spaceSpace and Physics

The Solar Wind Is Hotter Than Expected And Now We May Know Why


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockApr 21 2020, 15:03 UTC

Artist impression of the solar wind and magnetic field interactions. NASA's Scientific Visualization Studio/Anil Rao/Univ. of Colorado/MAVEN/NASA GSFC

The Sun is constantly ejecting a stream of charged particles that we call the solar wind. As this plasma expands through space it cools down, but not nearly as much as the laws of thermodynamics predicts. A more complex effect might be at work and scientists think they finally have an idea what that is.

“People have been studying the solar wind since its discovery in 1959, but there are many important properties of this plasma which are still not well understood,” Stas Boldyrev, professor of physics at the University of Wisconsin – Madison and lead author of the study, said in a statement. “Initially, researchers thought the solar wind has to cool down very rapidly as it expands from the Sun, but satellite measurements show that as it reaches the Earth, its temperature is 10 times larger than expected. So, a fundamental question is: Why doesn’t it cool down?”


The idea is simple. As a gas expands it should cool down. Think of using a spray deodorant, for example. The compressed gas that was previously at room temperature expands and cools, and the canister has less gas (so less pressure) so also cools down. Pressure, volume, and temperature are all connected. So as the solar wind expands, if it was an ideal thermodynamic gas it should cool down in a certain way. 

As reported in the Proceedings of the National Academy of Sciences, the team proposes a scenario that explains why the solar wind doesn't follow the idealized case, pointing the finger at the electromagnetic interactions between the plasma components. Electrons, in particular, are to blame, they suggest.

Particles thrown out by the Sun are a mixture of positively charged ions and negatively charged very light electrons. These tend to escape very quickly away from our star, with the positive particles trailing behind. As the solar wind expands, it tends to have a bit of an electric charge, with a negative front and a positive rear. Given that opposite charges attract, some of the electrons, the ones moving slower, will slow down even more, becoming “trapped” in this expansion.


To test this, the team used a Mirror Machine, a type of fusion reactor. In this device plasma is kept in a chamber, the entrance and exit to which are bottlenecked, so the vast majority of particles are reflected back into the chamber (hence the name). The focus of the researchers was on those particles that did manage to escape, and how they were spreading the heat.

It turned out that the escaping population distributes the heat slowly to the trapped population so as they move quickly across the solar wind they don’t lose as much heat as a simple thermodynamic expansion would suggest.

“In the solar wind, the hot electrons stream from the Sun to very large distances, losing their energy very slowly and distributing it to the trapped population,” Boldyrev said. “It turns out that our results agree very well with measurements of the temperature profile of the solar wind and they may explain why the electron temperature declines with the distance so slowly.”


The team was impressed with how well the mirror machine recreated this solar wind scenario, and they will continue to use it as a proxy for it, hoping to discover some properties that solar astronomers can then look for in the solar wind.

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