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" 

---------------------------------------------------------
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

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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 &amp; 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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
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&amp;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.

---------------------------------------------------------
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.

---------------------------------------------------------
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 &amp; Yang (1988) and
  Ruderman &amp; 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 &amp; Yang
  (\cite{hollweg1988}) and Ruderman &amp; 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.