389 research outputs found
On the heliolatitudinal variation of the galactic cosmic-ray intensity. Comparison with Ulysses measurements
International audienceWe study the dependence of cosmic rays with heliolatitude using a simple method and compare the results with the actual data from Ulysses and IMP spacecraft. We reproduce the galactic cosmic-ray heliographic latitudinal intensity variations, applying a semi-empirical, 2-D diffusion-convection model for the cosmic-ray transport in the interplanetary space. This model is a modification of our previous 1-D model (Exarhos and Moussas, 2001) and includes not only the radial diffusion of the cosmic-ray particles but also the latitudinal diffusion. Dividing the interplanetary region into "spherical magnetic sectors" (a small heliolatitudinal extension of a spherical magnetized solar wind plasma shell) that travel into the interplanetary space at the solar wind velocity, we calculate the cosmic-ray intensity for different heliographic latitudes as a series of successive intensity drops that all these "spherical magnetic sectors" between the Sun and the heliospheric termination shock cause the unmodulated galactic cosmic-ray intensity. Our results are compared with the Ulysses cosmic-ray measurements obtained during the first pole-to-pole passage from mid-1994 to mid-1995
On signal-noise decomposition of timeseries using the continuous wavelet transform: Application to sunspot index
We show that the continuous wavelet transform can provide a unique
decomposition of a timeseries in to 'signal-like' and 'noise-like' components:
From the overall wavelet spectrum two mutually independent skeleton spectra
can be extracted, allowing the separate detection and monitoring in even
non-stationary timeseries of the evolution of (a) both stable but also
transient, evolving periodicities, such as the output of low dimensional
dynamical systems and (b) scale-invariant structures, such as discontinuities,
self-similar structures or noise. An indicative application to the
monthly-averaged sunspot index reveals, apart from the well-known 11-year
periodicity, 3 of its harmonics, the 2-year periodicity (quasi-biennial
oscillation, QBO) and several more (some of which detected previously in
various solar, earth-solar connection and climate indices), here proposed being
just harmonics of the QBO, in all supporting the double-cycle solar magnetic
dynamo model (Benevolenskaya, 1998, 2000). The scale maximal spectrum reveals
the presence of 1/f fluctuations with timescales up to 1 year in the sunspot
number, indicating that the solar magnetic configurations involved in the
transient solar activity phenomena with those characteristic timescales are in
a self-organized-critical state (SOC), as previously proposed for the solar
flare occurence (Lu and Hamilton, 1991).Comment: 22 pages, 2 figure
Spatial & Temporal Characteristics of Ha flares during the period 1975-2002 (comparison with SXR flares)
Although the energetic phenomena of the Sun (flares, coronal mass injections
etc.) exhibit intermittent stochastic behavior in their rate of occurrence,
they are well correlated to the variations of the solar cycle. In this work we
study the spatial and temporal characteristics of transient solar activity in
an attempt to statistically interpret the evolution of these phenomena through
the solar cycle, in terms of the self-organized criticality theory.Comment: Recent Advances in Astronomy and Astrophysics: 7th International
Conference of the Hellenic Astronomical Society. AIP Conference Proceedings,
Volume 848, pp. 194-198 (2006
Extraction of Fraud Schemes from Trade Series
2000 Mathematics Subject Classification: 62H30, 62M10, 62M20, 62P20, 94A13.It is very often the case that the patterns of a fraudulent activity in trade are hidden within existing trade data time series. Furthermore, with the advent of powerful and affordable computing hardware, relatively big amounts of available trade data can be quickly analyzed with a view to assisting antifraud investigations in this field. In this paper, based on the availability of such import/export data series, we present a statistical method for the identification of potential fraud schemes, by extracting and highlighting those cases which lend themselves to further investigation by anti-fraud domain experts. The proposed method consists in applying time series analysis for prediction purposes, calculating the resulting significant deviations, and finally clustering time series with similar patterns together, thus identifying suspect or abnormal cases
