A new type of accessible environmental influences on neutrino oscillation

Considering a new type of environment influences, we use a two-energy-level (nu (1)-nu (2)) quantum system to investigate neutrino oscillations in med ium. Besides the matter effects derived by Wolfenstein, there may exist ext ra terms due to a unitary evolution of the system between pure and mixed st ates, so the evolution equation is modified obviously. We show that the ext ra terms may play some role and induce observable effects in solar neutrino problem, especially, in the long baseline neutrino oscillation experiments which are under serious consideration recently if the parameters fall into a suitable region.