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Original Message
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Terrestrial atmospheric effects induced by counterstreaming dense interstellar cloud material
A. Yeghikyan1 - H. Fahr2
1 - Byurakan Astrophysical Observatory, 378433, Byurakan, Armenia
2 - Institute of Astrophysics and Extraterrestrial Research (IAER), University of Bonn, Auf dem Huegel 71, 53121 Bonn, Germany
Received 15 April 2004 / Accepted 26 May 2004
Abstract
The Solar System during its life has travelled more than 10 times through dense interstellar clouds with particle concentrations of 102-103 and more, compressing the heliosphere to heliopause dimensions smaller than 1 AU and thus bringing the Earth in immediate contact with the interstellar matter. For cloud concentrations greater than of 102 , the flowing interstellar material even at the Earth`s orbit remains completely shielded from solar wind protons and would only be subject to solar photoionization processes. We have developed a 2D-two-fluid gas-dynamical numerical code to describe the hydrodynamical behavior of the incoming interstellar gas near the Earth, taking into account both the photoionization and the gravity of the Sun. As we show, the resulting strongly increased neutral hydrogen fluxes ranging from 109 to 1011 cause substantial changes in the terrestrial atmosphere. During the phase of the immersion into the cloud the resulting flux of neutral hydrogen incident on the terrestrial atmosphere in the steady state would be balanced by the upward escape flux of H-atoms and the downward flux of water molecules, which is the product of the atmospheric hydrogen-oxygen chemistry via even-odd reaction schemes. In that case hydrogen acts as a chemical agent to remove oxygen atoms and to cause ozone concentration reductions above 50 km by a factor of 1.5 at the stratopause to about a factor of 1000 and more at the mesopause. Thus, depending on the specific encounter parameters the high mixing ratio of hydrogen in the Earth's atmosphere may substantially decrease the ozone concentration in the mesosphere and may trigger an ice age of relatively long duration.
1 Introduction
From time to time, the Solar System on its galactic itinerary encounters various galactic objects, e.g. spiral arms (Leitch & Vasisht 1998; Shaviv 2003), star clusters and associations (Innanen 1996), galactic diffuse clouds (H I) and giant molecular clouds (GMC) (Talbot & Newman 1977), etc. Although all encounter probabilities are finite, only a few of them are high enough to make it worthwhile to consider them. All of these mentioned events correspond to different mean travel times of the solar system between consecutive encounters with the corresponding objects, e.g. depending on their distributions in the galactic plane, their sizes and their peculiar velocities. For example, neutral H I clouds, having a mean number density in the range from 10 to 100 and a radius of about a few pc, are objects fairly frequently encountered by the Sun, perhaps over 100 times since its birth 4.6 Gyr ago. The more dense GMCs, having densities of 103 or more, probably must have been encountered by the Sun about 5-10 times (see e.g., Talbot & Newman 1977). When such events happen (especially in the case of GMCs) the solar wind expansion region must be reduced to small scales, and the flow of solar coronal matter hence must be deflected into the heliotail within distances of less than 1 AU. Thus the Earth under these conditions should inevitably be immersed in the direct flow of the cloud material (see Yeghikyan & Fahr 2003), at least during its upwind orbital passage. Concerning this aspect it is interesting to note that Wimmer-Schweingruber & Bochsler (2000) have recently given clear hints that gas constituents implanted in cristalline surface layers of lunar soil grains can be taken as a record of encounters with dense interstellar clouds.
[link to www.aanda.org]
Heliospheric Response to Different Possible Interstellar
Environments
Hans-Reinhard M¨uller1
Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755.
[email protected]
Priscilla C. Frisch
Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637.
[email protected]
Vladimir Florinski and Gary P. Zank
Institute of Geophysics and Planetary Physics, University of California, Riverside, CA 92521.
[email protected], [email protected]
ABSTRACT
At present, the heliosphere is embedded in a warm low density interstellar cloud that belongs
to a cloud system flowing through the local standard of rest with a velocity near ∼18 km s−1. The
velocity structure of the nearest interstellar material (ISM), combined with theoretical models
of the local interstellar cloud (LIC), suggest that the Sun passes through cloudlets on timescales
of ≤ 103–104 yr, so the heliosphere has been, and will be, exposed to different interstellar environments
over time. By means of a multi-fluid model that treats plasma and neutral hydrogen
self-consistently, the interaction of the solar wind with a variety of partially ionized ISM is investigated,
with the focus on low density cloudlets such as are currently near the Sun. Under
the assumption that the basic solar wind parameters remain/were as they are today, a range of
ISM parameters (from cold neutral to hot ionized, with various densities and velocities) is considered.
In response to different interstellar boundary conditions, the heliospheric size and structure
change, as does the abundance of interstellar and secondary neutrals in the inner heliosphere,
and the cosmic ray level in the vicinity of Earth. Some empirical relations between interstellar
parameters and heliospheric boundary locations, as well as neutral densities, are extracted from
the models.
