453 research outputs found
A revised radiometric calibration for the Hinode/EIS instrument
A preliminary assessment of the in-flight radiometric calibration of the
Hinode EUV Imaging Spectrometer (EIS) is presented. This is done with the line
ratio technique applied to a wide range of observations of the quiet Sun,
active regions and flares from 2006 until 2012. The best diagnostic lines and
the relevant atomic data are discussed in detail. Radiances over the quiet Sun
are also considered, with comparisons with previous measurements. Some
departures in the shapes of the ground calibration responsivities are found at
the start of the mission. These shapes do not change significantly over time,
with the exception of the shorter wavelengths of the EIS short-wavelength (SW)
channel, which shows some degradation. The sensitivity of the SW channel at
longer wavelengths does not show significant degradation, while that of the
long-wavelength (LW) channel shows a significant degradation with time. By the
beginning of 2010 the responsivity of the LW channel was already a factor of
two or more lower than the values measured on the ground. A first-order
correction is proposed. With this correction, the main ratios of lines in the
two channels become constant to within a relative 20%, and the He II 256 A
radiances over the quiet Sun also become constant over time. This correction
removes long-standing discrepancies for a number of lines and ions, in
particular those involving the strongest Fe X, Fe XIII, Fe XIV, Fe XVII, and Fe
XXIV lines, where discrepancies of factors of more than two were found. These
results have important implications for various EIS science analyses, in
particular for measurements of temperatures, emission measures and elemental
abundances.Comment: Accepted for publication in A&A (under minor revision
The EUV spectrum of the Sun: SOHO CDS NIS radiances during solar cycle 23
For the first time, we present and discuss EUV radiances of the solar
transition region (TR) and corona obtained during a solar cycle. The
measurements were obtained with the SOHO/coronal diagnostic spectrometer (CDS)
during the period from 1996 to 2010. We find that limb-brightening
significantly affects any characterisation of the solar radiances. We present
the limb-brightening function for the main lines and find that it does not
change measurably during the cycle. We confirm earlier findings that the
radiance histogram of the cooler lines have a well defined, log-normal
quiet-Sun component, although our results differ from previous ones. The width
of the lowest-radiance log-normal distribution is constant along the cycle.
Both the analysis of the centre-to-limb variation and of the radiance
statistical distribution point to a constant QS emission along solar cycle 23.
Lines formed above 1 MK are dramatically affected by the presence of active
regions, and indeed, no "quiet Sun" region can be defined during periods of
maximum activity. Much of the irradiance variability in lines formed below 1.5
MK is due to a change in the emitting area. For hotter lines, the emitting area
saturates to almost 100% of full solar disk at the maximum of activity, while
simultaneously the emission due to active regions increases by more than an
order of magnitude. We show that structures around active regions, sometimes
referred to as dark halos or dark canopies, are common and discuss their
similarities and differences with coronal holes. In particular, we show how
they are well visible in TR lines, contrary to coronal holes.Comment: 15 pages, 11 figures, 2 tables; in press in: Astronomy & Astrophysic
Modelling ion populations in astrophysical plasmas: carbon in the solar transition region
The aim of this work is to improve the modelling of ion populations in higher
density, lower temperature astrophysical plasmas, of the type commonly found in
lower solar and stellar atmospheres. Ion population models for these regions
frequently employ the coronal approximation, which assumes conditions more
suitable to the upper solar atmosphere, where high temperatures and lower
densities prevail. Using the coronal approximation for modelling the solar
transition region gives theoretical lines intensities for the Li-like and
Na-like isoelectronic sequences which can be factors of 2-5 times lower than
observed. The works of Burgess & Summers (1969) and Nussbaumer & Storey (1975)
showed the important part ions in excited levels play when included in the
modelling. Their models, however, used approximations for the atomic rates to
determine the ion balance. Presented here is the first stage in updating these
earlier models of carbon by using rates from up-to-date atomic calculations and
more recent photo-ionising radiances for the quiet Sun. Where such atomic rates
