Diagram illustrating a DA (hydrogen-dominated) white dwarf’s atmospheric structure and dynamics of incoming polluting ⁴⁰Ca ions. Initially, the incoming particle arrived at the top of the atmosphere with velocity v_in, punching through the atmosphere with length L_stop, which is a small fraction of the atmospheric pressure scale height. Within the atmosphere, the particle’s motion can be affected by collisions with atmospheric electrons and ions (characterized by frequencies f_e and f_H for collisions with electrons and hydrogen, respectively) or cyclotron motion from the background (characterized by the gyrofrequency f_gyro). There may be a convection zone, which may be beneath or within the photosphere. The bulk motion of particles in the atmosphere can be described by the spreading timescale τ_spread and the sinking timescale τ_sink. When τ_sink ≳ τ_spread, the initial pollution spot can spread globally before sinking, but a pollution gradient remains across the surface. When τ_sink ≫ τ_spread, the white dwarf atmosphere will be homogeneously mixed. τ_conv is the vertical convective turnover timescale.