Solar neutrino oscillations and bounds on neutrino magnetic moment and solar magnetic field

If the observed deficit of solar neutrinos is due to neutrino oscillations, neutrino conversions caused by the interaction of their transition magnetic moments with the solar magnetic field (spin-flavour precession) can still be present at a subdominant level. In that case, the combined action of neutrino oscillations and spin-flavour precession can lead to a small but observable flux of electron antineutrinos coming from the sun. Non-observation of these nuebar's could set limits on neutrino transition moment \mu and the strength and coordinate dependence of the solar magnetic field B_\perp. The sensitivity of the nuebar flux to the product \mu B_\perp is strongest in the case of the vacuum oscillation (VO) solution of the solar neutrino problem; in the case of the LOW solution, it is weaker, and it is the weakest for the LMA solution. For different solutions, different characteristics of the solar magnetic field B_\perp(r) are probed: for VO solution, the nuebar flux is determined by the integral of B_\perp(r) over the solar convective zone, for LOW solution it depends on the derivative of B_\perp(r) at the surface of the sun, and for LMA it is determined by the magnitude of B_\perp in the neutrino production region