code: def s_l(l, alpha):
      
      a2 = alpha**2
      c_a = 0.547
      s_l = np.sqrt(old_div(((c_a**(2. * l)) * a2), ((l + 1.) * (2. * l + 1.))))
      return s_l
docs: get sigma as a function of degree l from Constable and Parker (1988)