# Cold and warm electrons at comet 67P

## TL;DR

This study presents in situ measurements of electron temperatures at comet 67P, revealing persistent warm electrons and intermittent cold electron pulses, which support models of collisional cooling and plasma-neutral coupling in the comet's coma.

## Contribution

First in situ measurements showing coexistence of warm and cold electrons at comet 67P, confirming collisional cooling and plasma filamentation predicted by models.

## Key findings

- Warm electrons (~5-10 eV) dominate the plasma.
- Cold electron pulses appear near perihelion and are short-lived.
- Cold electrons are always observed with warm electrons, indicating collisional cooling.

## Abstract

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. We present in situ measurements of plasma density, electron temperature and spacecraft potential by the Rosetta Langmuir probe instrument, 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. The warm (5--10 eV) electron population 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.