are not readily available, in the case of electron-impact direct ionisation and
excitation--auto-ionisation, new calculations have been made and compared to
theoretical and experimental studies. The effects each atomic process has on
the ion populations as density changes is demonstrated, and final results from
the modelling are compared to the earlier works. Lastly, the new results for
ion populations are used to predict line intensities for the solar transition
region in the quiet Sun, and these are compared with predictions from
coronal-approximation modelling and with observations. Significant improvements
in the predicted line intensities are seen in comparison to those obtained from
zero-density modelling of carbon.Comment: Draft accepted by A&A, 13 pages, 15 figure
The EUV spectrum of the Sun: quiet and active Sun irradiances and chemical composition
We benchmark new atomic data against a selection of irradiances obtained from
medium-resolution quiet Sun spectra in the EUV, from 60 to 1040 \AA. We use as
a baseline the irradiances measured during solar minimum on 2008 April 14 by
the prototype (PEVE) of the Solar Dynamics Observatory Extreme ultraviolet
Variability Experiment (EVE). We take into account some inconsistencies in the
PEVE data, using flight EVE data and irradiances we obtained from Solar &
Heliospheric Observatory (SoHO) Coronal Diagnostics Spectrometer (CDS) data. We
perform a differential emission measure and find overall excellent agreement
(to within the accuracy of the observations, about 20%) between predicted and
measured irradiances in most cases, although we point out several problems with
the currently available ion charge state distributions. We used the
photospheric chemical abundances of Asplund et al. (2009). The new atomic data
are nearly complete in this spectral range, for medium-resolution irradiance
spectra. Finally, we use observations of the active Sun in 1969 to show that
also in that case the composition of the solar corona up to 1 MK is nearly
photospheric. Variations of a factor of 2 are present for higher-temperature
plasma, which is emitted within active regions.These results are in excellent
agreement with our previous findings.Comment: Accepted for publication in A&
A Constrained Transport Method for the Solution of the Resistive Relativistic MHD Equations
We describe a novel Godunov-type numerical method for solving the equations
of resistive relativistic magnetohydrodynamics. In the proposed approach, the
spatial components of both magnetic and electric fields are located at zone
interfaces and are evolved using the constrained transport formalism. Direct
application of Stokes' theorem to Faraday's and Ampere's laws ensures that the
resulting discretization is divergence-free for the magnetic field and
charge-conserving for the electric field. Hydrodynamic variables retain,
instead, the usual zone-centred representation commonly adopted in
finite-volume schemes. Temporal discretization is based on Runge-Kutta
implicit-explicit (IMEX) schemes in order to resolve the temporal scale
disparity introduced by the stiff source term in Ampere's law. The implicit
step is accomplished by means of an improved and more efficient Newton-Broyden
multidimensional root-finding algorithm. The explicit step relies on a
multidimensional Riemann solver to compute the line-averaged electric and
magnetic fields at zone edges and it employs a one-dimensional Riemann solver
at zone interfaces to update zone-centred hydrodynamic quantities. For the
latter, we introduce a five-wave solver based on the frozen limit of the
relaxation system whereby the solution to the Riemann problem can be decomposed
into an outer Maxwell solver and an inner hydrodynamic solver. A number of
numerical benchmarks demonstrate that our method is superior in stability and
robustness to the more popular charge-conserving divergence cleaning approach
where both primary electric and magnetic fields are zone-centered. In addition,
the employment of a less diffusive Riemann solver noticeably improves the
accuracy of the computations.Comment: 25 pages, 14 figure
General relativistic models for rotating magnetized neutron stars in conformally flat spacetime
The extraordinary energetic activity of magnetars is usually explained in
terms of dissipation of a huge internal magnetic field of the order of
G. How such a strong magnetic field can originate during the
formation of a neutron star is still subject of active research. An important
role can be played by fast rotation: if magnetars are born as millisecond
rotators dynamo mechanisms may efficiently amplify the magnetic field inherited
from the progenitor star during the collapse. In this case, the combination of
rapid rotation and strong magnetic field determine the right physical condition
