The solar wind is ever enticing, providing as it does a highly variable stream of charged particles moving out from the Sun at speeds up to 800 kilometers per second. Finding ways to harness that energy for propulsive purposes is tricky, although a good deal of work has gone into designs like magsails, where a loop of superconducting wire creates the magnetic field needed to interact with this ‘wind.’ But given its ragged variability, the sail metaphor makes us imagine a ship constantly pummeled by gusts in varying degrees of intensity, constantly adjusting sail to maintain course and stability. And it’s hard to keep the metaphor working when we factor in solar flares or coronal mass ejections.

We can lose the superconducting loop if we create a plasma cloud of charged particles around the craft for the same purpose. Or maybe we can use an electric ‘sail,’ enabled by long tethers that deflect solar wind ions. All of these ideas cope with a solar wind that, near the Sun, may be moving at tens of kilometers per second but accelerating rapidly with distance, so that it can reach its highest speeds at 10 solar radii and more. Different conditions in the corona can produce major variations in these velocities.

Obviously it behooves us to learn as much as we can about the solar wind even as we continue to investigate less turbulent options like solar sails (driven by photon momentum) and their beam-driven lightsail cousins. A new paper in Science is a useful step in nailing down the process of how the solar wind is energized once it has left the Sun itself. The work comes out of the Smithsonian Astrophysical Observatory (SAO), which is part of the Center for Astrophysics | Harvard & Smithsonian (CfA), and it bores into the question of ‘switchbacks’ in the solar wind that have been thought to deposit energy.

At the heart of the process are Alfvén waves, named after Hannes Alfvén (1908-1995), a Nobel-winning Swedish scientist and engineer whose work is at the heart of the discipline known as magnetohydrodynamics. What Alfvén more or less defined was the study of the interactions of magnetic behavior in plasmas. Alfvén waves move along magnetic field lines imparting energy and momentum that nourishes the solar wind. Kinks in the magnetic field known as ‘switchbacks’ are crucial here. These sudden deflections of the magnetic field quickly snap back to their original position. Although not fully understood, switchbacks are thought to be closely involved with the Alfvén wave phenomenon.

Image: Artist’s illustration of the solar wind flowing from the Sun measured by Parker Solar Probe near the edge of the corona and later with Solar Orbiter at a larger distance during a spacecraft alignment. The solar wind contains magnetic switchbacks, or large amplitude magnetic waves, near Parker Solar Probe that disappear farther from the Sun where Solar Orbiter is located. Credit: Image background: NASA Goddard/CIL/Adriana Manrique Gutierrez, Spacecraft images: NASA/ESA.

Data from two spacecraft have now clarified the role of these switchbacks. The Parker Solar Probe readily detected them in the solar wind, but data from ESA’s Solar Orbiter mission added crucial context. The two craft, one designed to penetrate the solar corona, the other working at larger distances, came into alignment in February of 2022 so that they observed the same solar wind stream in the scope of two days of observations. CfA’s Samuel Badman is a co-author of the study:

“We didn’t initially realize that Parker and Solar Orbiter were measuring the same thing at all. Parker saw this slower plasma near the Sun that was full of switchback waves, and then Solar Orbiter recorded a fast stream which had received heat and with very little wave activity. When we connected the two, that was a real eureka moment.”

So we had a theoretical process of energy movement through the corona and the solar wind in which Alfvén waves transported energy, but now we have data charting the interaction of the waves with the solar wind over time. The authors argue that the switchback phenomena pumps energy into the process of heating and acceleration sufficient to drive the fastest streams of the solar wind. Indeed, John Belcher (MIT), not a part of the study, considers this to be a ‘classic paper’ that demonstrates the fulfillment of one of the Parker Solar Probe’s main goals.

Such work has ramifications that will be amplified over time as we continue to investigate the environment close to the Sun and the solar wind that grows out of it. The findings will help clarify how future craft might be designed to take advantage of solar wind activity, but will also provide insights into the behavior of any sailcraft we send into close solar passes to achieve high velocity gravitational slingshots to the outer system. Always bear in mind that heliophysics plays directly into our thinking about the system’s outer edges and the evolution of spacecraft designed to explore them.

The paper is Rivera et al., “In situ observations of large-amplitude Alfvén waves heating and accelerating the solar wind,” Science Vol 385, Issue 6712 (29 August 2024), p. 962-966 (abstract).