138 research outputs found
Magnetic Connectivity between Active Regions 10987, 10988, and 10989 by Means of Nonlinear Force-Free Field Extrapolation
Extrapolation codes for modelling the magnetic field in the corona in
cartesian geometry do not take the curvature of the Sun's surface into account
and can only be applied to relatively small areas, \textit{e.g.}, a single
active region. We apply a method for nonlinear force-free coronal magnetic
field modelling of photospheric vector magnetograms in spherical geometry which
allows us to study the connectivity between multi-active regions. We use vector
magnetograph data from the Synoptic Optical Long-term Investigations of the Sun
survey (SOLIS)/Vector Spectromagnetograph(VSM) to model the coronal magnetic
field, where we study three neighbouring magnetically connected active regions
(ARs: 10987, 10988, 10989) observed on 28, 29, and 30 March 2008, respectively.
We compare the magnetic field topologies and the magnetic energy densities and
study the connectivities between the active regions(ARs). We have studied the
time evolution of magnetic field over the period of three days and found no
major changes in topologies as there was no major eruption event. From this
study we have concluded that active regions are much more connected
magnetically than the electric current.Comment: Solar Physic
Magnetohydrodynamics dynamical relaxation of coronal magnetic fields. I. Parallel untwisted magnetic fields in 2D
Context. For the last thirty years, most of the studies on the relaxation of
stressed magnetic fields in the solar environment have onlyconsidered the
Lorentz force, neglecting plasma contributions, and therefore, limiting every
equilibrium to that of a force-free field. Aims. Here we begin a study of the
non-resistive evolution of finite beta plasmas and their relaxation to
magnetohydrostatic states, where magnetic forces are balanced by
plasma-pressure gradients, by using a simple 2D scenario involving a
hydromagnetic disturbance to a uniform magnetic field. The final equilibrium
state is predicted as a function of the initial disturbances, with aims to
demonstrate what happens to the plasma during the relaxation process and to see
what effects it has on the final equilibrium state. Methods. A set of numerical
experiments are run using a full MHD code, with the relaxation driven by
magnetoacoustic waves damped by viscous effects. The numerical results are
compared with analytical calculations made within the linear regime, in which
the whole process must remain adiabatic. Particular attention is paid to the
thermodynamic behaviour of the plasma during the relaxation. Results. The
analytical predictions for the final non force-free equilibrium depend only on
the initial perturbations and the total pressure of the system. It is found
that these predictions hold surprisingly well even for amplitudes of the
perturbation far outside the linear regime. Conclusions. Including the effects
of a finite plasma beta in relaxation experiments leads to significant
differences from the force-free case
Relativistic analysis of magnetoelectric crystals: extracting a new 4-dimensional P odd and T odd pseudoscalar from Cr_2 O_3 data
Earlier, the linear magnetoelectric effect of chromium sesquioxide Cr_2 O_3
has been determined experimentally as a function of temperature. One measures
the electric field-induced magnetization on Cr_2 O_3 crystals or the magnetic
field-induced polarization. From the magnetoelectric moduli of Cr_2 O_3 we
extract a 4-dimensional relativistic invariant pseudoscalar \tilde\alpha. It is
temperature dependent and of the order of ~ 10^{-4} Y_0, with Y_0 as vacuum
admittance. We show that the new pseudoscalar \tilde\alpha is odd under parity
transformation and odd under time inversion. Moreover, \tilde\alpha is for Cr_2
O_3 what Tellegen's gyrator is for two port theory, the axion field for axion
electrodynamics, and the PEMC (perfect electromagnetic conductor) for
electrical engineering.Comment: 6 pages latex, 3 figures, accepted by Physics Letters
Stereoscopic Polar Plume Reconstructions from Stereo/Secchi Images
We present stereoscopic reconstructions of the location and inclination of
polar plumes of two data sets based on the two simultaneously recorded images
taken by the EUVI telescopes in the SECCHI instrument package onboard the
\emph{STEREO (Solar TErrestrial RElations Observatory)} spacecraft. The ten
plumes investigated show a superradial expansion in the coronal hole in 3D
which is consistent with the 2D results. Their deviations from the local
meridian planes are rather small with an average of . By
comparing the reconstructed plumes with a dipole field with its axis along the
solar rotation axis, it is found that plumes are inclined more horizontally
