The solar neutrino problem. Structure, variability of the Sun (an astrophysicist point of view)

Presentation of the so-called ''standard model''. For the chlorine experiment, which detects mainly the neutrinos from boron, the prediction, including a minor correction to the p-p cross section, is 12 SNU's; for the gallium experiment, which detects mainly the neutrinos from the p-p reaction, the prediction is 134 SNU's. The different ways of explaining the discrepancy with the observed flux are discussed; non standard models, MSW interaction with solar matter, WIMP's models). The compatibility with astrophysical constraints favours the MSW effect. The present understanding of the relation between magnetic fields and solar activity is presented and the possibility of a variation of the MSW effect, due to changes in the solar structure, is discussed. Unless some new physics is involved (like the magnetic moment of the neutrino), the discussion casts some doubts on the reality of the variation of the solar neutrino flux and therefore on the connection with solar activity