1. INTRODUCTION
The heliosphere is a low density cavity that
is carved out from the local interstellar medium
(LISM) by the solar wind. The size and particle
content of the heliosphere are determined by the solar wind – LISM interaction, and they
vary in response to the Galactic environment of
the Sun as the Sun and interstellar clouds move
through space. The path of the Sun has taken us
through the Local Bubble void, and we have recently ( <∼
103 −105 yr ago, depending on
cloud shapes and densities) entered a clumpy flow
of low density interstellar material (Frisch 1994).
This clumpy flow, the “cluster of local interstellar
cloudlets” (CLIC), is flowing away from the
Sco-Cen association and extends 10–30 pc into the
Galactic center hemisphere and <∼
3 pc for many
directions in the anticenter hemisphere.
6. For encounters with a high density interstellar
cloud (∼15 cm−3, about 50 times the
contemporary value), the particle fluxes arriving
at Earth orbit, including interstellar
neutrals, neutral solar wind, and cosmic rays
will increase markedly. These changes potentially
affect Earth’s atmosphere and its
climate. The changes in particle fluxes just
due to a higher interstellar velocity are less
pronounced.
7. For the period when the Sun was embedded
in the Local Bubble, particle fluxes were
reduced substantially. Secondary particles
like anomalous cosmic rays and neutral solar
wind were entirely absent, and the galactic
cosmic ray flux arriving at Earth was comparable
to the contemporary flux, or even reduced,
depending on the modulation model.
[link to astro.uchicago.edu]
Neutral hydrogen surveys have been made to search for dense cloudlets within the Local Bubble, but none have ever been detected by this means, at least in non-ionized hydrogen gas to which the 21-centimeter observations are the most sensitive. 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 www.astronomycafe.net]
Scientists from the Space Research Centre of the Polish Academy of Sciences, Los Alamos National Laboratory, Southwest Research Institute, and Boston University suggest that the ribbon of enhanced emissions of energetic neutral atoms, discovered last year by the NASA Small Explorer satellite IBEX, could be explained by a geometric effect coming up because of the approach of the Sun to the boundary between the Local Cloud of interstellar gas and another cloud of a very hot gas called the Local Bubble. If this hypothesis is correct, IBEX is catching matter from a hot neighboring interstellar cloud, which the Sun might enter in a hundred years.
First full-sky maps of the emissions of energetic neutral atoms (ENA), obtained last year by IBEX, showed a surprising arc-like feature called the Ribbon. This astonishing discovery was later announced by NASA as one of the most important findings in space exploration made in 2009. Shortly after the discovery six hypotheses were proposed to explain the Ribbon, all of them predicting its relation to processes going on within the heliosphere or in its neighborhood. In a paper recently published in the Astrophysical Journal Letters, a Polish-US team of scientists led by Prof. Stan Grzedzielski from the Space Research Centre of the Polish Academy of Sciences in Warsaw, Poland, offers a different explanation. "We observe the Ribbon," says Grzedzielski "because the Sun is approaching a boundary between our Local Cloud of interstellar gas and another cloud of a very hot and turbulent gas."
[link to www.sciencedaily.com]
December 23, 2009: The solar system is passing through an interstellar cloud that physics says should not exist. In the Dec. 24th issue of Nature, a team of scientists reveal how NASA's Voyager spacecraft have solved the mystery.
Astronomers call the cloud we're running into now the Local Interstellar Cloud or "Local Fluff" for short. It's about 30 light years wide and contains a wispy mixture of hydrogen and helium atoms at a temperature of 6000 C. The existential mystery of the Fluff has to do with its surroundings. About 10 million years ago, a cluster of supernovas exploded nearby, creating a giant bubble of million-degree gas. The Fluff is completely surrounded by this high-pressure supernova exhaust and should be crushed or dispersed by it.
"The observed temperature and density of the local cloud do not provide enough pressure to resist the 'crushing action' of the hot gas around it," says Opher.
*snip*
The Fluff is held at bay just beyond the edge of the solar system by the sun's magnetic field, which is inflated by solar wind into a magnetic bubble more than 10 billion km wide. Called the "heliosphere," this bubble acts as a shield that helps protect the inner solar system from galactic cosmic rays and interstellar clouds. The two Voyagers are located in the outermost layer of the heliosphere, or "heliosheath," where the solar wind is slowed by the pressure of interstellar gas.
*snip*
The fact that the Fluff is strongly magnetized means that other clouds in the galactic neighborhood could be, too. Eventually, the solar system will run into some of them, and their strong magnetic fields could compress the heliosphere even more than it is compressed now. Additional compression could allow more cosmic rays to reach the inner solar system, possibly affecting terrestrial climate and the ability of astronauts to travel safely through space. On the other hand, astronauts wouldn't have to travel so far because interstellar space would be closer than ever. These events would play out on time scales of tens to hundreds of thousands of years, which is how long it takes for the solar system to move from one cloud to the next.
"There could be interesting times ahead!" says Opher.
[link to science.nasa.gov]
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