Shielded by the wind: the influence of the interstellar medium on the environment of Earth

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Abstract

We report on the recent studies on the long-term influence of cosmic rays on the Earth's environment. While on short time-scales solar activity is the driver for atmospheric changes suspected to be due to cosmic rays, for long time-scales the heliosphere, i.e. the circumsolar region occupied by the expanding part of the Sun's atmosphere, has to be considered. The heliosphere is identified as an important shield against interstellar influences and hazards. It has been demonstrated by quantitative modelling that a change of the interstellar medium surrounding the heliosphere as a result of the Sun's quasi-Keplerian motion around the galactic center triggers significant changes of planetary environments caused by enhanced fluxes of neutral atoms as well as by the increased cosmic ray fluxes. We give a compilation of recent space science results of interest to the atmospheric science community.

Introduction

Many studies have revealed that numerous parameters play a role for the environment of Earth (see, e.g. the review by Scherer, 2000). The particular relevance of astrophysical parameters is evident from the obvious importance of the terrestrial elemental abundance as a result of nucleosynthesis in stars, of the chemo-physical conditions on Earth as a result of the nature of the Sun and the structure of the solar system, or of the length of a day and the seasons as a result of the Earth's rotation around its axis and around the Sun, respectively. Examples for rather catastrophic astrophysical events are cometary impacts (e.g. Rampino, 1997).

There are astrophysical parameters of less obvious but potentially high significance. Amongst those is, first, the long-term solar activity which can be monitored using the variation in the abundances of the cosmogenic elements (Beer et al., 1998; Beer, 2000) or historical sunspot data (Solanki et al., 2000). Second, since the paper by Ney (1959) it has been suspected that cosmic rays (CRs) play a role in the climate of Earth (see the recent review by Tinsley, 2000). Also, temperature changes due to Forbush decreases have been studied by Pudovkin and Veretenenko (1995) and the correlation between CRs and the global cloud coverage has been discussed by Svensmark and Friis-Christensen (1997), Svensmark (1998), Feynman and Ruzmaikin (1999), and Bazilevskaya et al. (2000). Third, the Sun's passage through galactic spiral arms during its orbit around the galactic center may cause mass extinctions (Leitch and Vasisht, 1998). Fourth, the heliosphere, i.e. the circumsolar region filled with the plasma of the continuously expanding part of the solar corona, shields the inner solar system from the harsh interstellar environment (e.g. Zank and Frisch, 1999; Scherer, 2000).

In the following, we report on the recent work regarding interstellar influences on the Earth's environment and the heliospheric shielding effects.

Section snippets

The solar galactic neighborhood

Besides the permanent danger of quasi-singular events like a supernova explosion (Ruderman, 1974) or a gamma-ray burst (Thorsett, 1995, see also Dar et al., 1998) in the vicinity of the solar system, changes in the Sun's galactic neighborhood result from its quasi-Keplerian revolution around the center of the Galaxy (Reid et al., 1999), its non-Keplerian oscillation through the galactic mid-plane (Innanen et al., 1978) and its motion relative to the nearby stars (Frisch 1997, Frisch 1998) as

Modelling the interaction between the LISM and the heliosphere

The expanding part of the solar corona, the supersonic solar wind, cannot flow unhindered arbitrarily far away from the Sun but has to adapt to the conditions in the LISM. The mutual interaction of the stellar and the interstellar medium defines the extent of the heliosphere. This interaction can be described within the framework of hydrodynamics and kinetic theory:∂tρi+·(ρiui)=Qρ,i,∂tiui)+·(ρiuiui+PiI)=Qm,i,∂tρi2ui2+Piγi−1+·ρi2ui2ui+γiuiPiγi−1=Qe,i,∂fj∂t=·(κfj)−U·fj+·U3∂fjlnp+Sj(r

Increased neutral atom flux

The expected shrinkage of the heliosphere within a denser interstellar environment weakens the shielding for the planets against the LISM. Assuming that the degree of ionization remains unchanged in a ten times denser cloud, neutral atom fluxes in the heliosphere will be ten times higher. The reduction of the neutral hydrogen flux due to charge exchange with protons in the region between the heliosphere and the bow shock cannot compensate this density increase. Consequently, significantly more

Résumé

It has been shown recently that the entry of the Sun into a denser interstellar cloud will cause the heliosphere to shrink significantly. This shrinkage will facilitate the entry of neutral atoms as well as cosmic-rays into the inner heliosphere. The neutral atoms, mainly hydrogen, will influence the chemistry of planetary atmospheres, in particular the ozone concentration of the terrestrial atmosphere. Furthermore, the (high-energy) cosmic-rays that will reach ground level will have a direct

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