34 research outputs found
A quantitative test of Jones NTC beaming theory using CLUSTER constellation
Non-thermal continuum (NTC) radiation is, with auroral kilometric radiation (AKR), one of the two electromagnetic emissions generated within the Earth's magnetosphere and radiated into space. The location of the source of NTC has been sought for several decades, with only limited success. The constellation formed by the four CLUSTER spacecraft provides the possibility of triangulation in the vicinity of the source, thus allowing progress in source localisation, while simultaneously revealing the beaming properties of NTC radio sources. <br><br> We present a case event showing two beams localised on opposite sides of the magnetic equator. At any selected frequency, triangulation points to a single region source of small size. Its position is compatible with the range of possible loci of sources predicted by the radio window theory of Jones (1982) in a frame of constraints relaxed from the simple sketch proposed in early works. The analysis of similar observations from the Dynamics Explorer 1 by Jones et al. (1987) enabled the authors to claim validation of the radio window theory. CLUSTER observations, however, reveal a large beaming cone angle projected onto the ecliptic plane, a feature unobservable by Dynamics Explorer which had a different spin axis orientation. According to the radio window theory, such a large observed cone angle can only be formed by a series of point sources, each beaming in a narrow cone angle. This study demonstrates the difficulty of validating NTC linear generation mechanisms using global beaming properties alone
Reconnaissance automatique de fréquences de résonance à partir des données satellitaires de la mission spatiale CLUSTER
Dans cette communication, nous présentons une méthode automatique pour l'identification de la fréquence de résonance d'un plasma spatial. Les données satellitaires sont issues de deux instruments de mesure. Elles se présentent sous la forme d'un spectrogramme bruité et d'un signal. Ce signal permet de procéder automatiquement à un découpage adapté en temps des spectrogrammes et à une analyse en fréquence de chaque bande temporelle. La méthode mise en place exploite la distance de Fréchet comme critère de ressemblance. Une courbe de l'évolution de la fréquence plasma au cours du temps est ensuite reconstruite. Le logiciel développé permet l'étude systématique, dans certaines régions de la magnétosphère terrestre, des spectrogrammes de l'expérience WHISPER de la mission spatiale CLUSTER
Nuclear expansion with excitation
The expansion of an isolated hot spherical nucleus with excitation energy and
its caloric curve are studied in a thermodynamic model with the SkM* force as
the nuclear effective two-body interaction. The calculated results are shown to
compare well with the recent experimental data from energetic nuclear
collisions. The fluctuations in temperature and density are also studied. They
are seen to build up very rapidly beyond an excitation energy of 9 MeV/u.
Volume-conserving quadrupole deformation in addition to expansion indicates,
however, nuclear disassembly above an excitation energy of 4 MeV/uComment: 17 pages, 5 figures, revtex4; calculations with deformation adde
Modulation of NTC frequencies by Pc5 ULF pulsations : experimental test of the generation mechanism and magnetoseismology of the emitting surface
Nonthermal continuum (NTC) radiation is believed to be emitted by the conversion of an electrostatic wave into an electromagnetic one, which takes place at the Earth's magnetic equator. It is generally accepted that the frequency of the electrostatic wave at the source meets a local characteristic frequency placed in between two multiples of the electron cyclotron frequency, fce, which results in emission of a narrow band frequency element. In an event on 14 August 2003, we compare oscillations of the central frequency of distinct NTC frequency elements observed from Cluster orbiting near perigee, with simultaneous Pc5 Ultra Low Frequency (ULF) pulsations in the magnetic field observed from the same platform. The latter magnetic perturbations are interpreted as magnetohydrodynamic poloidal waves, where fundamental and second harmonic modes coexist. The NTC oscillation and the fundamental wave have similar periods, but are phase shifted by a quarter of period. From the correlation between both signals, and the proximity of the NTC source (localized via triangulation) with Cluster, we infer that the poloidal perturbations are spatially uniform between the source and the satellites. From the phase shift between signals, we conclude that the electrostatic wave which converts into NTC is mainly governed by the plasma density, affected by movements of the magnetic field lines. Furthermore, we demonstrate that the observations can be used to perform a magnetoseismology of the emitting surface. The results show a steepening of the plasmapause density profile near the satellites, which can be responsible for the generation of NTC emission
Nuclei beyond the drip line
In a Thomas-Fermi model, calculations are presented for nuclei beyond the
nuclear drip line at zero temperature. These nuclei are in equilibrium by the
presence of an external gas, as may be envisaged in the astrophysical scenario.