A Study of Halo Coronal Mass Ejections and Related Flare and Radio Burst Observations in Solar Cycle 23
We present a statistical study of dynamical and kinetic characteristics of
CMEs which show temporal and spatial association with flares and type II radio
bursts or complex radio events of type II bursts and type IV continua. This
study is based on a set of earth-directed full halo CMEs occurring during the
present solar cycle, with data from the Large Angle Spectrometric Coronagraphs
(LASCO) and Extreme-Ultraviolet Imaging Telescope (EIT) aboard the Solar and
Heliospheric Observatory (SOHO) mission and the Magnetic Fields Investigation
(MFI) and 3-D Plasma and Energetic Particle Analyzer Investigation experiment
on board the WIND spacecraft.Comment: Recent Advances in Astronomy and Astrophysics: 7th International
Conference of the Hellenic Astronomical Society. AIP Conference Proceedings,
Volume 848, pp. 218-223 (2006
Coronal shocks associated with CMEs and flares and their space weather consequences
We study the geoeffectiveness of a sample of complex events; each includes a
coronal type II burst, accompanied by a GOES SXR flare and LASCO CME. The radio
bursts were recorded by the ARTEMIS-IV radio spectrograph, in the 100-650 MHz
range; the GOES SXR flares and SOHO/LASCO CMEs, were obtained from the Solar
Geophysical Data (SGD) and the LASCO catalogue respectively. These are compared
with changes of solar wind parameters and geomagnetic indices in order to
establish a relationship between solar energetic events and their effects on
geomagnetic activity.Comment: Universal Heliophysical Processes, Proceedings of the International
Astronomical Union, IAU Symposium, Volume 257, p. 61-6
Cosmic Rays X. The cosmic ray knee and beyond: Diffusive acceleration at oblique shocks
Our purpose is to evaluate the rate of the maximum energy and the
acceleration rate that cosmic rays acquire in the non-relativistic diffusive
shock acceleration as it could apply during their lifetime in various
astrophysical sites, where highly oblique shocks exist. We examine numerically
(using Monte Carlo simulations) the effect of the diffusion coefficients on the
energy gain and the acceleration rate, by testing the role between the
obliquity of the magnetic field to the shock normal, and the significance of
both perpendicular cross-field diffusion and parallel diffusion coefficients to
the acceleration rate. We find (and justify previous analytical work - Jokipii
1987) that in highly oblique shocks the smaller the perpendicular diffusion
gets compared to the parallel diffusion coefficient values, the greater the
energy gain of the cosmic rays to be obtained. An explanation of the cosmic ray
spectrum in high energies, between eV and about eV is
claimed, as we estimate the upper limit of energy that cosmic rays could gain
in plausible astrophysical regimes; interpreted by the scenario of cosmic rays
which are injected by three different kind of sources, (a) supernovae which
explode into the interstellar medium, (b) Red Supergiants, and (c) Wolf-Rayet
stars, where the two latter explode into their pre-supernovae winds.Comment: Accepted in Astronomy and Astrophysics, 9 pages, 8 figures (for the
'Cosmic Rays' series papers
Shock and statistical acceleration of energetic particles in the interplanetary medium
Definite evidence for particle acceleration in the solar wind came around a decade ago. Two likely sources are known to exist: particles may be accelerated by the turbulence resulting from the superposition of Alfven and Magnetosonic waves (Statistical Acceleration) or they may be accelerated directly at shock fronts formed by the interaction of fast and slow solar wind (CIR's) or by traveling shocks due to sporadic coronal mass ejections. Naurally both mechanisms may be operative. In this work the acceleration problem was tackled numerically using Helios 1 and 2 data to create a realistic representation of the Heliospheric plasma. Two 24 hour samples were used: one where there are only wave like fluctuations of the field (Day 90 Helios 1) and another with a shock present in it (Day 92 of Helios 2) both in 1976 during the STIP 2 interval. Transport coefficients in energy space have been calculated for particles injected in each sample and the effect of the shock studied in detail
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