Probe of Solar Neutrino Magnetic Moments through Spin-Flavor Precession: Resonance Structure and Antineutrino Appearance

We investigate solar-neutrino spin--flavor precession (SFP) induced by magnetic moments in the three-active-flavor framework. For Majorana neutrinos, SFP can convert solar neutrinos into antineutrinos of different active flavors. In the Dirac case, SFP instead produces sterile right-handed states and can lead to the disappearance of active neutrinos. Using the full 6 times6 Hamiltonians and GS98 and AGSS09 solar profiles, we examine propagation-eigenvalue crossings at B=0 and the projected magnetic couplings between the corresponding states. For normal mass ordering and 1 at most E ν/MeV at most20, we confirm the absence of finite-density Majorana crossings. A magnetically coupled Dirac crossing emerges above approximately 12 MeV but involves only a subdominant electron-flavor component, limiting resonant disappearance. Nonresonant Majorana conversion nevertheless offers a distinctive lepton-number-violating solar ν e signal, motivating our sensitivity study for the Jinping Neutrino Experiment. For a proposed 3 kt detector operating for five to ten years, we project a 90% C.L. sensitivity of P(ν e to ν e) (0.85-1.3) times10-5. In the μ 12-only benchmark, optimistic solar-core transverse magnetic fields of B=7-10 MG imply a reach of μ 12 (2.3-4.1) times10-13,μ B, numerically below existing direct-scattering limits and commonly quoted stellar-cooling bounds. This could enable Jinping to provide one of the most stringent projected terrestrial sensitivities to Majorana transition magnetic moments.