Auroras have long dazzled sky watchers but befuddled physicists for a very long time. Their exact cause remains unclear, but it is widely believed that electrons get yanked away from the magnetosphere and smashed into the ionosphere, where this collision of atoms and molecules engender green, red, and blue lights in the sky, as discovered by Robert J. Strangeway. The main reason for scooping up and accelerating auroral electrons are ‘Alfvén waves, which are simply the ionic oscillations in plasma that proliferate along the magnetic field lines. A major triggering factor of these oscillations are magnetic storms, that herald auroral displays, similarly, it has been theoretically believed that these waves should be able to propagate electrons in their path.
A major question that scientists were trying to resolve was, ‘Why do electrons accelerate to Earth where they trigger Auroras? Thus, producing such a wonderful display of colorful and characteristic light in the polar skies.
The study published in the journal Nature Communications by James Schroeder and a team of physicists at the University of Iowa concludes a decade-long quest to validate experimentally, exact physical mechanisms for the acceleration of electrons by Alfvén waves under conditions that correspond to Earth’s auroral magnetosphere.
To prove this a research team lead by Jim Schroeder of Wheaton College used the Large Plasma Device (LAPD) at the University of California to get a closer look at the phenomenon using a 20-meter long vacuum cylindrical chamber with a diameter of 1 meter, with a strong magnetic field. Physicist Troy Carter said, “This challenging experiment required a measurement of the very small population of electrons moving down the LAPD chamber at nearly the same speed as the Alfvén waves, numbering less than one in a thousand of the electrons in the plasma”.
The team successfully produced Alfvén waves in the plasma inside the LAPD, and concurrently calculated the distribution of electron velocity under conditions relevant to the formation of auroras, thus proving that Alfvén waves transfer energy to electrons with resonance with the waves, with a ‘phase velocity.
The calculations exposed that this tiny population of electrons undergo ‘resonant acceleration’ by the electric field of the Alfvén waves, just like a surfer catching a wave and being continually accelerated as the surfer moves along the wave. This process is called ‘Landau damping’ because the transfer of energy from the wave to the particle dampens the wave, preventing any instability from emerging. Also, the signature generated by the electron velocity was the signature of Landau damping, suggesting the occurrence of resonance acceleration.
To prove their finding, the team compared their conclusions to a model aurora, demonstrating that the energization rate of the electron was consistent with Landau damping in real life.
In conclusion, the observed oscillations are fundamentally related to the mechanism that’s thought to drive auroras: resonant electron acceleration. If electrons can be jolted bypassing Alfvén waves, then they should be able to get picked up and boosted by the plasma disturbances as well. Schroeder and colleagues are designing follow-up experiments at UCLA to hunt for electrons surfing Alfvén waves.
Auroral phenomena, electrostatics, magnetic storms, plasma, planetary magnetic fields, Aurora, plasma physics, phase velocity, large plasma device