explanation blue bibcodes open ADS page with paths to full text
Author name code: andries
ADS astronomy entries on 2022-09-14
author:"Andries, Jesse"
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Title: Performance assessment and verification of space weather
operational service provision at SIDC
Authors: de Patoul, Judith; Andries, Jesse; Maneva, Yana; Loumou,
Konstantina
2022cosp...44.3436D Altcode:
Since many years, the SIDC (Solar Influences Data analysis Centre)
at the Royal Observatory of Belgium is performing operational space
weather monitoring and providing public space weather services that
are being disseminated through different categories, such as SSA, RWC,
and ICAO. The strength and focus at SIDC are the interaction between
Operations and Research. Operations support fundamental research by
using data from operational activities and fundamental research in turn
leads to improving forecasting capabilities. We will discuss here the
validation and verification methods used at SIDC as well as internal
procedures (1) to quantitatively measure the quality of the services,
(2) to improve the quality of service and the inter portability between
R2O and O2R, and (3) to guide the user and provide knowledge on what to
expect regarding these services, and in particular how to minimize the
discrepancy between the user expectations and actual service provision
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Title: STCE participation in space weather services for aviation
Authors: Maneva, Yana; Andries, Jesse; Verstringe, Freek; de Patoul,
Judith; Loumou, Konstantina
2022cosp...44.3495M Altcode:
Since more than two years the Solar-Terrestrial Centre of Excellence
(STCE) at Brussels, Belgium has become part of one of the global
operational centres providing 24/7 real-time space weather services
to the international civil aviation organization (ICAO). In this
presentation we will discuss the operational infrastructure utilized
and developed at STCE for the continuous space weather services within
the three main domains of interest for aviation, namely, satellite
navigation, radiation dose at different flight levels and long-distance
radio communication. We will focus on the implemented data monitoring
tools, the automated procedures for warnings based on predefined event
thresholds, describe automated software for generation of space weather
advisories and discuss some internal operational workflows for data
dissemination, storage and provision. We will present the remaining
challenges with respect to integration of this new service with
our existing space weather forecast operations and the continuously
changing income data formats due to lack of predefined standards for
space weather service provision in the aviation sector. Finally, some
thoughts will be given on ongoing data standardisations and possible
future solutions.
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Title: The International Space Environment Service
Authors: Andries, Jesse; Ishii, Mamoru
2021cosp...43E2384A Altcode:
Since 1962 the International Space Environment Service is an
organisation that brings together Space Weather Service Centres around
the globe. Promoting data exchange and cooperation between it members,
it strives to enhance capabilities in Space Weather forecasting and
services worldwide. We will present an overview and of the organisation
and its members.
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Title: A Comparison of Flare Forecasting Methods. IV. Evaluating
Consecutive-day Forecasting Patterns
Authors: Park, Sung-Hong; Leka, K. D.; Kusano, Kanya; Andries, Jesse;
Barnes, Graham; Bingham, Suzy; Bloomfield, D. Shaun; McCloskey,
Aoife E.; Delouille, Veronique; Falconer, David; Gallagher, Peter T.;
Georgoulis, Manolis K.; Kubo, Yuki; Lee, Kangjin; Lee, Sangwoo; Lobzin,
Vasily; Mun, JunChul; Murray, Sophie A.; Hamad Nageem, Tarek A. M.;
Qahwaji, Rami; Sharpe, Michael; Steenburgh, R. A.; Steward, Graham;
Terkildsen, Michael
2020ApJ...890..124P Altcode: 2020arXiv200102808P
A crucial challenge to successful flare prediction is
forecasting periods that transition between "flare-quiet" and
"flare-active." Building on earlier studies in this series in which we
describe the methodology, details, and results of flare forecasting
comparison efforts, we focus here on patterns of forecast outcomes
(success and failure) over multiday periods. A novel analysis is
developed to evaluate forecasting success in the context of catching
the first event of flare-active periods and, conversely, correctly
predicting declining flare activity. We demonstrate these evaluation
methods graphically and quantitatively as they provide both quick
comparative evaluations and options for detailed analysis. For the
testing interval 2016-2017, we determine the relative frequency
distribution of two-day dichotomous forecast outcomes for three
different event histories (I.e., event/event, no-event/event, and
event/no-event) and use it to highlight performance differences between
forecasting methods. A trend is identified across all forecasting
methods that a high/low forecast probability on day 1 remains high/low
on day 2, even though flaring activity is transitioning. For M-class
and larger flares, we find that explicitly including persistence or
prior flare history in computing forecasts helps to improve overall
forecast performance. It is also found that using magnetic/modern
data leads to improvement in catching the first-event/first-no-event
transitions. Finally, 15% of major (I.e., M-class or above) flare
days over the testing interval were effectively missed due to a lack
of observations from instruments away from the Earth-Sun line.
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Title: Community-wide Space Weather Scoreboards: Facilitating the
Validation of Real-time CME, Flare, and SEP Forecasts
Authors: Taktakishvili, A.; Mays, M. L.; Andries, J.; Bingham, S.;
Dierckxsens, M.; Jones, J. T.; Kuznetsova, M. M.; Marsh, M. S.; Murray,
S. A.; Mullinix, R.; Owens, M. J.; Riley, P.; Semones, E.; Wiegand, C.
2019AGUFMSM31C3178T Altcode:
Testing predictive capabilities before event onset is important
and especially relevant for validating space weather models. We
describe three real-time forecast validation projects facilitated
by the CCMC via forecast collection "scoreboards": (1) CME
arrival time and geomagnetic storm strength, (2) flare occurrence
probability, (3) SEP onset, duration, peak flux, probability,
and overall profile, and (4) IMF at L1. The scoreboards enable
world-wide community involvement in real-time predictions, foster
community validation projects, and ultimately help researchers
improve their forecasts. <P />The "CME arrival time scoreboard" (<A
href="https://kauai.ccmc.gsfc.nasa.gov/CMEscoreboard/">https://kauai.ccmc.gsfc.nasa.gov/CMEscoreboard/
</A>) provides a central location for the community to: submit
their CME arrival time forecast in real-time, quickly view
all forecasts at once in real-time, and compare forecasting
methods when the event has arrived. <P />The "Flare Scoreboard" (<A
href="http://ccmc.gsfc.nasa.gov/challenges/flare.php">http://ccmc.gsfc.nasa.gov/challenges/flare.php
</A>) project is led by Trinity College Dublin and the UK
Met Office. The full disk and active region flare forecasts
can currently be viewed on an interactive display overlaid
on an SDO/AIA or HMI image and is paired with a display
of flare probability time series. <P />The "SEP Scoreboard" (<A
href="http://ccmc.gsfc.nasa.gov/challenges/sep.php">http://ccmc.gsfc.nasa.gov/challenges/sep.php
</A>) project is led by BIRA-IASB and the UK Met Office. The
SEP scoreboard captures SEP onset, duration, peak flux,
probability, all-clear, and overall profile. In 2018,
Johnson Space Center's Space Radiation Analysis Group
has become involved in the SEP scoreboard in support of
upcoming human exploration missions. <P />The "Bz Scoreboard" (<A
href="http://ccmc.gsfc.nasa.gov/challenges/bz.php">http://ccmc.gsfc.nasa.gov/challenges/bz.php
</A>) is the latest project and is led by Predictive Science and
University of Reading. It is currently in planning phase and will be
designed as an automated system to gather real-time interplanetary
magnetic field forecasts at L1 and evaluate their accuracy.
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Title: PECASUS, European Space Weather Service Network for Aviation
Authors: Harri, A. M.; Kauristie, K.; Andries, J.; Gibbs, M.; Beck,
P.; Berdermann, J.; Perrone, L.; van den Oord, B.; Berghmans, D.;
Bergeot, N.; De Donder, E.; Latocha, M.; Dierckxsens, M.; Haralambous,
H.; Stanislawska, I. M.; Wilken, V.; Romano, V.; Kriegel, M.;
Österberg, K.
2019AGUFMSA33D3168H Altcode:
The PECASUS Consortium (European Consortium for Aviation Space weather
User Services) will provide a space weather service focusing on the
dissemination of warning messages ('advisories') towards aviation
actors and corresponds to extreme space weather events with impact
on aviation GNSS systems, HF communication and radiation levels at
flight altitudes. In November 2018 ICAO (International Civil Aviation
Organization) designated three global space weather service centers
to be operated by the European PECASUS consortium, by United States
and by the consortium of Australia, Canada, France and Japan. <P
/>PECASUS was set-up as a consortium bringing together a number of
European partners with proven space weather service capabilities. The
PECASUS consortium is coordinated by FMI (Finland) who is also the
ultimate responsible for communications towards the aviation sector. The
Advisory Messages are produced by STCE (Belgium) on the basis of expert
interpretation and data streams produced by DLR (Germany), INGV (Italy),
Seibersdorf Laboratories (Austria), STCE (Belgium), SRC (Poland) and
FU (Cyprus). In addition, the MetOffice (UK) will act as a resilience
node in case of a major failure in the network, while the KNMI (The
Netherlands) will take care of user liaison and monitor the PECASUS
performance. <P />The PECASUS Consortium was audited in February 2018
by space weather and operational management experts, nominated by the
World Meteorological Organisation (WMO). The audit addressed a broad
spectrum of criteria under Institutional, Operational, Technical and
Communication/ Dissemination categories. PECASUS was declared fully
compliant in all ICAO/WMO criteria with no areas for improvement
identified. <P />In this presentation we will describe the coordinated
action of three ICAO Space Weather Centers, PECASUS network and its
operations, and the vision of the PECASUS team to move forward. User
interactions such as education and training, user feedback at ESWW,
product and performance verification are part of PECASUS operations.
