Bartol Research Institute, University of Delaware, Newark, DE 19716
Department of Astronomy and Astrophysics, University of Chicago, IL 60637
Received 1998 November 12; accepted 1999 January 27

The interaction of the heliosphere with interstellar clouds has attracted interest since the late 1920s, with a view to explaining apparent both quasi-periodic climate “catastrophes” as well as periodic mass extinctions. Until recently, however, models describing the solar windÈlocal interstellar medium (LISM)interaction self-consistently had not been developed. Here we describe the results of a two-dimensional simulation of the interaction between the heliosphere and an interstellar cloud with the same properties as currently, except that the H0 density is increased from the present value of n(H0)D0.2 cm~3 to 10 cm~3. The mutual interaction of interstellar neutral hydrogen and plasma is included. The heliospheric cavity is reduced considerably in size (approximately 10È14 AU to the termination shock in the upstream direction) and is highly dynamical. The interplanetary environment at the orbit of the Earth changes markedly, with the density of interstellar H0 increasing to D2 cm~3. The termination shock itself experiences periods where it disappears, reforms, and disappears again. Considerable mixing of the shocked solar wind and LISM occurs because of Rayleigh-TaylorÈlike instabilities at the nose, driven by ion-neutral friction. Implications of two anomalously high concentrations of 10Be found in Antarctic ice cores, corresponding to 33,000 and 60,000 yr ago, and the absence of prior similar events are discussed in terms of density enhancements in the surrounding interstellar cloud. The calculation presented here supports past speculation that the Galactic environment of the Sun moderates the interplanetary environment at the orbit of the Earth and possibly also the terrestrial climate.

The Galactic environment of the Sun is regulated by the properties of the interstellar cloud surrounding the solar system.
Over the past century, many conjectures have appeared in the scientiÐc literature linking encounters with dense interstellar clouds to possible climate changes on Earth.

For these suggestions to have substance, however, it must first be shown that the interplanetary environment of the Earth varies with changing properties of the surrounding interstellar cloud. It has been shown that in the past the Galactic environment of the Sun has changed as a function of time and that the cloud complex sweeping past the Sun now has an order of magnitude more like that of nearby interstellar gas in the upwind direction than that of the downwind direction (Frisch & York 1986; FR). Therefore the sensitivity of the heliosphere to variations in the boundary conditions imposed by the local interstellar medium (LISM)justify closer examination. It is the purpose of this paper to show that even a moderate alteration in the density of the cloud surrounding the solar system can yield substantial variations to the interplanetary environment in the inner heliosphere.

The implications of high interstellar neutral densities at 1 AU for the terrestrial environment are interesting.

Is there evidence that the Earth has passed through a dense interstellar cloud in the prehistoric or historic past, or that it might do so in the near future?

…A third kind of evidence is the cosmic-ray record implied by the spikes in the 10Be record found in Antarctic ice core samples at ages corresponding to 33,000 and 60,000 yr ago(the D1 and D2 events, respectively).

Raisbeck et al. (1987) and Sonett, MorÐll, & Jokipii (1987) suggest that an increase in the cosmic-ray Ñux on the EarthÏs atmosphere could lead to an enhancement in the precipitated beryllium.

There are two possible ways that the cosmic-ray Ñux at 1 AU can be increased by the interaction of the solar wind with a dense interstellar cloud. The Ðrst is that the increased pickup ion population increases the anomalous cosmic-ray population. The second is that the reduction in the size of the heliospheric cavity implies that Galactic cosmic rays are no longer modulated signiÐcantly by an extended solar wind and so, at 1 AU, the Earth samples the full spectrum.

In addition, enhanced cosmic-ray fluxes at 1 AU may alter the global electric circuit, since the cosmic-ray flux is the dominant source of conductivity in the lower atmosphere (Roble 1985) ; the global electrical circuit has been postulated to play a role in the terrestrial climate (Tinsley 1994, 1997).


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