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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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.

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.

The "Flare Scoreboard" (http://ccmc.gsfc.nasa.gov/challenges/flare.php ) 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.

The "SEP Scoreboard" (http://ccmc.gsfc.nasa.gov/challenges/sep.php ) 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.

The "Bz Scoreboard" (http://ccmc.gsfc.nasa.gov/challenges/bz.php ) 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. Bibcode: 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.

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.

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.

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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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.

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.

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.

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.

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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Title: Surface Alfvén Waves in Solar Flux Tubes Authors: Goossens, M.; Andries, J.; Soler, R.; Van Doorsselaere, T.; Arregui, I.; Terradas, J. Bibcode: 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. Bibcode: 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.
Aims: We study resonant Alfvén waves in partially ionized plasmas.
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.
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.
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. Bibcode: 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. Bibcode: 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.
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.
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.
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.
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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
Aims: The goal of this work is to study and determine the nature of MHD kink waves.
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.
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 (kz 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.
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. Bibcode: 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. Bibcode: 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.
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.
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.
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/k2 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2007A&A...471..311V Altcode: Context: How coronal loops are heated to their observed temperatures is the subject of a long standing debate.
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.
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.
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. Bibcode: 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.
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).
Methods: We calculate, using analytical expressions, the amplitude and the energy of the oscillation of the magnetic tube for different kinds of initial excitations.
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.
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. Bibcode: 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.
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.
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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2005A&A...441..361A Altcode: Resonantly damped fast kink quasi-modes are computed in fully

resistive magnetohydrodynamics (MHD) for two-dimensional equilibrium

models. The equilibrium model is a straight cylindrically symmetric flux

tube with a plasma density that is non-uniform both across and along the

loop. The non-uniform layer across the loop is not

restricted to be thin, but its thickness can reach values up to the

loop diameter.

Our results indicate that

the period and damping of coronal loop oscillations mainly depend on the density contrast

and the inhomogeneity length-scale and are independent of the details of

longitudinal stratification, depending on the weighted mean density,

weighted with the wave energy. For fully non-uniform loops, quasi-modes can

interact with resistive Alfvén eigenmodes leading to avoided crossings

and gaps in the complex frequency plane. The present study extends previous

studies on coronal loop oscillations in one-dimensional equilibrium models

with thick boundary layers and in equilibria with longitudinally stratified loops under the

thin boundary approximation, and allow for a better comparison between

observations and theory raising the prospect of coronal seismology using

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. Bibcode: 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. Bibcode: 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 (~107 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 Bibcode: 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. Bibcode: 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.

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. Bibcode: 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. Bibcode: 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. Bibcode: 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.

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. Bibcode: 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. Bibcode: 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 Bibcode: 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 ne(r) and line-of-sight integration to the cross-sectional fluxes F(r) observed with TRACE 171 Å. This way we model the internal (ni) and external electron density (ne) 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=ne/ni. 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, χLEDA=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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.