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Title: A Comparison of Flare Forecasting Methods. III. Systematic
Behaviors of Operational Solar Flare Forecasting Systems
Authors: Leka, K. D.; Park, Sung-Hong; Kusano, Kanya; Andries, Jesse;
Barnes, Graham; Bingham, Suzy; Bloomfield, D. Shaun; McCloskey,
Aoife E.; Delouille, Veronique; Falconer, David; Gallagher, Peter
T.; Georgoulis, Manolis K.; Kubo, Yuki; Lee, Kangjin; Lee, Sangwoo;
Lobzin, Vasily; Mun, JunChul; Murray, Sophie A.; Hamad Nageem, Tarek
A. M.; Qahwaji, Rami; Sharpe, Michael; Steenburgh, Robert A.; Steward,
Graham; Terkildsen, Michael
2019ApJ...881..101L Altcode: 2019arXiv190702909L
A workshop was recently held at Nagoya University (2017 October
31-November 2), sponsored by the Center for International Collaborative
Research, at the Institute for Space-Earth Environmental Research,
Nagoya University, Japan, to quantitatively compare the performance
of today’s operational solar flare forecasting facilities. Building
upon Paper I of this series, in Paper II we described the participating
methods for this latest comparison effort, the evaluation methodology,
and presented quantitative comparisons. In this paper, we focus on
the behavior and performance of the methods when evaluated in the
context of broad implementation differences. Acknowledging the short
testing interval available and the small number of methods available,
we do find that forecast performance: (1) appears to improve by
including persistence or prior flare activity, region evolution,
and a human “forecaster in the loop” (2) is hurt by restricting
data to disk-center observations; (3) may benefit from long-term
statistics but mostly when then combined with modern data sources
and statistical approaches. These trends are arguably weak and must
be viewed with numerous caveats, as discussed both here and in Paper
II. Following this present work, in Paper IV (Park et al. 2019) we
will present a novel analysis method to evaluate temporal patterns of
forecasting errors of both types (i.e., misses and false alarms). Hence,
most importantly, with this series of papers, we demonstrate the
techniques for facilitating comparisons in the interest of establishing
performance-positive methodologies.
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Title: A Comparison of Flare Forecasting Methods. II. Benchmarks,
Metrics, and Performance Results for Operational Solar Flare
Forecasting Systems
Authors: Leka, K. D.; Park, Sung-Hong; Kusano, Kanya; Andries, Jesse;
Barnes, Graham; Bingham, Suzy; Bloomfield, D. Shaun; McCloskey,
Aoife E.; Delouille, Veronique; Falconer, David; Gallagher, Peter
T.; Georgoulis, Manolis K.; Kubo, Yuki; Lee, Kangjin; Lee, Sangwoo;
Lobzin, Vasily; Mun, JunChul; Murray, Sophie A.; Hamad Nageem, Tarek
A. M.; Qahwaji, Rami; Sharpe, Michael; Steenburgh, Robert A.; Steward,
Graham; Terkildsen, Michael
2019ApJS..243...36L Altcode: 2019arXiv190702905L
Solar flares are extremely energetic phenomena in our solar
system. Their impulsive and often drastic radiative increases,
particularly at short wavelengths, bring immediate impacts that motivate
solar physics and space weather research to understand solar flares
to the point of being able to forecast them. As data and algorithms
improve dramatically, questions must be asked concerning how well the
forecasting performs; crucially, we must ask how to rigorously measure
performance in order to critically gauge any improvements. Building
upon earlier-developed methodology of Paper I (Barnes et al. 2016),
international representatives of regional warning centers and
research facilities assembled in 2017 at the Institute for Space-Earth
Environmental Research, Nagoya University, Japan to, for the first time,
directly compare the performance of operational solar flare forecasting
methods. Multiple quantitative evaluation metrics are employed, with the
focus and discussion on evaluation methodologies given the restrictions
of operational forecasting. Numerous methods performed consistently
above the “no-skill” level, although which method scored top marks
is decisively a function of flare event definition and the metric
used; there was no single winner. Following in this paper series, we
ask why the performances differ by examining implementation details
(Leka et al. 2019), and then we present a novel analysis method to
evaluate temporal patterns of forecasting errors in Paper IV (Park
et al. 2019). With these works, this team presents a well-defined and
robust methodology for evaluating solar flare forecasting methods in
both research and operational frameworks and today’s performance
benchmarks against which improvements and new methods may be compared.
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Title: Benchmarking CME Arrival Time and Impact: Progress on Metadata,
Metrics, and Events
Authors: Verbeke, C.; Mays, M. L.; Temmer, M.; Bingham, S.; Steenburgh,
R.; Dumbović, M.; Núñez, M.; Jian, L. K.; Hess, P.; Wiegand, C.;
Taktakishvili, A.; Andries, J.
2019SpWea..17....6V Altcode: 2018arXiv181110695V
Accurate forecasting of the arrival time and subsequent geomagnetic
impacts of coronal mass ejections (CMEs) at Earth is an important
objective for space weather forecasting agencies. Recently, the CME
Arrival and Impact working team has made significant progress toward
defining community-agreed metrics and validation methods to assess
the current state of CME modeling capabilities. This will allow the
community to quantify our current capabilities and track progress in
models over time. First, it is crucial that the community focuses
on the collection of the necessary metadata for transparency and
reproducibility of results. Concerning CME arrival and impact we have
identified six different metadata types: 3-D CME measurement, model
description, model input, CME (non)arrival observation, model output
data, and metrics and validation methods. Second, the working team
has also identified a validation time period, where all events within
the following two periods will be considered: 1 January 2011 to 31
December 2012 and January 2015 to 31 December 2015. Those two periods
amount to a total of about 100 hit events at Earth and a large amount
of misses. Considering a time period will remove any bias in selecting
events and the event set will represent a sample set that will not be
biased by user selection. Lastly, we have defined the basic metrics and
skill scores that the CME Arrival and Impact working team will focus on.
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Title: SSA SWE Network user test campaigns: validating SSA space
weather prototype service performance together with end users
Authors: Chabanski, Sophie; Crosby, Norma Bock; Glover, Alexi; De
Donder, Erwin; Andries, Jesse; Perry, Chris; Borries, Claudia
2018cosp...42E.544C Altcode:
In this presentation we summarize the preparation, execution, results
and conclusions of user test campaigns run by the SSA Space Weather
Coordination Centre (SSCC) for validating and improving the products
and services of the SSA Space Weather Service Network. <P />In the
frame of the Space Weather (SWE) Segment of its Space Situational
Awareness (SSA) programme, the European Space Agency (ESA) is
developing the SSA Space Weather Service Network which now provides
access to 21 prototype end-user driven services through its SWE Portal
(http://swe.ssa.esa.int/). These services are built upon a pool of
around 150 products provided to the Network by ∼40 Expert Groups
who themselves are organised into five expert service centres: Solar
Weather, Heliospheric Weather, Space Radiation, Ionospheric Weather
and Geomagnetic conditions. The Network also includes a Data Centre and
the SSA Space Weather Coordination Centre (SSCC) located at the Space
Pole in Brussels which provides first line end-user support via the
SWE Service Helpdesk and overall service availability monitoring. <P
/>The SWE services are distributed over eight different service
domains: Spacecraft Design, Spacecraft Operation, Human Space Flight,
Trans-ionospheric Radio Link, Space Surveillance & Tracking,
Non-space System Operation, and General Data Services. <P />To
demonstrate and validate the services and gain further insight into
end users' needs, the SSA Space Weather Coordination Centre (SSCC)
has run a number of dedicated user test campaigns in specific high
priority domains such as Spacecraft Operations, Aviation and GNSS
services. The results of these campaigns are then analysed and the
resulting recommendations utilised by ESA in the context of future
service improvements. <P />After a brief description of the context
and organisation of the test user campaigns, this presentation will
give examples of the resulting tailored SWE bulletins and describe
some of the key findings from these campaigns.
