Two spacecraft have captured waves that can heat and accelerate the solar wind.

Since the dawn of the space age, when robotic probes first left our atmosphere, scientists have known that the solar wind—a stream of charged particles emitted from the Sun’s atmosphere—accelerates as it blows into the solar system.Serial Number: August 18, 2017) Theoretical calculations show that the temperature of the solar wind will drop as it expands into space. This drop does occur, but measurements show that it occurs more slowly than expected.

Observations from Earth have previously found Alfvén waves oscillating near the Sun. These waves are oscillations in the magnetic field of plasma coming from the Sun. Sometimes they are so large that they flip over in what is called a “switchback.”Date January 15, 2021) The observed Alfvén waves had enough energy to explain two long-standing mysteries about the speed and temperature of the solar wind, but direct evidence was still lacking.

Enter Parker Solar Probe and Solar Orbiter. In late February 2022, Parker was passing through a region about one-fifth the distance between the Sun and Mercury, right where the switchbacking Alfvén waves are fluttering. Coincidentally, Solar Orbiter passed through the same plasma stream two days later in the orbit of Venus.

“Both spacecraft intercepted the same solar wind, allowing us to quantify the energy of these waves,” said Jamie Rivera, a heliophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Parker measured the plasma stream to be moving at about 1.4 million kilometers per hour, while Solar Orbiter found it to be moving at 1.8 million kilometers per hour. The Solar Orbiter plasma was also about 200,000 degrees Celsius, three times hotter than theoretical estimates would suggest. The Alfvén waves disappeared in the meantime. This disappearance would have injected just the right amount of energy into the solar wind to account for the speed and temperature increases measured by Solar Orbiter, Rivera and her colleagues calculated.

Sam Badman, a solar physicist at the Center for Astrophysics, says the effect is similar to flapping your hands in a wind tunnel, creating waves that mix with the surrounding air.

But not everyone is entirely convinced that the mystery has been solved. Some scientists say the team may not have taken into account the complexity of the solar wind, which means the two probes may not have intercepted the same plasma stream.

Rivera and Badman agree that these measurements are difficult, but they believe they have provided several confirmations of their observations, including finding exactly the same amount of helium in the stream the spacecraft passed through. The researchers say they plan to further confirm their findings by exploring the detailed physics of the energy transfer between Alfvén waves and the solar wind.