Cold electrons in comets ejecting weak gas | Monthly Notices of the Royal Astronomical Society

Throughout the Rosetta mission, cold electrons (<1eV) were measured in the coma of comet 67P/Churyumov-Gerasimenko. Comet electrons are produced by photolysis at ∼10eV or by electron-impact ionization collisions. The colder electron population is formed by cooling the hotter population through inelastic electron-neutral collisions. Assuming radial electron ejection, the electrons collide with the neutral gas coma below the electron exophase, which forms only in perihelion, high-emission conditions near the cometary surface.K > 3 × 10²⁷s⁻¹) however, cooler populations were identified at lower gas emissions (K < 10²⁶s⁻¹), when the internal coma is not expected to be collisional. Using a 3D collisional model of electrons in a comet, we investigate the cooling of electrons in a comet ejecting a weak gas. Electron trajectories are extended by being trapped in an ambipolar electric field and rotating around magnetic field lines. This increases the probability that electrons undergo inelastic collisions with the coma and become cooler. We demonstrate that a cold electron population can form and persist under weak outgassing conditions (K = 10²⁶s⁻¹), once the 3D electron dynamics is calculated. Cold electrons are produced in the inner coma by electron-neutral collisions and transported towards the tail. E × B We measure the effectiveness of the problem in driving drift electron cooling, typically 100 times higher than trajectories expected from ballistic radial ejection. Based on collisional simulations, we define an estimate for a region where a cold electron population can form, bounded by an electron-cooling exophase. This estimate is in good agreement with cold electron measurements by the Rosetta Plasma Consortium.