Author name code: vandenoord
ADS astronomy entries on 2022-09-14
author:"van den Oord, G.H.J."
------------------------------------------------------------------------
Title: Towards Open and FAIR Hydrological Modelling with eWaterCycle
Authors: Drost, Niels; Aerts, Jerom P. M.; Alidoost, Fakhereh; Andela,
Bouwe; Camphuijsen, Jaro; van de Giesen, Nick; Hut, Rolf; Hutton,
Eric; Kalverla, Peter; van den Oord, Gijs; Pelupessy, Inti; Smeets,
Stef; Verhoeven, Stefan; van Werkhoven, Ben
Bibcode: 2021EGUGA..23.7797D
Altcode:
The eWaterCycle platform (https://www.ewatercycle.org/) is a fully
Open Source system designed explicitly to advance the state of Open and
FAIR Hydrological modelling. While working with Hydrologists to create
a fully Open and FAIR comparison study, we noticed that many ad-hoc
tools and scripts are used to create input (forcing, parameters) for a
hydrological model from the source datasets such as climate reanalysis
and land-use data. To make this part of the modelling process better
reproducible and more transparent we have created a common forcing
input processing pipeline based on an existing climate model analysis
tool: ESMValTool (https://www.esmvaltool.org/). Using ESMValTool,
the eWaterCycle platform can perform commonly required preprocessing
steps such as cropping, re-gridding, and variable derivation in a
standardized manner. If needed, it also allows for custom steps for
a hydrological model. Our pre-processing pipeline directly supports
commonly used datasets such as ERA-5, ERA-Interim, and CMIP climate
model data, and creates ready-to-run forcing data for a number of
Hydrological models.Besides creating forcing data, the eWaterCycle
platform allows scientists to run Hydrological models in a standardized
way using Jupyter notebooks, wrapping the models inside a container
environment, and interfacing to these using BMI, the Basic Model
Interface (https://bmi.readthedocs.io/). The container environment
(based on Docker) stores the entire software stack, including the
operating system and libraries, in such a way that a model run can
be reproduced using an identical software environment on any other
computer.The reproducible processing of forcing and a reproducible
software environment are important steps towards our goal of fully
reproducible, Open, and FAIR Hydrological modelling. Ultimately, we hope
to make it possible to fully reproduce a hydrological model experiment
from data pre-processing to analysis, using only a few clicks.
Title: XIOS-GRIB compare
Authors: van den Oord, Gijs
Bibcode: 2021zndo...4906175V
Altcode:
Python scripts to compare IFS GRIB files and XIOS NetCDF files.
Title: IFS CY43R3 and XIOS 2.0 integration
Authors: Yepes-Arbós, Xavier; van den Oord, Gijs
Bibcode: 2021zndo...4905832Y
Altcode:
This project implements the integration between the IFS CY43R3 model
and the XIOS 2.0 I/O server.
Title: Towards Reproducible Hydrological Modelling with eWaterCycle
Authors: Drost, N.; Aerts, J. P. M.; Alidoost, F.; Andela, B.;
Camphuijsen, J.; Dzigan, Y.; Van De Giesen, N.; Hut, R.; Hutton, E.;
Kalverla, P.; van Meersbergen, M.; van den Oord, G.; Pelupessy, I.;
Verhoeven, S.; Weel, B.; van Werkhoven, B.
Bibcode: 2020AGUFMIN016..15D
Altcode:
The eWaterCycle platform( https://www.ewatercycle.org/
) is a fully Open Source system designed explicitly to advance the
state of Open and FAIR Hydrological modelling. Reproducibility is a key
ingredient of FAIR, and one of the driving principles of eWaterCycle. While working with Hydrologists to create a fully Open and FAIR
comparison study, we noticed that many ad-hoc tools and scripts are
used to create input (forcing, parameters) for a hydrological model
from the source datasets such as climate reanalysis and land-use
data. To make this part of the modelling process better reproducible
and more transparent we have created a common forcing input processing
pipeline based on an existing climate model analysis tool: ESMValTool
( https://www.esmvaltool.org/
). Using ESMValTool the eWaterCycle platform can perform
commonly required pre-processing steps such as cropping, re-gridding,
and variable derivation in a standardized manner. If needed, it also
allows for custom steps for a Hydrological model. Our pre-processing
pipeline directly supports commonly used datasets such as ERA-5,
ERA-Interim, and CMIP climate model data, and creates ready-to-run
forcing data for a number of Hydrological models. Besides
creating forcing data, the eWaterCycle platform allows scientists
to run Hydrological models in a standardized way using Jupyter
notebooks, wrapping the models inside a container environment,
and interfacing to these using BMI, the Basic Model Interface (
https://bmi.readthedocs.io/
). The container environment (based on Docker) stores the entire
software stack, including the operating system and libraries, in such
a way that a model run can be reproduced using an identical software
environment on any other computer. The reproducible processing of
forcing and a reproducible software environment are important steps
towards our goal of fully reproducible, Open, and FAIR Hydrological
modelling. Ultimately, we hope to make it possible to fully reproduce
a Hydrological model experiment from data pre-processing to analysis,
using only a few clicks.
Title: HyMUSE: a multi-model framework for Hydrology.
Authors: Pelupessy, Inti; Drost, Niels; Hut, Rolf; Aerts, Jerome;
Portegies Zwart, Simon; van Elteren, Arjen; van den Oord, Gijs;
van Werkhoven, Ben; Verhoeven, Stefan; Weel, Berend
Bibcode: 2019EGUGA..2114271P
Altcode:
We present the Hydrological Multipurpose Software Environment (HyMUSE),
a Python framework for Hydrological model simulations, that will
form part of the computational core of the eWaterCycle project. The
eWaterCycle II project aims to develop a platform for researchers
to easily develop and use hydrological models and deploy them on
HPC resources. HyMUSE is being developed at the Netherlands eScience
Center using technology developed in the AMUSE and OMUSE projects, which
where developed for the astrophysical and oceanographic domains. HyMUSE
presents the user with a homogeneous interface to different hydrological
simulation codes. For this it provides a number of services such
as unit conversion, encapsulation of the internal model data to a
common object oriented representation and maintaining the simulation
in a consistent state. HyMUSE can be accessed from within the online
notebook environment of the eWaterCycle toolset, where researchers can
explore, adapt and collaborate on simulations models. The use cases
for HyMUSE range from running simple numerical experiments with single
codes and the addition of data analysis tools in model runs, to running
large model run ensembles or setting up coupled solvers for problems
where different types of physics interact. In addition to AMUSE-type
low level interfaces, it is possible in HyMUSE, to interface directly
with the CSDMS Basic Model interface (BMI). For this we have developed
an interoperability layer for the BMI, that can optionally route its
low level communication through the newly developed GRPC4BMI library,
which enables models to be run on remote HPC resources. We discuss the
current status of the project, and the medium and long term development
goals, as well as giving example applications.
Title: Ece2Cmor3
Authors: van den Oord, Gijs
Bibcode: 2017zndo...1051094V
Altcode:
ece2cmor3 processes your EC-Earth output data to be CMIP6-compliant. It
postprocesses the atmosphere output and adds the correct metadata to
all netcdf files for publishing on your egsf data node.
Title: Ozone monitoring instrument flight-model on-ground and
inflight calibration
Authors: Dobber, Marcel; Dirksen, Ruud; Levelt, Pieternel F.; van den
Oord, Gijsbertus; Jaross, Glen; Kowalewski, Matt; Mount, George H.;
Heath, Donald; Hilsenrath, Ernest; Cebula, Richard
Bibcode: 2017SPIE10568E..0CD
Altcode:
The Ozone Monitoring Instrument (OMI) is an ultravioletvisible imaging
spectrograph that uses two-dimensional CCD detectors to register both
the spectrum and the swath perpendicular to the flight direction. This
allows having a 114 degrees wide swath combined with an unprecedented
small ground pixel (nominally 13 x 24 km2), which in turn
enables global daily ground coverage with high spatial resolution. The
OMI instrument is part of NASA's EOSAURA satellite, which will be
launched in the second half of 2004. The on-ground calibration of the
instrument was performed in 2002. This paper presents and discusses
results for a number of selected topics from the on-ground calibration:
the radiometric calibration, the spectral calibration and spectral
slit function calibration. A new method for accurately calibrating
spectral slit functions, based on an echelle grating optical stimulus,
is discussed. The in-flight calibration and trend monitoring approach
and facilities are discussed.
Title: In-flight calibration of the ozone monitoring instrument
Authors: Dirksen, Ruud; Dobber, Marcel; Voors, Robert; Kleipool,
Quintus; van den Oord, Gijsbertus; Levelt, Pieternel
Bibcode: 2017SPIE10567E..24D
Altcode:
This paper discusses various aspects of the on-ground and in-flight
calibration of the OMI instrument.
Title: Sensitivity analysis of a new SWIR-channel measuring
tropospheric CH 4 and CO from space
Authors: Jongma, Rienk T.; Gloudemans, Annemieke M. S.; Hoogeveen,
Ruud W. M.; Aben, Ilse; de Vries, Johan; Escudero-Sanz, Isabel;
van den Oord, Gijsbertus; Levelt, Pieternel F.
Bibcode: 2006SPIE.6302E..14J
Altcode: 2006SPIE.6302E..29J
In preparation for future atmospheric space missions a consortium
of Dutch organizations is performing design studies on a nadir
viewing grating-based imaging spectrometer using OMI and SCIAMACHY
heritage. The spectrometer measures selected species (O 3,
NO II, HCHO, H IIO, SO II, aerosols
(optical depth, type and absorption index), CO and CH4) with sensitivity
down to the Earth's surface, thus addressing science issues on air
quality and climate. It includes 3 UV-VIS channels continuously covering
the 270-490 nm range, a NIR-channel covering the 710-775 nm range,
and a SWIR-channel covering the 2305-2385 nm range. This instrument
concept is, named TROPOMI, part of the TRAQ-mission proposal to ESA in
response to the Call for Earth Explorer Ideas 2005, and, named TROPI,
part of the CAMEO-proposal prepared for the US NRC decadal study-call
on Earth science and applications from space. The SWIR-channel is
optional in the TROPOMI/TRAQ instrument and included as baseline in
the TROPI/CAMEO instrument. This paper focuses on derivation of the
instrument requirements of the SWIR-channel by presenting the results
of retrieval studies. Synthetic detector spectra are generated by
the combination of a forward model and an instrument simulator that
includes the properties of state-of-the-art detector technology. The
synthetic spectra are input to the CO and CH 4 IMLM
retrieval algorithm originally developed for SCIAMACHY. The required
accuracy of the Level-2 SWIR data products defines the main instrument
parameters like spectral resolution and sampling, telescope aperture,
detector temperature, and optical bench temperature. The impact of
selected calibration and retrieval errors on the Level-2 products has
been characterized. The current status of the SWIR-channel optical
design with its demanding requirements on ground-pixel size, spectral
resolution, and signal-to-noise ratio will be presented.
