Parker Solar Probe uncovers mystery of 'fast' solar winds

And just in time for the solar maximum, when wind-generating coronal holes like to point right at Earth

Earthlings have unraveled yet another solar mystery after NASA's Parker Solar Probe managed to detect what scientists believe to be structures inside coronal holes responsible for "fast" solar winds.

A team of researchers led by UC Berkeley's Stuart Bale and James Drake of the University of Maryland-College Park said they've detected streams of high-energy particles that match the supergranulation flows within the Sun's coronal holes.

On most of the Sun's surface the corona keeps magnetic field lines looping back onto the surface. Holes in the corona are spaces where the field lines don't loop back, instead ejecting high-energy particles into space at intense speeds. We already knew that, but to understand more we had to fly closer to the Sun than even the Parker probe had managed to make it until recently.

"It's like seeing jets of water emanating from a showerhead through the blast of water hitting you in the face," the institution said of the winds being generated at the surface.

By identifying areas of supergranulation – large convection cells where the fast solar winds actually originate – Bale and his team believe they've also determined how the fast solar winds are actually being formed, through a concept called magnetic reconnection.

The convection cells, Bale explained, meet and "drag the magnetic field in their path into this downward kind of funnel. The magnetic field becomes very intensified there because it's just jammed," he said.

That jamming effect is where the bursts of energy that cause the fast solar wind get their start, the team believes. "It doesn't just come from everywhere in a coronal hole, it's substructured within coronal holes to these supergranulation cells. It comes from these little bundles of magnetic energy that are associated with the convection flows," Bale said.

"The big conclusion is that it's magnetic reconnection within these funnel structures that's providing the energy source of the fast solar wind," Bale said.

ESA's own Solar Orbiter late last year identified the origin of solar switchbacks, furthering our understanding of contrasting "slow" solar winds.

Just in time for the solar maximum

The discovery of how fast solar winds form couldn't come at a better time because the Sun's 11-year activity cycle is approaching its most energetic period, meaning more solar storms that could be a major problem for our increasingly electrified society.

The coronal holes where fast solar winds originate are typically confined to the Sun's poles, and so don't usually pose much of a threat to Earth. When the Sun's magnetic field flips every 11 years during its active period, however, coronal holes appear all over its surface, and could be aimed right at us.

"Understanding the mechanism behind the Sun's wind … affects our ability to understand how the Sun releases energy and drives geomagnetic storms, which are a threat to our communication networks," Drake warned.

The Parker Solar Probe is on a slow approach course to the Sun's surface and will make its closest approach of just under 4 million miles in the peak of the solar maximum in 2025, at which point it will be fried by the star. Between now and then Bale and his team hope to gather evidence needed to solidify their conclusions, but they admit the approaching solar maximum will make things a bit unpredictable.

"There was some consternation at the beginning of the solar probe mission that we're going to launch this thing right into the quietest, most dull part of the solar cycle. But I think without that, we would never have understood this. It would have been just too messy," Bale concluded. ®

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