THE LOCAL BUBBLE - CURRENT STATE OF OBSERVATIONS AND MODELS

Recently, observations with the ROSAT PSPC instrument and the spectro- meters onboard the EUVE satellite have given new detailed information on the structure and physical conditions of the Local Bubble. From the early rocket experiments, and in particular from the WISCONSIN Survey , the existence of a diffuse hot gas in the vicinity of the solar syst em, extending out to about 100 pc, has been inferred in order to expla in the emission below 0.3 keV. The higher angular resolution and sensi tivity of ROSAT made it possible to use diffuse neutral clouds as targ ets for shadowing the soft X-ray background. Thus, in some directions, more than half of the flux in the 0.25 keV band appears to come from outside the Local Bubble. Further, measurements of the diffuse EUV in the LISM, show surprisingly few emission lines. These findings are in conflict with the standard LHB model, which assumes a local hot (T sim ilar to 10(6) K) plasma in CIE. Model calculations, based on the non-e quilibrium cooling of an expanding plasma, show a promising way of rec onciling all available observations. Thus the present temperature with in the LB may be as low as 4 x 10(4) K and its number density as large as 2 x 10(-2) cm(-3), giving a total pressure that is roughly in agre ement with the Local Cloud.