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Title: Provision of space weather bulletins in support to ESA missions
Authors: Kruglanski, Michel; Devos, Andy; Calders, Stijn; De Donder,
Erwin; Berghmans, David; Andries, Jesse; Crosby, Norma; Dierckxsens,
Mark; Glover, Alexi
2017EGUGA..1918071K Altcode:
The SSCC (SSA Space Weather Coordination Centre) is the focal point for
user support of the ESA Space Situational Awareness (SSA) Program Space
Weather Service Network (http://swe.ssa.esa.int/) and offers first line
support to end-users. Its activities are gradually expanding as more
products and tools are incorporated in the associated Network. Here
we report on the tailoring of the network resources at the SSCC in
order to produce space weather forecast notifications in support to
selected ESA missions. With the help of forecasters from the Expert
Service Centres (ECSs) for Solar Weather and for Space Radiation,
the SSCC already has provided dedicated notifications for various
missions including the launch window and the L2 insertion manoeuvre of
the GAIA mission, for the last aerobraking campaign of Venus Express,
during the trajectory around Comet 67P/Churyumov-Gerasimenko of the
Rosetta mission and the landing of Philae lander, for the flight of
the experimental IXV space plane, and for the launch LISA Pathfinder
mission and during the operations phase at L1. Examples of space weather
notification will be given. Those exercises are performed in a test
and evaluation context helping to evaluate the network capabilities
and to identify recommendations for improvement.
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Title: Community-wide space weather Scoreboards: Facilitating the
Validation of Real-time CME, Flare, and SEP Forecasts
Authors: Mullinix, R.; Mays, M. L.; Kuznetsova, M. M.; Andries,
J.; Bingham, S.; Bloomfield, D.; Boblitt, J. M.; Crosby, N. B.;
Dierckxsens, M.; Guerra, J. A.; Leka, K. D.; Marsh, M. S.; Murray,
S.; Wiegand, C.
2016AGUFMSH11C2256M Altcode:
Confidence assessment of predictive space weather models ultimately
determines the value of forecasts for end users. Testing predictive
capabilities before event onset is important and especially relevant
for validating space weather models. This poster presents three
real-time forecast validation projects facilitated by the CCMC via
forecast collection "scoreboards": (1) CME arrival time and geomagnetic
storm strength, (2) flare occurrence probability, and (3) SEP onset,
duration, peak flux, probability, and overall profile. The CME,
Flare, and SEP scoreboards enable world-wide community involvement
in real-time predictions, foster community validation projects,
and ultimately help researchers improve their CME, flare, and
SEP forecasts. All CME, Flare, SEP forecast modelers and experts
worldwide are invited to advise or participate in this effort. The
flare and SEP systems are automated such that model developers can
routinely upload their predictions to an anonymous ftp and the data
is accessible to anyone via an API. The "CME arrival time scoreboard"
(https://kauai.ccmc.gsfc.nasa.gov/CMEscoreboard/) provides a central
location for the community to: submit their CME arrival time forecast
in real-time, quickly view all forecasts at once in real-time, and
compare forecasting methods when the event has arrived. There are
currently 19 registered CME arrival time prediction methods. The "Flare
Scoreboard" (http://ccmc.gsfc.nasa.gov/challenges/flare.php) project
is led by the UK Met Office.The full disk and active region flare
forecasts can currently be viewed on an interactive display overlaid
on an SDO/AIA or HMI image of the Sun and will be dynamically paired
with a display of flare probability time series (coming soon). The
"SEP Scoreboard" (http://ccmc.gsfc.nasa.gov/challenges/sep.php)
project is led by BIRA-IASB and the UK Met Office. SEP forecasts can
be roughly divided into three categories: continuous/Probabilistic,
solar event triggered, non near real-time. The SEP scoreboard will
focus on real-time forecasts, however the SEP scoreboard team can
also coordinate a set of historical events for a "SEP challenge" with
different models, particularly those physics-based models in the third
category that are not ready or relevant for real-time modeling.
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Title: Energy Propagation by Transverse Waves in Multiple Flux Tube
Systems Using Filling Factors
Authors: Van Doorsselaere, T.; Gijsen, S. E.; Andries, J.; Verth, G.
2014ApJ...795...18V Altcode:
In the last few years, it has been found that transverse waves are
present at all times in coronal loops or spicules. Their energy has
been estimated with an expression derived for bulk Alfvén waves in
homogeneous media, with correspondingly uniform wave energy density and
flux. The kink mode, however, is localized in space with the energy
density and flux dependent on the position in the cross-sectional
plane. The more relevant quantities for the kink mode are the integrals
of the energy density and flux over the cross-sectional plane. The
present paper provides an approximation to the energy propagated by
kink modes in an ensemble of flux tubes by means of combining the
analysis of single flux tube kink oscillations with a filling factor
for the tube cross-sectional area. This finally allows one to compare
the expressions for energy flux of Alfvén waves with an ensemble
of kink waves. We find that the correction factor for the energy in
kink waves, compared to the bulk Alfvén waves, is between f and 2f,
where f is the density filling factor of the ensemble of flux tubes.
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Title: Wave Energy Deposition in the Solar Corona
Authors: Van Doorsselaere, Tom; Goossens, Marcel; Verth, Gary; Soler,
Roberto; Gijsen, Stief; Andries, Jesse
2014cosp...40E3464V Altcode:
Recently, a significant amount of transverse wave energy has been
estimated propagating along solar atmospheric magnetic fields. However,
these estimates have been made with the classic bulk Alfven wave
model which assumes a homogeneous plasma. In this talk, the kinetic,
magnetic, and total energy densities and the flux of energy are first
computed for transverse MHD waves in one-dimensional cylindrical flux
tube models with a piecewise constant density profile. There are
fundamental deviations from the properties for classic bulk Alfven
waves. (1) There is no local equipartition between kinetic and magnetic
energy. (2) The flux of energy and the velocity of energy transfer have,
in addition to a component parallel to the magnetic field, components
in the planes normal to the magnetic field. (3) The energy densities
and the flux of energy vary spatially, contrary to the case of classic
bulk Alfven waves. This last property is then used to connect the
energy flux in such a simple model to the energy flux in multiple flux
tube systems. We use the plasma filling factor f to derive an ad-hoc
formula for estimating the energy that is propagated in bundles of
loops. We find that the energy flux in kink waves is lower than the
energy computed from a bulk Alfven wave interpretation, by a factor
that is (approximately) between f and 2f. We consider some geometric
models to quantify this correction factor.
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Title: On the non-existence of a cut-off frequency for the propagation
of kink-modes on an isothermal slender stratified tube
Authors: Andries, Jesse
2014cosp...40E..99A Altcode:
Recently (Andries and Cally, ApJ, 743, 164, 2011) a fairly general
analytic theory was established for the dispersion and scattering
of magnetohydrodynamic waves by longitudinally stratified flux
tubes. The theory provides a common framework for, and synthesis of,
many previous studies of flux tube oscillations that were carried out
under various simplifying assumptions. In particular we illustrated
the unifying theoretical framework underlying both the description of
waves scattered by flux tubes and the dispersion of waves carried along
flux tubes. In the present contribution we will investigate in more
detail the propagation of the kink-mode on a slender flux tube in a
stratified atmosphere. Spruit (A&A, 98, 155-160, 1981) has studied
this situation before and discussed both the appearance of 'buoyancy'
terms in the kink-mode and the reduction to a Klein-Gordon equation
in the case of an isothermally stratified medium with the associated
cut-off frequency preventing the propagation of kink modes towards
the corona for low frequencies. We show in convincing detail why the
derivation by Spruit (1983) is erroneous and conclude that there is no
cut-off frequency for the vertical propagation of kink-modes along a
flux tube. The non-existence of the cut-off frequency implies that,
regardless of the frequency, kink-modes may propagate along a flux
tube from the photosphere towards the corona. How much of the energy
is effectively transmitted into the corona, however, remains subject
of further study.
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Title: Surface Alfvén Waves in Solar Flux Tubes
Authors: Goossens, M.; Andries, J.; Soler, R.; Van Doorsselaere, T.;
Arregui, I.; Terradas, J.
2012ApJ...753..111G Altcode: 2012arXiv1205.0935G
Magnetohydrodynamic (MHD) waves are ubiquitous in the solar
atmosphere. Alfvén waves and magneto-sonic waves are particular
classes of MHD waves. These wave modes are clearly different and have
pure properties in uniform plasmas of infinite extent only. Due to
plasma non-uniformity, MHD waves have mixed properties and cannot
be classified as pure Alfvén or magneto-sonic waves. However,
vorticity is a quantity unequivocally related to Alfvén waves as
compression is for magneto-sonic waves. Here, we investigate MHD waves
superimposed on a one-dimensional non-uniform straight cylinder with
constant magnetic field. For a piecewise constant density profile,
we find that the fundamental radial modes of the non-axisymmetric
waves have the same properties as surface Alfvén waves at a true
discontinuity in density. Contrary to the classic Alfvén waves in
a uniform plasma of infinite extent, vorticity is zero everywhere
except at the cylinder boundary. If the discontinuity in density is
replaced with a continuous variation of density, vorticity is spread
out over the whole interval with non-uniform density. The fundamental
radial modes of the non-axisymmetric waves do not need compression
to exist unlike the radial overtones. In thin magnetic cylinders,
the fundamental radial modes of the non-axisymmetric waves with phase
velocities between the internal and the external Alfvén velocities
can be considered as surface Alfvén waves. On the contrary, the radial
overtones can be related to fast-like magneto-sonic modes.
