Since the 1960s, astronomers have wondered how the Sun’s supersonic “solar wind,” a stream of energetic particles that flows into the solar system, continues to gain energy after it leaves the Sun. In addition to helping scientists better predict solar activity and space weather, such information also helps us understand the mysteries of the universe elsewhere and how stars like the Sun and stellar winds elsewhere operate.
Since the 1960s, astronomers have wondered how the Sun’s supersonic “solar wind,” a stream of energetic particles that flows into the solar system, continues to gain energy after it leaves the Sun. Now, thanks to a lucky coincidence between the NASA and ESA (European Space Agency)/NASA spacecraft currently studying the Sun, they may have found the answer — knowledge that is a crucial piece of the puzzle that helps scientists better predict solar activity between the Sun and Earth.
A paper published in the August 30, 2024, issue of the journal Science provides compelling evidence that the fastest solar winds are caused by magnetic “switches,” or large bends in the magnetic field near the Sun.
“Our study addresses the huge open question of how the solar wind is charged and helps us understand how the Sun affects its environment and, ultimately, Earth,” said Yeimi Rivera, co-leader of the study and a research scientist at the Smithsonian Astrophysical Observatory, part of the Harvard Center for Astrophysics. “If this process is happening in our local star, it is very likely that it is powering winds from other stars throughout the Milky Way galaxy and beyond, and could have implications for the habitability of exoplanets.”
Previously, NASA’s Parker Solar Probe found that these reversals are common throughout the solar wind. Parker, which became the first spacecraft to enter the Sun’s magnetic atmosphere in 2021, allowed scientists to determine that reversals are more pronounced and more powerful near the Sun. However, until now, scientists had no experimental evidence that the phenomenon actually releases enough energy to play a significant role in the solar wind.
Parker is designed to fly through the Sun’s atmosphere, or “corona.” ESA and NASA’s Solar Orbiter mission is also in an orbit that passes relatively close to the Sun, measuring the solar wind at great distances.
The discovery was made possible by a coincidence in February 2022 that allowed Parker Solar Probe and Solar Orbiter to measure the same solar wind flow two days apart. Solar Orbiter was nearly halfway to the Sun, while Parker was skirting the edge of the Sun’s magnetic atmosphere.
“We didn’t realize at first that Parker and Solar Orbiter were measuring the same thing. Parker saw this slower plasma near the sun that was full of backward waves, and then Solar Orbiter saw a fast flow that was warm and had very little wave activity,” said Samuel Badman, an astrophysicist at the Center for Astrophysics and another co-leader of the study. “When we connected the two, it was a real eureka moment.”
Scientists have long known that energy moves through the sun’s corona and solar wind, at least in part, via so-called “Alfvén waves.” These waves carry energy through plasma, the superheated state of matter that makes up the solar wind.
But how much Alfvén waves evolve and interact with the solar wind between the Sun and Earth couldn’t be measured — until these two missions were sent closer to the Sun than ever before at the same time. Now scientists can directly determine how much energy is stored in the magnetic and speed fluctuations of these waves near the corona, and how much less energy is carried by waves farther from the Sun.
A new study shows that Alfvén waves in the form of backward waves provide enough energy to explain the heating and acceleration seen in the faster solar wind as it moves away from the Sun.
“It took more than half a century to confirm that the acceleration and heating of Alfvén waves are important processes, and that they happen in much the same way we think,” said John Belcher, a professor emeritus at the Massachusetts Institute of Technology who co-discovered Alfvén waves in the solar wind but was not involved in this research.
In addition to helping scientists better predict solar activity and space weather, such information also helps us understand the mysteries of the Universe elsewhere and how stars like the Sun and stellar winds everywhere operate.
“This discovery is one key piece of the puzzle to answer a 50-year-old question about how the solar wind is accelerated and heated in the inner heliosphere, bringing us closer to one of the Parker Solar Probe mission’s key science goals,” said Adam Szabo, Parker Solar Probe’s principal investigator at NASA.
