382 research outputs found
Azimuthal expansion of high-latitude auroral arcs
International audienceWe used the TV auroral observations in Barentsburg (78.05° N 14.12° E) in Spitsbergen archipelago, together with the data of the CUTLASS HF radars and the POLAR satellite images to study azimuthal (in the east-west direction) expansion of the high-latitude auroral arcs. It is shown that the east or west edge of the arc moved in the same direction as the convection flow, westward in the pre-midnight sector and eastward in the post-midnight sector. The velocity of arc expansion was of the order of 2.5 km/s, which is 2?3 times larger than the convection velocity measured in the arc vicinity and 2?3 times smaller than the velocity of the bright patches propagating along the arc. The arc expanded from the active auroras seen from the POLAR satellite around midnight as a region of enhanced luminosity, which might be the auroral bulge or WTS. The pole- or equatorward drift of the arcs occurred at the velocity of the order of 100 m/s that was close to the convection velocity in the same direction. These experimental results can be well explained in terms of the interchange (or flute) instability
Lithium-6 from Solar Flares
By introducing a hitherto ignored Li-6 producing process, due to accelerated
He-3 reactions with He-4, we show that accelerated particle interactions in
solar flares produce much more Li-6 than Li-7. By normalizing our calculations
to gamma-ray data we demonstrate that the Li-6 produced in solar flares,
combined with photospheric Li-7, can account for the recently determined solar
wind lithium isotopic ratio, obtained from measurements in lunar soil, provided
that the bulk of the flare produced lithium is evacuated by the solar wind.
Further research in this area could provide unique information on a variety of
problems, including solar atmospheric transport and mixing, solar convection
and the lithium depletion issue, and solar wind and solar particle
acceleration.Comment: latex 9 pages, 2 figures, ApJ Letters in pres
Attractive instability of oppositely charged membranes induced by charge density fluctuations
We predict the conditions under which two oppositely charged membranes show a
dynamic, attractive instability. Two layers with unequal charges of opposite
sign can repel or be stable when in close proximity. However, dynamic charge
density fluctuations can induce an attractive instability and thus facilitate
fusion. We predict the dominant instability modes and timescales and show how
these are controlled by the relative charge and membrane viscosities. These
dynamic instabilities may be the precursors of membrane fusion in systems where
artificial vesicles are engulfed by biological cells of opposite charge
Effect of an electric field on nucleation and growth of crystals
The effect of the electric field strength on nucleation and growth of the crystals of ammonium halides and alkali metal sulfates has been studied. The optimal electric field strength for NH[4]Cl and NH[4]Br crystals was found to be 15 kV/cm, and for NH[4]I, it equaled 10 kV/cm. No effect of the electric field strength on the crystal growth was found for alkali metal sulfates. This difference is analyzed in terms of the crystal growth thermodynamics. In case, when the electric field is small and the Gibbs energy is of a significant value, the influence of the electric field at the crystal growth is negligible. A method to estimate the critical radius of homogeneous nucleation of the crystal is suggested
Identification of species of the genus Acer L. using vegetation indices calculated from the hyperspectral images of leaves
Selection of the most suitable spectral vegetation indices which are applicable to the remote sensing of the forest species composition and status, is an important task aimed at the evaluation of the large-scale plant communities. There are 80 vegetation indices have been collected in the present work using the hyperspectral data, including that for the Acer platanoides L., A. saccharinum L. and A. pseudoplatanus
Curvature-Induced Defect Unbinding in Toroidal Geometries
Toroidal templates such as vesicles with hexatic bond orientational order are
discussed. The total energy including disclination charges is explicitly
computed for hexatic order embedded in a toroidal geometry. Related results
apply for tilt or nematic order on the torus in the one Frank constant
approximation. Although there is no topological necessity for defects in the
ground state, we find that excess disclination defects are nevertheless
energetically favored for fat torii or moderate vesicle sizes. Some
experimental consequences are discussed.Comment: 12 pages, 15 eps figure
Combined optical, EISCAT and magnetic observations of the omega bands/Ps6 pulsations and an auroral torch in the late morning hours: a case study
Light Element Evolution and Cosmic Ray Energetics
Using cosmic-ray energetics as a discriminator, we investigate evolutionary
models of LiBeB. We employ a Monte Carlo code which incorporates the delayed
mixing into the ISM both of the synthesized Fe, due to its incorporation into
high velocity dust grains, and of the cosmic-ray produced LiBeB, due to the
transport of the cosmic rays. We normalize the LiBeB production to the integral
energy imparted to cosmic rays per supernova. Models in which the cosmic rays
are accelerated mainly out of the average ISM significantly under predict the
measured Be abundance of the early Galaxy, the increase in [O/Fe] with
decreasing [Fe/H] notwithstanding. We suggest that this increase could be due
to the delayed mixing of the Fe. But, if the cosmic-ray metals are accelerated
out of supernova ejecta enriched superbubbles, the measured Be abundances are
consistent with a cosmic-ray acceleration efficiency that is in very good
agreement with the current epoch data. We also find that neither the above
cosmic-ray origin models nor a model employing low energy cosmic rays
originating from the supernovae of only very massive progenitors can account
for the Li data at values of [Fe/H] below 2.Comment: latex 19 pages, 2 tables, 10 eps figures, uses aastex.cls natbib.sty
Submitted to the Astrophysical Journa
Nuclear Interaction Gamma-Ray Lines from the Galactic Center Region
Aims. The accretion of stars onto the central supermassive black hole at the
center of the Milky Way is predicted to generate large fluxes of
subrelativistic ions in the Galactic center region. We analyze the intensity,
shape and spatial distribution of de-excitation gamma-ray lines produced by
nuclear interactions of these energetic particles with the ambient medium.
Methods. We first estimate the amount and mean kinetic energy of particles
released from the central black hole during star disruption. We then calculate
from a kinetic equation the energy and spatial distributions of these particles
in the Galactic center region. These particle distributions are then used to
derive the characteristics of the main nuclear interaction gamma-ray lines.
Results. Because the time period of star capture by the supermassive black
hole is expected to be shorter than the lifetime of the ejected fast particles
against Coulomb losses, the gamma-ray emission is predicted to be stationary.
We find that the nuclear de-excitation lines should be emitted from a region of
maximum 5 angular radius. The total gamma-ray line flux below 8 MeV is
calculated to be photons cm s. The most
promising lines for detection are those at 4.44 and 6.2 MeV, with a
predicted flux in each line of photons cm s.
Unfortunately, it is unlikely that this emission can be detected with the
INTEGRAL observatory. But the predicted line intensities appear to be within
reach of future gamma-ray space instruments. A future detection of
de-excitation gamma-ray lines from the Galactic center region would provide
unique information on the high-energy processes induced by the central
supermassive black hole and the physical conditions of the emitting region.Comment: 7 pages, 5 figures, accepted for publication in A&
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