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Title: Resonant Alfvén waves in partially ionized plasmas of the
solar atmosphere
Authors: Soler, R.; Andries, J.; Goossens, M.
2012A&A...537A..84S Altcode: 2011arXiv1111.4134S
Context. Magnetohydrodynamic (MHD) waves are ubiquitous in the solar
atmosphere. In magnetic waveguides resonant absorption due to plasma
inhomogeneity naturally transfers wave energy from large-scale motions
to small-scale motions. In the cooler parts of the solar atmosphere
as, e.g., the chromosphere, effects due to partial ionization may
be relevant for wave dynamics and heating. <BR /> Aims: We study
resonant Alfvén waves in partially ionized plasmas. <BR /> Methods:
We use the multifluid equations in the cold plasma approximation. We
investigate propagating resonant MHD waves in partially ionized flux
tubes. We use approximate analytical theory based on normal modes in
the thin tube and thin boundary approximations along with numerical
eigenvalue computations. <BR /> Results: We find that the jumps of
the wave perturbations across the resonant layer are the same as in
fully ionized plasmas. The damping length due to resonant absorption is
inversely proportional to the frequency, while that due to ion-neutral
collisions is inversely proportional to the square of the frequency. For
observed frequencies in the solar atmosphere, the amplitude of MHD
kink waves is more efficiently damped by resonant absorption than by
ion-neutral collisions. <BR /> Conclusions: Most of the energy carried
by chromospheric kink waves is converted into localized azimuthal
Alfvén waves that can deposit energy in the coronal medium. The
dissipation of wave energy in the chromosphere due to ion-neutral
collisions is only effective for high-frequency waves. The chromosphere
acts as a filter for kink waves with periods shorter than 10 s.
---------------------------------------------------------
Title: On the Dispersion and Scattering of Magnetohydrodynamic Waves
by Longitudinally Stratified Flux Tubes
Authors: Andries, J.; Cally, P. S.
2011ApJ...743..164A Altcode:
We provide a fairly general analytic theory for the dispersion and
scattering of magnetohydrodynamic waves by longitudinally stratified
flux tubes. The theory provides a common framework for, and synthesis
of, many previous studies of flux tube oscillations that were
carried out under various simplifying assumptions. The present theory
focuses on making only a minimal number of assumptions. As a result
it thus provides an analytical treatment of several generalizations of
existing tube oscillation models. The most important practical cases are
inclusion of plasma pressure and possibly buoyancy effects in models
of straight non-diverging tubes as applied in coronal seismology, and
relaxation of the "thin tube" approximation in oscillation models of
diverging tubes as applied both in the context of p-mode scattering
and coronal seismology. In particular, it illustrates the unifying
theoretical framework underlying both the description of waves scattered
by flux tubes and the dispersion of waves carried along flux tubes.
---------------------------------------------------------
Title: Linear coupling between fast and slow MHD waves due to
line-tying effects
Authors: Terradas, J.; Andries, J.; Verwichte, E.
2011A&A...527A.132T Altcode: 2010arXiv1011.0936T
Context. Oscillations in coronal loops are usually interpreted in terms
of uncoupled magnetohydrodynamic (MHD) waves. Examples of these waves
are standing transverse motions, interpreted as the kink MHD modes, and
propagating slow modes, commonly reported at the loop footpoints. <BR
/> Aims: Here we study a simple system in which fast and slow MHD
waves are coupled. The goal is to understand the fingerprints of
the coupling when boundary conditions are imposed. <BR /> Methods:
The reflection problem of a fast and slow MHD wave interacting with a
rigid boundary, representing the line-tying effect of the photosphere,
is analytically investigated. Both propagating and standing waves
are analysed and the time-dependent problem of the excitation of these
waves is considered. <BR /> Results: An obliquely incident fast MHD wave
on the photosphere inevitably generates a slow mode. The frequency of
the generated slow mode at the photosphere is exactly the same as the
frequency of the incident fast MHD mode, but its wavelength is much
smaller, assuming that the sound speed is slower than the Alfvén
speed. <BR /> Conclusions: The main signatures of the generated slow
wave are density fluctuations at the loop footpoints. We have derived
a simple formula that relates the velocity amplitude of the transverse
standing mode with the density enhancements at the footpoints due to
the driven slow modes. Using these results it is shown that there is
possible evidence in the observations of the coupling between these
two modes.
---------------------------------------------------------
Title: Resonant Absorption as Mode Conversion?
Authors: Cally, P. S.; Andries, J.
2010SoPh..266...17C Altcode: 2010arXiv1007.1808C; 2010SoPh..tmp..151C
Resonant absorption and mode conversion are both extensively studied
mechanisms for wave "absorption" in solar magnetohydrodynamics
(MHD). But are they really distinct? We re-examine a well-known simple
resonant absorption model in a cold MHD plasma that places the resonance
inside an evanescent region. The normal mode solutions display the
standard singular resonant features. However, these same normal modes
may be used to construct a ray bundle which very clearly undergoes
mode conversion to an Alfvén wave with no singularities. We therefore
conclude that resonant absorption and mode conversion are in fact the
same thing, at least for this model problem. The prime distinguishing
characteristic that determines which of the two descriptions is
most natural in a given circumstance is whether the converted wave
can provide a net escape of energy from the conversion/absorption
region of physical space. If it cannot, it is forced to run away in
wavenumber space instead, thereby generating the arbitrarily small
scales in situ that we recognize as fundamental to resonant absorption
and phase mixing. On the other hand, if the converted wave takes net
energy away, singularities do not develop, though phase mixing may
still develop with distance as the wave recedes.
---------------------------------------------------------
Title: Coronal Seismology by Means of Kink Oscillation Overtones
Authors: Andries, J.; van Doorsselaere, T.; Roberts, B.; Verth, G.;
Verwichte, E.; Erdélyi, R.
2009SSRv..149....3A Altcode:
The detection of overtones of coronal loop kink oscillations has been
an important advance in the development of coronal seismology. It has
significantly increased the potential of coronal seismology and has
thus initiated important theoretical and observational improvements. New
detections of overtones have been made and a reduction of the error bars
has been obtained. The efforts of theoreticians to extend eigenmode
studies to more general coronal loop models is no longer a matter
of checking the robustness of the model but now also allows for the
estimation of certain equilibrium parameters. The frequencies of
the detected (longitudinal) overtones are in particular sensitive to
changes in the equilibrium properties along the loop, especially the
density and the magnetic field expansion. Also, attempts have been
made to use the limited longitudinal resolution in combination with
the theoretical eigenmodes as an additional seismological tool.
---------------------------------------------------------
Title: On the nature of kink MHD waves in magnetic flux tubes
Authors: Goossens, M.; Terradas, J.; Andries, J.; Arregui, I.;
Ballester, J. L.
2009A&A...503..213G Altcode: 2009arXiv0905.0425G
Context: Magnetohydrodynamic (MHD) waves are often reported in the
solar atmosphere and usually classified as slow, fast, or Alfvén. The
possibility that these waves have mixed properties is often ignored. <BR
/>Aims: The goal of this work is to study and determine the nature
of MHD kink waves. <BR />Methods: This is done by calculating the
frequency, the damping rate and the eigenfunctions of MHD kink waves
for three widely different MHD waves cases: a compressible pressure-less
plasma, an incompressible plasma and a compressible plasma which allows
for MHD radiation. <BR />Results: In all three cases the frequency and
the damping rate are for practical purposes the same as they differ
at most by terms proportional to (k<SUB>z</SUB> R)^2. In the magnetic
flux tube the kink waves are in all three cases, to a high degree of
accuracy incompressible waves with negligible pressure perturbations
and with mainly horizontal motions. The main restoring force of kink
waves in the magnetised flux tube is the magnetic tension force. The
total pressure gradient force cannot be neglected except when the
frequency of the kink wave is equal or slightly differs from the local
Alfvén frequency, i.e. in the resonant layer. <BR />Conclusions:
Kink waves are very robust and do not care about the details of the
MHD wave environment. The adjective fast is not the correct adjective
to characterise kink waves. If an adjective is to be used it should
be Alfvénic. However, it is better to realize that kink waves have
mixed properties and cannot be put in one single box.
---------------------------------------------------------
Title: The Nature of Kink MHD Waves in Magnetic Flux Tubes
Authors: Goossens, Marcel; Terradas, J.; Andries, J.; Arregui, I.;
Ballester, J.
