Parker Solar Probe, the fastest spacecraft ever that will skirt the surface of the sun, lifts off from Cape Canaveral this weekend.
On a March day in 1223 BC, the moon blocked out the sun. The eclipse cast an enormous shadow across the Earth that happened to pass over Ugarit, an ancient port city in present-day Syria, plunging the inhabitants into darkness for a little more than two minutes.
This may well have been the first total solar eclipse anyone bothered to document. On an ancient clay tablet discovered in 1948, an inscription in the Ugaritic language reads: “On the day of the new moon, in the month of Hiyar, the Sun was put to shame, and went down in the daytime, with Mars in attendance.”
The awestruck ancients likely beheld great tendrils of glowing red plasma enveloped in a wispy gray haze as they snuck out from behind the moon. These tendrils, called solar prominences, are only visible to the naked eye during a total solar eclipse.
As humanity’s obsession with our life-giving star pressed on—measuring its position with ancient stones to charting it’s path across the sky to observing it with telescopes—we learned that prominences often accompany solar eruptions of supercharged particles that rain down on the planets, creating the aurora borealis on Earth and stripping bare the atmosphere of Mars. “They are relatively unstable. They get to a certain point, and they just explode,” says Robert Decker, a research physicist at Johns Hopkins University’s Applied Physics Laboratory (APL).
But for all we’ve learned since that fateful eclipse 3,000 years ago, many things about our home star still mystify scientists. This weekend, on August 11, NASA plans to launch a spacecraft called the Parker Solar Probe to travel into the sun’s corona for a long-awaited closer look.
Magnetism Rules Everything Around Me
One of the greatest mysteries of the sun is the extraordinary temperature of its corona. This mass of plasma stretches millions of miles outward from the solar surface, as if it were the sun’s atmosphere. Somehow, it’s heated to millions of degrees Kelvin, much higher than the sun’s surface (called the photosphere), which burns at a comparatively balmy 6,000 degrees K.
“So you get a lot of heat pumped into both the electrons and the ions [of the corona], and we’re not sure how this happens,” says Decker, who serves as the Deputy Project Scientist on NASA’s Parker Solar Probe.
What we do know is that the sun’s magnetic field is responsible for essentially all of the other processes and phenomena that we see emanating from our host star. In many ways, the magnetic field of the sun is similar to the Earth’s. Driven by an internal dynamo process—where swirling superheated material at the core generates electromagnetic currents—the magnetic field flows inward at one of the poles and outward at the other.
Occasionally, the magnetic field flips, flowing inward and outward at opposite poles. The last time this happened on Earth was 780,000 years ago (though on average it occurs about every 250,000 years). On the sun, however, the magnetic field reverses every 11 years like clockwork. This process determines the solar cycles, where solar activity grows from a relatively calm minimum up to a fierce solar maximum full of active convection and eruptive flares. (We are currently in the solar minimum between solar cycle 24 and 25, which will grow to solar maximum before calming down again and entering solar cycle 26.)
Despite the regularity of these cycles, magnetic activity on the sun is absolute chaos. The primary field is only part of the picture. Smaller, less predictable, turbulent, and largely random magnetic fields dance across the solar surface and propagate out into space. Particularly during solar maximum, intense magnetic fields drive flares of energy as well as much larger explosive events associated with prominences known as coronal mass ejections(CMEs).
The Parker probe will try to get a handle on all this madness. To do so, it will carry an instrument appropriately named FIELDS. This set of magnetometers will measure magnetic variation throughout the spacecraft’s orbit, which rangers from out beyond Venus to a close distance of 9 solar radii, or only 3.85 million miles from the photosphere. These measurements of the slithering, arching, intertwining magnetic fields of the sun could help solve the solar mysteries that have plagued particle physicists for decades.
The Wind of the Sun
Parker Solar Probe is named for Eugene Parker, a solar physicist who predicted the solar wind and coined the term. (It is the first spacecraft named after a living person, and 91-year-old Parker will watch his namesake probe launch from Cape Canaveral.)
Whatever mystery action heats the corona also accelerates solar particles to extreme supersonic speeds of up to a million mph. These particles, primarily composed of electrons, protons, and helium nuclei (known as alpha particles), fly all over the solar system. In many ways, they define the solar system, creating a vast bubble known as the heliosphere.
