What are solar neutrinos?
The Sun is the most powerful neutrino sources in our circumstances. The energy source of the Sun shining is a fusion reaction in the center of the Sun. Four hydrogen nuclei (i.e. protons) form a helium-4 nuclei (two protons and two neutrons) via the fusion reaction, in which fusion energy, two positrons and two electron neutrinos are also released.（4p → He + 2e+ + 2νe + fusion energy）The electron neutrinos generated in this reaction are called as "solar neutrinos". The solar neutrino flux, on the Earth, per one second, per one square centimeter, is about 66 billion.
It will take about 100,000 years to appear the heat, generated by the fusion reaction in the center of the Sun, on the solar surface. On the other hand, solar neutrinos, born in the center of the Sun, will arrive in approximately 8 minutes to Earth, since neutrinos are very hard to interact. In other words, we see 10 million years ago solar activity in the light, but in the neutrino we are able to observe the current activities of the center of the Sun.
Lack of solar neutrinos
Observation of solar neutrinos was began with Homestake experiment by R. Davis and so on from the late 1960s in US. In the Homestake experiment, solar neutrino flux was measured via the production rate of argon atoms from the neutrino reaction of chlorine atoms. It was impossible to measure the neutrino direction. As a results of the experiment, the observed production rate was about 1/3 of the expected value from Standard Solar Model (SSM). There were several questions on this result: does it really capture the neutrinos coming from the Sun?, why is the neutrino flux small?, is SSM correct?, does neutrinos have finite masses and then neutrino oscillation occur?, and so on. This problem was called as "solar neutrino problem". It annoyed the researchers for many years.
In 1988, the solar neutrino results other than the Homestake experiment was firstly reported from the Kamiokande II experiment group. Kamiokande, which was the former experiment of Super-Kamiokande, was able to measure neutrino coming direction in real time. It was firstly shown by Kamiokande that the observed neutrinos were coming from the direction of the Sun. However, the observed solar neutrinos flux was about half of the expected value from SSM. Therefore, the solar neutrino problem still remained.
Solving the solar neutrino problem, then unraveling further questions
In June 2000, Super-Kamiokande has reported the observation result of the solar neutrino flux with a highest accuracy than ever before. As a result, the observed solar neutrino flux was about 45% of the expected flux in SSM with more than 99.9% confidence level, suggesting the solar neutrino problem was caused by a neutrino oscillation. Furthermore, measuring energy distribution of solar neutrinos and time variation of the solar neutrinos in day time and night time with high accuracy, a large limit on the neutrino oscillation parameter (mass difference and mixing angle) area was obtained. It was also shown that the mixing between neutrinos are large. In June 2001, a combined analysis of solar neutrino observations between Super-Kamiokande and SNO experiment in Canada, showed a reliable evidence that a neutrino oscillation really occurred. In addition, it was confirmed at the same time that the neutrino flux calculated from SSM was correct.
Although the problem that observed solar neutrino flux looked smaller than SSM was solved by neutrino oscillation, there are still unsolved questions in nature of neutrinos or the burning mechanism of the Sun (SSM). For example, they are true values of the solar neutrino oscillation parameters (mass difference and mixing angle), confirmation of the Earth's matter effect on solar neutrinos, elucidation of the chemical composition of the solar interior, and so on. Towards unraveling these questions, Super-Kamiokande continues to measure solar neutrinos with more precision and high statistical accuracy.