Title: In-Flight Calibration of the Ozone Monitoring Instrument
Authors: Dirksen, R.; Dobber, M.; Voors, R.; Kleipool, Q.; van den
Oord, G.; Levelt, P.
Bibcode: 2006ESASP.621E..84D
Altcode: 2006spop.confE..84D
No abstract at ADS
Title: Ozone Monitoring Instrument flight-model on-ground and
in-flight calibration
Authors: Dobber, Marcel; Dirksen, Ruud; Levelt, Pieternel F.; van den
Oord, Gijsbertus; Jaross, Glen; Kowalewski, Matt; Mount, George H.;
Heath, Donald; Hilsenrath, Ernest; Cebula, Richard
Bibcode: 2004ESASP.554...89D
Altcode: 2004icso.conf...89D
The Ozone Monitoring Instrument (OMI) is an ultraviolet-visible imaging
spectrograph that uses two-dimensional CCD detectors to register both
the spectrum and the swath perpendicular to the flight direction. This
allows having a 114 degrees wide swath combined with an unprecedented
small ground pixel (nominally 13×24 km2), which in turn
enables global daily ground coverage with high spatial resolution. The
OMI instrument is part of NASA's EOS-AURA satellite, which will be
launched in the second half of 2004. The on-ground calibration of the
instrument was performed in 2002. This paper presents and discusses
results for a number of selected topics from the on-ground calibration:
the radiometric calibration, the spectral calibration and spectral
slit function calibration. A new method for accurately calibrating
spectral slit functions, based on an echelle grating optical stimulus,
is discussed. The in-flight calibration and trend monitoring approach
and facilities are discussed.
Title: EOS-Aura's Ozone Monitoring Instrument (OMI): Performance
and science goals
Authors: Levelt, P. F.; van den Oord, G. H. J.; Veefkind, J. P.;
de Haan, J. F.; Hilsenrath, E.; Bhartia, P. K.; Leppelmeier, G. W.;
Maelkki, A.
Bibcode: 2004cosp...35.2822L
Altcode: 2004cosp.meet.2822L
The Ozone Monitoring Instrument (OMI) will fly on NASA's EOS-AURA
satellite, now scheduled for launch in June 2004. OMI is a UV/VIS,
nadir viewing spectrometer that will provide near global coverage of
solar backscatter radiances in one day. OMI has several technological
advances, but has heritage from the TOMS, SBUV, GOME, GOMOS and
SCIAMACHY. OMI employs a wide field telescope to image a 2600 km wide
Earth swath onto two CCDs, which detect light from two wavelength
bands (UV and VIS). Using the wavelength range 270 to 500 nm with
a 0.5 nm resolution, OMI will measure several key parameters for
stratospheric and tropospheric chemistry and for climate research,
including O3, NO2, SO2, OClO, HCHO,
BrO, UVB, aerosols, and cloud heights and fraction. Combining OMI data
with the other Aura instruments will allow derivation of tropospheric
gases important for air quality studies. OMI's high spatial resolution
(13 x 24 km2) will allow observation between clouds, thus
giving better penetration into the troposphere than any other UV/VIS
backscatter instrument flown to date. Proven TOMS and DOAS retrieval
algorithms for trace gases will be employed as well as new ones,
which will take advantage of experience gained by both European and
US scientists. OMI ozone data will be of the same quality as TOMS to
insure continuity of ozone trends detected to date. Tropospheric ozone
will be retrieved using two algorithms developed for TOMS but tailored
for OMI and using ozone profiles from all Aura instruments. Clouds
heights will be retrieved using two different algorithms. Some of
these algorithms are discussed in other EGS sessions, while all OMI
algorithms are available at http://eospso.gsfc.nasa.gov/. In this
paper performance characteristics with respect to meeting the science
requirements will be presented.
Title: Ozone Monitoring Instrument: system description and test
results
Authors: te Plate, Maurice B.; Draaisma, F.; de Vries, Johan; van
den Oord, Gijsbertus H.
Bibcode: 2001SPIE.4540..122T
Altcode:
The Ozone Monitoring Instrument (OMI) is a nadir viewing wide field
imaging spectrometer for ozone monitoring. The instrument is the
Dutch/Finnish contribution to the NASA EOS-AURA mission. OMI observes
earth's back scattered radiation in two spectral channels: the UV
channel (270 nm - 350 nm) and the VIS channel (350 nm - 500 nm). Each
channel employs a CCD detector (576 X 780 px). The extreme wide field
of view of 114 degrees, equal to a swath wide of 2600 km, is obtained
by an all reflective telecentric telescope and enables global ozone
coverage in one day. Other key features are the spectral range (270
nm - 500 nm) and resolution (spectral sampling distance 0.15 - 0.32
nm/px), the application of a polarization scrambler and its compact
design (400 X 300 X 500 mm). Excellent stray light performance in the
UV channel is obtained by an elegant opto-mechanical design of the UV
optics where the UV wavelength range is split in two parts with separate
optical paths and the separate spectra are imaged on one CCD. Onboard
calibration includes a white light source, LEDs, and multi-surface
solar-calibration diffuser. The OMI-EOS project follows a Proto-Flight
approach, supported by breadboards and engineering qualification models
on parts and sub-system level. In order to increase confidence in the
design, the instrument development model was built. During intensive
testing critical performance parameters were checked , e.g. UV stray
light behavior, polarization sensitivity, distortion, spatial and
spectral ranges and resolutions.
Title: Exploring the dynamical nature of the lower solar chromosphere
Authors: Doyle, J. G.; van den Oord, G. H. J.; O'Shea, E.; Banerjee, D.
Bibcode: 1999A&A...347..335D
Altcode:
We examine spectral time-series of two lower-chromospheric lines (N i
1319 Ä and C ii 1335 Ä) observed with the SUMER instrument on the SOHO
spacecraft. We point out differences between (intensity and velocity)
power spectra of network and internetwork regions and argue that the
behaviour resembles that of Ca ii power spectra. No significant phase
differences are found between the intensities of both lines. However,
when phase spectra are averaged along the slit there is some evidence
that the C ii intensity lags that of N i by 16 sec near 3 mHz. Intensity
power spectra of C ii are affected at higher frequencies by streams
of emitting structures. Using contrast-enhanced time slices we show
that 1) there exists a grain-like pattern which is found in both
network and internetwork regions; 2) streams of supersonically moving
structures probably outline a wave interference pattern; 3) the sizes
of structures observed in N i are smaller than when observed in C ii. At
various points our findings disagree with earlier results from SUMER. A
cookbook formalism is presented to derive confidence levels for power,
phase, gain and coherency spectra.
Title: Stellar Flares: Theory and Observations
Authors: van den Oord, G. H. J.
Bibcode: 1999ASPC..158..189V
Altcode: 1999ssa..conf..189V
No abstract at ADS
Title: Dynamical Nature of the Quiet Solar Outer Atmosphere
Authors: Banerjee, D.; Doyle, J.; O'Shea, E.; van den Oord, G.
Bibcode: 1999ASPC..183..389B
Altcode: 1999hrsp.conf..389B
No abstract at ADS
Title: The oscillatory nature of the upper solar atmosphere.
Authors: O'Shea, E.; Doyle, J. G.; Banerjee, D.; van den Oord, G. H. J.
Bibcode: 1999joso.proc..128O
Altcode:
The authors examine time series spectral data obtained by the SUMER
instrument onboard SOHO. Lines of N V 1242 Å, Si II 1533 Å, C IV 1548
Å, C II 1037 Å and O VI 1037.6 Å were measured and found to show
intensity oscillations. The oscillations were usually observed to occur
in small regions along the slit, limited to a few arcsec. N V 1242 Å
typically showed frequencies of between 1 - 2 mHz. The Si II and C IV
lines observed simultaneously were found to have differing oscillation
frequency ranges of 2 - 8 mHz and 2 - 4 mHz respectively. The C II
and O VI lines also measured simultaneously were both found to have
oscillation frequency ranges of 2 - 4 mHz. In addition a time lag
of ≡50 seconds was measured between the intensity oscillations in
these two lines, perhaps signifying a propagating wave from the low
temperature C II line region to the higher temperature O VI region of
the atmosphere.
Title: The effect of delays on filament oscillations and stability
Authors: van den Oord, G. H. J.; Schutgens, N. A. J.; Kuperus, M.
Bibcode: 1998A&A...339..225V
Altcode:
We discuss the linear response of a filament to perturbations, taking
the finite communication time between the filament and the photosphere
into account. The finite communication time introduces delays in the
system. Recently Schutgens (1997ab) investigated the solutions of the
delay equation for vertical perturbations. In this paper we expand his
analysis by considering also horizontal and coupled oscillations. The
latter occur in asymmetric coronal fields. We also discuss the effect
of Alfven wave emission on filament oscillations and show that wave
emission is important for stabilizing filaments. We introduce a fairly
straightforward method to study the solutions of delay equations
as a function of the filament-photosphere communication time. A
solution can be described by a linear combination of damped harmonic
oscillations each characterized by a frequency, a damping/growth
time and, accordingly, a quality factor. As a secondary result of
our analysis we show that, within the context of line current models,
Kippenhahn/Schlüter-type filament equilibria can never be stable in
the horizontal and the vertical direction at the same time but we also
demonstrate that Kuperus/Raadu-type equilibria can account for both
an inverse or a normal polarity signature. The diagnostic value of our
analysis for determining, e.g., the filament current from observations
of oscillating filaments is discussed.
