Evidence from Helioradiology of an Inner Tachocline in the Sun

One can make a good case that the Rieger and related solar oscillations may be understood as r-mode oscillations. The frequencies of these oscillations fit the formula 2*m*nu/(l*(l+1)), where l and m (two of the three spherical harmonic indices) take the values m = 1, l = 2,3,4,..., (the third index n does not affect the mode frequency), and the sidereal rotation frequency nu = 13.7 year-1. This value of the rotation frequency indicates that these oscillations occur in the tachocline, which separates the radiative zone from the convection zone. There is now strong evidence that beta decay rates are not constant, and there is a persuasive case that the Sun is responsible for variations in these rates. This new source of solar observational data, for which we propose the name "helioradiology," yields evidence that the solar core rotates more slowly than the radiative zone. This raises the possibility that there is a second, "inner," tachocline that separates the core from the radiative zone. This possibility in turn suggests that there may be a second group of "Rieger-like" oscillations that have their origin in this inner tachocline. We present evidence for such oscillations, derived from helioradiology data and also from the long-term diameter measurements acquired at the Mount Wilson Observatory.