The mechanism for the formation of the “slow” solar wind has been revealed. The results of observations by Solar Probe support the theory that the “slow” solar wind consists of matter that is usually trapped in the so-called closed regions of the corona, where magnetic field lines usually remain closed
An international team of astronomers has used the Solar Orbiter probe to discover the first physical evidence that the so-called slow solar wind is generated in closed regions of the Sun’s corona as a result of magnetic field lines breaking and reconnecting, the press service of the British University of Northumbria reported.
“Data collected by Solar Orbiter indicated that the “slow” solar wind flows originate from regions of the Sun where the closed and open corona regions meet. This supports the theory that the formation of the “slow” solar wind is linked to the reconnection of magnetic field lines, which allows matter to “escape” from closed regions of the corona,” the report says.
This conclusion was reached by a group of astronomers led by Udo Schüle, a research fellow at the Institute for Solar System Research in Göttingen, while studying data collected by the Solar Orbiter probe in March 2022. At that time, as scientists note, the spacecraft was at a distance of only 0.5 astronomical units from the Sun (the average distance between the Earth and the star), which allowed scientists to study the structure of two streams of the “slow” solar wind at once.
In their study, scientists took advantage of the fact that the proportions of magnesium, neon, and some other heavy elements in the matter of the Sun’s corona differ significantly for different regions. This allows them to determine where a particular solar wind flow originated and to uncover the mechanisms of its formation. Guided by this idea, scientists measured the proportions of magnesium and neon in five solar wind flows using Solar Orbiter instruments.
The scientists’ calculations indicated that the “slow” solar wind streams originated in areas of the Sun’s surface that are border areas between coronal holes and the rest of the sun’s corona. According to the researchers, this suggests that the “slow” wind is caused by magnetic field lines breaking and reconnecting in these areas of the corona, allowing matter from its inner regions to “escape” into outer space.
As the researchers note, the results of observations by Solar Probe support the theory that the “slow” solar wind consists of matter that is usually trapped in the so-called closed regions of the corona, where magnetic field lines usually remain closed. Their periodic ruptures and reconnections create conditions for the formation of very heterogeneous and unstable emissions of the “slow” solar wind, the researchers concluded.
Solar Orbiter is a joint project between NASA and the European Space Agency worth approximately $1.5 billion. The probe is equipped with ten different instruments, six of which will be constantly directed at the Sun, and four others are needed to study the state of the environment around the device itself. In addition, a special heat shield is installed on the probe, which allows the device to approach the Sun at a distance of up to 42 million km.
NASA has several missions studying how the Sun and solar storms affect Earth and space travel, and the International Space Station contributes to this research in several ways.
Improved solar measurements. The station’s Total and Spectral Solar Irradiance Sensor (TSIS) measures solar irradiance, the solar energy received by Earth, and solar spectral irradiance, a measure of the Sun’s energy in individual wavelengths. Knowing the magnitude and variability of solar irradiance improves our understanding of Earth’s climate, atmosphere, and oceans, and enables more accurate space weather forecasts. More accurate forecasts, in turn, can help protect people and satellites in space, as well as power transmission and radio communications on Earth.
The first five years of TSIS observations have demonstrated improved long-term spectral readings and lower uncertainty compared to measurements from NASA’s previous mission, the Solar Radiation and Climate satellite. The accuracy of TSIS observations can improve models of solar irradiance variability and contribute to the long-term record of solar irradiance.
ESA’s (European Space Agency) Solar Monitoring mission on the Columbus, or Solar, spacecraft has been collecting solar energy output data for more than a decade using three instruments covering most wavelengths of the electromagnetic spectrum. Different wavelengths of light emitted by the Sun are absorbed and affected by the Earth’s atmosphere and contribute to our climate and weather. This monitoring helps scientists see how solar radiation affects Earth and provides data to create models to predict its impact.
One of the instruments, the Solar Variable and Irradiance Monitor, covered the near ultraviolet, visible, and thermal parts of the spectrum and helped improve the accuracy of these measurements.
