4,5 . 1021 photons / (m². sec) - Surrounded by lonely atoms of: O / N / He / H - Environment: ρ ≈ 0 kg/m³ à p ≈ 0 Pa - SolarWinds - Limited availability of photons - In an Atmosphere of 80% N2 , 20% O2 , 0.1% H2 O - 1,13 ≤ ρ (kg/m³) ≤ 1,27 - + Katabatic winds à 100 ≤ p (Pa) ≤ 1000