measurable flow. However, if the plasma ions are heated by, e.g., ion cyclotron waves a thermodynamic equilibrium does not necessarily exist since the plasma is mainly cooled by a divergent electron heat flux and by inelastic electron collisions with the ions [@Tu:Marsch:1997]. Following their two-fluid approach, the sum $p=p_i+p_e$ of ion and electron pressure has to obey $$\frac{d}{dr}(p_i+p_e)(r) = -m_i n g_\odot \frac{R^2_\odot}{r^2}$$ Quasineutrality in this balance is insured by an ambipolar electric field which cancels when the momentum equations for ions and electrons are added. Here, $m_i$ is the mean ion mass, $n$ the plasma density and $g_\odot$ the gravity acceleration at the solar surface. For the total pressure we have $$p = p_i+p_e = n k_B (T_i+T_e) = 2n k_B T_\lambda$$ Insertion and integration yields, changing the variable $r$ to $h=r-R_\odot$ $$p = p_0 \exp {\left(-\frac{m_i g_\odot}{k_B}
                \int_0^h \frac{R^2_\odot}{T_\lambda
                (R_\odot+h')^2}\,dh'\right)}$$ For the small height range of our observations, we can neglect a possible height variation of $T_\lambda$ inside the plumes. Then $$n \simeq n_0 \exp {\left(-\frac{m_i g_\odot}{k_B T_\lambda}\;\frac{h}
         {(1+h/R_\odot)}\right)}
         = n_0 \exp {\left(-\frac{h}{\lambda_n(T_\lambda)\,(1+h/R_\odot)}\right)}
\label{equ:ne_h}$$ where the scale height $\lambda_n(T_\lambda)$ depends on $T_\lambda=(T_i+T_e)/2$ and $n_{\rm 0}$ is the density at $h=0$ taken to be the base of the corona. 