not only for the development of a powerful jet driven explosion, manifesting as
a gamma ray burst, but also for a copious gravitational waves emission. Strong
magnetic fields are indeed able to induce substantial quadrupolar deformations
in the star. In this paper we analyze the joint effect of rotation and
magnetization on the structure of a polytropic and axisymmetric neutron star,
within the ideal magneto-hydrodynamic regime. We will consider either purely
toroidal or purely poloidal magnetic field geometries. Through the sampling of
a large parameter space, we generalize previous results in literature,
inferring new quantitative relations that allow for a parametrization of the
induced deformation, that takes into account also the effects due to the
stellar compactness and the current distribution. Finally, in the case of
purely poloidal field, we also discuss how different prescriptions on the
surface charge distribution (a gauge freedom) modify the properties of the
surrounding electrosphere and its physical implications.Comment: 25 pages, 17 figures, 6 tables, accepted for publication in MNRA
Hinode EIS line widths in the quiet corona up to 1.5 Rsun
We present an analysis of several Hinode EIS observations of coronal line
widths in the quiet Sun, up to 1.5 Rsun radial distances. No significant
variations are found, which indicates no damping of Alfv\'en waves in the
quiescent corona.
However, the uncertainties in estimating the instrumental width mean that a
firm conclusion cannot be reached. We present a discussion of various EIS
instrumental issues and suggest that the strongest lines, from Fe XII at 193.5
and 195.1 A, have anomalous instrumental widths. We show how line widths in EIS
are uncertain when the signal is low, and that the instrumental variation along
the slit is also uncertain. We also found an anomalous decrease (up to 40%) in
the intensities of these lines in many off-limb and active region observations,
and suggest that this is due to opacity effects. We find that the most reliable
measurements are obtained from the weaker lines.Comment: Submitted to A&A, under revision - comments welcome
The role of currents distribution in general relativistic equilibria of magnetized neutron stars
Magnetic fields play a critical role in the phenomenology of neutron stars.
There is virtually no observable aspect which is not governed by them. Despite
this, only recently efforts have been done to model magnetic fields in the
correct general relativistic regime, characteristic of these compact objects.
In this work we present, for the first time a comprehensive and detailed
parameter study, in general relativity, of the role that the current
distribution, and the related magnetic field structure, have in determining the
precise structure of neutron stars. In particular, we show how the presence of
localized currents can modify the field strength at the stellar surface, and we
look for general trends, both in terms of energetic properties, and magnetic
field configurations. Here we verify that, among other things, for a large
class of different current distributions the resulting magnetic configurations
are always dominated by the poloidal component of the current.Comment: 14 pages, 13 figures, accepted for publication in MNRA
Modeling the structure of magnetic fields in Neutron Stars: from the interior to the magnetosphere
The phenomenology of the emission of pulsars and magnetars depends
dramatically on the structure and properties of their magnetic field. In
particular it is believed that the outbursting and flaring activity observed in
AXPs and SRGs is strongly related to their internal magnetic field. Recent
observations have moreover shown that charges are present in their
magnetospheres supporting the idea that their magnetic field is tightly twisted
in the vicinity of the star. In principle these objects offer a unique
opportunity to investigate physics in a regime beyond what can be obtained in
the laboratory. We will discuss the properties of equilibrium models of
magnetized neutron stars, and we will show how internal and external currents
can be related. These magnetic field configurations will be discussed
considering also their stability, relevant for their origin and possibly
connected to events like SNe and GRBs. We will also show what kind of
deformations they induce in the star, that could lead to emission of
gravitational waves. In the case of a twisted magnetosphere we will show how
the amount of twist regulates their general topology. A general formalism based
on the simultaneous numerical solution of the general relativistic
Grad-Shafranov equation and Einstein equations will be presented.Comment: 9 pages, 3 figures, Proceedings of the 10th International Conference
on Numerical Modeling of Space Plasma Flows, 8-12 June 2015, Avignon, Franc
- …