Title: Large-scale coronal heating by the small-scale magnetic field
of the Sun
Authors: Schrijver, C. J.; Title, A. M.; Harvey, K. L.; Sheeley,
N. R.; Wang, Y. -M.; van den Oord, G. H. J.; Shine, R. A.; Tarbell,
T. D.; Hurlburt, N. E.
Bibcode: 1998Natur.394..152S
Altcode:
Magnetic fields play a crucial role in heating the outer atmospheres
of the Sun and Sun-like stars, but the mechanisms by which magnetic
energy in the photosphere is converted to thermal energy in the corona
remain unclear. Observations show that magnetic fields emerge onto
the solar surface as bipolar regions with a broad range of length
scales. On large scales, the bipolar regions survive for months before
dispersing diffusively. On the smaller scales, individual bipolar
regions disappear within days but are continuously replenished by new
small flux concentrations, resulting in a sustained state of mixed
polarity. Here we determine the rate of emergence of these small
bipolar regions and we argue that the frequent magnetic reconnections
associated with these regions (an unavoidable consequence of continued
flux replacement) will heat the solar atmosphere. The model that
describes the details of these mixed-polarity regions is complementary
to the traditional diffusion model for large-scale flux dispersal and
a combination of the two should lead to a more complete understanding
of the role of magnetic fields in stellar atmospheres.
Title: Waves in the solar transition region
Authors: Doyle, J. G.; van den Oord, G. H. J.; O'Shea, E.; Banerjee, D.
Bibcode: 1998SoPh..181...51D
Altcode:
The power spectra for line intensities of several lines formed in the
upper transition region around 100000 to 250000 K are presented. A
period of ∼5 min is clearly present in lines due to Oiii, Oiv,
and Ov. In one dataset a period approaching 10 min is present for
∼40 min. The size of the emitting features is limited to ∼7 arc
sec squared. In all datasets examined, there is excess power below 4
mHz everywhere along the slit, although the observed periods do not
always come from the most intense regions. In ∼40% of instances clear
periods are observable in the 2-5 mHz range with the largest power peak
at 3.0 mHz. In all regions, the 5.0 mHz power peak is smaller. For the
frequencies investigated there are no significant time delays in any of
the datasets examined. This finding may not be entirely unexpected as
the formation temperatures of Oiii (∼100000 K) and Ov (∼250000 K)
may be too close in order to result in an observable phase shift.
Title: Evidence of Prominences on Cool Late-Type Stars (Review)
Authors: Byrne, P. B.; Eibe, M. T.; van den Oord, G. H. J.
Bibcode: 1998ASPC..150..227B
Altcode: 1998IAUCo.167..227B; 1998npsp.conf..227B
No abstract at ADS
Title: Prominence Oscillations and the Influence of the Distant
Photosphere
Authors: Schutgens, N. A. J.; Kuperus, M.; van den Oord, G. H. J.
Bibcode: 1998ASPC..150..147S
Altcode: 1998IAUCo.167..147S; 1998npsp.conf..147S
No abstract at ADS
Title: Prominence-like Clouds Near HK AQR
Authors: van den Oord, G. H. J.; Bryne, P. B.; Elbe, M. T.
Bibcode: 1998ASPC..150..251V
Altcode: 1998IAUCo.167..251V; 1998npsp.conf..251V
No abstract at ADS
Title: Randomly sampling the chromospheric peak power distribution.
Authors: Doyle, J. G.; van den Oord, G. H. J.; O'Shea, E.
Bibcode: 1997A&A...327..365D
Altcode:
We have analyzed the UV continuum light curves of 2535 pixels
obtained during fifteen separate observations of active regions
with the Ultraviolet Spectrometer and Polarimeter (UVSP) on board
the solar maximum mission in 1989. Specifically we have looked for
periodicities. In the power spectra of 738 light curves (i.e., 29%),
evidence was found for periodicities at multiple frequencies. For
each power spectrum, containing significant power, we determined the
frequency at which the maximum power is found. The distribution N(ν),
which describes the number of pixels having maximum power at frequency
ν, is strongly concentrated in the 2-5mHz band with a distinct maximum
at 3-3.5mHz. No pixels had their maximum power above 10mHz and only
a few had their maximum power in the 5-10mHz range. The oscillations
in the 2-5mHz range are probably related to evanescent acoustic waves
driven by the photospheric five minute oscillations. In the 2-5mHz band,
the distribution N(ν) resembles very much the power spectra as have
been observed near the temperature minimum. This suggests that the
photospheric power distribution can be interpreted as a probability
distribution for finding a given frequency in the UV continuum light
curves. In the 2-5mHz band the spread of the maximum powers and the
spread of the count rates, at any frequency, is much larger than above
5mHz where both are relatively constant. No clear correlation is found
between the maximum power and the count rate in a pixel. Furthermore,
no evidence is found for emission from the shocks which have recently
been invoked to explain the formation of CaII grains. Whether this is
due to temperature effects is unclear. Our general procedure adopted to
estimate the confidence level in the power spectrum of a light curve
containing photon noise can be equally applied to SUMER and CDS data
from SOHO.
Title: EUV spectroscopy of cool stars. III. Interpretation of EUVE
spectra in terms of quasi-static loops.
Authors: van den Oord, G. H. J.; Schrijver, C. J.; Camphens, M.;
Mewe, R.; Kaastra, J. S.
Bibcode: 1997A&A...326.1090V
Altcode:
We discuss the limitations of coronal spectroscopy to derive physical
parameters of stellar magnetic loops. We distinguish between the
intrinsic non-uniqueness of emitted spectra for models of quasi-static
coronal loops, and the supplemental ambiguity introduced by both
instrumental effects and spectral line formation. We demonstrate
that the spectrum emitted by loops with constant cross-sections is
the same for a large range of values of the conductive flux at the
base when the apex temperature is fixed. Because it is impossible to
estimate the conductive flux at the base from observations, it is also
impossible to determine the volume heating rate and the loop length
uniquely. For geometrically expanding (tapered) loops, the emitted
spectrum depends on the expansion and on the conductive flux at the
base, and there is a trade off between them without significant changes
in the spectrum. We show that loop length and heating rate can only be
derived if the density is known, but that even then a large intrinsic
uncertainty remains for these loop parameters. We conclude that there
is no unambiguous relationship between loop parameters and emitted
spectra: modeling the spectra as the sum of spectra from discrete loops
cannot result in a unique determination of coronal structure. Based
on spectra observed with the Extreme Ultra Violet Explorer (EUVE) we
find that quasi-static loop models allow adequate modeling of stellar
coronal spectra. We show that coronal loops on active cool stars must
expand with height. The minimum required areal expansion between base
and apex is not very large, lying between 2 and 5. For three stars
(α Cen, Capella and ξ UMa) the observations suggest the presence of
two distinct, dominant loop populations, while for χ^1^ Ori a single
population, characterized by a single apex temperature, suffices. The
high electron densities (10^12^-10^13^cm^-3^) for coronal components
on Capella and ξ UMa require abnormally large heating rates. It is
likely that these high densities are related to a multitude of small
volumes that are temporarily excited.
Title: ASCA and EUVE observations of II Pegasi: flaring and quiescent
coronal emission.
Authors: Mewe, R.; Kaastra, J. S.; van den Oord, G. H. J.; Vink, J.;
Tawara, Y.
Bibcode: 1997A&A...320..147M
Altcode:
We have analyzed X-ray and EUV spectra of both the quiescent and
flaring state of II Peg, obtained from observations with ASCA and
EUVE. Coronal temperature structure and abundances have been derived
from multi-temperature and differential emission measure (DEM) analyses
of the spectra. The abundances are non-solar; in the case of ASCA for
most elements (O, Ne, Mg, Si, S, Ar, Ca, Ni) we obtain abundances that
are consistent with about 1/2-1/5 of the solar photospheric abundances
of Anders and Grevesse (1989, Geochim. Cosmochim. Acta 53, 197),
but the Fe abundance is even lower, i.e. 0.1xSolar. The multi-T and
DEM fitting analysis shows that the quiescent EUVE and ASCA spectra
can be described by two temperature components: 4 and 10MK (EUVE),
10 and 20MK (ASCA). The two flares detected by EUVE and ASCA show
peak temperatures of 20 and >35MK, respectively. The latter flare
has a total energy (0.1-10keV) of 2.7x10^34^erg, a peak luminosity of
2.6x10^30^erg/s. There is evidence for an increase of a factor ~4 of
the iron abundance during the rise phase of the flare. Application of
a cooling model yields a loop height of about 8x10^10^cm and a plasma
density of 8x10^10^cm^-3^.
Title: Constraints on mass loss from dMe stars: theory and
observations.
Authors: van den Oord, G. H. J.; Doyle, J. G.
Bibcode: 1997A&A...319..578V
Altcode:
We show that the flux distribution of a wind from a cool star
differs considerably from what is predicted by the theory for mass
loss from hot stars. The differences are caused by the facts that 1)
the mass loss rates are lower, resulting in smaller optical depths
in the wind, and 2) for winds from cool stars the temperature of the
wind is higher than the temperature of the star while for winds from
hot stars the reverse holds. These differences result in substantial
modifications of the flux distribution and imply that care must be
exercised when applying the flux predictions by e.g. Wright and Barlow
(1975MNRAS.170...41W) to winds from cool stars. By using observational
constraints we show that the mass loss from cool dwarf stars equals at
most 10^-12^Msun_/yr. This is a factor hundred lower than
previous estimates. At this rate the mass loss from dMe stars is of
little importance for the enrichment of the interstellar medium. By
solving the radiative transfer equations for stellar winds from dMe
stars, we show that the inferred power-law flux distributions, based
on radio, JCMT and IRAS data, cannot be reconciled with the flux
distributions from a stellar wind of 10^-10^Msun_/yr as
was previously assumed. The maximum allowable mass loss rate is at
most a few times 10^-12^Msun_/yr which implies that the
fluxes observed with JCMT, IRAS, and in the future with ISO, require
a different interpretation than free-free emission from a stellar wind.
