Journal of Statistical Physics, Vol. 80, Nos. 1/2, 1995. p. 23-33.
A magnetotactic bacterium aligns itself along the magnetic field. When the field is reversed the bacterium makes a U-turn in the surrounding water. The turning is described by a Fokker-Planck equation for the angle teta, which is singular at the endpoints teta = 0 and ,teta = n. The time needed for turning can be found exactly as a first-passage time. The probability distribution itself can be found in terms of an approximation for low temperature. To cover the regions near the endpoints singular perturbation theory is needed.