The solar wind—a stream of charged particles blowing off the sun’s surface—affects the entire solar system. Yet its behavior remains largely enigmatic. Now a new effort has provided the most detailed long-term measurements ever made of the solar wind, helping researchers to model its ever-changing nature while assisting missions such as NASA’s recently-launched Parker Solar Probe that aim to further investigate solar wind secrets.
Heliophysicists have been observing the solar wind ever since 1959, when the Soviet Union’s Luna 1 spacecraft directly measured it while en route to the moon. Modern-day efforts rely on probes such as the Solar and Heliospheric Observatory (SOHO) that continuously monitor interplanetary space weather. But few studies have looked at the large-scale aspects of solar wind streams for more than a few days.
An international team, led by Korean researchers, downloaded data from the SOHO satellite between 1999 and 2010, corresponding to the length of the sun’s natural 11-year activity cycle, and employed image-processing algorithms to identify currents and jets within the solar wind, as well as measure its speed. They were able to monitor the solar wind out to 18 million kilometers (11 million miles), or about a third of the way to Mercury, and observe daily changes as well as year-long alterations over the sun’s entire surface. Their findings recently appeared in Physical Review Letters.
In the 1990s, NASA’s Ulysses spacecraft became the first probe to fly over the sun’s poles. Once it left the plane of Earth’s orbit, Ulysses discovered that the solar wind—which arrives as a chaotic mish-mash of fast and slow torrents at our planet—actually smoothed out into a uniformly fast stream when emanating from the sun’s highest and lowest latitudes. The effect was only seen during solar minimum, when the sun’s activity ebbs to a low point. The reason for this bizarre bimodal structure has never been explained.
The new detailed observations confirmed Ulysses’ findings and will help in constructing physical models of the solar wind that may give hints as to why it behaves this way, says heliophysicist Il-Hyun Cho of Kyung Hee University in Korea, lead author of the new paper.
The solar wind’s high speed presents another mystery. While the sun’s surface is a scorching 6,000 degrees Kelvin, its outermost atmosphere is even hotter, reaching one million degrees Kelvin. Astrophysicist Eugene Parker of the University of Chicago in Illinois, for whom the Parker probe is named, realized back in 1958 that the solar wind couldn’t just sit in such blazing temperatures and do nothing. It would be accelerated to supersonic speeds.
Probes have confirmed Parker’s prediction but also shown that the solar wind is far more complex than his simple models suggested. After leaving the sun, some solar wind streams reach speeds up to 2.7 million kilometers per hour (1.67 million mph) but the exact mechanism behind this acceleration is not well understood.
Cho and his colleagues found that the acceleration seems to be most intense somewhere within about 10 solar radii (6 million kilometers) from the sun, a region that will be directly explored by the Parker spacecraft in 2025. “I’m happy they’re seeing that,” says astrophysicist Justin Kasper of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts and principal investigator on one of Parker’s instruments. “This will be very easy for us to test.”
These results aren’t merely academic. The solar wind interacts with Earth’s magnetosphere and upper atmosphere and affects communication satellites in orbit. Cho hopes the measurements will allow researchers to make better predictions of space weather events, such as coronal mass ejections that explode from the sun’s surface, since the exact timing of when such phenomena reach the Earth depends in part on background solar wind speeds.