Title: Book reviews
Authors: Howard, R. F.; van den Oord, G. H. J.; Švestka, Z.
Bibcode: 1996SoPh..169..225H
Altcode:
No abstract at ADS
Title: Flare energetics: analysis of a large flare on YZ Canis
Minoris observed simultaneously in the ultraviolet, optical and radio.
Authors: van den Oord, G. H. J.; Doyle, J. G.; Rodono, M.; Gary,
D. E.; Henry, G. W.; Byrne, P. B.; Linsky, J. L.; Haisch, B. M.;
Pagano, I.; Leto, G.
Bibcode: 1996A&A...310..908V
Altcode:
The results of coordinated observations of the dMe star YZ CMi
at optical, UV and radio wavelengths during 3-7 February 1983 are
presented. YZ CMi showed repeated optical flaring with the largest
flare having a magnitude of 3.8 in the U-band. This flare coincided
with an IUE exposure which permits a comparison of the emission measure
curves of YZ CMi in its flaring and quiescent state. During the flare a
downward shift of the transition zone is observed while the radiative
losses in the range 10^4^-10^7^K strongly increase. The optical flare
is accompanied with a radio flare at 6cm, while at 20cm no emission
is detected. The flare is interpreted in terms of optically thick
synchrotron emission. We present a combined interpretation of the
optical/radio flare and show that the flare can be interpreted within
the context of solar two-ribbon/white-light flares. Special attention
is paid to the bombardment of dMe atmospheres by particle beams. We
show that the characteristic temperature of the heated atmosphere is
almost independent of the beam flux and lies within the range of solar
white-light flare temperatures. We also show that it is unlikely that
stellar flares emit black-body spectra. The fraction of accelerated
particles, as follows from our combined optical/radio interpretation
is in good agreement with the fraction determined by two-ribbon flare
reconnection models.
Title: DEM Analyses with the Utrecht Codes
Authors: Mewe, R.; van den Oord, G. H. J.; Schrijver, C. J.; Kaastra,
J. S.
Bibcode: 1996aeu..conf..553M
Altcode: 1996IAUCo.152..553M
No abstract at ADS
Title: EUV spectroscopy and coronal loop models
Authors: van den Oord, G. H. J.; Schrijver, C. J.; Mewe, R.; Kaastra,
J. S.
Bibcode: 1996ASPC..109..231V
Altcode: 1996csss....9..231V
No abstract at ADS
Title: Are Some Stellar Coronae Optically Thick?
Authors: Schrijver, C. J.; van den Oord, G. H. J.; Mewe, R.; Kaastra,
J. S.
Bibcode: 1996aeu..conf..121S
Altcode: 1996IAUCo.152..121S
No abstract at ADS
Title: Non-thermal Eimission Mechanisms in Stellar Corona
Authors: van den Oord, G. H. J.
Bibcode: 1996ASPC...93..263V
Altcode: 1996ress.conf..263V
No abstract at ADS
Title: Is the optical depth of stellar coronae really negligible?
Authors: Schrijver, C. J.; Mewe, R.; Kaastra, J. S.; van den Oord,
G. H. J.; Bruls, J. H. M. J.
Bibcode: 1996ASPC..109..289S
Altcode: 1996csss....9..289S
No abstract at ADS
Title: Coronal loops and their modeling (review)
Authors: van den Oord, G. H. J.; Zuccarello, F.
Bibcode: 1996IAUS..176..433V
Altcode:
No abstract at ADS
Title: EUV spectroscopy of cool stars. II. Coronal structure of
selected cool stars observed with the EUVE.
Authors: Schrijver, C. J.; Mewe, R.; van den Oord, G. H. J.; Kaastra,
J. S.
Bibcode: 1995A&A...302..438S
Altcode:
We analyze the coronal EUV spectra of seven cool stars, solar-like
single stars and components of RS CVn-like binaries, as observed with
the Spectrometers of the Extreme Ultraviolet Explorer (EUVE). The
observations cover the wavelength range of 60A up to 800A with a
resolution of λ/{DELTA}λ=~160-320. The data constrain the coronal
temperature structure between several hundred thousand Kelvin up
to roughly 10 million Kelvin through a differential emission measure
analysis. The resulting differential emission measure distributions show
distinct features from source to source, but the common properties are
a) a relatively weak emission from coronal plasma below about 1MK, b)
a dominant component somewhere between 2MK and about 10MK, often peaking
at solar-like coronal temperatures of 2 to 4MK, and c) in all cases but
χ^1^ Ori a very hot component in the formal solution with a temperature
exceeding several tens of million of Kelvin. This hot tail in the
differential emission measure distribution may reflect, as discussed
in this paper, one or several of the following sources or processes:
a real hot component, a reduced coronal abundance of heavy elements,
or scattering in some of the strongest coronal lines with subsequent
photon destruction upon impact on the lower, dense atmosphere. Coronal
electron densities of brightly emitting regions are constrained by an
analysis of ratios of density-sensitive iron lines. Strengths of Fe
XIX-Fe XXII lines (corresponding to a temperature range of T=6-11MK) for
α Aur, AU Mic (of which the spectrum is dominated by a large flare),
ξ UMa, and σ Gem suggest typical electron densities in the range
n_e_~10^12^-10^13^cm^-3^. Cooler Fe X and Fe XII-Fe XIV lines (T=1-2MK)
in the case of α CMi suggest n_e_~10^9^-10^10^cm^-3^. In general, the
electron densities of the hot 5-15MK components are some three orders
of magnitude larger than typical of the solar-like component around
2MK; the volume filling factors of the hot components are therefore
expected to be substantially smaller than those of the cooler component.
Title: The electrodynamics of beam/return current systems. The effect
on the accelerator.
Authors: van Oss, R. F.; van den Oord, G. H. J.
Bibcode: 1995A&A...299..297V
Altcode:
The initial value problem of the injection of an electron beam into a
plasma cylinder is investigated. Emphasis is put on the effect of the
return current on the acceleration region. The full set of Maxwell's
equations is solved in combination with Ohm's law. By splitting all
vector fields into their solenoidal and irrotational components,
the inductive and electrostatic response of the plasma electrons to
the beam injection can be described separately. It is found that the
effect of the return current on the beam acceleration region, located
somewhere in the plasma cylinder, is negligible on time scales shorter
than the time scale for magnetic diffusion. In realistic circumstances
the duration of the acceleration process is much shorter than this time
scale so that the accelerator does not experience any influence of the
return current. Several cylinder radii from the accelerator site our
solutions approach those obtained in earlier work where a self-similar
approach was used. The energy partitioning in the beam/return current
system is discussed.
Title: Book-Review - Fragmented Energy Release in Sun and Stars -
the Interface Between Magnetohydrodynamics and Plasma Physics
Authors: van den Oord, G. H. J.; Rijnbeek, R. P.
Bibcode: 1995Obs...115...94V
Altcode:
No abstract at ADS
Title: EUV spectroscopy of cool stars. I. The corona of α Centauri
observed with EUVE. R
Authors: Mewe, R.; Kaastra, J. S.; Schrijver, C. J.; van den Oord,
G. H. J.; Alkemade, F. J. M.
Bibcode: 1995A&A...296..477M
Altcode:
We perform an emission measure analysis of spectra of the coronae of
the cool star binary α Cen (A: HD 128620 (G2 V) + B: HD 128621 (K1 V))
as observed with the Spectrometers of the Extreme UltraViolet Explorer
(EUVE). These observations, covering the wavelength range of 60A up
to 800A with a resolution of λ/{DELTA}λ=~160-320, constrain the
coronal temperature structure between about 10^5^K up to roughly 10MK,
while some additional line and continuum information is available to
extend that coverage from several tens of thousands of Kelvin up to
several tens of millions of Kelvin, although the solution is poorly
constrained in the extended range. We performed a re-calibration of
the wavelength scale for each of the three instrumental pass bands by
comparing the model list of emission lines with the spectra of α Cen
and eight other cool stars. The high signal-to-noise ratio of the α
Cen spectrum yielded indications for a Lorentzian rather than a Gaussian
line profile for the MW passband. The observed spectrum was decomposed
into a linear combination of spectra emitted by isothermal plasmas in
thermal equilibrium, using the SPEX plasma emission code developed in
Utrecht. The resulting differential emission measure distribution shows:
a) emission from plasma below ~5MK with a broad peak around ~3MK and
with a tail extending down to about 0.5MK, b) very little emission from
plasma between 0.1MK and 0.5MK, c) emission from plasma below 0.1MK,
and d) possibly a hot component exceeding several tens of million of
Kelvin which, however, may (in part) have another origin than a hot
coronal plasma component. We propose that the hot component may be
an artifact of the corona being close to unit optical thickness for
resonant scattering in the strongest spectral lines, so that part
of the line photons can be destroyed by being scattered towards the
stellar surface while the plasma remains optically thin for continuum
photons. Diagnostics for the electron density n_e_, using Fe X, XII,
XIII, and XIV lines, yields values in the range 2-20x10^8^cm^-3^
at T=1-2MK.
Title: Books-Received - Fragmented Energy Release in Sun and Stars -
the Interface Between MHD and Plasma Physics
Authors: van den Oord, G. H. J.
Bibcode: 1994Sci...266..310V
Altcode:
No abstract at ADS
Title: The optical thickness of stellar coronae in the EUV lines.
Authors: Schrijver, C. J.; van den Oord, G. H. J.; Mewe, R.