than the dipole field. The lower the latitude is, the larger is the deviation
from the dipole field. The relationship between plumes and bright points has
been investigated and they are not always associated. For the first data set,
based on the 3D height of plumes and the electron density derived from
SUMER/\emph{SOHO} Si {\sc viii} line pair, we found that electron densities
along the plumes decrease with height above the solar surface. The temperature
obtained from the density scale height is 1.6 to 1.8 times larger than the
temperature obtained from Mg {\sc ix} line ratios. We attribute this
discrepancy to a deviation of the electron and the ion temperatures. Finally,
we have found that the outflow speeds studied in the O {\sc vi} line in the
plumes corrected by the angle between the line of sight and the plume
orientation are quite small with a maximum of 10 . It is
unlikely that plumes are a dominant contributor to the fast solar wind.Comment: 25 pages, 13 figure
Theory of Non-Reciprocal Optical Effects in Antiferromagnets: The Case Cr_2O_3
A microscopic model of non-reciprocal optical effects in antiferromagnets is
developed by considering the case of Cr_2O_3 where such effects have been
observed. These effects are due to a direct coupling between light and the
antiferromagnetic order parameter. This coupling is mediated by the spin-orbit
interaction and involves an interplay between the breaking of inversion
symmetry due to the antiferromagnetic order parameter and the trigonal field
contribution to the ligand field at the magnetic ion. We evaluate the matrix
elements relevant for the non-reciprocal second harmonic generation and
gyrotropic birefringence.Comment: accepted for publication in Phys. Rev.
Newly identified properties of surface acoustic power
The cause of enhanced acoustic power surrounding active regions, the acoustic
halo, is not as yet understood. We explore the properties of the enhanced
acoustic power observed near disk center from 21 to 27 January 2002, including
AR 9787. We find that (i) there exists a strong correlation of the enhanced
high frequency power with magnetic-field inclination, with greater power in
more horizontal fields, (ii) the frequency of the maximum enhancement increases
along with magnetic field strength, and (iii) the oscillations contributing to
the halos show modal ridges which are shifted to higher wavenumber at constant
frequency in comparison to the ridges of modes in the quiet-Sun.Comment: 16 pages, 10 figures, submitted to solar physic
Particle kinetic analysis of a polar jet from SECCHI COR data
Aims. We analyze coronagraph observations of a polar jet observed by the Sun
Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument
suite onboard the Solar TErrestrial RElations Observatory (STEREO) spacecraft.
Methods. In our analysis we compare the brightness distribution of the jet in
white-light coronagraph images with a dedicated kinetic particle model. We
obtain a consistent estimate of the time that the jet was launched from the
solar surface and an approximate initial velocity distribution in the jet
source. The method also allows us to check the consistency of the kinetic
model. In this first application, we consider only gravity as the dominant
force on the jet particles along the magnetic field. Results. We find that the
kinetic model explains the observed brightness evolution well. The derived
initiation time is consistent with the jet observations by the EUVI telescope
at various wavelengths. The initial particle velocity distribution is fitted by
Maxwellian distributions and we find deviations of the high energy tail from
the Maxwellian distributions. We estimate the jet's total electron content to
have a mass between 3.2 \times 1014 and 1.8 \times 1015 g. Mapping the
integrated particle number along the jet trajectory to its source region and
assuming a typical source region size, we obtain an initial electron density
between 8 \times 109 and 5 \times 1010 cm-3 that is characteristic for the
lower corona or the upper chromosphere. The total kinetic energy of all
particles in the jet source region amounts from 2.1 \times 1028 to 2.4 \times
1029 erg.Comment: A&A, in pres
LEMUR: Large European Module for solar Ultraviolet Research. European contribution to JAXA's Solar-C mission
Understanding the solar outer atmosphere requires concerted, simultaneous
solar observations from the visible to the vacuum ultraviolet (VUV) and soft
X-rays, at high spatial resolution (between 0.1" and 0.3"), at high temporal
resolution (on the order of 10 s, i.e., the time scale of chromospheric
dynamics), with a wide temperature coverage (0.01 MK to 20 MK, from the
chromosphere to the flaring corona), and the capability of measuring magnetic
fields through spectropolarimetry at visible and near-infrared wavelengths.
