def calc_growth_rate(tube):
    """the growth rate equation from Sultan 96

    Parameters
    ----------
    tube : (FluxTube)
        flux tube object

    Variables used
    ----------
    sig_F_P : (float)
        flux tube integrated pedersen cond. F region in mho
    sig_total : (float)
        flux tube integrated pedersen cond. total in mho
    V_P : (float)
        flux tube integrated vertical drift or drift at apex altitude m/s
    U_L : (float)
        flux tube integrated neutral wind perp. B in L direction in m/s
    g_e : (float)
        gravtiy at apex altitude in m/s^2
    nu_eff : (float)
        collision frequency in s-1
    K_F : (float)
        altitude gradient in density in m-1
    """
    sig_F_P = tube.sig_F
    sig_total = tube.sig_total
    V = tube.V
    U_L = tube.U
    g_e = tube.g
    nu_eff = tube.nu_ef
    K_F = tube.K
    R_T = tube.R

    gamma = sig_F_P / sig_total * (V - U_L + g_e/nu_eff) * K_F - R_T
    return gamma