The local interstellar medium

The local interstellar medium can be probed in different ways: by analyzing low energy X-ray data in the range 0.1-0.4 keV, where the radiation is absorbed by the interstellar gas at column densities in excess of about 10(20) cm(-2) - and can therefore be regarded as 'local', by determining the absorption of stellar emission spectra from nearby stars along their lines of sight by intervening gas and by direct in situ measurements of those components which penetrate the heliosphere sufficiently far, provided they can be distinguished from interplanetary material. The current status of these different investigations gives the following picture: the solar system is surrounded by a 'bubble' of hot gas (density 0.005 cm(-3) temperature 10(6) K) out to several tens of parsecs. More locally it is embedded in a small warm cloud of density 0.07 cm(-3), temperature 7000 K, column density similar to 5 X 10(17) cm(-2) - which gives a mass of about 0.1M circle dot. The transition to the heliosphere is governed by solar UV ionization, snowploughing of the interstellar gas by the outwardly expanding solar wind and the bow shock. The heliosphere is the region inside the solar wind terminal shock. Classically it would be regarded as not yet affected by (or aware of) the obstacle ahead. Practically, the existence of the interstellar medium makes itself felt even far inside the heliosphere by the penetration of neutral gas, dust, plasma waves, shock accelerated particles and cosmic rays. These are the local 'probes' of the interstellar medium.