6,742 research outputs found
A study of periodicities and recurrences in solar activity and cosmic ray modulation
The 154d periodicity was found in the cosmic ray intensity (RE) vs Flares, and some other peaks of coherency in the RC vs aa sub I, that when interpreted as aliased values, might correspond to recurring interplanetary magnetic field structures and solar wind streams. It cannot be excluded, however, that some of the correspondence with aa are of terrestrial origin. This study cannot be considered exhaustive due to the fact that other solar variables, such as polar hole size, are possibly correlated to cosmic ray intensities. However, the number of observations is small so that the interpretation of the results is very difficult
Astrophysical Ionizing Radiation and the Earth: A Brief Review and Census of Intermittent Intense Sources
Cosmic radiation backgrounds are a constraint on life, and their distribution
will affect the Galactic Habitable Zone. Life on Earth has developed in the
context of these backgrounds, and characterizing event rates will elaborate the
important influences. This in turn can be a base for comparison with other
potential life-bearing planets. In this review we estimate the intensities and
rates of occurrence of many kinds of strong radiation bursts by astrophysical
entities ranging from gamma-ray bursts at cosmological distances to the Sun
itself. Many of these present potential hazards to the biosphere: on timescales
long compared with human history, the probability of an event intense enough to
disrupt life on the land surface or in the oceans becomes large. We enumerate
the known sources of radiation and characterize their intensities at the Earth
and rates or upper limits on these quantities. When possible, we estimate a
"lethal interval", our best estimate of how often a major extinction-level
event is probable given the current state of knowledge; we base these estimates
on computed or expected depletion of stratospheric ozone. In general, moderate
level events are dominated by the Sun, but the far more severe infrequent
events are probably dominated by gamma-ray bursts and supernovae. We note for
the first time that so-called "short-hard" gamma-ray bursts are a substantial
threat, comparable in magnitude to supernovae and greater than that of the
higher-luminosity long bursts considered in most past work. Given their
precursors, short bursts may come with little or no warning.Comment: to be published in Astrobiolog
Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data
The MOST (Microvariability & Oscillations of STars) satellite obtains
ultraprecise photometry from space with high sampling rates and duty cycles.
Astronomical photometry or imaging missions in low Earth orbits, like MOST, are
especially sensitive to scattered light from Earthshine, and all these missions
have a common need to extract target information from voluminous data cubes.
They consist of upwards of hundreds of thousands of two-dimensional CCD frames
(or sub-rasters) containing from hundreds to millions of pixels each, where the
target information, superposed on background and instrumental effects, is
contained only in a subset of pixels (Fabry Images, defocussed images,
mini-spectra). We describe a novel reduction technique for such data cubes:
resolving linear correlations of target and background pixel intensities. This
stepwise multiple linear regression removes only those target variations which
are also detected in the background. The advantage of regression analysis
versus background subtraction is the appropriate scaling, taking into account
that the amount of contamination may differ from pixel to pixel. The
multivariate solution for all pairs of target/background pixels is minimally
invasive of the raw photometry while being very effective in reducing
contamination due to, e.g., stray light. The technique is tested and
demonstrated with both simulated oscillation signals and real MOST photometry.Comment: 16 pages, 23 figure
Long term time variability of cosmic rays and possible relevance to the development of life on Earth
An analysis is made of the manner in which the cosmic ray intensity at Earth
has varied over its existence and its possible relevance to both the origin and
the evolution of life. Much of the analysis relates to the 'high energy' cosmic
rays () and their variability due to the changing
proximity of the solar system to supernova remnants which are generally
believed to be responsible for most cosmic rays up to PeV energies. It is
pointed out that, on a statistical basis, there will have been considerable
variations in the likely 100 My between the Earth's biosphere reaching
reasonable stability and the onset of very elementary life. Interestingly,
there is the increasingly strong possibility that PeV cosmic rays are
responsible for the initiation of terrestrial lightning strokes and the
possibility arises of considerable increases in the frequency of lightnings and
thereby the formation of some of the complex molecules which are the 'building
blocks of life'. Attention is also given to the well known generation of the
oxides of nitrogen by lightning strokes which are poisonous to animal life but
helpful to plant growth; here, too, the violent swings of cosmic ray
intensities may have had relevance to evolutionary changes. A particular
variant of the cosmic ray acceleration model, put forward by us, predicts an
increase in lightning rate in the past and this has been sought in Korean
historical records. Finally, the time dependence of the overall cosmic ray
intensity, which manifests itself mainly at sub-10 GeV energies, has been
examined. The relevance of cosmic rays to the 'global electrical circuit'
points to the importance of this concept.Comment: 18 pages, 5 figures, accepted by 'Surveys in Geophysics
A measurement of the spectrum of cosmic ray electrons between 20 MeV and 4 BeV in 1968 - Further evidence for extensive time variations of this component
Cosmic ray electron intensity and spectrum measurements between 20 MeV and 4 BeV - 196
Spatial and Temporal Variability of the Gamma Radiation from Earth's Atmosphere during a Solar Cycle
The Solar Maximum Mission satellite's Gamma Ray Spectrometer observed Earth's
atmosphere for most of the period 1980-1989. Its 28deg orbit ensured that a
range of geomagnetic latitudes (geomagnetic cutoff rigidities) was sampled. We
measured the variation with time and rigidity of albedo gamma-ray lines at 1.6
MeV, 2.3 MeV and 4.4 MeV which are diagnostic of Galactic cosmic radiation
penetrating the cutoff and of the secondary neutrons produced in the
atmosphere. We found that the gamma-ray line intensities varied inversely with
solar activity and cutoff rigidity, as expected. The line ratio (1.6 MeV + 2.3
MeV)/4.4 MeV was remarkably constant (close to 0.39) at all times and
rigidities; the former two lines are produced by 5-10 MeV secondary neutrons
causing excitation and de-excitation of 14N, while the latter is produced by
more energetic (>20 MeV) neutrons inducing spallation. We infer that the shape
of the secondary neutron energy spectrum is virtually constant everywhere and
at all times. We also measured the intensity of the 0.511 MeV electron-positron
annihilation line. This line too varies with solar cycle and cutoff rigidity,
but its fall-off from low to high rigidity is less marked than that of the
nuclear lines. This results from the energy dependences of the cross sections
for positron production and for the hadronic processes which which produce
secondary neutrons.Comment: 11 pages, 12 figs., J. Geophys. Res. in pres
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