The electron pressure in the outer atmosphere of ϕ Eri (K2 V)

Observations of ε Eri (K2 V) have been made with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. The spectra obtained show a number of emission lines which can be used to determine, or place limits on, the electron density and pressure. Values of the electron pressure are required in order to make quantitative models of the transition region and inner corona from absolute line fluxes, and to constrain semi-empirical models of the chromosphere. Using line flux ratios in Si iii and O iv a mean electron pressure of Pe = NeTe = 4.8 × 1015 cm−3 K is derived. This value is compatible with the lower and upper limits to Pe found from flux ratios in C iii, O v and Fe xii. Some inconsistencies which may be because of small uncertainties in the atomic data used are discussed.