Bibcode: 1994A&A...289L..23S
Altcode:
Stellar coronae are commonly assumed to be optically thin. Recent
spectroscopic observations in the EUV, however, lead us to question
the validity of this approximation for strong lines. We argue that
scattering may significantly affect the strongest coronal lines in
coronae composed of magnetic loops, possibly embedded in a hot stellar
wind. Even if the average number of scatterings per photon in some
coronal lines is only of order unity, the relative line strenghts
and the line-to-continuum ratio can be significantly affected in
non-symmetric inhomogeneous atmospheres: photons in weak lines and
in the optically thin continuum escape without any scattering, but
strong lines can be weakened or enhanced depending on the balance
between outward traveling line photons that are scattered back toward
the stellar surface (if not lost by branching), there destroyed by
absorption, and downward traveling line photons that are scattered
upward and escape. We draw attention to the fact that line scattering
due to the non-negligible optical thickness in strong coronal lines
can have severe implications for differential emission measure models
and for abundance and density determinations, while it may serve as
a diagnostic for the existence of tenuous hot winds.
Title: Book Review: Whistler-mode waves in a hot plasma / Cambridge
U Press, 1993
Authors: van den Oord, G. H. J.
Bibcode: 1994SSRv...69..414V
Altcode: 1994SSRv...69..414S
No abstract at ADS
Title: Unusual flares at 360 and 609 MHz in the RS Canum
Venaticorum-system II Pegasi observed with the WSRT: coherent emission
from II Pegasi
Authors: van den Oord, G. H. J.; de Bruyn, A. G.
Bibcode: 1994A&A...286..181V
Altcode:
We report on the detection of flare and quiescent emission from the
RS CVn system II Peg at 360 and 609 MHz. The radio flare at 360 MHz
lasted for half an hour and showed strong, spikelike variability on
time scales probably shorter than the instrumental time resolution
of one minute. The emission is coherent and must occur at or near the
local plasma frequency in the source. Electron cyclotron maser emission
can be excluded because of the low magnetic field strengths required
at this observing frequency. Plasma emission by a moving exciter
(beam or shock) can be excluded on base of the available spectral
information. Plasma maser emission originating in a collection of
double layers is consistent with the available spectral information. We
discuss the role of free-free opacity which can dramatically lower
the observed brightness temperature and influence the shape of the
observed spectrum. The flare at 609 MHz was only observed during
the decay phase. This flare has the typical characteristics of the
incoherent radio flares previously observed in RS CVn systems except
for an unusual high percentage of circular polarization (90%) which on
a timescale of a few hours decreases to approximately 50%. We discuss
the most likely emission mechanisms but are not able to provide a
definite conclusion concerning the nature of the emission at 609 MHz.
Title: Fragmented Energy Release in Sun and Stars
Authors: van den Oord, G. H. J.
Bibcode: 1994SSRv...68....1V
Altcode:
No abstract at ADS
Title: Foreword
Authors: van den Oord, Bert; Kuijpers, Jan; Kuperus, Max; Benz, A. O.;
Brown, J. C.; Einaudi, G.; Kuperus, M.; Raadu, M. A.; Trottet, G.;
van den Oord, G. H. J.; Vlahos, L.; Zheleznyakov, V. V.; Wijburg,
Marion; Fletcher, Lyndsay; Volwerk, Martin
Bibcode: 1994SSRv...68D..17V
Altcode:
No abstract at ADS
Title: Book Review: Physics of solar and stellar coronae: G. S. Vaiana
memorial symposium / Kluwer, 1993
Authors: van den Oord, G. I. J.; van den Oord, G. H. J.
Bibcode: 1994SoPh..151..397V
Altcode: 1994SoPh..151..397L
No abstract at ADS
Title: The role of current sheets in the flare development
Authors: van den Oord, G. H. J.
Bibcode: 1993AdSpR..13i.143V
Altcode: 1993AdSpR..13..143V
A review is presented on the physics of energy release in current
sheets during flares. The process of energy storage is discussed and
it is shown how the evolution of the magnetic field may give rise to
the presence of current sheets. The presently known types of magnetic
energy release are discussed and attention is paid to the results of
analytical/numerical models and laboratory experiments. Recent results
on three-dimensional reconnection are summarized. The observational
evidence that flare energy release is a highly time-dependent process
puts severe constraints on the models and indicates that nonlinear
processes are important. The large scale magnetic structure in which
sheets are embedded is discussed for both compact and two-ribbon
flares. Apart from the MHD-picture of current sheets some plasma
physical aspects are discussed together with the energy partitioning
in sheets (acceleration/heating).
Title: Accretion disk flares in energetic radiation fields. A model
for hard X-rays from black hole candidates.
Authors: van Oss, R. F.; van den Oord, G. H. J.; Kuperus, M.
Bibcode: 1993A&A...270..275V
Altcode:
An investigation is conducted of the magnetic flares in the energetic
radiation field of an accretion-disk corona, whose X-ray emission
may be the source of the hard power-law component in the X-ray
spectra of galactic black-hole candidates in their 'high' spectral
state. The concept of the low plasma-beta accretion-disk corona (ADC),
which produces the hard X-rays via inverse Compton scattering of
lower-energy photons from the disk on enegetic electrons in the corona,
is elaborated. A spectrum is derived for the up-scattered radiation
from current sheets in the ADC; this spectrum mimics a power-law above
a critical photon energy.
Title: Accretion Disk Flares in Energetic Radiation Fields
Authors: van Oss, R. F.; van den Oord, G. H. J.; Kuperus, M.
Bibcode: 1993IAUS..157..217V
Altcode:
No abstract at ADS
Title: Emission up to 18 ke V in the Quiescent X-ray Spectrum of
II Peg
Authors: Doyle, J. G.; van den Oord, G. H. J.; Kellett, B. J.
Bibcode: 1993ASSL..183..311D
Altcode: 1993pssc.symp..311D
No abstract at ADS
Title: The Effect of Retardation on the Stability of Current Filaments
Authors: van den Oord, G. H. J.; Kuperus, M.
Bibcode: 1992SoPh..142..113V
Altcode:
We investigate the influence of the finite Alfvén velocity on the
evolution of an active region filament. In general, variations of
a current result in variations of the magnetic fields which spread
around with the Alfvén velocity. As a consequence of the fact that a
magnetic field can only change with the Alfvén velocity, a filament
will experience the photospheric boundary conditions as these were at
an Alfvén travel time back in time. The inclusion of this retardation
effect in the momentum equation of a filament leads effectively to
an extra force term. This force contribution acts in the direction in
which the filament moves and has therefore a destabilizing effect on
the filament. Because a moving filament acts as an antenna of Alfvén
waves, the filament loses energy by the emission process. This leads to
a radiative damping term in the equation of motion of the filament. In
general, the radiative damping will be sufficiently strong to counteract
the retardation instability. Numerical simulations show that during
the energy build-up phase a filament follows the van Tend-Kuperus
equilibrium curve. After the van Tend-Kuperus equilibrium has
disappeared the filament goes through a transient phase moving with a
sub-Alfvénic velocity upward. At greater heights the repulsive Lorentz
force of the photospheric surface current magnetic field is balanced
by the radiative damping, resulting in a decreasing filament velocity.
Title: Book Review: Basic plasma processes on the sun (IAU symposium
no. 142) / Kluwer, 1990
Authors: van den Oord, G. H. J.
Bibcode: 1992SSRv...61..424V
Altcode: 1992SSRv...61..424P
No abstract at ADS
Title: Retardation effects and the origin of filament oscillations.
Authors: Kuperus, M.; van den Oord, G. H. J.
Bibcode: 1992ESASP.346...39K
Altcode: 1992ssts.rept...39K
A filament current subject to a perturbation experiences the
photospheric boundary an Alfvén bounce time later. This retardation
effect gives rise to an extra force which causes the filament to
oscillate around an equilibrium position, emitting Alfvén waves which
damp the filament oscillations. The onset of filament oscillations is a
natural consequence of the presence of the inert photospheric boundary
and the finite Alfvén speed. Numerical simulations are presented that
show the evolution of a filament in a bipolar magnetic field as the
filament current slowly increases. The emission of waves during this
evolution is an important energy source for the coronal heating.
Title: Dynamic phenomena on the RS Canum Venaticorum binary II Pegasi
in August 1989. II. an interpretation.
Authors: Doyle, J. G.; van den Oord, G. H. J.; Kellett, B. J.
Bibcode: 1992A&A...262..533D
Altcode:
An analysis of the quiescent X-ray and radio observations and of the
X-ray observations of a major flare of II Pegasi is presented. For both
the flare and the quiescent emission, photometric, and spectroscopic
optical data are also discussed. The quiescent X-ray emission shows
a power-law tail up to 18 keV. It is shown that an interpretation
in terms of free-free emission by nonthernal particles requires
unrealistically large values for the number of particles involved
and for the energy input required to maintain the population. Basic
flare parameters like volume, loop length, and typical magnetic field
strength are derived. The observed flare decay on II Peg is compared
with the observed flare decay on Sigma CrB and Algol. It is shown
that the behavior of the temperature and emission measure during the
II Peg flare differs significantly from the observed behavior during
the Algol/sigma CrB flares.
Title: Small scale processes in solar and stellar plasmas.
Authors: van den Oord, G. H. J.
Bibcode: 1992ESASP.344...97V
Altcode: 1992spai.rept...97V
The author discusses the small scale processes which occur in the
magnetically dominated outer layers of the Sun. He discusses the
transport coefficients for the thermal and the electrical conductivity
in the classical limit and in the limit of anomalous processes. It
is demonstrated that the corona and the chromosphere must have a
filamentary structure. Various heating mechanisms which are based
on small scale dissipation of magnetic energy are discussed. The
observational requirements for improved modelling of the transition
region and the chromosphere are formulated and the consequences of
high resolution observations for flare models are pointed out.
Title: Electrodynamics of Electron Beams in Solar Flares
Authors: van den Oord, G. H. J.
Bibcode: 1992IrAJ...20..188V
Altcode:
The electrodynamical aspects of beam/return current systems during the
impulsive phase of solar flares are discussed. The relevant equations
for the electrostatic and the inductive response are given and the
solutions are briefly discussed. The conditions for beam propagation are
derived and it is shown that under certain conditions beam propagation
is inhibited leading to bulk plasma heating.
Title: The Quiescent X-ray Spectrum of the RS CVn Binary II Peg in
August 1989
Authors: Doyle, J. G.; Byrne, P. B.; van den Oord, G. H. J.; Kellett,
B. J.; Bromage, G. E.
Bibcode: 1992ASPC...26..356D
Altcode: 1992csss....7..356D
No abstract at ADS
Title: Coronal and transition region heating (review)
Authors: van den Oord, G. H. J.
