Neutrinos are elementary particles emitted in nuclear reactions like fission, fusion and radioactive decay. The sun emits vast numbers of neutrinos which can pass through the earth with little or no interaction.Solar neutrinos shine down on us during the day, and shine up on us during the night after passing through the earth almost no absorption. At the surface of the Earth, the flux is about 65 billion (6.5×1010) solar neutrinos, per second per square centimetre. That is smaller that the area of your finger nail.
About 91% of solar neutrons are produced by the proton-proton fusion to produce deuterium. Neutrinos from the proton-proton fusion have low energy up to 400 keV. The flux of low energy neutrons is about 7 x 1010 neutrinos/cm2s, i.e 70 billion neutrinos per square cm per second. A square centimetre is about the size of your small finger nail. There are several other processes producing neutrinos up to 18 MeV (see figures below).
Solar neutrinos are produced in the core of the Sun through various reactions, each of which occurs at a particular rate and leads to its own spectrum of neutrino energies.
The main contribution to the neutrino flux comes from the proton–proton chain. The reaction is:
All solar neutrinos are less than 20 MeV.
Cosmic Neutrinos
Neutrinos detected by the University of Tokyo’s underground Kamiokande experiment from the supernova SN 1987A in the Magellan cloud galaxy.
Cosmic neutrinos may be produced either in the vicinity of the cosmic-ray source or along the cosmic-ray propagation path, leading to the production of secondary unstable particles, which subsequently decay into neutrinos.
Ten neutrino events were detected in a deep mine neutrino detection facility in Japan which coincided with the observation of Supernova 1987A. They were detected within a time interval of about 15 seconds against a background of lower energy neutrino events. A similar facility, IMB in Ohio detected 8 neutrino events in 6 seconds. These observations were made 18 hours before the first optical sighting of the supernova. http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/sn87a.html#c2
Cosmic neutrinos can also be detected in natural bodies of water or ice by measuring the Cherenkov light induced by the passage of the charged particles that result from neutrino interactions in or near the detector.
On 12 February the KM3NeT Collaboration reported an exceptionally high-energy cosmic neutrino. The KM3NeT research infrastructure comprises two detector arrays of optical sensors at a depth of about 3,450 m in the Mediterranean Sea. Cosmic neutrinos are detected by measuring the Cherenkov light induced by the passage of the charged particles that result from neutrino interactions in or near the detector.
A muon detected by the KM3NeT Collaboration had an estimated energy of 120 (−60+110) petaelectronvolts (PeV). In light of its enormous energy and near-horizontal direction, the research team found that the muon most probably originated from the interaction of a neutrino of even higher energy in the vicinity of the detector. The energy of this event is much larger than any previously detected cosmic neutrino. (Observation of an ultra-high-energy cosmic neutrino with KM3NeT. Nature 638, 376–382 (2025). https://doi.org/10.1038/s41586-024-08543-1). An article in the Economist (15 Feb 2025) suggested the energy of this neutrino is about equivalent to the energy of a table tennis ball dropped from a height of 1 metre.