5,372 research outputs found
The Core-Wing Anomaly of Cool Ap Stars: Abnormal Balmer Profiles
Paper by Cowley et al. The Core-Wing Anomaly Etc. The profiles of H
in a number of cool Ap stars are anomalous. Broad wings, indicative of
temperatures in the range 7000-8000K end abruptly in narrow cores. The widths
of these cores are compatible with those of dwarfs with temperatures of 6000K
or lower. This profile has been known for Przybylski's star, but it is seen in
other cool Ap's. The H profile in several of these stars shows a similar
core-wing anomaly (CWA). In Przybylski's star, the CWA is probably present at
higher Balmer members. We are unable to account for these profiles within the
context of LTE and normal dwarf atmospheres. We conclude that the atmospheres
of these stars are not ``normal.'' This is contrary to a notion that has long
been held.Comment: 4 Pages 5 Figures. Submitted to Astronomy and Astrophysics 4 Dec.
200
The interaction between transpolar arcs and cusp spots
Transpolar arcs and cusp spots are both auroral phenomena which occur when
the interplanetary magnetic field is northward. Transpolar arcs are associated
with magnetic reconnection in the magnetotail, which closes magnetic flux and
results in a "wedge" of closed flux which remains trapped, embedded in the
magnetotail lobe. The cusp spot is an indicator of lobe reconnection at the
high-latitude magnetopause; in its simplest case, lobe reconnection
redistributes open flux without resulting in any net change in the open flux
content of the magnetosphere. We present observations of the two phenomena
interacting--i.e., a transpolar arc intersecting a cusp spot during part of its
lifetime. The significance of this observation is that lobe reconnection can
have the effect of opening closed magnetotail flux. We argue that such events
should not be rare
Realistic model atmosphere and revised abundances of the coolest Ap star HD101065
Among the known Ap stars, HD101065 is probably one of the most interesting
objects, demonstrating very rich spectra of rare-earth elements (REE). Strongly
peculiar photometric parameters of this star that can not be fully reproduced
by any modern theoretical calculations, even those accounting for realistic
chemistry of its atmosphere. In this study we investigate a role of missing REE
line opacity and construct a self-consistent atmospheric model based on
accurate abundance and chemical stratification analysis. We employed the
LLmodels stellar model atmosphere code together with DDAFit and SynthMag
software packages to derive homogeneous and stratified abundances for 52
chemical elements and to construct a self-consistent model of HD101065
atmosphere. The opacity in REE lines is accounted for in details, by using
up-to-date extensive theoretical calculations. We show that REE elements play a
key role in the radiative energy balance in the atmosphere of HD101065, leading
to the strong suppression of the Balmer jump and energy redistribution very
different from that of normal stars. Introducing new line lists of REEs allowed
us to reproduce, for the first time, spectral energy distribution of HD101065
and achieve a better agreement between the unusually small observed Str\"omgren
c1 index and the model predictions. Using combined photometric and
spectroscopic approaches and based on the iterative procedure of abundance and
stratification analysis we find effective temperature of HD101065 to be
Teff=6400K.Comment: Accepted by A&A, 13 pages (3 of Online Material), 10 figures, 4
table
Nonlinear phase mixing and phase-space cascade of entropy in gyrokinetic plasma turbulence
Electrostatic turbulence in weakly collisional, magnetized plasma can be
interpreted as a cascade of entropy in phase space, which is proposed as a
universal mechanism for dissipation of energy in magnetized plasma turbulence.
When the nonlinear decorrelation time at the scale of the thermal Larmor radius
is shorter than the collision time, a broad spectrum of fluctuations at
sub-Larmor scales is numerically found in velocity and position space, with
theoretically predicted scalings. The results are important because they
identify what is probably a universal Kolmogorov-like regime for kinetic
turbulence; and because any physical process that produces fluctuations of the
gyrophase-independent part of the distribution function may, via the entropy
cascade, result in turbulent heating at a rate that increases with the
fluctuation amplitude, but is independent of the collision frequency.Comment: Revtex, 4 pages, 3 figures; replaced to match published versio
Kinetic Simulations of Magnetized Turbulence in Astrophysical Plasmas
This letter presents the first ab initio, fully electromagnetic, kinetic
simulations of magnetized turbulence in a homogeneous, weakly collisional
plasma at the scale of the ion Larmor radius (ion gyroscale). Magnetic and
electric-field energy spectra show a break at the ion gyroscale; the spectral
slopes are consistent with scaling predictions for critically balanced
turbulence of Alfven waves above the ion gyroscale (spectral index -5/3) and of
kinetic Alfven waves below the ion gyroscale (spectral indices of -7/3 for
magnetic and -1/3 for electric fluctuations). This behavior is also
qualitatively consistent with in situ measurements of turbulence in the solar
wind. Our findings support the hypothesis that the frequencies of turbulent
fluctuations in the solar wind remain well below the ion cyclotron frequency
both above and below the ion gyroscale.Comment: 4 pages, 3 figures, submitted to Physical Review Letter
Gyrokinetic simulation of entropy cascade in two-dimensional electrostatic turbulence
Two-dimensional electrostatic turbulence in magnetized weakly-collisional
plasmas exhibits a cascade of entropy in phase space [Phys. Rev. Lett. 103,
015003 (2009)]. At scales smaller than the gyroradius, this cascade is
characterized by the dimensionless ratio D of the collision time to the eddy
turnover time measured at the scale of the thermal Larmor radius. When D >> 1,
a broad spectrum of fluctuations at sub-Larmor scales is found in both position
and velocity space. The distribution function develops structure as a function
of v_{perp}, the velocity coordinate perpendicular to the local magnetic field.
The cascade shows a local-scale nonlinear interaction in both position and
velocity spaces, and Kolmogorov's scaling theory can be extended into phase
space.Comment: 8 pages, 10 figures, Conference paper presented at 2009 Asia-Pacific
Plasma Theory Conference. Ver.2 includes corrected typos & updated reference
Risk mitigation for ITER by a prolonged and joint international
Prolonged operation of the Joint European Torus (JET) in a set-up involving all ITER partners will be beneficial for ITER. Experiments at JET with its ITER-like wall and using a D–T plasma mixture will help to mitigate risks in the ITER research plan. Training of the ITER operators, technicians and engineers at JET will safe valuable time when ITER comes into operation. Moreover, the way in which the future ITER experiments will be organized can already be experienced at JET, by imposing a similar organisational structure. This paper will present arguments in favour of an extension of JET and additionally briefly discuss a number of enhancements that will make experiments on JET even more relevant for ITER
- …