(Thanks goes out to Xenus for his informative post; see link)
The existence of a cloud or clouds near the Sun has, however, been established by what are called solar backscatter observations in which the lyman-alpha emission from the Sun is reflected back to the Earth from distant material outside the solar system. There is, apparently, a medium called the Local Fluff in which the solar system is embedded, which has a density of about 0.1 atoms/cc, a temperature of 10,000 K, and a relative velocity with respect to the solar system of about 20 km/sec based on a slight doppler shift in the reflected emission. McClintock and his coworkers in 1978 used data from the Copernicus satellite which involved measuring the Local Fluff towards stars with distances between 1.3 and 14 parsecs, and concluded that the Local Fluff extends about 3.5 parsecs. Frisch and York, in 1983, surveyed 140 stars out to several hundred parsecs from the Sun and detected a pattern of emission that indicated a dense cloud located about 17-35 parsecs from the Sun towards the Galactic Center in Sagittarius. In a 1983 Nature article ( vol 302, p. 806) Francesco Paresce proposed that the Local Fluff is the low density, ionized outer layers of this cloud, and that the Sun has just recently entered the outer regions of this dense cloud.
Astronomers Priscilla Frisch and Daniel Welty at the University of Chicago announced at the June, 1996 meeting of the American Astronomical Society ( see the New York Times, Science Supplement, June 18, issue) recapitulated the earlier proposal that the Sun may have already entered the Local Fluff a few thousand years ago. Observations by Dr. Jeffrey Linsky at the University of Colorado of 18 nearby stars indicated that the Local Fluff cloud surrounding the solar system was not a uniform cloud, but contained cloudlets of very different internal density with one of these located between the Sun and the nearby star Alpha Centauri.
Astronomers John Watson and David Meyer at Northwestern University have also discovered that in the Sun's vicinity, the interstellar medium is filled with many cloudlets with a size comparable to the solar system. Radio astronomers have also observed the phenomenon of interstellar scintillation in the radio signals from distant quasars, and deduced that the interstellar medium is far from smooth, but contains clumps and filaments at many different scales.
The solar system is, apparently, moving along a path that is certain to take us closer to the Sco-Cen expanding superbubble. The 'wall' between the Local Bubble and the Sco-Cen bubble now seems to consist of an increasing density of cloudlets of varying size and density. The Sun, after apparently spending many hundreds of millennia in quieter regions of the Local Bubble, is apparently now moving nearer one wall of this cavity towards us from the direction of Scorpio/Centaurus. Rather than a smooth wall of material, it consists of many individual pieces and cloudlets. When the solar system enters such a cloud, the first thing that will happen will be that the magnetic field of the Sun, which now extends perhaps 100 AU from the Sun and 2-3 times the orbit of Pluto, will be compressed back into the inner solar system depending on the density of the medium that the Sun encounters. When this happens, the Earth may be laid bare to an increased cosmic ray bombardment. To make matters worse, the Earth's magnetic field is itself decreasing as we enter the next field reversal era in a few thousand years. If the Earth's field is 'down' during the same time that the solar system has wandered into the new could, the cosmic ray flux at the Earth's surface could be many times higher than it now is.
The biological effects may not be so severe. We just don't really know. Fossil records show that in previous field reversals, there was hardly a sign of any biological impact caused by species extinctions or mutations. We don't really know when the last time it was that our solar system found itself in a dense interstellar cloud, so we cannot look at the fossil record to see what effects this might have had. Since all of the major extinctions seem to be related to tectonic activity, or to asteroid impacts, there isn't much left over to argue that there will be a dire effect of the next cloud passage upon the biosphere. If you believe our knowledge of the solar vicinity, the next cloud passage could happen within 20 - 50,000 years. I guess we will just have to wait and see.
link to http://www.astronomycafe.net]/
Interstellar Cloud Material: Contribution to Planetary Atmospheres
A statistical analysis of the properties of dense interstellar clouds indicates that the solar system has encountered at least a dozen clouds of sufficient density to cause planets to accumulate nonnegligible amounts of some isotopes. The effect is most pronounced for neon. This mechanism could be responsible for much of the neon in Earth's atmosphere.
link to http://www.sciencemag.org]/
ESA Sees Stardust Storms Heading For Solar System
Until ten years ago, most astronomers did not believe stardust could enter our Solar System. Then ESA's Ulysses spaceprobe discovered minute stardust particles leaking through the Sun's magnetic shield, into the realm of Earth and the other planets. Now, the same spaceprobe has shown that - flood of dusty particles is heading our way.
Astronomers still do not know whether the current stardust influx, apart from being favoured by the particular configuration of the Sun's magnetic field, is also enhanced by the thickness of the interstellar clouds into which the Solar System is moving. Currently located at the edge of what astronomers call the local interstellar cloud, our Sun is about to join our closest stellar neighbour Alpha Centauri in its cloud, which is less hot but denser.
link to http://www.spacedaily.com]/
Consequences of the Solar System passage through dense
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 (10–100AU) condensations with gas concentrations ranging up to 105 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 1AU, 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 possibility of solar encounters with dense interstellar clouds (IC) with particle concentrations about 10 − 1000 cm−3 and more, is of great interest in view of its possible effects upon the Earth. The question of whether dense IC would prevent the solar wind (SW) from reaching the Earth with the result of cloud material directly impacting the terrestrial atmosphere, as well as many other aspects of this complex problem were already discussed in the literature of the past (Fahr, 1968a, b; Talbot and Newman, 1977; Holzer, 1977; Fahr, 1980; Ripken and Fahr, 1981; Zank and Frisch, 1999; Scherer, 2000; Scherer et al. 2002, and references therein). Such a scenario is considered as possibly triggering global glaciations, depositions of a prebiotic material on the primordial Earth, possible ecological repercussions for the Earth due to an accretion of the cloud’s matter, and of course, bio-mass extinction (Yabushita and Allen, 1997). It should also be added that in fact the correlation between periods of glaciations and long-time variations in the accretion rate of interplanetary dust particles onto the Earth is revealed (Farley and Patterson, 1995).
Yabushita and Allen (1983) have argued that if the Sun should ever have passed through such dense core of GMC (nH = 105), then as much oxygen as is now present in the terrestrial atmosphere could have been removed, due to water formation.
We have also shown that large dust grains of the cloud, when present in countable abundance, serve as a source of neutrals, generated by the solar UV irradiation of dust particles, followed by a strong influence on the gas flow, by mass loading.
[link to http://www.ann-geophys.net/]