  We consider Langevin equation with dichotomously fluctuating diffusivity,
where the diffusion coefficient changes dichotomously in time, in order to
study fluctuations of time-averaged observables in temporary heterogeneous
diffusion process. We find that occupation time statistics is a powerful tool
for calculating the time-averaged mean square displacement in the model. We
show that the time-averaged diffusion coefficients are intrinsically random
when the mean sojourn time for one of the states diverges. Our model provides
anomalous fluctuations of time-averaged diffusivity, which have relevance to
large fluctuations of the diffusion coefficient in single-particle-tracking
experiments.
