def s_l(l, alpha):
    """
    get sigma as a function of degree l from Constable and Parker (1988)
    """
    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