Total Solar Irradiance Variability and the Solar Activity Cycle
  It is suggested that the solar variability is due to the perturbed nature of the solar core and this variability is provided by the variability of the solar neutrino flux from the solar neutrino detectors i.e., Homestake, Superkamiokande, SAGE and GALLEX-GNO. The solar neutrino flux in the standard solar model (SSM) was calculated on the assumption of L_nu (neutrino luminosity) = L_gamma (optical luminosity) which implies that if there is a change in optical luminosity then solar neutrino flux data will also be changed. An internal dynamo due to the cyclic variation of nuclear energy generation inside the core of the sun is responsible for the solar activity cycle was suggested and thus the internal magnetic field is also variable. Again the changes in the nuclear energy generation induce structural changes that result in variations of the global solar parameters i.e., luminosity, radius and temperatures etc. From the analysis of total solar irradiance (TSI) data during the year from 1970 to 2003 we have found five phases within the solar activity cycle. The first phase (I) starts before two years from the sunspot minimum. The second phase (II) starts at the time of sunspot minimum and phase (III) starts before 2/3 years from sunspot maximum whereas phase (IV) starts at sunspot maximum and fifth phase (V) starts at after 2-3 years from sunspot maximum. 
Probhas Raychaudhuri