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Our planet is constantly bathed in winds coming out of the scorching sphere at the center of our Solar System. But even though the Sun itself is so ridiculously hot, once the solar winds reach Earth, they are warmer than they should be, and we could finally know why.
We know that the particles that make up the plasma of the Sun’s heliosphere cool as they spread. The problem is that they seem to take their sweet time doing it, lowering the temperature much more slowly than the models predict.
“People have been studying the solar wind since its discovery in 1959, but there are many important properties of this plasma that are not yet well understood,” says physicist Stas Boldyrev of the University of Wisconsin-Madison.
“Initially, the researchers thought that the solar wind has to cool down very quickly as it expands from the Sun, but satellite measurements show that as it reaches Earth, its temperature is 10 times higher than expected.”
The research team used laboratory equipment to study plasma in motion, and now believes the answer to the problem lies in a sea trapped with electrons that can’t seem to escape the grip of the Sun.
It has long been assumed that the expansion process itself is subject to adiabatic laws, a term that simply means that thermal energy is not added to or removed from a system. This keeps the numbers nice and simple, but assumes that there are no places where energy enters or leaves the particle stream.
Unfortunately, an electron’s journey is anything but simple, pushed at the mercy of vast magnetic fields like a roller coaster from hell. This chaos leaves many opportunities for heat to pass from one side to the other.
To further complicate matters, thanks to their small mass, electrons gain a good advantage over heavier ions as they exit the Sun’s atmosphere, leaving a largely positive cloud of particles in their wake.
Finally, the increasing attraction between the two opposite charges takes over the inertia of those flying electrons, taking them back to the starting line where the magnetic fields once again wreak havoc on their paths.
“Such returning electrons are reflected away from the Sun, but again cannot escape due to the Sun’s attractive electrical force,” says Boldyrev.
“Therefore, their destiny is to bounce back and forth, creating a large population of the so-called trapped electrons.”
Boldyrev and his team recognized a similar electronic ping-pong game in their own lab, within a commonly used plasma-studying apparatus called a mirror machine.
A linear fusion reactor, or ‘mirror machine’. (Cary Forest)
Mirror machines don’t actually contain any mirrors. At least, not the bright, familiar type. Also known as magnetic mirrors or magnetic traps, these linear fusion devices are little more than long tubes with a bottleneck at each end.
Its reflective nature is created as the plasma currents passing through the bottle pinch at each end, altering the surrounding magnetic fields in such a way that the particles within the stream are reflected back inside.
“But some particles can escape, and when they do, they flow along expanding magnetic field lines outside the bottle,” says Boldyrev.
“Because physicists want to keep this plasma very hot, they want to find out how the temperature of the electrons escaping from the bottle outside this opening decreases.”
Or if you are Boldyrev and his team, those leaking electrons can be studied to better understand what is happening with our own solar wind.
He and his colleagues suggest that the population of trapped electrons in which the yo-yo plays an important role in the way in which the electrons distribute their thermal energy, changing the typical distributions of the speeds and temperatures of the particles in a predictable way.
“It turns out that our results agree very well with measurements of the solar wind temperature profile and may explain why the temperature of the electrons decreases with distance so slowly,” says Boldyrev.
Finding such a good match between the figures of the mirror machine and what we see in space suggests that there could be other solar phenomena worth studying in this way.
This research was published in PNAS.
