In matter, the mixing angle depends on the number density of electrons and the energy of the neutrinos: . As the neutrinos propagate through density-variant matter, changes – and with it, the flavors of the eigenstates. With antineutrinos, the conceptual point is the same but the effective charge that the weak interaction couples to (called weak isospin) has an opposite sign. If the electron density of matter changes along the path of neutrinos, the mixing of neutrinos grows to maximum at some value of the density, and then turns back; it leads to resonant conversion of one type of neutrinos to another one. The effect is important at the very large electron densities of the Sun where electron neutrinos are produced. The high-energy neutrinos seen, for example, in Sudbury Neutrino Observatory (SNO) and in Super-Kamiokande, are produced mainly as the higher mass eigenstate in matter , and remain as such as the density of solar material changes. Thus, the neutrinos of high energy leaving the Sun are in a vacuum propagation eigenstate, , that has a reduced overlap with the electron neutrino seen by charged current reactions in the detectors. Resonance in the MSW effect Neutrino flavor mixing experiences resonance and becomes maximal under certain conditions of the relationship between the vacuum oscillation length and the matter density-dependent refraction length , where is the Fermi coupling constant. The refraction length is understood as the distance over which the matter "phase" from the coherent scattering is equal to . The resonance condition is given by , which is when the neutrino system experiences resonance and the mixing becomes maximal. For very small , this condition becomes , that is, the eigenfrequency for a system of mixed neutrinos becomes approximately equal to the eigenfrequency of medium. The resonance density is informed by the resonance condition: and is directly related the number density of electrons . If vacuum density reaches the maximal value, , the resonance density goes to zero. In a medium with fluctuating density, itself fluctuates – the interval between its maximum and minimum values is called the resonance layer. Solar neutrinos and the MSW effect For high-energy solar neutrinos the MSW effect is important, and leads to the expectation that , where is the solar mixing angle. This was dramatically confirmed in the Sudbury Neutrino Observatory (SNO), which has resolved the solar neutrino problem. SNO measured the flux of solar