LOCAL INTERSTELLAR CLOUD ELECTRON-DENSITY FROM MAGNESIUM AND SODIUM IONIZATION - A COMPARISON

The ambient interstellar plasma density (i.e. the plasma density of th e interstellar medium surrounding the Sun) directly governs the struct ure and the size of our heliosphere. Information on this density can b e derived from the ionization states of the interstellar species which can be detected in absorption along the paths to the nearby stars, an d which can be shown to belong to the Local Interstellar Cloud (LIC). Echelle spectra around the resonance lines of neutral and singly ioniz ed magnesium have been obtained for the nearby star delta Cas with the Goddard High-Resolution Spectrograph (GHRS) on board the Hubble Space Telescope. While apparently a unique velocity component (a unique clo udlet) is detected in both lines of the MgII lambda lambda 2800 resona nce doublet, at the expected Doppler shift for the LIC, an extremely s mall lambda lambda 2853 Mg line is also detected at a Doppler shift co mpatible with the LIC motion, allowing a measurement of the LIC MgII/M gI ratio, here found to be 400 (-130,+190). This ratio implies a mean electron density of about 0.28 (-0.14,+0.34) cm(-3) along this line-of -sight, if equilibrium conditions prevail, and if T=7000K, when using the most recent recombination and charge-exchange rates. This MgII/MgI ratio is larger than for Sirius (R similar or equal to 220), which li es at 110 degrees from delta Cas, providing some evidence for an ioniz ation gradient in the local cloud. Such an electron density implies a surprisingly large ionization degree, and the upper range of the inter val is incompatible with the minimum size of our heliosphere. A second and independent way to derive the electron density along the path to delta Cas uses the simplicity of the LIC geometry in the sky region su rrounding the star, which allows an estimate of the H column-density t o the star, as well as previous ground-based CaII data, and the measur ed NaI/CaII ratio and calcium depletion in the LIC. The resulting most probable electron density at 7000K, 0.05 cm(-3), provides a new evide nce for a significant ionization degree of the LIC, but is a factor of four to five smaller than the value based on magnesium. The upper lim it of 0.19 cm(-3) remains consistent with the minimum size of our heli osphere. The existence of a common interval to the two determinations: n(e)=0.14-0.19 cm(-3) implies that ionization equilibrium within the LIC is not totally precluded. However, the lack of a real convergence deserves further observations, involving other interstellar species. T he common interval is compatible with the result of Frisch (1994), fro m anomalous C and O cosmic rays abundances, if carbon is not too much filtered at the heliospheric interface. On the other hand, the sodium- based most probable value is in agreement with neutral hydrogen decele ration at the heliospheric interface for the Baranov two-shocks model, as well as with the ionization degree of hy drogen implied by local E UV sources.