ESA and NASA Sent Two Spacecraft to the Sun. Together, They Successfully Solved a Long-Standing Puzzle: Why Solar Wind Doesn’t Cool Down

Space probes orbiting the Sun have helped astrophysicists solve a decades-old mystery: Where does the energy that heats and accelerates solar wind come from?

A decades-old mystery. Solar wind is a continuous flow of charged particles that emanate from the Sun. When these particles interact with our planet’s atmosphere, they can create auroras. Moreover, during more intense periods, solar storms can occur, impacting satellite communications and posing risks to astronauts.

After leaving the corona (the Sun’s outer layer), solar wind should cool and slow down as it expands, as expected from an expanding gas. However, decades of observations have shown that it doesn’t cool as fast as it should, and instead accelerates to speeds of 1.1 million mph. This suggests that something else is heating and accelerating it once it leaves the Sun.

Probes orbiting the Sun. The U.S. and Europe launched two spacecraft in 2018 and 2020 dedicated to studying the Sun. NASA’s Parker Solar Probe orbits very close to the star, specifically at the outermost edges of the solar corona, making it the fastest human-made object.

The Solar Orbiter from the European Space Agency (ESA) is farther away, but this distance has allowed scientists to compare measurements from both spacecraft as they passed through the same solar wind stream at different times, helping solve the mystery.

Alfvén waves. It’s important to understand Alfvén waves in the context of solar wind. When a gas is heated to extremely high temperatures, like those found in the Sun’s atmosphere, it becomes a plasma that is affected by magnetic fields. This results in the formation of magnetohydrodynamic waves that travel through the magnetic field lines of the magnetized plasma.

These waves can store and efficiently transport energy through the plasma. Swedish physicist Hannes Alfvén first predicted them in 1942, and he later received the Nobel Prize in Physics for his work in this field.

The solution to the puzzle. A recent study published in the journal Science has revealed that the Alfvén waves in solar wind have an amplitude large enough to provide the energy required to heat and accelerate solar wind flow.

The researchers compared measurements from the two spacecraft and confirmed that solar wind warms and accelerates due to the energy provided by Alfvén waves at different distances. These waves also dissipate as solar wind moves away from the Sun, converting some of their energy into heat which further warms solar wind.

Teamwork. This collaborative effort between NASA and ESA provides direct evidence that high-amplitude Alfvén waves play a significant role in heating and accelerating solar wind, explaining why it doesn’t cool as it moves away from the Sun. This confirms Alfvén’s suspicions, and the teamwork of the two space agencies made this discovery possible.

Image | European Space Agency

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