Acad Sinica, Inst Theoret Phys, Beijing 100080, Peoples R China
; Nankai Univ, Dept Phys, Tianjin 300071, Peoples R China
; Liaoning Univ, Dept Phys, Shenyang 110036, Peoples R China
; Acad Sinica, Inst High Energy Phys, Beijing 100039, Peoples R China
Considering a new type of environment influences, we use a two-energy-level (nu (1)-nu (2)) quantum system to investigate neutrino oscillations in medium. Besides the matter effects derived by Wolfenstein, there may exist extra terms due to a unitary evolution of the system between pure and mixed states, so the evolution equation is modified obviously. We show that the extra 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.