The Electron to Neutral Number Density Ratio as a Proxy for Processes at Play in the Coma of 67P

Abstract:

The electron to neutral number density ratio, n_e/n_N, can provide insights into processes at play in the inner coma of comet 67P. Based on a field free and chemistry free radial expansion model and from computed photoionization frequencies of dominant cometary neutrals we derive a benchmark expression of n_e/n_N as a function of cometocentric distance, r, and heliocentric distance, d. For mid-October 2014 when r=10 km and d=3.1 AU the model gives a ratio of n_e/n_N = 9.8×10^-7, which is in good agreement with the ratios of 1-2×10^-6 derived from observations by the dual Langmuir probe (LAP), the Mutual Impedance Probe (MIP) and the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA). Further comparisons with observations will be presented at the meeting. We discuss various effects/processes that can cause deviations between the benchmark n_e/n_N values and the observations. In particular, the presence of electric fields (e.g., the solar wind’s convective electric field and the ambipolar electric field) - accelerating the cometary ion population – can act to markedly reduce ratios from the benchmark values. On the other hand, additional plasma sources (electron impact ionization and charge exchange with the solar wind) act in the opposite way, i.e., towards increasing ratios. Near perihelion, and in particular in the near nucleus environment (tens of km), these particular effects are anticipated to be of less significance, while other processes become increasingly important. In particular, with a denser neutral background collisional electron cooling becomes more efficient, reducing the electron temperature and increasing the efficiency of dissociative recombination as a plasma neutralizing process. This, combined with solar EUV attenuation and possibly also nanograin charging, can set n_e/n_N ratios well below the benchmark values.