H.3.8 Induced environments

H.3.8.1. Photo and secondary electrons

The electron flux at the spacecraft surface can be determined from the incident UV and primary electron fluxes, multiplied by the yield for the surface in question. Away from the emitting surface the density can be calculated from the following [RD.115]:

(H-5)

where

N is the density (cm^-3);

N₀ is the density at emitter (cm^-3);

z is the distance from surface;

λ₀ is the shielding distance, calculated as the Debye length due to the emitted electrons.

Table H-9 gives typical photoelectron sheath parameters [RD.55].

H.3.8.2. Ionization of contaminant gasses

Once neutral gas is released into space by whatever mechanism, it becomes subject to photoionization and dissociation by solar UV and ionization by charge exchange with solar wind ions. Production of new ions can be calculated from the appropriate photoionization rates and charge exchange crosssections (from [RD.57]).

(H-6)

where

Q is the production rate, ions s^-1;

N_i is the ion density;

ν is the photoionization rate coefficient;

n_sw is the solar wind density;

v_sw is the solar wind velocity;

σ is the charge exchange coefficient.

Photoionization rates depend on the atom or molecule concerned, and UV intensity and spectrum. Huebner and Giguere [RD.56] have tabulated a number of rate coefficients for different species, for sunlight at 1 AU. As an example, some photoionization rates for common gasses are listed in Table H-10.

For H₂O, where the charge exchange coefficient is around 2,1 ⨉10^-19 m^-2 [RD.57], photoionization and charge exchange are comparable processes. However, all species and dissociation products need to be considered to calculate the total production of emitted ions.