Understanding the oblate shape of electron holes in magnetized plasmas

Regions of low electron density in magnetized plasmas, like the one surrounding Earth, are called plasma electron holes. These holes take on an oblate shape, shrinking along the direction of the magnetic field and stretching in the perpendicular direction. Ian Hutchinson shows that this effect cannot be understood through gyrokinetics, a simplified framework used to study magnetized plasmas, as previous studies have proposed. Hutchinson presents a theoretical argument supported by numerical simulation to show that the assumptions required for the gyrokinetic model don’t apply to electron holes.

Although electron holes have been predicted mathematically for decades, “their shape is only just beginning to be reliably measured,” Hutchinson said. Earlier studies speculated that the oblate aspect ratio of the electron holes is due to anisotropic shielding caused by gyrokinetics, which considers the average response of electrons in a magnetic field.

That speculation “was wrong and based on a misunderstanding of gyrokinetics,” said Hutchinson. Instead, he shows that certain implicit assumptions required by gyrokinetics don’t apply to electron holes. Without these assumptions, the anisotropic shielding predicted by gyrokinetics doesn’t appear.

Hutchinson verifies this finding computationally, using a particle in cell simulation to model electron holes in a volume of plasma. In these simulations, “the supposed anisotropic shielding didn’t occur,” he said.

Instead, he argues, hole shape is a result of spatial anisotropy of the trapped particle charge deficit.

Recent multi-satellite missions might provide spatially resolved data to verify Hutchinson’s findings. Soon, he said “we can begin to see observationally what the full story is about the shape of these plasma holes. With luck, this theory is slightly ahead of the observation.”

Source: “Oblate electron holes are not attributable to anisotropic shielding,” by I. H. Hutchinson, Physics of Plasmas (2021). The article can be accessed at https://doi.org/10.1063/5.0039233 .