“There are two aspects of the solar wind,” Decker says. “One is, how do they get so hot initially? And second, how are they then, in bulk, pushed to supersonic speeds to make the solar wind that we observe throughout the heliosphere?”
There are a couple prevailing theories to explain this process. The first, called nanoflares, was put forward by Eugene Parker himself.
“In the corona you have these magnetic fields or loops, and occasionally you have a situation where two magnetic fields come together and they’re oppositely directed,” Decker says. “When that happens, and there’s a plasma present, that magnetic field can annihilate. The magnetic field will disappear, but the energy goes into heating a plasma.”
The second prevailing theory of coronal heating involves waves of energy, known as Alfvén waves, traveling back and forth between the sun’s surface and corona. These , driven by electromagnetic and hydrodynamic forces (called magnetohydrodynamic forces when taken together), can transfer energy to particles while “gyrating around the magnetic field,” Decker says. “If the field is just at the right frequency at the particle, the particle will just sit there and pick up energy.”
To test these ideas, the Parker Solar Probe will carry a device called a Faraday cup as part of the Solar Wind Electrons Alphas and Protons (SWEAP) set of instruments. This cup, sitting outside of the protective heat shield, will trap solar wind particles to measure their properties, such as velocity, temperature, and density.
“It’s a very simple instrument. Basically it’s got grids on it that you can retard different energies coming in,” Decker says. “You can… control what energy range in that current you get, and from those measurements you can then reconstruct the flow velocity of the solar wind.”
You might be wondering how Parker could survive so close to the chaotic sun. While the particles of the corona are indeed superheated, the density of the particles is so low that the probe’s heat shield will reach temperatures of only 2,500 degrees F.
But there are other, even more extreme blasts of supercharged particles that Solar Probe will need to contend with. CMEs can create a kind of solar wind on steroids, generating shock waves that push energetic particles to relativistic speeds, or velocities that approach the speed of light.
“We’re going to be launching into a pretty quiet solar minimum,” Decker says. But during the course of the mission, it’s going to get a lot louder.
We Need To Be Ready
The sun gives, and it takes. Our star grants us energy and life, but it also presents an existential threat. Relativistic particles, when aligned with the Earth, not only create dazzling displays of green light in the atmosphere, but also could knock out electronics and take down power grids. “Parker Solar Probe is going to taste two different types of suns,” Decker says—a gentler one, and then an active one later in the cycle.
In 1859, eruptions from an angry sun lit up the Earth, generating bright auroras around the globe. The bombardment of particles, known as the Carrington Event, wreaked havoc on the newly installed telegraph systems of Europe and North America, even shocking some telegraph operators with jolts of electricity. Today, such an event could plunge our technological culture into chaos, frying satellites and costing trillions of dollars in damages.
“Particularly during solar maximum, you get a succession of flares and coronal mass ejections from the sun,” Decker says. “The ones you have to worry about are when you get either one after another. Or sometimes, they coalesce into one huge perturbation or irregularity—those you’ve got to watch out for.”
To study these eruptive events, the Parker Solar Probe will carry a tool called the Integrated Science Investigation of the Sun (ISʘIS, pronounced “ee-sis” and including the symbol for the sun in its acronym). ISʘIS will be capable of measuring relatively low-energy particles, as well as the high-energy particles that SWEAP cannot study in its Faraday cup.
“I’m hoping to see at least one or two strong CMEs during solar max, later in the mission when [Parker] is close to the sun,” Decker says. “One of the objectives of Solar Probe is also to fly through these regions where these particles are being accelerated from the shock waves.”
Although it might sound catastrophic to fly the spacecraft through a shock wave, the impact would not actually hit the spacecraft hard in a physical sense, again because the density of the particles is so low. Instead, these particles—including the heavier nuclei of things like carbon, oxygen, and iron—will slam into a stack of detectors on the ISʘIS instrument. Scientists can then measure how deep the particles penetrate into the detectors to determine their properties.
“These are exactly the same particles that pose a hazard to spacecraft and to astronauts,” Decker says. In the future, almost inevitably, they could cause a second Carrington Event with cataclysmic results. We need to be ready.
Predicting Explosions on the Sun
The true particle research breakthrough would be to predict events like CMEs well in advance. Today, Decker says, 10 to 20 minutes is about the best we can do. “You say, ‘oh, a flare has occurred, or a CME has occurred, now go hide.'”