Abstract Most power-full solar flare as the ones occurred on 23th February 1956, September 29th 1989, 28th October and on 2nd-4th November 2003 are sources of cosmic rays, X, gamma and neutrino bursts. These flares took place both on front or in the edge and in the hidden solar disk. The 4th November event was the most powerful X event in the highest known rank category X28 just at horizons. The observed and estimated total flare energy ( {E}FL ≃ {10}31÷ {10}33erg) should be a source of a prompt secondary neutrino burst originated, by proton-proton-pion production on the sun itself; a more delayed and spread neutrino flux signal arise by the solar charged flare particles reaching the terrestrial atmosphere. These first earliest prompt solar neutrino burst might be observed, in a few neutrino clustered events, in present or future largest neutrino underground detectors as Super-Kamiokande one, in time correlation with the X-Radio flare. The onset in time correlation has great statistical significance. Our first estimate on the neutrino number events detection at the Super-Kamiokande II Laboratory for horizontal or hidden flare is found to be few events: {N}_{eV_{&bar;{ν}e}}≃ 0.63{\etae}(&frac;{⟨ {E}ν⟩}{35 MeV}) (&frac;{⟨ {E}FL⟩}{{10}31 erg}); and {N}_{eV_{&bar;{ν}μ}} ≃ 3.58(&frac;{⟨ {E}_{{ν}μ}... ...&rm; MeV}) (&frac;{⟨ {E}FL⟩}{{10}31 &rm; erg}){\etaμ}, where {η}≃ 1, {E}_{{ν}μ} > 113MeV. Our first estimates of neutrino signals in largest underground detectors hint for few events in correlation with X, gamma, radio onser. Our approximated spectra for muons and taus from these rare solar eruption are shown over the most common background. The muon and tau signature is very peculiar and characteristic over electron and anti-electron neutrino fluxes. The rise of muon neutrinos will be detectable above the minimal muon threshold Eν ≃ 113MeV energy, or above the pion and Δ °r; thresholds ( Eν≃ 151 and 484MeV). Any large neutrino flare event record might also verify the expected neutrino flavour mixing leading to a few as well as a comparable, {ν}e, {ν}μ, &bar;{ν}e, &bar;{ν}μ energy fluence and spectra. The rarest tau appearence will be possible only for hardest solar neutrino energies above 3.471GeV.

Keywords Sun

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