Image credit: ESA/NASA
The scientists propose that the electron component of the solar wind plasma is heated by electrons streaming from the hot solar corona, and slowly lose their energy due to weak Coulomb collisions. Their theory reveals the role of the non-Maxwellian electron distribution function in the heating mechanism, and it may be valuable for interpreting the results of in situ satellite measurements.
Solar plasma is a molten combination of negatively-charged electrons and positively-charged ions. This hot plasma flows through space as solar wind as it escapes from the Sun's outer atmosphere, or Corona. The electrons, being lighter than ions, move around 40 times faster.
As more negatively-charged ions move away, the Sun takes on a positive charge, making it difficult for electrons to escape the Sun's pull. Electrons with less energy cannot escape the Sun's positive charge, so they are attracted back to the star. Some of the electrons may be knocked off their paths slightly by collisions with surrounding plasma.
To help explain the solar wind's temperature observations, the researchers developed "mirror machines" or plasma-filled magnetic field lines, formed as tubes with pinched ends. The pinch at the end acts like a mirror that reflects the particles back into the machine. But some particles can escape, and when they do, they stream along expanding magnetic field lines outside the bottle.
It was found that the very hot electrons escaping the bottle were capable of distributing their heat energy to trapped electrons. 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. It turns out that the results of the experiment agree very well with measurements of the temperature profile of the solar wind, and they may explain why the electron temperature declines so slowly with the distance.