reconnection events take place lower down near the Sun’s surface, the spacecraft will see the plasma right after they occur,” stated Goddard solar scientist Nicholeen Viall. “We have a chance to stick our thermometer right in the corona and watch the temperature rise. Compare that to studying plasma that was heated four days ago from Earth, where a lot of the 3D structures and time-sensitive information are washed out.”
This portion of the corona is a completely unexplored region, and researchers anticipate sights very different from anything observed earlier. Some consider the plasma there will be tenuous and wispy, similar to cirrus clouds. Or maybe it will seem like massive pipe cleaner-like structures radiating from the Sun.
“I’m pretty sure when we get that first round of data back, we’ll see the solar wind at lower altitudes near the Sun is spiky and impulsive,” stated Stuart Bale, astrophysicist from University of California, Berkeley, and FIELDS principal investigator. “I’d lay my money on the data being much more exciting than what we see near Earth.”
The data is highly complex—and from multiple instruments—and will take researchers some time to compose an explanation for coronal heating. Moreover, since the surface of the Sun is not smooth and changed throughout, Parker Solar Probe has to make a number of passes over the Sun to recite the complete story. However, researchers are convinced it has the tools to answer their questions.
The basic concept is that each proposed mechanism for heating has its own unique signature. In case the source of the extreme heat of the corona is Alfvén waves, FIELDS will detect their activity. As heavier ions are heated at varying rates, it seems that distinctive classes of particles interact with those waves in unique ways; SWEAP will characterize their specific interactions.
If nanoflares are behind the extreme heat, researchers anticipate observing jets of accelerated particles shooting out in opposite directions—a revealing sign of explosive magnetic reconnection. If magnetic reconnection occurs, they should also detect hot spots at which magnetic fields are quickly changing and heating the surrounding plasma.
Discoveries Lie Ahead
Solar scientists are buzzing with an eagerness and excitement: Parker Solar Probe’s mission is a turning point in the history of astrophysics, and they have a real chance of revealing the mysteries that have confounded the field for more than one and a half centuries.
By assembling the inner workings of the corona, researchers can gain in-depth insights into the dynamics that trigger space weather events, shaping conditions in near-Earth space. However, the applications of this science can be applied even beyond the solar system. The Sun opens the door for gaining insights of stars—specifically those that also demonstrate Sun-like heating—that could probably foster habitable environments but are too very far to ever investigate. Moreover, throwing light on the basic physics of plasmas could probably teach researchers a lot in relation to how plasmas behave in other parts of the universe, such as in clusters of galaxies or surrounding black holes.
It is also completely possible that researchers do not know much of the greatest discoveries to happen. It is difficult to estimate how solving coronal heating will transform the knowledge of the space around humans; however, basic discoveries such as this have the potential to transform science and technology forever. Parker Solar Probe’s journey takes the curiosity of humans to a never-before-observed region of the solar system, where every observation is a possible discovery.
“I’m almost certain we’ll discover new phenomena we don’t know anything about now, and that’s very exciting for us,” stated Raouafi. “Parker Solar Probe will make history by helping us understand coronal heating—as well as solar wind acceleration and solar energetic particles—but I think it also has the potential to steer the direction of solar physics’ future.”
Discovering the Sun's Mysteriously Hot Atmosphere
The coronal heating problem remains one of the greatest unanswered questions in astrophysics. Learn how astronomers first discovered evidence for this mystery during an eclipse in the 1800s, and what scientists today think could explain it. (Image credit: NASA’s Goddard Space Flight Center/Joy Ng)