Abstract
Abstract. Several consequences of the passage of the solar system through dense interstellar molecular clouds are discussed. These clouds, dense (more than 100 cm-3), cold (10–50 K) and extended (larger than 1 pc), are characterized by a gas-to-dust mass ratio of about 100, by a specific power grain size spectrum (grain radii usually cover the range 0.001–3 micron) and by an average dust-to-gas number density ratio of about 10-12. Frequently these clouds contain small-scale (10–100 AU) condensations with gas concentrations ranging up to 10 5 cm-3. At their casual passage over the solar system they exert pressures very much enhanced with respect to today’s standards. Under these conditions it will occur that the Earth is exposed directly to the interstellar flow. It is shown first that even close to the Sun, at 1 AU, the cloud’s matter is only partly ionized and should mainly interact with the solar wind by charge exchange processes. Dust particles of the cloud serve as a source of neutrals, generated by the solar UV irradiation of dust grains, causing the evaporation of icy materials. The release of neutral atoms from dust grains is then followed by strong influences on the solar wind plasma flow. The behavior of the neutral gas inflow parameters is investigated by a 2-D hydrodynamic approach to model the interaction processes. Because of a reduction of the heliospheric dimension down to 1 AU, direct influence of the cloud’s matter to the terrestrial environment and atmosphere could be envisaged.Key words. Interplanetary physics (heliopause and solar wind termination; interplanetary dust; interstellar gas)
Highlights
The interstellar medium (ISM) in our galaxy, a mixture of gas and dust is known to be highly inhomogeneous with respect to density and temperature
Observations of real comets show at 1 AU the values ZE 1 · 1016 mol · cm−2· s−1 apply both for production rates of water and atomic hydrogen (Delsemme, 1982)
In this paper we have shown that efficient dust evaporation processes increase the inflow density and push the heliopause down to the Earth, with the result that counterstreaming interstellar clouds (IC) material directly influences the Earth
Summary
The interstellar medium (ISM) in our galaxy, a mixture of gas and dust is known to be highly inhomogeneous with respect to density and temperature. Adopting mean values for the parameters DIC and N (2 and 5 pc, and 15 · 10−5 and 0.14 · 10−5 pc−3 for the HI and GMC, respectively, (Talbot and Newman, 1977), as well as for the v ∼ 20 km/s, (Scoville and Sanders, 1986), one may find 27 Myr and 460 Myr as the mean travel time between encounters of the Sun with the neutral HIand molecular H2-clouds, respectively. Pauls et al (1995) used a time-dependent hydrodynamical model to describe the mutual influence of the two components, but the effects of photoionization and gravity were ignored This code was used to investigate the global structure of the heliosphere embedded in an environment with a density of neutral hydrogen increased from the present value ni ∼ 0.1 cm−3 to 10 cm−3, i.e. by a factor of 100 (Zank and Frisch, 1999). In the following we will adopt the present parameters of the solar output, which is valid for the most frequent events under consideration (encounters with IC during last 2–3 Gyr), while more specific models should be developed for the more earlier ones
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