Bibcode: 1992sccw.conf...11V
Altcode:
No abstract at ADS
Title: Inference of 'bumps' in electron energy spectra from high
resolution X-ray bremsstrahlung spectra
Authors: Brown, J. C.; MacKinnon, A. L.; van den Oord, G. H. J.;
Trottet, G.
Bibcode: 1991A&A...242L..13B
Altcode:
Regions of positive slope in an electron speed distribution function
fs(v) are shown to result in sharp downward 'knees' (spectral
index increases) in the emitted bremsstrahlung spectra. For Kramers'
cross-section, a criterion is established for the detectability of such
a 'bump' in the source electron spectrum from the photon spectrum,
realizable from state of the art X-ray spectrometry, and applying a
fortiori for more exact cross-sections. Inference of bumps in the thick
target injection spectrum, however, demands spectral resolution far
beyond present capabilities. Physical implications of bumps in fs(v)
are discussed.
Title: Effect of Electron Beams during Solar Flares
Authors: Aboudarham, J.; Henoux, J. C.; Brown, J. C.; van den Oord,
G. H. J.; van Driel-Gesztelyi, L.; Gerlei, O.
Bibcode: 1990SoPh..130..243A
Altcode:
Electron bombardment of the solar atmosphere has two effects: one is
to enhance hydrogen recombination emission, the other is to increase
the opacity via an increase of H− population. The first
effect is the most important in the upper part of the atmosphere and
the second in the lower part. We predict that, when enhanced absorption
dominates in the part of the atmosphere where radiation originates,
there will be a decrease in the white-light emission, leading to a
`negative flare', or what we call a `Black-Light Flare'. This phenomenon
occurs only for a short duration, not more than ∼ 20 s. `Black-Light
Flares' have already been observed in the case of flare stars and we
suggest here that they could also be present on the Sun, just prior
to a White-Light Flare.
Title: Observations of Beam Propagation
Authors: Pick, M.; van den Oord, G. H. J.
Bibcode: 1990SoPh..130...83P
Altcode:
We discuss the observational evidence for propagating electron
distributions in the solar corona as obtained from type III
observations. The location of the beam acceleration region together with
the effects of the ambient medium on beam propagation are discussed in
the framework of recent observational and theoretical developments. The
results of combined X-ray and radio observations obtained during and
after the Solar Maximum Year are summarized. Special attention is
paid to the effect of the small-scale structure of the corona on the
observed radio emission. We outline various desirable developments
which are necessary to improve the understanding of the behaviour of
charged particle beams in magnetized plasmas.
Title: The electrodynamics of beam/return current systems in the
solar corona
Authors: van den Oord, G. H. J.
Bibcode: 1990A&A...234..496V
Altcode:
The response of a uniform plasma to the propagation of a prescribed
electron beam in the approximation of decoupled solenoidal and
irrotational fields is studied. It is shown that the electrostatic and
inductive response of a plasma are governed by two independent sets of
equations and that both are important for the current neutralization
process. It is also found that the electrostatic response accounts for
neutralization of the irrotational part of the beam current while the
inductive response leads to neutralization of the solenoidal part of
the beam current.
Title: Black and white flares?
Authors: Henoux, J. -C.; Aboudarham, J.; Brown, J. C.; van den Oord,
G. H. J.; van Driel-Gesztelyi, L.
Bibcode: 1990A&A...233..577H
Altcode:
It is shown that the nonthermal ionization of a solar flare atmosphere
by beam bombardment can result, in the initial stages of a flare, in
dimunitions in the continuum solar flare brightness (termed 'black
light flares'). The most favorable conditions for observing this
diagnostic phenomenon are investigated in terms of flare location and
wavelength. In terms of bombarding beam parameters, electron beams
of large peak flux are shown to be the most favorable as are target
atmospheres with minimal preheating.
Title: A periodicity in the flaring rate on the eclipsing binary
YY Geminorum.
Authors: Doyle, J. G.; Butler, C. J.; van den Oord, G. H. J.; Kiang, T.
Bibcode: 1990A&A...232...83D
Altcode:
Five flares were observed on the late-type binary YY Gem in March 1988
during a total monitoring time of 408 min. The flares were unusual
in that there is a periodicity in the occurrence of four of them,
these being separated by 48 + or - 3 min. Considering the flares to
be formed as a stochastic process, it is found that the probability
of these events occurring by chance is 0.5 percent. Modeling indicates
that for quite reasonable input parameters, the flare periodicity can
be explained in terms of filament oscillations. The only requirement
is that there should be a filament at these heights where the magnetic
field drops inversely proportional to the height.
Title: Beam Heating in Solar Flares: Electrons or Protons?
Authors: Brown, J. C.; Karlicky, M.; MacKinnon, A. L.; van den Oord,
G. H. J.
Bibcode: 1990ApJS...73..343B
Altcode:
The current status of electron and proton beam models as candidates for
the impulsive phase heating of solar flares is discussed in relation to
observational constants and theoretical difficulties. It is concluded
that, while the electron beam model for flare heating still faces
theoretical and observational problems, the problems faced by low
and high energy proton beam models are no less serious, and there are
facets of proton models which have not yet been studied. At the present,
the electron beam model remains the most viable and best developed of
heating model candidates.
Title: Negative Flares on the Sun
Authors: van Driel-Gesztelyi, L.; Brown, J. C.; Hénoux, J. -C.;
Aboudarham, J.; van den Oord, G. H. J.; Gerlei, O.; Fárnik, F.
Bibcode: 1990PDHO....7..202V
Altcode: 1990dysu.conf..202V; 1990ESPM....6..202V
The authors showed that under certain conditions at the beginning
of a solar white-light flare (WLF) event an electron beam can cause
a transient darkening before the WLF emission starts. They propose
optimal conditions for possible observation of solar negative flares and
present an observation, which may be the first identified observation
of such events.
Title: Beam/Return Current Systems in the Solar Corona.
Authors: van den Oord, G. H. J.
Bibcode: 1990ppsa.conf..265V
Altcode:
No abstract at ADS
Title: A Model for the Observed Periodicity in the Flaring Rate
on Yy-Geminorum
Authors: Doyle, J. G.; van den Oord, G. H. J.; Butler, C. J.; Kiang, T.
Bibcode: 1990IAUS..137..325D
Altcode:
No abstract at ADS
Title: Ultraviolet flares on II Pegasi.
Authors: Doyle, J. G.; Byrne, P. B.; van den Oord, G. H. J.
Bibcode: 1989A&A...224..153D
Altcode:
UV spectroscopic observations of the RS CVn star, II Peg, were obtained
in February 1983. The observations show evidence for flare activity
in greatly enhanced chromospheric and transition region emission
lines. The flare radiation is isolated and the electron pressures of
the mean quiescent atmosphere and the two flares are derived from the
intersystem line flux ratios. Estimates are given of the total flare
power output over the whole atmosphere and the surface filling factor
for the flares. The implications of the observations for studies of
flares on RS CVn stars are considered.
Title: The X-ray flare and the quiescent emission from Algol as
detected by EXOSAT.
Authors: van den Oord, G. H. J.; Mewe, R.
Bibcode: 1989A&A...213..245V
Altcode:
The Algol system was observed on August 18/19,1983 with different
combinations of three instruments aboard EXOSAT: the low-energy
channel multiplier array, the medium-energy proportional counters,
and the transmission grating spectrometer. The X-ray flare observed on
August 19 was analyzed and the decay phase was studied. It is shown
that during this phase the flare cools quasistatically according to
the scaling laws and analytical expressions for the time-evolution of
the relevant physical quantities are presented.
Title: A study of the decay phase of an X-ray flare on Algol.
Authors: Mewe, R.; van den Oord, G. H. J.; Jakimiec, J.
Bibcode: 1989sasf.confP.123M
Altcode: 1988sasf.conf..123M; 1989IAUCo.104P.123M
The authors have re-analyzed the X-ray flare on Algol which was observed
with EXOSAT (White et al., 1986). The common practice of estimating
loop volume and length from the decay time of the flare is discussed
extensively. The authors show that during the decay phase of the flare
both scaling laws for coronal loops are valid. This implies a unique
determination of loop volume and length and allows a check whether
additional heating occurs in the decay phase of a flare.
Title: Optical flares from the dwarf M star V577 MON (Gliese 234 AB =
Ross 614).
Authors: Doyle, J. G.; van den Oord, G. H. J.; Butler, C. J.
Bibcode: 1989A&A...208..208D
Altcode:
The paper describes a search for flaring on the Balmer emission line
star, Gliese 234 AB. Ten flares were detected in a total monitoring time
of 4.6 h, implying a very high level of activity. The time-averaged
flare energy in the Johnson U-band is slightly greater than measured
previously but due to the small number of hours of observations it
is best to use the mean of these two measurements. Relative energies
are given for the U, B, V, R, and I bands for a 3.8 mag U-band flare
which had a 45 s rise time and 20 min decay time. Of the various models
fitted to the flare data, bound-free emission seems the most promising.
Title: An analysis of the continuum light in the 3500 - 8500 Å region
from a flare observed on the dwarf M star Gliese 234AB (= V577 Mon).
Authors: Doyle, J. G.; van den Oord, G. H. J.; Butler, C. J.
Bibcode: 1989sasf.confP..91D
Altcode: 1989IAUCo.104P..91D; 1988sasf.conf...91D
Relative energies are given for the U,B,V,R and I bands for a 3.8
magnitude U-band flare observed on the dwarf dMe star Gl 234 AB on 28
Feb. 1985. This flare had a 45 second rise time and 20 minute decay
time. The total flare energy from all five bands during the flare
was 7×1031erg, 34% of this total was from the U-band
and 20% from the two near infrared R and I bands. The energy density
(per frequency interval) implied a rising continuum towards the red,
however this only lasted for approximately 20 - 40 seconds, i.e. during
the impulsive phase, afterwhich the excess flare emission could not
be detected in the near infrared bands. Of the various models fitted
to the flare data (i.e. optical synchrotron, bound-free emission and
free-free emission), bound-free emission seems the most promising.
