What has the Rosetta probe taught us about comet tails?

Comets are distinct among celestial objects for their bright tails, created from a cloud of surrounding dust and ionized particles called a coma. The interactions of electrons and protons in this cometary atmosphere are little understood by scientists. Using measurements from the space probe Rosetta, which took information on Kuiper-belt-originating comet 67P/Churyumov-Gersimenko in 2014, Sishtla et al. simulated and analyzed electron trajectories in the coma of a weakly outgassing comet.

The researchers discovered the presence of a phenomenon known as “electron trapping.” They found that as a comet moves through the magnetic field from the sun, an electric field is generated which then traps and accelerates passing electrons. The scientists believe this phenomenon could be occurring in planetary magnetospheres and solar corona as well.

“Our work is a small step forward to a better understanding of cometary physics,” said author Stefano Markidis. “The same trapping and acceleration mechanisms we identified in our work might be present in proximity other larger astrophysical objects such as moons and planets.”

The team simulated the trajectories of a billion electrons and ions in a cometary atmosphere. Among the billion particles, they analyzed and identified trajectories which showed the boundary between the low-energy trapped electrons and the passing electrons which may be accelerated along the magnetic field.

So far, the computer only modelled a weakly-outgassing comet, where the particles do not collide frequently. “We are now studying how the trapping and acceleration mechanisms vary when including particle collisions in our simulations and changing the number of particles ejected by the comet,” said Markidis. These future simulations will apply to comets with a higher outgassing rate and with a higher density coma.

Source: “Electron trapping in the coma of a weakly outgassing comet,” by Chaitanya Prasad Sishtla, Andrey Divin, Jan Deca, Vyacheslav Olshevsky, and Stefano Markidis, Physics of Plasmas (2019). The article can be accessed at https://doi.org/10.1063/1.5115456 .