Photophoresis on particles hotter/colder than the ambient gas in the   free molecular flow
  Aerosol particles experience significant photophoretic forces at low pressure. Previous work assumed the average particle temperature to be very close to the gas temperature. This might not always be the case. If the particle temperature or the thermal radiation field differs significantly from the gas temperature (optically thin gases), given approximations overestimate the photophoretic force by an order of magnitude on average with maximum errors up to more than three magnitudes. We therefore developed a new general approximation which on average only differs by 1 % from the true value. 
C. Loesche, G. Wurm, T. Jankowski, M. Kuepper
cond-mat.stat-mech astro-ph.SR physics.flu-dyn