Solar neutrinos are neutrinos which are produced in the Sun by nuclear fusion. Most of the neutrinos which reach and pass through the Earth today originate in our Sun - that is, they are solar neutrinos.
- About Neutrinos -
A neutrino is an extremely small particle of matter which has no electrical charge (unlike the larger positively charged protons and negatively charged electrons within atoms) and very little mass, which travels at extremely high velocities (close to but not at the speed of light, which matter cannot attain), and which can therefore travel straight through most other matter. Initially after their existence was theoretically proposed, neutrinos were actually assumed to have no mass at all; moreover, if they had more than extremely negligible mass, physical models showed that they would add so much mass to the universe that it would collapse due to the increased force of gravity.
In nature, neutrinos originate in stars (including our Sun) and supernovae, background radiation due to geological and chemical processes within the Earth, and by cosmic rays striking the upper atmosphere. However, they can also be artificially created by human technology, namely using nuclear fission reactors and particle accelerators. Detecting these particles is very difficult and generally requires the construction of large surfaces on which to detect collisions. The resulting "neutrino observatories" have been constructed in Sudbury, Canada, using heavy water; Japan (Super-Kamiokande), using pure water; and Antarctica (IceCube), using an ice sheet.
Neutrinos come in three types, known in quantum physics parlance as "flavours": electron neutrinos, muon neutrinos, and tau neutrinos. However, under certain conditions they can also change between these flavours or oscillate.
- About Solar Neutrinos -
Solar neutrinos are those neutrinos which have been produced by nuclear fusion within the Sun. Essentially, stars such as our Sun are simply enormous nuclear fusion reactors, fusing hydrogen into helium. (In several billion years, our Sun will run out of sufficient hydrogen in its core to maintain this process, and go into its death throes, first as a red giant and then as a white dwarf.) In what is known as the proton-proton chain reaction, four protons and two electrons combine to form a helium atom plus two electron neutrinos.
Enormous numbers of solar neutrinos are produced by the Sun on a continuous basis. Every second, several trillion of them will pass through the space occupied by the average human body alone. An extremely small percentage of these will actually collide with particles; it is these rare collisions which the sensitive neutrino observatories are designed to monitor.
Initially, solar neutrinos were also the subject of a minor astrophysical mystery, known as the solar neutrino problem. Given the standard understanding of stars (known as the Standard Solar Model) and the above chemical formula, the Sun should be producing about three times as many solar neutrinos as were originally detected by the first neutrino observatories capable of doing so. This clearly meant that either the theoretical understanding of the Sun or the theoretical understanding of neutrinos was incorrect. For the moment, the solution has been to hypothesize that neutrons can "oscillate," or change between flavours. Since there are three flavours of neutrons and about one-third as many neutrons as expected, this would mean that two-thirds of solar neutrinos, by the time they reach Earth, are simply no longer electron neutrinos, as they were when first produced in the Sun.
However, as research into neutrons continues, other revisions to the prevailing theories about this extremely small particle will almost certainly be made.