Energetics and structure of multispecies solar coronal loops

The temperature and density profiles of multispecies quiescent solar corona l loops containing hydrogen, helium, and heavier species are investigated u sing a numerical model for steady-state force and energy balance. The model loop follows a semicircular magnetic field line anchored in the chromosphe re, and it contains low-beta plasma, The model allows for species-dependent heating. The electrons, protons, and helium ions are taken to be in therma l equilibrium and form the dominant plasma component. In nonisothermal regi ons (i.e., in the presence of steep transition-region temperature gradients ), the outward thermal force induces an inward polarization electric field along the loop; in nearly isothermal (i.e., VT small) coronal regions, the electric field is outward to counterbalance gravity. The pressure gradient is negative for the protons, although in many cases it is positive for heav ier ions. The thermal force can induce local minor ion overdensities. Gravi tational settling may deplete the heavy ion densities, especially in the lo nger loops, and can occur if the settling timescale is short compared with the loop lifetime and the turbulent mixing timescale. The calculated loop a bundances vary with the loop parameters; if the FIP effect is present in th e assumed base abundances, the models can alter it for some combinations of parameters. We show that in order to reach heavy ion temperatures of simil ar to 10(7)-10(8) K, the collisional energy transfer rate per particle, and therefore the required heat input per ion, is similar to 10(-8)-10(-7) erg s s(-1).