2009SPD....40.1306G Altcode:
We examine the nature of MHD kink waves. This is done by determining
the frequency, the damping rate and the eigenfunctions of MHD kink waves
for three widely different MHD waves cases: a compressible pressure-less
plasma, an incompressible plasma and a compressible plasma with non-zero
plasma pressure which allows for MHD radiation. The overall conclusion
is that kink waves are very robust and do not care about the details
of the MHD wave environment. In all three cases the frequency and the
damping rate are for most practical purposes the same. In the magnetic
flux tube the kink waves are in all three cases, to a high degree of
accuracy incompressible waves with negligible pressure perturbations
and with mainly horizontal motions. The main restoring force of kink
waves in the magnetized flux tube is the magnetic tension force. The
gradient pressure force cannot be neglected except when the frequency
of the kink wave is equal or slightly differs from the local Alfvén
frequency, i.e. in the resonant layer. In a non-magnetic external
plasma the wave is of course acoustic. The adjective fast is not the
correct adjective to characterize kink waves. If an adjective is to
be used it should be Alfvénic. However, it is better to realize that
kink waves have mixed properties and cannot be put in one single box.
---------------------------------------------------------
Title: The influence of longitudinal density variation in coronal
loops on the eigenfunctions of kink-oscillation overtones
Authors: Andries, J.; Arregui, I.; Goossens, M.
2009A&A...497..265A Altcode:
Context: As coronal loops are spatially at least partially resolved
in the longitudinal direction, attempts have been made to use
the longitudinal profiles of the oscillation amplitudes as a
seismological tool. <BR />Aims: We aim to derive simple formulae to
assess which oscillation modes and which quantities of the oscillation
(displacement or compression) are most prone to modifications induced
by stratification of the equilibrium density along the loop. We
furthermore clarify and quantify that the potential of such a method
could be enhanced if observational profiles of the compression in the
oscillations could be determined. <BR />Methods: By means of a linear
expansion in the longitudinal stratification along with the “thin
tube” approximation, the modifications to the eigenfunctions are
calculated analytically. The results are validated by direct numerical
computations. <BR />Results: Higher axial overtones are found to be
more affected by equilibrium stratification and hence would provide a
much better tool if observed. For the k-1th overtone the compression
is found to be around (k + 2)^2/k<SUP>2</SUP> times more sensitive to
longitudinal density variation than the displacement. While the linear
formulae do give a good indication of the strength of the effects
of longitudinal density stratification, the numerical computations
indicate that the corrections to the approximate analytical results
are significant and cannot be neglected under the expected coronal
conditions.
---------------------------------------------------------
Title: Nonlinear Instability of Kink Oscillations due to Shear Motions
Authors: Terradas, J.; Andries, J.; Goossens, M.; Arregui, I.; Oliver,
R.; Ballester, J. L.
2008ApJ...687L.115T Altcode: 2008arXiv0809.3664T
First results from a high-resolution three-dimensional nonlinear
numerical study of the kink oscillation are presented. We show in
detail the development of a shear instability in an untwisted line-tied
magnetic flux tube. The instability produces significant deformations
of the tube boundary. An extended transition layer may naturally evolve
as a result of the shear instability at a sharp transition between
the flux tube and the external medium. We also discuss the possible
effects of the instability on the process of resonant absorption when an
inhomogeneous layer is included in the model. One of the implications
of these results is that the azimuthal component of the magnetic field
of a stable flux tube in the solar corona, needed to prevent the shear
instability, is probably constrained to be in a very specific range.
---------------------------------------------------------
Title: Resonant Absorption in Complicated Plasma Configurations:
Applications to Multistranded Coronal Loop Oscillations
Authors: Terradas, J.; Arregui, I.; Oliver, R.; Ballester, J. L.;
Andries, J.; Goossens, M.
2008ApJ...679.1611T Altcode: 2008arXiv0802.0591T
We study the excitation and damping of transverse oscillations in a
multistranded model of a straight line-tied coronal loop. The transverse
geometry of our equilibrium configuration is quite irregular and
more realistic than the usual cylindrical loop model. By numerically
solving the time-dependent ideal magnetohydrodynamic equations in
two dimensions, we show how the global motion of the whole bundle of
strands, excited by an external disturbance, is converted into localized
Alfvénic motions due to the process of resonant absorption. This
process produces the attenuation of the transverse oscillations. At
any location in the structure, two dominant frequencies are found:
the frequency of the global mode or quasi-mode, and the local Alfvén
frequency. We find that the mechanism of mode conversion, due to the
coupling between fast and Alfvén waves, is not compromised by the
complicated geometry of the model. We also show that it is possible to
have energy conversion not only at the external edge of the composite
loop, but also inside the structure. The implications of these results
and their relationship with the observations are discussed.
---------------------------------------------------------
Title: On the Excitation of Leaky Modes in Cylindrical Loops
Authors: Terradas, J.; Andries, J.; Goossens, M.
2007SoPh..246..231T Altcode:
The role of leaky waves in the coronal loop oscillations observed by
TRACE is not yet clearly understood. In this work, the excitation of
fast waves in solar coronal loops modelled as dense plasma cylindrical
tubes in a uniform straight magnetic field is investigated. We study
the trapped and especially leaky modes (whose energy escapes from
the tube) that result from an initial disturbance by solving the
time-dependent problem numerically. We find that the stationary state
of the tube motion is given by the trapped normal modes. By contrast,
the transient behaviour between the initial and the stationary phase
is dominated by wave leakage. The so-called trig leaky modes are
clearly identified since the transient behaviour shows periods and
damping times that are in agreement with the values calculated from the
normal-mode analysis. Consequently, these radiating modes have physical
significance. However, we have not found any evidence for the excitation
of other types of modes, such as the principal leaky kink mode.
---------------------------------------------------------
Title: Observational evidence favors a resistive wave heating
mechanism for coronal loops over a viscous phenomenon
Authors: Van Doorsselaere, T.; Andries, J.; Poedts, S.
2007A&A...471..311V Altcode:
Context: How coronal loops are heated to their observed temperatures
is the subject of a long standing debate. <BR />Aims: Observational
evidence exists that the heating in coronal loops mainly occurs near the
loop footpoints. In this article, analytically and numerically obtained
heating profiles produced by resonantly damped waves are compared to
the observationally estimated profiles. <BR />Methods: To do that,
the predicted heating profiles are fitted with an exponential heating
function, which was also used to fit the observations. The results
of both fits, the estimated heating scale heights, are compared to
determine the viability of resonant absorption as a heating mechanism
for coronal loops. <BR />Results: Two results are obtained. It is shown
that any wave heating mechanism (i.e. not just resonant absorption)
should be dominated by a resistive (and not a viscous) phenomenon in
order to accomodate the constraint of footpoint heating. Additionally
it is demonstrated that the analytically and numerically estimated
heating scale heights for the resonant absorption damping mechanism
fit the observations very well.
---------------------------------------------------------
Title: Coronal loop oscillations: energy considerations and initial
value problem
Authors: Terradas, J.; Andries, J.; Goossens, M.
2007A&A...469.1135T Altcode:
Context: Flares and eruptions in the solar corona generate
oscillations of loops which have been interpreted as eigenmodes
(mainly the fundamental kink mode, although other modes can also be
excited). From the theoretical point of view the excitation of the
tube eigenmodes due to an initial disturbance has not been studied in
much detail. <BR />Aims: The main aim of this work is to calculate for
a given initial disturbance the amount of energy that is deposited
in the trapped fast mode oscillation, how it depends on the initial
perturbation and how it is distributed among the different eigenmodes
(kink and fluting and also the longitudinal harmonics). <BR />Methods:
We calculate, using analytical expressions, the amplitude and the energy
of the oscillation of the magnetic tube for different kinds of initial
excitations. <BR />Results: We find that external excitations deposit
a small amount of energy in the tube. We show that fluting modes have
quite small energies in comparison with the energy of the kink mode
(around three orders of magnitude for the first fluting mode). On
the contrary, the longitudinal fundamental mode and the longitudinal
harmonics have energies of the same order of magnitude. In addition, we
find that the loop length and density contrast can be important factors
that determine the amount of energy that is trapped by the loop. <BR
/>Conclusions: The energy deposited in loops is typically six orders
of magnitude smaller than the energy of the initial disturbance (for
external excitations). However, it strongly depends on the distance
of the initial perturbation and also on the loop properties (length
and density). Fluting modes in coronal loops are very difficult to
excite. Longitudinal harmonics are in principle more easily excited.
---------------------------------------------------------
Title: MHD seismology of coronal loops using the period and damping
of quasi-mode kink oscillations
Authors: Arregui, I.; Andries, J.; Van Doorsselaere, T.; Goossens,
M.; Poedts, S.
2007A&A...463..333A Altcode:
Aims:We combine the magnetohydrodynamic (MHD) theory of resonantly
damped quasi-mode kink oscillations with observational estimates of
the period and damping of transverse coronal loop oscillations to
extract information on physical parameters in oscillating loops. <BR
/>Methods: A numerical study of the quasi-mode period and damping,
in one-dimensional fully non-uniform flux tubes, is used to obtain
equilibrium models that reproduce the observed periods and damping
rates. This scheme is applied to 11 loop oscillation events. <BR
/>Results: When only the damping rate is used, the valid equilibrium
models form a one-dimensional solution curve in the two-dimensional
parameter space (density contrast, transverse inhomogeneity
length-scale). Lower limits to the transverse inhomogeneity are
obtained in the limit of high contrast loops. When both the period and
the damping rate are used, the equilibrium Alfvén speed (or Alfvén
travel time) comes into play. The valid equilibrium models then form
a one-dimensional solution curve in the three-dimensional parameter
space (density contrast, transverse inhomogeneity length-scale, Alfvén
speed or Alfvén travel time). The projection of these solutions onto
the Alfvén speed axis is found to be constrained to a rather limited
interval. Upper limits to the internal Alfvén speed are derived for
9 of the 11 analysed events.
