page_title: Solar neutrino
section_title: Observed data
hierarchical_section_title: Solar neutrino / Observed data
caption_attribution_description: Flux of solar neutrinos when they arrive at the Earth as a function of their energy. Each color represents a different fusion reaction in which the neutrinos are produced within the core of the sun, as detailed in the legend. Data has been obtained from Bahcall, John; Serenelli, Aldo (2005). "New solar opacities, abundances, helioseismology, and neutrino fluxes" (PDF). Astrophys. J. 621: L85–L88. doi:10.1086/428929..
context_page_description: Electron neutrinos are produced in the Sun as a product of nuclear fusion. Solar neutrinos constitute by far the largest flux of neutrinos from natural sources observed on Earth, as compared with e.g. atmospheric neutrinos or the diffuse supernova neutrino background.
context_section_description: The highest flux of solar neutrinos come directly from the proton-proton interaction, and have a low energy, up to 400 keV. There are also several other significant production mechanisms, with energies up to 18 MeV. From the Earth, the amount of neutrino flux at Earth is around 7·10¹⁰ particles·cm⁻²·s ⁻¹. The number of neutrinos can be predicted with great confidence by the Standard Solar Model. However, the number of electron neutrinos detected on Earth was only  of the predicted number, and this was known as the “solar neutrino problem”.
  The absence of electron neutrinos, and the fact that it is one of three known types of neutrino, eventually prompted the idea of neutrino oscillation and the fact that neutrinos can change flavour. This was confirmed when the total flux of solar neutrinos of all types was measured and it agreed with the earlier predictions of the expected flux of only electron neutrinos, as seen by Sudbury Neutrino Observatory. The fact that the electron neutrinos could spontaneously change in flight through empty space also confirmed that neutrinos must have mass. Solar models additionally predict the location within the Sun's core where solar neutrinos should originate from, depending on the nuclear fusion reaction which leads to their production. Future neutrino detectors will be able to detect the incoming direction of these neutrinos with enough precision to measure this effect.
  The energy spectrum of solar neutrinos is also predicted by solar models. It is essential to know this energy spectrum because different neutrino detection experiments are sensitive to different neutrino energy ranges. The Homestake Experiment used chlorine and was most sensitive to solar neutrinos produced by the decay of the beryllium isotope ⁷Be. The Sudbury Neutrino Observatory is most sensitive to solar neutrinos produced by ⁸B. The detectors that use gallium are most sensitive to the solar neutrinos produced by the proton-proton chain reaction process, however they were not able to observe this contribution separately. The observation of the neutrinos from the basic reaction of this chain, proton-proton fusion in deuterium, was achieved for the first time by Borexino in 2014. In 2012 the same collaboration reported detecting low-energy neutrinos for the proton-electron-proton (pep reaction) that produces 1 in 400 deuterium nuclei in the sun. The detector contained 100 metric tons of liquid and saw on average 3 events each day (due to ¹¹C production) from this relatively uncommon thermonuclear reaction.
  Note that Borexino measured neutrinos of several energies; in this manner they have demonstrated experimentally, for the first time, the pattern of solar neutrino oscillations predicted by the theory. Neutrinos can trigger nuclear reactions. By looking at ancient ores of various ages that have been exposed to solar neutrinos over geologic time, it may be possible to interrogate the luminosity of the Sun over time, which, according to the Standard Solar Model, has changed over the eons as the (presently) inert byproduct helium has accumulated in its core.
caption: The greatest number of solar neutrinos are direct products of the proton-proton reaction (tall, dark blue curve on the left). They have a low energy – only reaching up to 400 keV. There are several other significant production mechanisms, with energies up to 18 MeV.[3]
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