Cold and warm electrons at comet 67P/Churyumov-Gerasimenko

Abstract

Context. Strong electron cooling on the neutral gas in cometary comae has been predicted for a long time, but actual measurements of low electron temperature are scarce. Aims. Our aim is to demonstrate the existence of cold electrons in the inner coma of comet 67P and show filamentation of this plasma. Methods. In situ measurements of plasma density, electron temperature and spacecraft potential were carried out by the Rosetta Langmuir probe instrument, LAP. We also performed analytical modelling of the expanding two-temperature electron gas. Results. LAP data acquired within a few hundred km from the nucleus are dominated by a warm component with electron temperature typically 5-10 eV at all heliocentric distances covered (1.25 to 3.83 AU). A cold component, with temperature no higher than about 0.1 eV, appears in the data as short (few to few tens of seconds) pulses of high probe current, indicating local enhancement of plasma density as well as a decrease in electron temperature. These pulses first appeared around 3 AU and were seen for longer periods close to perihelion. The general pattern of pulse appearance follows that of neutral gas and plasma density. We have not identified any periods with only cold electrons present. The electron flux to Rosetta was always dominated by higher energies, driving the spacecraft potential to order −10 V. Conclusions. The warm (5-10 eV) electron population observed throughout the mission is interpreted as electrons retaining the energy they obtained when released in the ionisation process. The sometimes observed cold populations with electron temperatures below 0.1 eV verify collisional cooling in the coma. The cold electrons were only observed together with the warm population. The general appearance of the cold population appears to be consistent with a Haser-like model, implicitly supporting also the coupling of ions to the neutral gas. The expanding cold plasma is unstable, forming filaments that we observe as pulses.

Electrostatic Potential Radiated by a Pulsating Charge in a Two‐Electron Temperature Plasma

“…For this case, with the sensitivity of the comparison method, n h =1500 ± 150 cm −3 , n c ≃2730 ± 150 cm −3 , T h =6 ± 1.5 eV, and T c =0.06 ± 0.01 eV. The derived temperatures are consistent with the typical temperatures of warm and cold cometary electrons measured with the Langmuir Probe during the Rosetta mission (Eriksson et al, ).…”

Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P

“…Finally, we have found that a population of cold ( 0.1 eV) electrons, first shown by Eriksson et al () to be intermittently present at Rosetta, is in fact observed consistently throughout the diamagnetic cavity. Already immediately outside the cavity, sweeps lacking a signature of cold electrons begin to turn up intermittently.…”

“…In addition to these warm electrons, clear signatures of cold ( 0.1 eV) electrons have also been observed by LAP (Engelhardt et al, ; Eriksson et al, ) and MIP (Gilet et al, ). In LAP, these show up in high‐time‐resolution current measurements at fixed bias voltage in the form pulses of typical duration between a few seconds and a few minutes, and in bias voltage sweeps in the form of very steep slopes in the current‐voltage curve at high positive bias voltages (to be discussed further below).…”

Plasma density structures at comet 67P/Churyumov–Gerasimenko

“…In this paper, we investigate the pulses of high plasma density observed outside the diamagnetic cavity by Eriksson et al (2017) while Hajra et al (2018) studied plasma density pulses inside the diamagnetic cavity. These pulses, where the density is higher than the background density have a relatively short duration, typically a few seconds to a few tens of seconds, as seen from the spacecraft.…”