---------------------------------------------------------
Title: Time Dependent Simulations of 2D Coronal Loop Models
Authors: van Doorsselaere, T.; Poedts, S.; Andries, J.; Arregui, I.
2006ESASP.617E.113V Altcode: 2006soho...17E.113V
No abstract at ADS
---------------------------------------------------------
Title: Seismology of Coronal Loops Using the Period and Damping of
Quasi-Mode Kink Oscillations
Authors: Arregui, I.; Andries, J.; Van Doorsselaere, T.; Goossens,
M.; Poedts, S.
2006ESASP.617E..81A Altcode: 2006soho...17E..81A
No abstract at ADS
---------------------------------------------------------
Title: Seismology of Transversely Oscillating Loops Using Periods
and Damping Times
Authors: Goossens, Marcel; Arregui, I.; Andries, J.; Van Doorsselaere,
T.
2006SPD....37.1804G Altcode: 2006BAAS...38..247G
Periods and damping times of quasi-mode fundamental kink oscillations
have been computed for non-uniform cylindrical models of coronal
loops. The radial inhomogeneity length-scale, the density contrast
and the internal Alfvén velocity are three equilibrium quantities
that determine the theoretical values of the period and damping
times in 1-D equilibrium models. From a seismological point of view
this means that observed values of period and damping time can be
recovered by an infinite number of equilibrium models. In other words,
observed values of period and damping time of the fundamental kink
oscillation mode do not allow a unique identification of even a 1-D
equilibrium model. Only if there is additional information on one of
the three equilibrium quantities, can we use the observed values of
period and damping time to determine the two remaining equilibrium
quantities. However, it is not all bad news. It turns out that, even
without additional information, we can determine upper limits to the
internal Alfvén velocity. We apply this scheme to the set of 11 loop
oscillation events studied in Goossens et al. 2002 and find constraints
on the equilibrium parameters for these 11 events.
---------------------------------------------------------
Title: Damping of magnetohydrodynamic waves by resonant absorption
in the solar atmosphere
Authors: Goossens, M.; Andries, J.; Arregui, I.
2006RSPTA.364..433G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Quasi-mode damping in two-dimensional fully non-uniform
coronal loops
Authors: Arregui, I.; Van Doorsselaere, T.; Andries, J.; Goossens,
M.; Poedts, S.
2006RSPTA.364..529A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Seismology of Coronal Loops Using Resonant Absorption
Authors: Arregui, I.; van Doorsselaere, T.; Andries, J.; Goossens,
M.; Poedts, S.
2005ESASP.600E..21A Altcode: 2005dysu.confE..21A; 2005ESPM...11...21A
No abstract at ADS
---------------------------------------------------------
Title: The Dynamic Sun: Challenges for Theory and Observations
Authors: Danesy, D.; Poedts, S.; de Groof, A.; Andries, J.
2005ESASP.600E....D Altcode: 2005dysu.confE....D; 2005ESPM...11.....D
No abstract at ADS
---------------------------------------------------------
Title: Building a Time Dependent Code to Simulate Oscillations of
Line-Tied Coronal Loops
Authors: van Doorsselaere, T.; Poedts, S.; Arregui, I.; Andries, J.
2005ESASP.600E..83V Altcode: 2005dysu.confE..83V; 2005ESPM...11...83V
No abstract at ADS
---------------------------------------------------------
Title: Dynamics of Coronal Loop Oscillations Recent Improvements
and Computational Aspects
Authors: van Doorsselaere, T.; Arregui, I.; Andries, J.; Goossens,
M.; Poedts, S.
2005SSRv..121...79V Altcode:
We will discuss the observed, heavily damped transversal oscillations
of coronal loops. These oscillations are often modeled as transversal
kink oscillations in a cylinder. Several features are added to the
classical cylindrical model. In our models we include loop curvature,
longitudinal density stratification, and highly inhomogeneous radial
density profiles. In this paper, we will first give an overview of
recently obtained results, both analytically and numerically. After
that, we shed a light on the computational aspects of the modeling
process. In particular, we will focus on the parallellization of the
numerical codes.
---------------------------------------------------------
Title: Dynamics of Coronal Loop Oscillations
Authors: van Doorsselaere, T.; Arregui, I.; Andries, J.; Goossens,
M.; Poedts, S.
2005ESASP.596E..44V Altcode: 2005ccmf.confE..44V
No abstract at ADS
---------------------------------------------------------
Title: Resonantly damped fast MHD kink modes in longitudinally
stratified tubes with thick non-uniform transitional layers
Authors: Arregui, I.; Van Doorsselaere, T.; Andries, J.; Goossens,
M.; Kimpe, D.
2005A&A...441..361A Altcode:
Resonantly damped fast kink quasi-modes are computed in fully <P
/>resistive magnetohydrodynamics (MHD) for two-dimensional equilibrium
<P />models. The equilibrium model is a straight cylindrically symmetric
flux <P />tube with a plasma density that is non-uniform both across
and along the <P />loop. The non-uniform layer across the loop is not
<P />restricted to be thin, but its thickness can reach values up to
the <P />loop diameter. <P />Our results indicate that <P />the period
and damping of coronal loop oscillations mainly depend on the density
contrast <P />and the inhomogeneity length-scale and are independent
of the details of <P />longitudinal stratification, depending on the
weighted mean density, <P />weighted with the wave energy. For fully
non-uniform loops, quasi-modes can <P />interact with resistive Alfvén
eigenmodes leading to avoided crossings <P />and gaps in the complex
frequency plane. The present study extends previous <P />studies on
coronal loop oscillations in one-dimensional equilibrium models <P
/>with thick boundary layers and in equilibria with longitudinally
stratified loops under the <P />thin boundary approximation, and allow
for a better comparison between <P />observations and theory raising
the prospect of coronal seismology using <P />the time damping of
coronal loop oscillations.
---------------------------------------------------------
Title: Solar coronal loop oscillations: theory of resonantly damped
oscillations and comparison with observations
Authors: Goossens, M.; Andries, J.; Arregui, I.; Doorsselaere, T. V.;
Poedts, S.
2005AIPC..784..114G Altcode:
One of the proposed damping mechanisms of coronal transverse loop
oscillations in the kink mode is resonant absorption as a result
of the spatial variation of the Alfvén velocity in the equilibrium
configuration. Analytical expressions for the period and the damping
time exist for 1-D cylindrical equilibrium models with thin non-uniform
transitional layers. Comparison with observations indicates that the
assumption of thin non-uniform transitional layers is not a very
accurate approximation of reality. This contributions starts with
a short review of observations on transverse oscillations in solar
coronal loops. Then it presents results on periods and damping times
of resonantly damped kink mode oscillations for (i) fully non-uniform
1-D cylindrical equilibrium models in which the equilibrium quantities
vary in the radial direction across the magnetic field from the centre
of the loop up to its boundary and (ii) non-uniform 2-D cylindrical
equilibrium models in which the equilibrium quantities vary both in the
radial direction across the magnetic field and in the axial direction
along the magnetic field. An important point is that the periods and
damping times obtained for these fully non-uniform models can differ
substantially from those obtained for thin non-uniform transitional
layers. This contribution then reports on a consistency test between
theory and observations showing that there is a very good agreement
within the observational inaccuracies.
---------------------------------------------------------
Title: Damping of phase-mixed slow magneto-acoustic waves: Real
or apparent?
Authors: Voitenko, Y.; Andries, J.; Copil, P. D.; Goossens, M.
2005A&A...437L..47V Altcode:
The propagation of slow magnetoacoustic waves along a multithreaded
coronal loop is modelled analytically by means of a ray tracing
method. It is shown how cross field gradients build up due to phase
mixing. The cross field gradients can enhance shear viscosity so
that it dominates over compressive viscosity. Nevertheless the short
dissipation distances (~10<SUP>7</SUP> m) observed for slow waves in
coronal loops require very small cross field length scales which imply
a filamentary structure on scales at least three orders of magnitude
below the current detection limit of TRACE and close to the limit where
magnetohydrodynamic (MHD) theory breaks down. The observed dissipation
distances can alternatively be explained by phase mixing in its ideal
regime, where the apparent damping is due to the spatial integration
of the phase mixed amplitudes by the observation.