Simultaneous spectroscopic measurements sampling the entire temperature range
are particularly important.
These requirements are fulfilled by the Japanese Solar-C mission (Plan B),
composed of a spacecraft in a geosynchronous orbit with a payload providing a
significant improvement of imaging and spectropolarimetric capabilities in the
UV, visible, and near-infrared with respect to what is available today and
foreseen in the near future.
The Large European Module for solar Ultraviolet Research (LEMUR), described
in this paper, is a large VUV telescope feeding a scientific payload of
high-resolution imaging spectrographs and cameras. LEMUR consists of two major
components: a VUV solar telescope with a 30 cm diameter mirror and a focal
length of 3.6 m, and a focal-plane package composed of VUV spectrometers
covering six carefully chosen wavelength ranges between 17 and 127 nm. The
LEMUR slit covers 280" on the Sun with 0.14" per pixel sampling. In addition,
LEMUR is capable of measuring mass flows velocities (line shifts) down to 2
km/s or better.
LEMUR has been proposed to ESA as the European contribution to the Solar C
mission.Comment: 35 pages, 14 figures. To appear on Experimental Astronom
Relativistic nature of a magnetoelectric modulus of Cr_2O_3-crystals: a new 4-dimensional pseudoscalar and its measurement
Earlier, the magnetoelectric effect of chromium sesquioxide Cr_2O_3 has been
determined experimentally as a function of temperature. One measures the
electric field-induced magnetization on Cr_2O_3 crystals or the magnetic
field-induced polarization. From the magnetoelectric moduli of Cr_2O_3 we
extract a 4-dimensional relativistic invariant pseudoscalar
. It is temperature dependent and of the order of
10^{-4}/Z_0, with Z_0 as vacuum impedance. We show that the new pseudoscalar is
odd under parity transformation and odd under time inversion. Moreover,
is for Cr_2O_3 what Tellegen's gyrator is for two port
theory, the axion field for axion electrodynamics, and the PEMC (perfect
electromagnetic conductor) for electrical engineering.Comment: Revtex, 36 pages, 9 figures (submitted in low resolution, better
quality figures are available from the authors
Magneto-static modeling from SUNRISE/IMaX : application to an active region observed with SUNRISE II
Magneto-static models may overcome some of the issues facing force-free magnetic field extrapolations. So far they have seen limited use and have faced problems when applied to quiet-Sun data. Here we present a first application to an active region. We use solar vector magnetic field measurements gathered by the IMaX polarimeter during the flight of the \sunrise{} balloon-borne solar observatory in June 2013 as boundary condition for a magneto-static model of the higher solar atmosphere above an active region. The IMaX data are embedded in active region vector magnetograms observed with SDO/HMI. This work continues our magneto-static extrapolation approach, which has been applied earlier ({\it Paper I}) to a quiet Sun region observed with \sunrise{} I. In an active region the signal-to-noise-ratio in the measured Stokes parameters is considerably higher than in the quiet Sun and consequently the IMaX measurements of the horizontal photospheric magnetic field allow us to specify the free parameters of the model in a special class of linear magneto-static equilibria. The high spatial resolution of IMaX (110-130 km, pixel size 40 km) enables us to model the non-force-free layer between the photosphere and the mid chromosphere vertically by about 50 grid points. In our approach we can incorporate some aspects of the mixed beta layer of photosphere and chromosphere, e.g., taking a finite Lorentz force into account, which was not possible with lower resolution photospheric measurements in the past. The linear model does not, however, permit to model intrinsic nonlinear structures like strongly localized electric currents.PostprintPeer reviewe
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