13,671 research outputs found
Consequences of a Change in the Galactic Environment of the Sun
The interaction of the heliosphere with interstellar clouds has attracted
interest since the late 1920's, both with a view to explaining apparent
quasi-periodic climate "catastrophes" as well as periodic mass extinctions.
Until recently, however, models describing the solar wind - local interstellar
medium (LISM) interaction self-consistently had not been developed. Here, we
describe the results of a two-dimensional (2D) simulation of the interaction
between the heliosphere and an interstellar cloud with the same properties as
currently, except that the neutral H density is increased from the present
value of n(H) ~ 0.2 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 10 - 14 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 H
increasing to ~2 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 due to Rayleigh-Taylor-like instabilities at the
nose, driven by ion-neutral friction. Implications for two anomalously high
concentrations of 10Be found in Antarctic ice cores 33 kya and 60 kya, 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.Comment: 23 pages, 2 color plates (jpg), 3 figures (eps
Is the Sun Embedded in a Typical Interstellar Cloud?
The physical properties and kinematics of the partially ionized interstellar
material near the Sun are typical of warm diffuse clouds in the solar vicinity.
The interstellar magnetic field at the heliosphere and the kinematics of nearby
clouds are naturally explained in terms of the S1 superbubble shell. The
interstellar radiation field at the Sun appears to be harder than the field
ionizing ambient diffuse gas, which may be a consequence of the low opacity of
the tiny cloud surrounding the heliosphere. The spatial context of the Local
Bubble is consistent with our location in the Orion spur.Comment: "From the Outer Heliosphere to the Local Bubble", held at
International Space Sciences Institute, October 200
Material studies related to lunar surface exploration. Volume 4 - Preliminary studies for the design of engineering probes Final report, 6 Mar. 1967 - 30 Jun. 1968
Preliminary design of engineering probes for studying lunar surface material propertie
Locality and stability of the cascades of two-dimensional turbulence
We investigate and clarify the notion of locality as it pertains to the
cascades of two-dimensional turbulence. The mathematical framework underlying
our analysis is the infinite system of balance equations that govern the
generalized unfused structure functions, first introduced by L'vov and
Procaccia. As a point of departure we use a revised version of the system of
hypotheses that was proposed by Frisch for three-dimensional turbulence. We
show that both the enstrophy cascade and the inverse energy cascade are local
in the sense of non-perturbative statistical locality. We also investigate the
stability conditions for both cascades. We have shown that statistical
stability with respect to forcing applies unconditionally for the inverse
energy cascade. For the enstrophy cascade, statistical stability requires
large-scale dissipation and a vanishing downscale energy dissipation. A careful
discussion of the subtle notion of locality is given at the end of the paper.Comment: v2: 23 pages; 4 figures; minor revisions; resubmitted to Phys. Rev.
Material studies related to lunar surface exploration Technical summary report, 6 Mar. 1967 - 30 Jun. 1968
Summary of research studies on lunar surface material propertie
Material studies related to lunar surface exploration. Volume 1 - Lunar soil mechanics and soil properties Final report, 6 Mar. 1967 - 30 Jun. 1968
Lunar soil mechanics and properties for structural engineering aspects of lunar spacecraft landings and surface exploratio
Time-variability in the Interstellar Boundary Conditions of the Heliosphere: Effect of the Solar Journey on the Galactic Cosmic Ray Flux at Earth
During the solar journey through galactic space, variations in the physical
properties of the surrounding interstellar medium (ISM) modify the heliosphere
and modulate the flux of galactic cosmic rays (GCR) at the surface of the
Earth, with consequences for the terrestrial record of cosmogenic
radionuclides. One phenomenon that needs studying is the effect on cosmogenic
isotope production of changing anomalous cosmic ray fluxes at Earth due to
variable interstellar ionizations. The possible range of interstellar ram
pressures and ionization levels in the low density solar environment generate
dramatically different possible heliosphere configurations, with a wide range
of particle fluxes of interstellar neutrals, their secondary products, and GCRs
arriving at Earth. Simple models of the distribution and densities of ISM in
the downwind direction give cloud transition timescales that can be directly
compared with cosmogenic radionuclide geologic records. Both the interstellar
data and cosmogenic radionuclide data are consistent with cloud transitions
during the Holocene, with large and assumption-dependent uncertainties. The
geomagnetic timeline derived from cosmic ray fluxes at Earth may require
adjustment to account for the disappearance of anomalous cosmic rays when the
Sun is immersed in ionized gas.Comment: Submitted to Space Sciences Review
The Velocity Distribution of the Nearest Interstellar Gas
The bulk flow velocity for the cluster of interstellar cloudlets within about
30 pc of the Sun is determined from optical and ultraviolet absorption line
data, after omitting from the sample stars with circumstellar disks or variable
emission lines and the active variable HR 1099. Ninety-six velocity components
towards the remaining 60 stars yield a streaming velocity through the local
standard of rest of -17.0+/-4.6 km/s, with an upstream direction of l=2.3 deg,
b=-5.2 deg (using Hipparcos values for the solar apex motion). The velocity
dispersion of the interstellar matter (ISM) within 30 pc is consistent with
that of nearby diffuse clouds, but present statistics are inadequate to
distinguish between a Gaussian or exponential distribution about the bulk flow
velocity. The upstream direction of the bulk flow vector suggests an origin
associated with the Loop I supernova remnant. Groupings of component velocities
by region are seen, indicating regional departures from the bulk flow velocity
or possibly separate clouds. The absorption components from the cloudlet
feeding ISM into the solar system form one of the regional features. The
nominal gradient between the velocities of upstream and downstream gas may be
an artifact of the Sun's location near the edge of the local cloud complex. The
Sun may emerge from the surrounding gas-patch within several thousand years.Comment: Typographical errors corrected; Five tables, seven figures;
Astrophysical Journal, in pres
Material studies related to lunar surface exploration. Volume 2 - Application of geophysical and geotechnical methods to lunar sites exploration Final report, 6 Mar. 1967 - 30 Jun. 1968
Geophysical and geotechnical methods for lunar landing site soil property studie
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