Scientists can now observe how particles heat up

Scientists have now received a closer look at how protons in solar winds are heated, since solar wind is much hotter than expected, and there are several theories explaining why with no real evidence to back them up. But an expedition to the Sun has provided the evidence needed.

Fouad Sahraoui, lead author of the study and a scientist at the CNRS-Ecole Polytechnique-UPMC in France, along with co-author Melvyn Goldstein, chief of the Geospace Physics Laboratory at NASA's Goddard Space Flight Center in Greenbelt, Maryland, have made it possible to observe and understand how the solar wind is millions of degrees Celsius. 

This solar wind surrounds the Sun and is made up of electrons and protons that are constantly turbulent. This turbulence can result in long streaming jets, or much smaller movements where charged particles create miniature orbits. In the midst of this chaos, electric currents and magnetic waves pass through disturbing the particles further. The solar wind can move quickly at 750 kilometers per second. Combined with its temperatures at millions of degrees Celsius, the solar wind defies all theories - until now.

To further observe the solar wind, the ESA/NASA Cluster was sent into space. The Cluster consists of four identical spacecraft flying together in formation in order to obtain 3-dimensional snapshots of the particles surrounding the Sun. 

"We had a perfect window of 50 minutes," said Goldstein. "It was a time when the four Cluster spacecraft were so close together they could watch movements in the solar wind at a scale small enough that it was possible to observe the heating of protons through turbulence directly for the first time."

The heating process works similarly to ocean waves, where turbulence "cascades" into smaller turbulence. The energy in ocean wave cascades adds a small amount of heat from the friction of particles passing one another, which warms the water a bit. In the case of the solar wind, charged particles heat up in a similar way but do not encounter this type of friction. 

"Unlike the usual fluids of everyday life, plasmas possess electric and magnetic fields generated by the motions of protons and electrons," said Sahraoui. "This changes much of the intuitive images that we get from observing conventional fluids."

While the exact nature of the waves doing the heating and the exact methods of energy transfer are not "completely established," scientists did find that electric and magnetic fields in the plasma play major roles in the particles' heating. With the help of previous solar wind data indicating the effects and length of the magnetic waves, scientists were able to conclude that the magnetic waves begin as long wavelength fluctuations, eventually losing energy as they get shorter over a certain time period. This loss of energy transfers energy to solar wind particles, which heats them up. 

"We can see that not all the energy is dissipated by protons," said Sahraoui. "The remaining energy in the wave continues its journey toward smaller scales, wavelengths of about two kilometers long. At that point, electrons in turn get heated."

This study was published in Physical Review Letters

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