We find that there is a limiting asymmetry beyond which these nuclei can no
longer be made stable.Comment: Physical Review C (in press), 1 ReVteX file for text, 4 PS-files for
figure
Thomas-Fermi theory for atomic nuclei revisited
The recently developed semiclassical variational Wigner-Kirkwood (VWK)
approach is applied to finite nuclei using external potentials and
self-consistent mean fields derived from Skyrme interactions and from
relativistic mean field theory. VWK consists of the Thomas-Fermi part plus a
pure, perturbative hbar^2 correction. In external potentials, VWK passes
through the average of the quantal values of the accumulated level density and
total energy as a function of the Fermi energy. However, there is a problem of
overbinding when the energy per particle is displayed as a function of the
particle number. The situation is analyzed comparing spherical and deformed
harmonic oscillator potentials. In the self-consistent case, we show for Skyrme
forces that VWK binding energies are very close to those obtained from extended
Thomas-Fermi functionals of hbar^4 order, pointing to the rapid convergence of
the VWK theory. This satisfying result, however, does not cure the overbinding
problem, i.e., the semiclassical energies show more binding than they should.
This feature is more pronounced in the case of Skyrme forces than with the
relativistic mean field approach. However, even in the latter case the shell
correction energy for e.g. 208Pb turns out to be only -6 MeV what is about a
factor two or three off the generally accepted value. As an ad hoc remedy,
increasing the kinetic energy by 2.5%, leads to shell correction energies well
acceptable throughout the periodic table. The general importance of the present
studies for other finite Fermi systems, self-bound or in external potentials,
is pointed out.Comment: 37 pages, 14 figures, revtex
Isospin-rich nuclei in neutron star matter
Stability of nuclei beyond the drip lines in the presence of an enveloping
gas of nucleons and electrons, as prevailing in the inner crust of a neutron
star, is studied in the temperature-dependent Thomas-Fermi framework. A
limiting asymmetry in the isospin space beyond which nuclei cannot exist
emerges from the calculations. The ambient conditions like temperature, baryon
density and neutrino concentration under which these exotic nuclear systems can
be formed are studied in some detail.Comment: Submitted to Phy. Rev. C: Revtex version of manuscript 22 pages and
10 PS-files for figure
Isospin-rich nuclei in neutron star matter
Stability of nuclei beyond the drip lines in the presence of an enveloping
gas of nucleons and electrons, as prevailing in the inner crust of a neutron
star, is studied in the temperature-dependent Thomas-Fermi framework. A
limiting asymmetry in the isospin space beyond which nuclei cannot exist
emerges from the calculations. The ambient conditions like temperature, baryon
density and neutrino concentration under which these exotic nuclear systems can
be formed are studied in some detail.Comment: Submitted to Phy. Rev. C: Revtex version of manuscript 22 pages and
10 PS-files for figure
Studies in the statistical and thermal properties of hadronic matter under some extreme conditions
The thermal and statistical properties of hadronic matter under some extreme
conditions are investigated using an exactly solvable canonical ensemble model.
A unified model describing both the fragmentation of nuclei and the thermal
properties of hadronic matter is developed. Simple expressions are obtained for
quantities such as the hadronic equation of state, specific heat,
compressibility, entropy, and excitation energy as a function of temperature
and density. These expressions encompass the fermionic aspect of nucleons, such
as degeneracy pressure and Fermi energy at low temperatures and the ideal gas
laws at high temperatures and low density. Expressions are developed which
connect these two extremes with behavior that resembles an ideal Bose gas with
its associated Bose condensation. In the thermodynamic limit, an infinite
cluster exists below a certain critical condition in a manner similar to the
sudden appearance of the infinite cluster in percolation theory. The importance
of multiplicity fluctuations is discussed and some recent data from the EOS
collaboration on critical point behavior of nuclei can be accounted for using
simple expressions obtained from the model.Comment: 22 pages, revtex, includes 6 figures, submitted to Phys. Rev.