Title: A combined radio and X-ray observation of Algol.
Authors: van den Oord, G. H. J.; Kuijpers, J.; White, N. E.; van der
Hulst, J. M.; Culhane, J. L.
Bibcode: 1989A&A...209..296V
Altcode:
The detection of Algol (Beta Per) at 1.4 GHz is reported. The eclipsing
phase of Algol has been observed in radio and at X-ray energies. The
radio emission is explained as synchrotron radiation from electrons
with energies of about 1 MeV. The magnetic field strength is of the
order of tens of Gauss. The radio and X-ray emission are cospatial but
the energy in the radio component is 1,000,000 times smaller than that
of the X-ray component. The simultaneous observation indicates that
an energetic particle component can be present in a quiescent X-ray
emitting corona.
Title: Energetic flares on RS CVn stars.
Authors: Doyle, J. G.; Byrne, P. B.; van den Oord, G. H. J.
Bibcode: 1989sasf.confP.115D
Altcode: 1988sasf.conf..115D; 1989IAUCo.104P.115D
Ultraviolet spectroscopic observations of the RS CVn star II Peg in
February 1983 show evidence for flare activity in greatly enhanced
chromospheric and transition region emission lines. The total
radiative losses from the chromosphere and transition region during
the flare is 3.1×1035erg. Over the whole atmosphere the
authors estimate total radiative losses of 2.4×1036erg,
(excluding hydrogen line radiation). At flare peak, the flare radiated
1.5×1032erg s-1. Adopting a two-ribbon flare
model, where the filament is located between the two stars of the
system, one can have 1039(l/Rsun) erg of magnetic
energy available, where l is the filament length. Therefore, only a
small fraction of this magnetic energy need be converted into heating
of the flare plasma.
Title: Time-dependent heating phenomena in coronal loops.
Authors: van den Oord, G. H. J.; Barstow, M. A.
Bibcode: 1988A&A...207...89V
Altcode:
The authors have investigated the behaviour of coronal loops subject
to flare-like heating. They pay attention to the conflicting results
obtained by Kuin and Martens (1982) and Craig and Schulkes (1985). They
show that the description of Kuin and Martens is correct and extend
it for a larger range of loop temperatures. Further, they show that
the evolution of the loop is governed by the history of the heating
function. Under certain conditions individual heat depositions are
masked. Quasi-periodic variations observed with EXOSAT in the light
curves of the RS CVn system HR 1099 (Barstow, 1985) are explained in
terms of time-dependent heating. This observation serves as a case study
for the theoretical results obtained in this paper and represents the
first detection of flare-like heating in the corona of an RS CVn system.
Title: The possible role of proton beams in flares on dwarf stars.
Authors: van den Oord, G. H. J.
Bibcode: 1988A&A...207..101V
Altcode:
The author has investigated the possible role of proton beams during
flares on dwarf stars. Simultaneous observations of flares at optical
and X-ray wavelengths indicate that a gradual phase is sometimes
absent. This can be explained when ionization due to the beam energy
deposition plays an important role. Two well observed optical/X-ray
flares on BY Dra and on YZ CMi are discussed. It is shown that optical
flares on dMe stars require energy deposition in the chromosphere by
beams instead of by a conductive flux, in order to be consistent with
the observations. The absence of correlation between optical flares
and microwave emission can be explained if during flares the energy
for the optical flare is carried to the chromosphere by proton beams.
Title: Filament support and flares in binaries
Authors: van den Oord, G. H. J.
Bibcode: 1988A&A...205..167V
Altcode:
The possibilities for filament support in a binary are investigated. A
description in terms of currents, the sources of the magnetic field is
employed. Several advantages of this method are discussed. An extended
discussion is presented on the stability of a filament in a RS Canum
Venaticorum system. Various locations for stable filament configurations
are identified, and analytical expressions for the amount of stored
energy are presented. It is shown that sufficient energy can be stored
to explain the largest flares in RS CVn and Algol systems.
Title: An EXOSAT observation of an X-ray flare and quiescent emission
from the RS CVn binary sigma2 CrB.
Authors: van den Oord, G. H. J.; Mewe, R.; Brinkman, A. C.
Bibcode: 1988A&A...205..181V
Altcode:
The RS CVn binary sigma2 Coronae Borealis was observed with
medium-energy and low-energy detectors and the transmission grating
spectrometer aboard EXOSAT, and a strong X-ray flare rising within
about five minutes and lasting more than two hours was detected. The
temperature rises steeply within three minutes from 19 MK to a value
well above 70 MK, while the emission measure attains a maximum of
6 x 10 to the 53rd/cu cm five minutes later. The e-folding time is
about half an hour. The flare volume and density are estimated and,
on the basis of the short rise time of the flare, can be classified
as a compact flare. The flare energy requirements are discussed,
and it is found that current-driven turbulence can produce such fast
heating. An equivalent-circuit description is presented.
Title: Rotational modulation and flares on RS CVn and BY DRA
systems. VII. simultaneous X-ray, radio and optical data for the
dMe star YZ CMi on 4/5 March 1985.
Authors: Doyle, J. G.; Butler, C. J.; Bryne, P. B.; van den Oord,
G. H. J.
Bibcode: 1988A&A...193..229D
Altcode:
Broad-band photometric observations of YZ CMi show a 1.2mag U-band flare
at 19:55 UT on 4 March 1985, however, simultaneous X-ray observations
from EXOSAT show no detectable increase. Two possible explanations
for the lack of an X-ray flare may be i) a flare occurring low in the
chromosphere, where the chromosphere may not have been coupled to
the transition region and therefore the corona via magnetic fields
or ii) proton beam heating as opposed to electron beams. During the
flare, optical spectra show excess emission in the wings of all the
hydrogen Balmer lines. Interpreted in terms of mass flows would imply
material moving at ∼300 km s-1 simultaneously to the
blue and red or alternatively random mass motions with a velocity
of a similar magnitude. At flare maximum, all the Balmer lines show
excess emission in the wings with Hγ and Hσ showing symmetrically
broadened lines while higher members of the series such as Hζ and
Hη show predominately red shifted material. Assuming a single loop
flare, an interpretation in terms of directed mass flows would imply a
loop of length ∼2-3 109 cm, however this would place the
material in the corona where we should have observed it in X-rays. An
alternative explanation, also assuming directed mass flows could
involve several small flare kernels, which brighten successively,
thus producing a broadened profile. An explanation similar to this
has been suggested to explain the excess emission seen in the wings
of Hα during solar flares. The total optical flare energy was ∼6
1031 erg in this event, with the Balmer lines contributing
∼10% to the energy in the U-band during the flare. Many short-lived
bursts or micro-flares were detected in both the Johnson U-band and the
Balmer emission lines. The probability for a correlation between these
two data-sets to have occurred by chance was less than 10-5
. The coronal X-ray flux show only a moderate dependence on Balmer
line emission and none on the U-band variations. Thus for YZ CMi not
only does the large optical flare at 19:55 UT produce no significant
X-ray emission but also the low level optical flares, which are seen
almost continuously, are similarly absent in soft X-rays. Several X-ray
flares/micro-flares were observed, however the majority of these did not
show an optical counter-part. The largest of these X-ray events were at
18:00 UT and 23:30 UT, lasting an hour and 1 1/2 h respectively. Their
energy in the 0.02-2 keV range were 9.1 1031 erg and 2.1
1032 erg respectively. No optical data was available for
the first large X-ray flare, while the second event had only limited
coverage. The photometric data implied an optical energy of at least
2.6 1031 erg. The Balmer line contribute at least 10%
to the U-band energy in both the 19:55 UT and 23:30 UT flares. Radio
observations at 326 MHz give an upper limit of 4 mJy for the flux.
Title: Stellar flares
Authors: van den Oord, Gijsbertus Hendrikus Jozef Bert
Bibcode: 1987PhDT........47V
Altcode:
No abstract at ADS
Title: Stellar flares
Authors: van den Oord, Gijsbertus H. J.
Bibcode: 1987stfl.book.....V
Altcode:
No abstract at ADS
Title: A Large X-ray Flare on σ2 Coronae Borealis Observed
with EXOSAT
Authors: van den Oord, G. H. J.; Mewe, R.; Brinkman, A. C.
Bibcode: 1987LNP...291..494V
Altcode: 1987LNP87.291..494V; 1987csss....5..494V
We report the observation of a compact flare on σ2
Coronae Borealis. The energy requirements for the flare are discussed
and we find that current-driven turbulence can produce heating fast
enough. During the initial phase of the flare classical transport
theory is not applicable.
Title: Calculated X-radiation from optically thin plasmas. VI -
Improved calculations for continuum emission and approximation
formulae for nonrelativistic average Gaunt actors.
Authors: Mewe, R.; Lemen, J. R.; van den Oord, G. H. J.
Bibcode: 1986A&AS...65..511M
Altcode:
The continuum spectrum of an optically thin plasma whose electrons
have a Maxwellian energy distribution is calculated for the wavelength
range 1 - 1000 Å and temperature range 0.01 - 100 MK. The effect of
recombination to excited states on the free-bound emission is taken
into account. Simple analytic approximations to the total free-free,
free-bound and two-photon Gaunt factors are presented for various
wavelength and temperature intervals covering the whole considered
range. A simple computer routine for calculating the continuum emission
with the aid of this approximation is given.
Title: Simultaneous optical and X-ray observations of a flare on
BY Draconis.
