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 $6\times 6$ Hamiltonians and GS98 and AGSS09 solar profiles, we examine propagation-eigenvalue crossings at $B_{\perp }=0$ and the projected magnetic couplings between the corresponding states. For normal mass ordering and $1\le E_{\nu }\mathrm{/}\mathrm{M}\mathrm{e}\mathrm{V}\le 20$, we confirm the absence of finite-density Majorana crossings. A magnetically coupled Dirac crossing emerges above approximately $12\mathrm{M}\mathrm{e}\mathrm{V}$ but involves only a subdominant electron-flavor component, limiting resonant disappearance. Nonresonant Majorana conversion nevertheless offers a distinctive lepton-number-violating solar ${\bar{\nu }}_e$ signal, motivating our sensitivity study for the Jinping Neutrino Experiment. For a proposed $3\mathrm{k}\mathrm{t}$ detector operating for five to ten years, we project a 90% C.L. sensitivity of $P({\nu }_e\to {\bar{\nu }}_e)\simeq (0.85-1.3)\times {10}^{-5}$. In the ${\mu }_{12}$-only benchmark, optimistic solar-core transverse magnetic fields of $B_{\perp }=7-10\mathrm{M}\mathrm{G}$ imply a reach of $\mathrm{\mid }{\mu }_{12}\mathrm{\mid }\simeq (2.3-4.1)\times {10}^{-13}{\mu }_B$, 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.