---------------------------------------------------------
Title: Determination of the Coronal Density Stratification from the
Observation of Harmonic Coronal Loop Oscillations
Authors: Andries, Jesse; Arregui, Inigo; Goossens, Marcel
2005ApJ...624L..57A Altcode:
The recent detection of multiple harmonic standing transverse
oscillations in coronal loops by Verwichte et al. is of special
importance, as it allows one to obtain information on the longitudinal
density variation in loops. Verwichte et al. detected the simultaneous
presence of both the fundamental and the first-overtone mode in two
coronal loops. Here we point out that the ratio of the period of the
fundamental mode to the period of the overtone mode differs from 2
in loops with longitudinal density stratification. Conversely, the
difference between this ratio and 2 can be used as a seismological
tool to obtain information about the density scale height in loops.
---------------------------------------------------------
Title: Coronal loop oscillations. Calculation of resonantly damped
MHD quasi-mode kink oscillations of longitudinally stratified loops
Authors: Andries, J.; Goossens, M.; Hollweg, J. V.; Arregui, I.;
Van Doorsselaere, T.
2005A&A...430.1109A Altcode:
The observed coronal loop oscillations and their damping are often
theoretically described by the use of a very simple coronal loop
model, viz. a straight, longitudinally invariant, axi-symmetric, and
pressureless flux tube with a different density inside and outside
of the loop. In this paper we generalize the model by including
longitudinal density stratification and we examine how the longitudinal
density stratification alters the linear eigenmodes of the system,
their oscillation frequencies, and the damping rates by resonant
absorption. <P />Appendix A is only available in electronic form at
http://www.edpsciences.org
---------------------------------------------------------
Title: The Effect of Curvature on Quasi-Modes in Coronal Loops
Authors: van Doorsselaere, T.; Debosscher, A.; Andries, J.; Poedts, S.
2004ESASP.575..448V Altcode: 2004soho...15..448V
No abstract at ADS
---------------------------------------------------------
Title: Numerical Solutions for Resonantly Damped MHD Quasi-Modes in
Two-Dimensional Coronal Loops
Authors: Arregui, I.; van Doorsselaere, T.; Andries, J.; Goossens, M.
2004ESASP.575...85A Altcode: 2004soho...15...85A
No abstract at ADS
---------------------------------------------------------
Title: The effect of curvature on quasi-modes in coronal loops
Authors: Van Doorsselaere, T.; Debosscher, A.; Andries, J.; Poedts, S.
2004A&A...424.1065V Altcode:
This paper studies quasi-mode oscillations in models of coronal loops
that include longitudinal curvature. Using a toroidal coordinate system
to incorporate curvature in a basic coronal loop model, the linearized
ideal MHD equations are solved for the plasma-β=0. As a result of
the curvature, quasi-modes with different poloidal wave numbers are
coupled resulting in modifications of the frequencies. However, for
small curvature, only the coupling of quasi-modes with a neighbouring
poloidal wave number remains in first order. In addition, the quasi-mode
frequencies are unchanged up to first order in the curvature. The
imaginary part of the frequency, however, does change in first order,
and quasi-modes are slightly more damped in realistically curved coronal
loop configurations. <P />Appendix A is only available in electronic
form at http://www.edpsciences.org
---------------------------------------------------------
Title: Damping of Coronal Loop Oscillations: Calculation of Resonantly
Damped Kink Oscillations of One-dimensional Nonuniform Loops
Authors: Van Doorsselaere, T.; Andries, J.; Poedts, S.; Goossens, M.
2004ApJ...606.1223V Altcode:
The analytic study of coronal loop oscillations in equilibrium states
with thin nonuniform boundary layers is extended by a numerical
investigation for one-dimensional nonuniform equilibrium states. The
frequency and the damping time of the ideal kink quasi mode are
calculated in fully resistive MHD. In this numerical investigation there
is no need to adopt the assumption of a thin nonuniform boundary layer,
which is essential for analytic theory. An important realization is
that analytical expressions for the damping rate that are equivalent
for thin nonuniform layers give results differing by a factor of 2
when they are used for thick nonuniform layers. Analytical theory for
thin nonuniform layers does not allow us to discriminate between these
analytical expressions. The dependence of the complex frequency of the
kink mode on the width of the nonuniform layer, on the length of the
loop, and on the density contrast between the internal and the external
region is studied and is compared with analytical theory, which is valid
only for thin boundaries. Our numerical results enable us to show that
there exists an analytical expression for thin nonuniform layers that
might be used as a qualitative tool for extrapolation into the regime
of thick nonuniform layers. However, when the width of the nonuniform
layer is varied, the differences between our numerical results and the
results obtained with the version of the analytical approximation that
can be extended into the regime of thick nonuniform layers are still
as large as 25%.
---------------------------------------------------------
Title: Damping of Coronal Loop Oscillations
Authors: van Doorsselaere, T.; Andries, J.; Poedt, S.; Goossens, M.
2004ESASP.547..453V Altcode: 2004soho...13..453V
The analytic study of coronal loop oscillations in equilibrium states
with thin nonuniform boundary layers is extended by a numerical
investigation for 1D nonuniform equilibrium states. The frequency and
the damping time of the ideal kink quasi-mode are calculated in fully
resistive MHD. In this numerical investigation there is no need to adopt
the assumption of a thin nonuniform boundary layer which is essential
for analytic theory. The dependence of the complex frequency of the
kink mode on the width of the nonuniform layer, the length of the loop
and the density contrast between the internal and the external region
is studied and is compared with analytical theory which is only valid
for thin boundaries. When the width of the nonuniformlayer is varied,
the differences between our numerical results and the results obtained
with analytical formula, still amount up to 25%.
---------------------------------------------------------
Title: Observational Tests of Damping by Resonant Absorption in
Coronal Loop Oscillations
Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Andries,
Jesse; Goossens, Marcel; Van Doorsselaere, Tom
2003ApJ...598.1375A Altcode: 2003astro.ph..9470A
One of the proposed damping mechanisms of coronal (transverse)
loop oscillations in the kink mode is resonant absorption as a
result of the Alfvén speed variation at the outer boundary of
coronal loops. Analytical expressions for the period and damping
time exist for loop models with thin nonuniform boundaries. They
predict a linear dependency of the ratio of the damping time to the
period on the thickness of the nonuniform boundary layer. Ruderman and
Roberts used a sinusoidal variation of the density in the nonuniform
boundary layer and obtained the corresponding analytical expression
for the damping time. Here we measure the thickness of the nonuniform
layer in oscillating loops for 11 events, by forward-fitting of the
cross-sectional density profile n<SUB>e</SUB>(r) and line-of-sight
integration to the cross-sectional fluxes F(r) observed with
TRACE 171 Å. This way we model the internal (n<SUB>i</SUB>) and
external electron density (n<SUB>e</SUB>) of the coronal plasma in
oscillating loops. This allows us to test the theoretically predicted
damping rates for thin boundaries as a function of the density ratio
χ=n<SUB>e</SUB>/n<SUB>i</SUB>. Since the observations show that
the loops have nonuniform density profiles, we also use numerical
results for damping rates to determine the value of χ for the
loops. We find that the density ratio predicted by the damping time,
χ<SUB>LEDA</SUB>=0.53+/-0.12, is a factor of ~1.2-3.5 higher than the
density ratio estimated from the background fluxes, χ=0.30+/-0.16. The
lower densities modeled from the background fluxes are likely to be
a consequence of the neglected hotter plasma that is not detected
with the TRACE 171 Å filter. Taking these corrections into account,
resonant absorption predicts damping times of kink-mode oscillations
that are commensurable with the observed ones and provides a new
diagnostic of the density contrast of oscillating loops.
---------------------------------------------------------
Title: Influence of a uniform coronal magnetic field on solar p-modes
Authors: Vanlommel, P.; Debosscher, A.; Andries, J.; Goossens, M.
2002ESASP.506..893V Altcode: 2002svco.conf..893V; 2002ESPM...10..893V
The influence of a constant coronal magnetic field on solar global
oscillations is investigated for a simple planar equilibrium model. The
model consists of an atmosphere with a constant horizontal magnetic
field on top of an unmagnetized solar interior. The focus is on the
possible resonant coupling of global solar oscillation modes to local
slow continuum modes of the atmosphere and the consequent damping of the
global oscillations. The physical process of resonant absorption of the
acoustic modes with frequency in the cusp continuum is mathematically
completely described by the ideal MHD differential equations which
for this particular equilibrium model reduce to the hypergeometric
differential equation.
---------------------------------------------------------
Title: Damping of coronal loop oscillations by resonant absorption
of quasi-mode kink oscillations
Authors: Goossens, M.; Andries, J.; Aschwanden, M. J.
2002ESASP.506..629G Altcode: 2002svco.conf..629G; 2002ESPM...10..629G
Damped quasi-mode kink oscillations in cylindrical flux tubes are
capable of explaining the observed rapid damping of the coronal loop
oscillations when the ratio of the inhomogneity length scale to the
radius of the loop is allowed to vary from loop to loop. They do not
need to invoke anomalously low Reynolds numbers. The theoretical
expressions for the decay time by Hollweg & Yang (1988) and
Ruderman & Roberts (2002) are used to estimate the ratio of
the length scale of inhomogneity compared to the loop radius for a
collection of loop oscillations.