Authors: de Jager, C.; Heise, J.; Avgoloupis, S.; Cutispoto, G.;
Kieboom, K.; Herr, R. B.; Landini, M.; Langerwerf, A. F.; Mavridis,
L. N.; Melkonian, A. S.; Molenaar, R.; Monsignori-Fossi, B. C.;
Nations, H. L.; Pallavicini, R.; Piirola, V.; Rodono, M.; Seeds,
M. A.; van den Oord, G. H. J.; Vilhu, O.; Waelkens, C.
Bibcode: 1986A&A...156...95D
Altcode:
The authors present a first report on a campaign for simultaneous
observations in the visual, radio and X-ray ranges of stellar flares
on the binary BY Dra. During two nights of observations they observed
one significant flare, simultaneously in soft X-rays and visible
wavelengths, and one or two smaller bursts. The main flare impulsively
heated an area of ⪉2×107km2 of the stellar
photosphere to a brightness temperature ⪆25000K, during about 5
min, and generated hot plasma, emitting soft X-rays. This emission
reached maximum after the impulsive burst and lasted for about an
hour; it covered a much larger area on the star than the optical
emission. There was no significant hard X-ray component. The authors
ascribe the optical continuous emission to the flare's impulsive phase,
and the soft X-ray emission to the gradual phase.
Title: An EXOSAT Observation of Quiescent and Flare Coronal X-Ray
Emission from Algol
Authors: White, N. E.; Culhane, J. L.; Parmar, A. N.; Kellett, B. J.;
Kahn, S.; van den Oord, G. H. J.; Kuijpers, J.
Bibcode: 1986ApJ...301..262W
Altcode:
X-ray emission from the Algol system is believed to originate in a
corona associated with the K star. The authors have used the EXOSAT
Observatory to make a 35 hr continuous observation centered on the
occultation of the K star by the B star primary. The spectrum of
the quiescent emission in the 1 - 10 keV band gives a temperature of
2.5×107K. This spectrum, extrapolated to lower energies,
can account for more than 80% of the observed count rate. No obvious
X-ray eclipse was seen. An X-ray flare was detected with a rise time
of ≡1700 s and an exponential decay of ≡7000 s. The 0.1 - 10 keV
peak luminosity was 1.4×1031ergs s-1. The peak
temperature was 6×107K, with an iron K line confirming
the thermal character of the emission.
Title: Flares: solar and stellar. Proceedings of an RAL workshop on
astronomy and astrophysics, held at Abingdon, UK, 19 - 21 May 1986.
Authors: Gondhalekar, P. M.; van den Oord, G. H. J.
Bibcode: 1986fssp.book.....G
Altcode:
No abstract at ADS
Title: The Modelling of Solar Two-Ribbon Flares
Authors: van den Oord, G. H. J.; Kaastra, J. S.
Bibcode: 1986mrt..conf..165V
Altcode:
No abstract at ADS
Title: Calculated X-Radiation from Optically Thin Plasmas - Part Five
Authors: Mewe, R.; Gronenschild, E. H. B. M.; van den Oord, G. H. J.
Bibcode: 1985A&AS...62..197M
Altcode: 2009A&A...500..301M
The spectral line list in a previously developed computer code has
been updated and extended with many new lines up to 300 A. These lines
comprise: H- and He-like n = 1-2 lines, together with their satellites
which are produced by innershell transitions: 2l-nl-prime (n = 3, 4)
transitions in Fe XVII-XXIV (10-17 A); 2s-2p lines in Fe XVIII-XXIII and
Ni XX-XXV (90-140 A); 3l-nl-prime (n = 3, 4) transitions in Fe VIII-XVI
and Ni X-XIV (50-200 A), and many lines from Li to Ne isoelectronic
sequences of elements C to Ca (20-300 A). Intensities of 2131 spectral
lines are tabulated over the temperature range 3 x 10 to the 4th -
10 to the 9th K.
Title: Variations in the Radio Flux of the Hypergiant P Cygni
(B1 Ia+)
Authors: van den Oord, G. H. J.; Waters, L. B. F. M.; Lamers,
H. J. G. L. M.; Abbott, D. C.; Bieging, J. H.; Churchwell, E.
Bibcode: 1985ASSL..116..111V
Altcode: 1985rst..conf..111V
The radio flux of P Cygni is variable on a time scale of months or
shorter. The variations are interpreted as variations in the degree
of ionization of the wind due to the ejection of shells which shield
the wind from the ionizing radiation of the star. The observed time
scale agrees with the recombination time scale of the wind.
Title: Observations of Steady Anomalous Magnetic Heating in Thin
Current Sheets
Authors: Martens, P. C. H.; van den Oord, G. H. J.; Hoyng, P.
Bibcode: 1985SoPh...96..253M
Altcode:
A faint steadily emitting loop-like structure has been observed
by HXIS in its low energy channels (3.5-8.0 keV) on November 5/6,
1980. These HXIS observations have permitted us to follow the thermal
evolution of this loop for a period of about 15 hr and from this study
we conclude that only a fraction of 0.1% of the volume of the loop is
steadily heated at the rather large rate of 0.6 erg cm-3
s-1. We interpret this heating as the dissipation of magnetic
fields in thin current sheets and we find that the dissipation with
classical resistivity is very unlikely, while ion-kinetic tearing, as
proposed by Galeev et al. (1981), suits the observations very well. The
enhancement of the resistivity over the classical resistivity then
turns out to be a factor 4 × 104. Dissipation in extremely
thin sheets via the ion-acoustic instability (Duijveman et al., 1981)
cannot be completely excluded when the cross-field heat conductivity
is anomalously enhanced by a factor 400.
Title: An EXOSAT Observation of the Morphology of the Coronal X-Ray
Emission from Algol
Authors: White, N. E.; Culhane, J. L.; Parmar, A. N.; Kellett, B.;
Kahn, S.; van den Oord, G. H. J.; Kuijpers, J.
Bibcode: 1985SSRv...40...25W
Altcode:
The X-ray emission from Algol is thought to originate in a corona
associated with the K star in this system. We report the results of a 35
hr continuous EXOSAT observation through secondary optical eclipse that
was designed to measure the structure of the corona. No obvious X-ray
eclipse was seen. The spectrum measured by the ME gives a temperature of
2.5 × 107 K, consistent with the hard component previously
seen by the Einstein SSS. The soft component previously reported by
the SSS would only contribute at most 25% to the count rate seen in
the LE (used with Al/P). The lack of a hard X-ray eclipse indicates the
dimensions of the higher temperature emission region to be comparable
to or greater than the size of the K star. An X-ray flare was detected
with a peak luminosity of 1.4 × 1031 erg s-1
and a total duration of 8 hours. The peak temperature was 5.0 keV with
an emission measure of 9.4 × 1053 cm-3. The
thermal nature of the flare is confirmed by the detection of an iron
line with an EW of ∼2 keV. By equating the observed decay time of
the flare to a known cooling law gives a dimension for the flaring
loop of ∼0.3 stellar radii. This is much smaller than the dimensions
of the hard component inferred from the lack of an eclipse. It seems
probable that the flare occurred in one of the loops responsible for
the lower temperature component seen by the SSS.
Title: A simultaneous X-ray and radio observation of a flare from
Algol
Authors: Parmar, A. N.; Culhane, J. L.; White, N. E.; van den Oord,
G. H. J.
Bibcode: 1985AdSpR...5c..69P
Altcode: 1985AdSpR...5...69P
An X-ray flare was observed from Algol using the low and medium energy
detectors on the European Space Agency's EXOSAT observatory. Spectra
obtained during the flare are well fitted by thermal continua while
an Fe XXV emission feature was also detected. The strength of this
feature indicates a cosmic abundance for iron. The data indicate that
the flare occurred in a loop of height approximately 0.25 of the K
star radius & with a magnetic field >300 Gauss.
Title: Shock-driven mass loss of stars.
Authors: de Jager, C.; van den Oord, G. H. J.
Bibcode: 1984mlao.conf...59D
Altcode:
The authors suggest that shocks, fed by multi-mode pulsational motion
of the photospheres of supergiants are the basic mechanism for the
mass loss of cool and medium type supergiants. They review the present
theoretical situation, and describe the implementations of the observed
transient mass loss component of the pulsating Beta Cephei stars BW
Vul and σ Sco.
Title: The pulsation of the outer layers of the BET CEP star SIG Sco.
Authors: Burger, M.; de Jager, C.; van den Oord, G. H. J.
Bibcode: 1982A&A...109..289B
Altcode:
The pulsation of the outer layers of the Beta Cephei-type variable σ
Sco is investigated using 17 ultraviolet spectrograms (1200-2000 A)
obtained during seven hours of observations with the International
Ultraviolet Explorer. A pulsational model established earlier for
BW Vul is confirmed. According to this model, after a period of
photospheric rest the outer layers expand suddenly. The acceleration
for the outermost layers, investigated with the resonance lines of C
IV and Si IV, is found to be larger than for the photosphere. During
the phase of deceleration that follows, no falling down of the C
IV absorbing layers is observed; their velocity component remains
directed outward. A transient stellar wind component sets in just
before the star has its minimum radius (phase, approximately 0.9);
it is strongest at a phase of approximately 0.2. The observations are
interpreted as suggesting the mechanism of pulsational driven mass loss.
Title: The pulsation of the outer layers of the BET Cep-type variable
BW Vul.
Authors: Burger, M.; de Jager, C.; van den Oord, G. H. J.; Sato, N.
Bibcode: 1982A&A...107..320B
Altcode:
Eleven high-resolution ultraviolet spectrograms of BW Vul were obtained
by means of the IUE over 1.6 periods to compare the pulsation of this
star as observed in the strong far-UV resonance lines with observations
of the UV lines of photospheric origin. The radial velocity curve of
the C IV lines is different from the photospheric one, showing that
both the photosphere and the C IV layers are accelerated upwards
impulsively. After about one hour, the acceleration decreases to
zero, and then the atmosphere falls with a nearly constant downward
acceleration of 2100 cm/sq s. The varying asymmetry indicates mass loss
due to the pulsation of the atmosphere, superimposed on the continuous
mass loss of the star.
Title: Shock-driven Mass Loss of Stars
Authors: de Jager, C.; van den Oord, G. H. J.
Bibcode: 1982mlao.conf...59D
Altcode:
No abstract at ADS
Title: The Outer Layers of the Beta-Cephei Type Variables
Bw-Vulpeculae and Omicron-Scorpii
Authors: Burger, M.; de Jager, C.; van den Oord, G. H. J.; Groupe
Etoiles Variables de L'Observatoire de Nice
Bibcode: 1981pbs..work..181B
Altcode:
No abstract at ADS