The SOLar SPECtral Irradiance Measurement instrument covers higher ranges of the solar spectrum. Its observations revealed significant differences from previous reference solar spectra and models. The researchers also reported that the repeated observations allowed them to determine a reference spectrum for the first year of the SOLAR mission that corresponded to the solar minimum before solar cycle 24.
Solar activity waxes and wanes in roughly 11-year cycles. The current solar cycle 25 began in December 2019, and scientists predicted solar activity would peak between January and October 2024, which included the May storms.
The third instrument, SOLar Auto-Calibrating EUV/UV Spectrometers, measured the portion of the solar spectrum between the extreme ultraviolet and the ultraviolet. Much of this high-energy radiation is absorbed by the upper atmosphere, making it impossible to measure from the ground. The results showed that these instruments could overcome the sensitivity degradation seen in other solar instruments and provide more efficient data collection.
For the Canadian Space Agency’s AuroraMAX project, crew members photographed the Northern Lights over Yellowknife , Canada, between the fall of 2011 and late spring of 2012. The space images, coordinated with a network of ground-based observatories across Canada, were used in an interactive exhibit at the Arts and Science Festival to spark public interest in how solar activity affects the Earth. The project also provides a live stream of the Northern Lights online each September through April.
The CubeSat miniature X-ray solar spectrometer measured variations in solar X-ray activity to help scientists understand how it affects Earth’s upper atmosphere. Solar X-ray activity increases during solar flares. Students at the University of Colorado’s Atmospheric Space Physics Laboratory built the satellite, which launched from the space station in early 2016.
Better data helps scientists understand how solar events affect satellites, human missions, and infrastructure in space and on Earth. Ongoing efforts to measure how Earth’s atmosphere responds to solar storms are an important part of NASA’s plans for the Artemis mission to the moon and subsequent missions to Mars.
ESA’s Solar Orbiter has for the first time linked measurements of the solar wind around a spacecraft with high-resolution images of the Sun’s surface at close range. This success opens a new avenue for solar physicists to study the source regions of the solar wind.
The solar wind is an endless rain of electrically charged particles emitted by the Sun. It is highly variable, changing its characteristics, such as speed, density, and composition, depending on what part of the Sun’s surface it comes from.
However, despite decades of research, some aspects of the solar wind’s origins remain poorly understood. And by the time the wind reaches Earth, many of its details are blurred, making it nearly impossible to trace it to specific areas on the sun’s surface.
As the solar wind moves through the solar system, it interacts with celestial bodies and spacecraft. These interactions range from the beneficial, such as the auroras on our planet, to the extremely destructive, where solar storms can interfere with or even damage electrical systems on Earth or in spacecraft.
Understanding the solar wind is therefore a priority for solar physicists. A key goal of the Solar Orbiter mission was to link the solar wind around the spacecraft to its sources on the Sun. This new result, using data from Solar Orbiter’s first close encounter with the Sun, shows that this is possible, a key goal of the mission, and a new way to study the origins of the solar wind.
Solar Orbiter can make these connections because it has both on-board and remote instruments. On-board instruments measure solar wind plasma and the magnetic field around the spacecraft, while remote-sensing instruments take images and other data about the Sun itself. The tricky part is that the cameras show the Sun as it appears now, while on-board instruments show the state of the solar wind that was released from the Sun’s surface days earlier. That’s because it takes time for solar wind particles to reach the spacecraft.
To link the two data sets, astronomers use online software called the Magnetic Connectivity Tool, which was developed to support the Solar Orbiter mission. The input data for the connectivity tool comes from the Global Oscillation Network Group, a series of six solar telescopes scattered around the world that constantly monitor oscillations on the sun’s surface. Based on these observations, a computer model calculates how the solar wind spreads throughout the solar system.
“You can predict where on the Sun’s surface you think Solar Orbiter will land a few days in advance,” says Stephanie Yardley of Northumbria University in the UK, who is lead author of a paper announcing the findings.
The team selected observation targets on the solar surface and used a magnetic communications instrument to predict when the spacecraft would fly through the solar wind that is released from these surfaces. Solar Orbiter’s unique suite of instruments, which includes both in-situ and remote sensing measurements, and its orbit that brings it close to the Sun, were specifically designed to allow this type of scientific communications to be attempted.
