  Rapidly rotating and strongly magnetized protoneutron stars (PNSs) created in
core-collapse supernovae can drive relativistic magnetized winds. Ions and
neutrons can be co-accelerated while they remain coupled through elastic
collisions. We investigate the nucleosynthesis and subsequent nuclear
disintegration, and find that relativistic neutrons can be generated in such
magnetized winds. Upon eventual decoupling, resulting inelastic collisions with
ejecta lead to pion production, resulting in $0.1-10\,{\rm GeV}$ neutrinos.
Following this scenario presented in Murase, Dasgupta & Thompson, Phys. Rev. D,
89, 043012 (2014), we numerically calculate the spectra and light curves of
quasithermal neutrino emission and find that power-law tails are formed without
cosmic-ray acceleration. In the event of a Galactic supernova, $\sim 10-1000$
neutrino events could be detected with Hyper-Kamiokande, KM3Net-ORCA and
IceCube-Upgrade for PNSs with surface magnetic field $B_{\rm dip}\sim
10^{13-15}\,{\rm G}$ and initial spin period $P_i \sim 1-30\,{\rm ms}$.
Successful detection will enable us to study supernovae as multienergy neutrino
sources and may provide clues to the roles of PNSs in diverse classes of
transients.