---------------------------------------------------------
Title: Coronal loop oscillations. An interpretation in terms of
resonant absorption of quasi-mode kink oscillations
Authors: Goossens, M.; Andries, J.; Aschwanden, M. J.
2002A&A...394L..39G Altcode:
Damped quasi-mode kink oscillations in cylindrical flux tubes are
capable of explaining the observed rapid damping of the coronal
loop oscillations when the ratio of the inhomogeneity length scale
to the radius of the loop is allowed to vary from loop to loop,
without the need to invoke anomalously low Reynolds numbers. The
theoretical expressions for the decay time by Hollweg & Yang
(\cite{hollweg1988}) and Ruderman & Roberts (\cite{ruderman2002})
are used to estimate the ratio of the length scale of inhomogeneity
compared to the loop radius for a collection of loop oscillations.
---------------------------------------------------------
Title: Waves and oscillations in magnetic fields
Authors: Goossens, Marcel; de Groof, Anik; Andries, Jesse
2002ESASP.505..137G Altcode: 2002solm.conf..137G; 2002IAUCo.188..137G
This paper gives an overview of the theory of MHD waves in magnetic
plasma configurations in the solar atmosphere. The emphasis is on basic
properties that are independent of specific equilibrium models but
are rather related to the intrinsic structuring and non-uniformity
of the plasma. The discussion is confined to MHD waves in uniform
and 1-d cylindrical equilibrium models of magnetic flux tubes with
a straight magnetic field. These models contain sufficient physics
for understanding basic properties of MHD waves and still allow for
a relatively straightforward and transparent mathematical analysis.
---------------------------------------------------------
Title: Frame dependence of the negative energy wave formula!?
Authors: Andries, Jesse; Goossens, Marcel
2002ESASP.505..341A Altcode: 2002solm.conf..341A; 2002IAUCo.188..341A
In this paper we show that the classical negative energy wave
(N.E.W.) formula is frame dependent. By analogy with a simple mechanical
problem, we show that the negligible second order perturbations
become energetically important and function as an energy source for
the linear waves.
---------------------------------------------------------
Title: Influence of a uniform coronal magnetic field on solar p modes:
coupling to slow resonant MHD waves
Authors: Vanlommel, P.; Debosscher, A.; Andries, J.; Goossens, M.
2002SoPh..205....1V Altcode:
The influence of a constant coronal magnetic field on solar global
oscillations is investigated for a simple planar equilibrium model. The
model consists of an atmosphere with a constant horizontal magnetic
field and a constant sound speed, on top of an adiabatic interior
having a linear temperature profile. The focus is on the possible
resonant coupling of global solar oscillation modes to local slow
continuum modes of the atmosphere and the consequent damping of
the global oscillations. In order to avoid Alfvén resonances, the
analysis is restricted to propagation parallel to the coronal magnetic
field. Parallel propagating oscillation modes in this equilibrium model
have already been studied by Evans and Roberts (1990). However, they
avoided the resonant coupling to slow continuum modes by a special
choice of the temperature profile. The physical process of resonant
absorption of the acoustic modes with frequency in the cusp continuum
is mathematically completely described by the ideal MHD differential
equations which for this particular equilibrium model reduce to the
hypergeometric differential equation. The resonant layer is correctly
dealt with in ideal MHD by a proper treatment of the logarithmical
branch cut of the hypergeometric function. The result of the resonant
coupling with cusp waves is twofold. The eigenfrequencies become
complex and the real part of the frequency is shifted. The shift of
the real part of the frequency is not negligible and within the limit
of observational accuracy. This indicates that resonant interactions
should definitely be taken into account when calculating the frequencies
of the global solar oscillations.
---------------------------------------------------------
Title: Influence of a Uniform Coronal Magnetic Field on Solar p Modes:
Coupling to Slow Resonant MHD Waves
Authors: Vanlommel, P.; Debosscher, A.; Andries, J.; Goossens, M.
2002ASPC..259..480V Altcode: 2002rnpp.conf..480V; 2002IAUCo.185..480V
No abstract at ADS
---------------------------------------------------------
Title: The influence of resonant MHD wave coupling in the boundary
layer on the reflection and transmission process
Authors: Andries, J.; Goossens, M.
2001A&A...375.1100A Altcode:
When a wave is incident on an inhomogeneous boundary layer separating
two plasmas in relative motion, it is often argued that the resonant
absorption rate can become negative when there is a sufficiently large
velocity shear, thus giving energy back to the wave. However such
treatment fails to distinguish the two energy exchange processes that
are operative: extraction of wave energy by dissipative processes around
the resonance point and energy exchange between the wave and the flow
of the medium. By aid of the proper wave energy definitions recently
discussed by Walker (\cite{walker}), rather than by the conventional
concept of “negative energy" waves we reveal the important dependence
of the resonant amplification process on the precise structure
of the boundary layer profiles. We conclude that stratification in
boundary layers is very important not only because the resulting wave
coupling can cause resonant instabilities for velocity shears below
the Kelvin-Helmholtz threshold, but also because the coupling can
stabilize the Kelvin-Helmholtz instability above this threshold.
---------------------------------------------------------
Title: Kelvin-Helmholtz instabilities and resonant flow instabilities
for a coronal plume model with plasma pressure
Authors: Andries, J.; Goossens, M.
2001A&A...368.1083A Altcode:
In this paper we continue the study of the effect of the velocity shear
between the coronal plume and the interplume region on the spectrum
of MHD waves trapped in the plume. In Andries et al. (\cite{andries})
we have illustrated the concept of resonant flow instability of the
trapped modes both in a 1-D slab model and a 1-D cylindrical model for
a coronal plume in which plasma-pressure was neglected. The important
result of that paper was that the threshold values of the velocity
shear are significantly smaller for resonant instability than for
Kelvin-Helmholtz instability to occur. The aim of this paper is to
study the effect of plasma pressure on the eigenmodes of the plume. As
expected we find slow waves in addition to the fast waves. Furthermore
there are two different types of Kelvin-Helmholtz instability. Along
with the fact that now not only Alfvén but also slow resonances can
occur this all leads to a wide variety of ranges of the velocity shear
for which instability can be present. Estimates of these ranges for
different equilibrium quantities can be obtained without going through
the elaborate numerical procedures of calculating the eigenmodes. We
show that the instability that will most probably occur in coronal
plumes is due to an Alfvén resonance of slow body modes. These
instabilities could lead to disruption of the coronal plumes and to
the mixing with interplume plasma. However we point out that there
might be a strong dependence of the resonant flow instability upon
the velocity profile that is to be investigated further.
---------------------------------------------------------
Title: Kelvin-Helmholtz instability and resonant flow instability
in a 1-dimensional coronal plume model
Authors: Andries, J.; Goossens, M.
2000AIPC..537..136A Altcode: 2000wdss.conf..136A
In a previous paper we have illustrated the concept of resonant flow
instability of the trapped modes both in a 1-D slab model and a 1-D
cylindrical model for a coronal plume in a cold plasma. We found that
much larger values of the velocity shear are needed for Kelvin-Helmholtz
than for resonant instability to occur. The aim of this paper is to
study the effect of a non-zero plasma pressure on the eigenmodes of
the plume structure. We show that the instability most probably to
occur in coronal plumes is due to the resonant coupling of slow body
modes to local resonant Alfvén waves. These instabilities could lead
to disruption of the coronal plumes and to the mixing with interplume
plasma. .
---------------------------------------------------------
Title: Modified Kelvin-Helmholtz Instabilities and Resonant Flow
Instabilities in a One-dimensional Coronal Plume Model: Results for
Plasma β=0
Authors: Andries, J.; Tirry, W. J.; Goossens, M.
2000ApJ...531..561A Altcode:
In this manuscript we study the effect of the velocity shear in the mass
flow between the coronal plume structure and the interplume region on
the spectrum of magnetohydrodynamic (MHD) waves trapped in the plume
structure. To illustrate the concept of resonant flow instability of
the trapped modes, we consider both a one-dimensional slab model and
a one-dimensional cylindrical model for a coronal plume. A nonuniform
intermediate region between the plume and the interplume region is taken
into account so that the waves can be subject to resonant absorption. We
show how the resonance can lead to instability of the trapped modes and
that this resonant instability, which is physically distinct from the
nonresonant Kelvin-Helmholtz instability, occurs for velocity shears
significantly below the Kelvin-Helmholtz threshold. These resonant flow
instabilities could lead to disruption of the coronal plumes and mixing
with interplume plasma. The dependence of the critical velocity shear
for which the resonant instability occurs on the difference between
plume and interplume density is investigated. Our results seem to
suggest that resonant flow instability in a pressureless plasma will
only appear for rather high density contrasts. However, it is clearly
shown that the velocity shear needed for Kelvin-Helmholtz instability
to occur is too high for all density contrasts. Hence, Kelvin-Helmholtz
instability will not be operative in pressureless coronal plumes.