The data was collected between March 1 and 9, 2022, when Solar Orbiter was about 75 million km from the Sun, about half the distance of Earth from the Sun.
In general, there are two types of solar wind: fast solar wind, moving at speeds greater than 500 km/s, and slow solar wind, moving at speeds less than 500 km/s.
While the fast solar wind is known to originate from magnetic features known as coronal holes that funnel solar wind into space, the origins of the slow solar wind are still poorly understood. It is known to be associated with “active regions” on the Sun where sunspots appear, but the details are elusive. Sunspots are cooler regions in the Sun’s photosphere where intense magnetic fields become twisted and concentrated. They indicate active regions of the Sun, often responsible for solar flares and eruptions.
To prove the team’s ability to link the slow solar wind measured in situ to its origin on the Sun’s surface, the spacecraft had to fly through the magnetic field associated with the edge of either a coronal hole or a sunspot complex. This allowed the team to observe how the solar wind changed its speed — from fast to slow or vice versa — and other properties, confirming that they were looking at the right area. In the end, they got the perfect combination of both types of features together.
“Solar Orbiter flew past the coronal hole and the active region, and we saw fast solar wind flows followed by slow ones. We saw a lot of complexity that we could relate to the source regions,” says Stephanie. This included changes in the composition and temperature of these specific regions.
By analyzing the different solar wind streams detected by Solar Orbiter, the team clearly showed that the solar wind still has “fingerprints” left by its different source regions, making it easier for solar radiation physicists to trace the streams back to their origins on the Sun.
FURST will obtain the first high-resolution spectra of the “Sun as a star” in the vacuum ultraviolet (VUV), a wavelength of light that is absorbed by the Earth’s atmosphere, meaning it can only be observed from space. Astronomers have studied other stars in the vacuum ultraviolet using orbiting telescopes, but these instruments are too sensitive to be pointed at the Sun. Recent advances in high-resolution VUV spectroscopy now make it possible to make the same observations of our own star, the Sun.
What if we compared the Sun to other stars NASA has studied over the years? Would it still be so unique? The Full-sun Ultraviolet Spectrograph (FURST) rocket is designed to answer these questions when it launches aboard a Black Brant IX sounding rocket on August 11 from White Sands Missile Range in New Mexico. NASA and partners canceled the first attempt to launch the FURST Sounding Rocket mission on August 11 due to problems with its cooling systems, but are awaiting a more successful next attempt.
“When we talk about the ‘Sun as a star,’ we treat it like any other star in the night sky, not as a unique object that we rely on to sustain life on Earth. It’s so exciting to study the Sun from this perspective,” said Adam Kobelski, FURST’s principal investigator and a research astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama.
Because the Hubble telescope is too sensitive to point at Earth’s Sun, new instruments were needed to obtain a spectrum of the entire Sun that would be of similar quality to Hubble’s observations of other stars. NASA’s Marshall Space Flight Center built the camera, avionics, and designed and built a new calibration system for the FURST mission. Montana State University (MSU), which is leading the FURST mission in partnership with Marshall, built the optical system, which includes seven optical elements that will feed the camera, which will essentially create seven exposures covering the entire range of ultraviolet wavelengths.
Charles Kankelborg, a professor of heliophysics at Moscow State University and the principal investigator of FURST, described the mission as a very close collaboration with wide-ranging implications.
“Our mission will obtain the first far-ultraviolet spectrum of the Sun as a star,” Kankelborg said. “This is a key piece of information that has been missing for decades. With it, we will put the Sun in the context of other stars.”
FURST will be Marshall’s third launch for NASA’s Sounding Rocket Program in five months, making 2024 an active year for the program. Like the Hi-C Flare mission that launched in April, the sounding rocket will flare and open during flight to allow FURST to observe the Sun for about five minutes before closing and falling back to Earth’s surface. Marshall’s team members will be able to calibrate instruments during launch and flight, as well as retrieve data during the flight and shortly after landing.