How a NASA Probe Solved a Scorching Solar Mystery

Our solar is the very most life like-noticed principal particular person in your entire universe.

We see it every day. For centuries, scientists have tracked the dark spots dappling its bright face, while in recent decades, telescopes in space and on Earth have examined sunlight in wavelengths spanning the electromagnetic spectrum. Experiments have also sampled the Sun’s atmosphere, captured puffs of the solar wind, collected solar neutrinos and high-energy particles, and mapped our star’s magnetic field — or tried to, since we have yet to clearly observe the polar regions that are key to understanding the Sun’s interior magnetic structure.

For all that scrutiny, however, one critical question remained embarrassingly unsolved. At its surface, the Sun is a toasty 6,000 degrees Celsius. But the outer layers of its atmosphere, called the corona, can be a blistering — and perplexing — 1 million degrees hotter.

You also can uncover that searing sheath of gas for the duration of a entire represent voltaic eclipse, as took situation on April 8 above a swath of North The United States. When you were in the walk of totality, that you simply would be able to uncover the corona as a refined halo in the route of the moon-shadowed solar.

This year, that halo appeared different from the actual person seen during the ideal North American eclipse in 2017. Not only is the Sun more active now, but you were observing a structure that we—the scientists who study our home star—have at best come close to understanding. Observing the Sun from afar was not sufficient for us to determine what heats the corona. To resolve this and other mysteries, we needed a solar-grazing spacecraft.

That spacecraft—NASA’s Parker Solar Probe—launched in 2018. As it loops around the Sun, dipping in and out of the solar corona, it has collected data that reveals how small-scale magnetic activity in the solar atmosphere makes the solar corona extraordinarily hot.

NASA Space Technology From Surface to Sheath

To begin to understand that scorching corona, we now have to consider magnetic fields.

The Sun’s magnetic engine, called the solar dynamo, lies about 200,000 kilometers below the Sun’s surface. As it churns, that engine drives solar activity which waxes and wanes over cycles of roughly 11 years. When the Sun is more active, solar flares, sunspots and outbursts increase in intensity and frequency (as is occurring now, approaching solar maximum).

At the Sun’s surface, magnetic fields emerge at the boundaries of churning convective cells, identified as supergranules, which look like bubbles in a pan of boiling oil on the stove. The constantly boiling solar surface concentrates and strengthens these magnetic fields at the cells’ edges. These amplified fields then produce transient jets and nanoflares as they interact with solar plasma.

Magnetic fields can also erupt through the Sun’s surface and produce larger-scale phenomena. In regions where the field is strong, you find dark sunspots and large magnetic loops. In most locations, especially in the lower solar corona and near sunspots, these magnetic arcs are “closed,” with both ends attached to the Sun. These closed loops come in a variety of sizes — from tiny ones to the dramatic, blazing arcs seen during eclipses.

In various locations, these loops are torn open. The Sun’s searing corona is the source of a supersonic solar wind — streams of charged particles that form a vast protective bubble around the solar system called the heliosphere, which extends far beyond the known planets. These particles carry magnetic fields with them, sometimes all the way into deep space. When that happens, the magnetic loop stretches to the edge of the heliosphere, forming what is called an “open” magnetic field.

We knew that somehow these magnetic processes must be working together to heat the corona — but how?

Over time, scientists proposed many explanations for the extremely hot corona. Most of these treated the solar atmosphere as a fluid, explaining heat transfer as it would occur in a fluid — through chaotic, turbulent cascades that carry heat from large reservoirs into smaller regions. Others suggested that magnetic waves originating at the Sun’s surface continuously wiggle and deposit heat into the atmosphere, or that, at the particle level, some form of kinetic instability is at work.

In 1988, Eugene Parker, a University of Chicago astrophysicist, argued that convection at the solar surface — these churning cells — could tangle magnetic fields that extend into the corona, thereby building up and storing magnetic energy in the solar atmosphere. When these field lines inevitably snap and reconnect, he said, the stored magnetic energy could be transferred into the solar atmosphere. There, the energy would heat the atmosphere to high temperatures, leading to nanoflares. (Parker was also responsible for a hypothesis from 1958 suggesting that the superheated corona is the source of the solar wind. Though widely ridiculed at the time, Parker’s idea was correct and foundational to the field of heliophysics.)

Parker’s belief made sense, but we didn’t have enough data to test or falsify any of the explanations, including his. The ways in which we were studying the sun simply weren’t up to the challenge.

