Waves could be heating up the solar wind, and two spacecraft caught them in action

Waves traveling through the solar wind could hold the key to solving a decades-long mystery, according to an analysis of satellite observations.

Small jets of plasma are seen streaming out from the Sun in this image taken by the NASA/ESA Solar Orbiter spacecraft. Credit: ESA and NASA/Solar Orbiter/EUI Team

Since the 1950s, scientists have known that the Sun emits a stream of particles in the form of a huge wind that carries away the Earth's mass every 150 years. This proton wind sweeps past comets, sculpting their tails into windsocks millions of miles across. Near Earth's orbit, the solar wind fills a volume equivalent to that of a sugar cube with five to ten particles that travel outward at speeds of between 250 and 700 kilometers per second (559,000 and 1.57 million miles per hour).

In recent decades, scientists have learned a lot about this wind, including what drives it. Satellite observations have revealed myriad tiny flares near the Sun’s surface, when the star’s magnetic field lines curl and snap like rubber bands. This release of energy can heat the Sun’s outer atmosphere (or corona) to about 2 million degrees Fahrenheit (one million degrees Celsius), propelling the solar wind far into space.

Yet some mysteries of the wind stubbornly persist. Among them is a temperature anomaly: The laws of physics dictate that as the wind moves away from the Sun and expands outward, it should slow down and cool. But observations indicate that the wind is not cooling as much as it should. The energy balance was off: some unexplained source is acting like a heating zone in a stove, preventing the solar wind from cooling as much as expected.

An analysis by Yeimy Rivera of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and colleagues, published today in Sciencesuggests that this energy puzzle may be solved. The key ingredient is the waves propagating outward through the solar wind and the energy they carry and release.

Making waves

We are all familiar with sound waves from everyday life. A powerful sound wave of more than 150 decibels (such as from a jet engine or a firecracker) carries enough energy to physically rupture our eardrums. Ordinary sound waves are carried by ordinary gases or even solid matter.

But when it comes to a plasma, electric currents and magnetic fields can pass through the gas, creating different kinds of waves. Plasmas can be set in motion, with their charged particles bouncing back and forth under the influence of electricity and magnetism. Plasma waves come in many forms, but one type can be visualized as magnetic field lines vibrating, like a guitar string being plucked. These oscillations are called Alfvén waves, after Hans Alfvén, who received the Nobel Prize in Physics in 1970 for discovering them.

Scientists have long wondered whether Alfvén waves could explain the mysterious heating of the solar wind. Perhaps the natural dissipation of Alfvén waves releases energy into the solar wind, like ocean waves crashing against the shore.

Rivera and his colleagues were able to test this hypothesis with data from two satellites orbiting and monitoring the Sun: NASA’s Parker Solar Probe and the European Space Agency/NASA Solar Orbiter mission. In February 2022, this duo reached a rare alignment, with Parker Solar Probe close to the Sun (within its outer corona) and Solar Orbiter farther away and directly downwind, near the orbit of Venus. This allowed scientists to observe the arrival of Alfvén waves at Parker Probe and then observe those same waves as they passed by Solar Orbiter, 45 hours later.

Parker Solar Probe detected large Alfvén waves, called “switchbacks,” in which the local magnetic field produced by sources closer to the Sun’s surface rapidly changes direction. But by the time they reached Solar Orbiter, these waves were much weaker. By comparing the data, the team calculated that the dissipated energy matched the expected warming of the solar wind.

With this discovery, the overall dynamics of the solar wind may have been resolved. However, nature often achieves the same ends using many different approaches, so what has worked in one particular corner of the solar wind may not apply everywhere in a system as vast as the solar system.