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Solar neutrinos, neutrinos from the Sun, were the first detected on Earth in the 1960s after well-established models of fusion predicted their existence and practical detection methods were devised. After relic neutrinos (those from very soon after the Big Bang), which are presumed to be by far the most common neutrinos passing through Earth's neighborhood, solar neutrinos are the next the most common. The bulk of relic neutrinos have by far the lowest kinetic energy (KE), and the bulk of solar neutrinos, the next lowest, have up to about 400 keV. KE is an indicator for determining whether a neutrino is one of these or is from some other astronomical source.
The first neutrino observatories were aimed at detecting solar neutrinos, but some newer ones clearly aim to detect those from other astronomical sources. There are many solar neutrinos to detect, but their low energy makes it a challenge to detect them. The solar neutrino unit (SNU) is a unit devised to relate interactions used to detect neutrinos to the rate of neutrinos from the Sun, and reflects the low probability of a particular solar neutrino producing the detectable interaction.
The solar neutrino problem was an anomaly in the results of detections of solar neutrinos: detections were only a fraction of what theory had predicted. The theory of neutrino oscillation could explain some of the deficit but not all. The Mikheyev-Smirnov-Wolfenstein effect (MSW effect) explained it, and eventual observation of the effect's predicted ratio of neutrino flavors confirmed that effect. The detection of solar neutrinos along with this confirmation is considered a detailed confirmation that the Sun's energy is the product of fusion at the solar core.