NASA Space Technology A Fresh Hope

The turning point came in 2005, when many solar scientists met in Whistler, British Columbia. I became the meeting’s chair, a role I deliberately assumed in an attempt to integrate the often-disjointed approaches of the communities studying the sun and the solar wind.

Until then, the solar community had mostly focused on distant observations of the sun, made by ground-based telescopes, rockets, or satellites such as SOHO, a mission led by the European Space Agency (ESA) that had been launched not too long ago and is still operating. The solar-wind community, on the other hand, was busy collecting and analyzing samples of the extended corona using satellites such as NASA’s Advanced Composition Explorer and Ulysses, a joint ESA/NASA mission that flew over the sun’s poles. Our goal for this conference was to merge the often-siloed results from these modern observatories and see if that could help solve the mystery of the hot corona and how it accelerates the solar wind.

At this point, we knew that solar magnetism was behaving in ways we weren’t expecting. SOHO data had shown that globally, the solar magnetic field was far more variable than we had imagined. And the particles comprising the solar wind, as measured near Earth, had unusual compositional patterns that didn’t make sense if the wind was emanating straight from the Sun’s surface, as had been predicted. It seemed that some form of magnetic activity in the solar atmosphere was producing that wind — and the corona’s heat — but we didn’t have the models to explain how it worked.

The discussions in the meeting were prolonged and intense, but they laid the basis for a key decision: There was an absolute need to produce observations closer to the Sun with a mission notionally called Solar Probe. A model of that spacecraft — a probe that could withstand the harshness of the near-solar atmosphere — was at the front of the meeting room, and after four decades of considering it, we were going to make it a reality. In 2017, shortly after I joined NASA as the head of science, the agency renamed the mission after Eugene Parker, consistent with my advice. It was now Parker Solar Probe.

NASA Space Technology Touching the Solar

Eugene Parker watched as Parker Solar Probe launched from Cape Canaveral in 2018 and rumbled into the sky atop a Delta IV Heavy rocket. After the liftoff he thanked me for the honor of having his name on this spacecraft and added, in an uncommon moment of directness, that he only wished a few of these bastards — colleagues who’d derided his ideas and nearly cost him his career — were still alive to see this.

The spacecraft used Venus flybys to sling itself successively closer to the Sun, and on April 28, 2021 it touched the corona for the first time. It was now the closest spacecraft to our star and the fastest human-made object ever launched. (Indeed, last month it passed by the Sun for the 18th time at a speed that could get you from Washington, D.C., to Los Angeles in about 20 seconds, and from the Earth to the Moon in 36 minutes.)

As hoped, the spacecraft’s near-Sun observations were groundbreaking for our understanding of coronal heating. The observations solved the problem by decoding magnetic signatures in the near-Sun solar wind — a key to studying how the coronal furnace works.

From near Earth, the solar wind seems like a turbulent fluid that is loosely linked to the Sun at only the largest scales. But from up close, its structure directly reflects the structures on the solar surface. Rather than being a disorganized fluid, the near-Sun solar plasma whooshes outward in streamlets that typically match the sizes of the convective supergranules on the Sun’s surface — the cells around which magnetic fields concentrate, expand and burst out into the corona.

During each solar orbit, the spacecraft zoomed through these streamlets, and it detected a telltale fingerprint of magnetic activity that permeated the plasma and pointed to a source for the corona’s heat. Called “switchbacks,” these fingerprints are S-shaped structures formed by transient reversals in the locally measured magnetic field. Such switchbacks indicate (at least, according to most scientists) when closed magnetic loops collide with open magnetic loops and join with them, during what is known as an interchange reconnection event. As with champagne in a bottle, the only way to release energy and plasma from a tangled, closed magnetic loop is to uncork it by breaking it open and reconnecting it with an open field line. These reconnection events generate heat and fling solar material into space — thus warming the corona and accelerating particles in the solar wind.

Although some scientists aren’t entirely convinced the problem is solved, the field is now converging on the conclusion that Parker’s 1988 explanation was correct. Coronal heating in the corona relies on magnetic fields at microscopic scales. Convective granules on the solar surface concentrate magnetic fields at their edges and trigger a chain of events that, through subsequent magnetic interactions in the atmosphere, leads to the supersonic solar wind and the million-degree temperatures we observe.

Later this year, Parker Solar Probe will break its own record and fly even closer to the Sun. Another journey into the inferno and back, in search of more answers to enduring solar mysteries.