Fourth annual CHARM meeting

4 Jun 2015, 10:00 → 5 Jun 2015, 17:00 UTC

RMI meeting room (IASB-BIRA (RMI Royal Meteorologic Institute meeting room))

Graciela Lopez Rosson (IASB-BIRA), Kris Borremans (IASB-BIRA), Sofia Moschou (IASB-BIRA and KULeuven), Viviane Pierrard (IASB-BIRA), Yuriy Voitenko (IASB-BIRA)

Five Belgian research teams and two international partners work together to face Contemporary physical challenges in Heliospheric and AstRophyscial Models (or CHARM).

The origin of this network comes from recognition of each team's weakness and finding colleague teams to fill the gaps. This way, common grounds across the disciplines have been identified, varying from solar, magnetospheric and heliospheric, to galactic or cosmologically driven curiosities.

Thursday, 4 June

10:00 → 10:50 Welcome

Introduction: Presentation of CHARM project and partners

10:00 Welcome

Speaker: Tea and coffee

10:15 Introduction CHARM meeting

Speaker: Viviane Pierrard (Belgian Institute for Space Aeronomy)

10:25 CHARM project

Speaker: Prof. Rony Keppens (CmPA, Department of Mathematics, KU Leuven)

10:50 → 13:00 Session 1: Coupling models and tools

Coupling models and tools

10:55 On the effect of inner magnetic structure on the evolution of CMEs in the inner heliosphere

Centre for mathematical Plasma-Astrophysics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium

Speaker: Prof. Stefaan Poedts (KU Leuven)

11:20 Virtual solar prominences

We present numerical simulations with MPI-AMRVAC of solar prominences, embedded in a realistically stratified solar atmosphere. The prominences show vigorous nonlinear magnetoconvective motions, as their density contrast triggers Rayleigh-Taylor/interchange instability dynamics. We present virtual SDO/AIA views, and contrast our findings with observational ones.

Speaker: Prof. Rony Keppens (CmPA, Department of Mathematics, KU Leuven)

11:45 Coronal rain in magnetic arcades: Rebound shocks, Limit cycles, and Shear flows

We extend our earlier multidimensional, magnetohydrodynamic simulations of coronal rain occurring in magnetic arcades with higher resolution, grid-adaptive computations covering a much longer ($>6$ hour) timespan. We quantify how in-situ forming blob-like condensations grow along and across field lines and show that rain showers can occur in limit cycles, here demonstrated for the first time in 2.5D setups. We discuss dynamical, multi-dimensional aspects of the rebound shocks generated by the siphon inflows and quantify the thermodynamics of a prominence-corona-transition-region like structure surrounding the blobs. We point out the correlation between condensation rates and the cross-sectional size of loop systems where catastrophic cooling takes place. We also study the variations of the typical number density, kinetic energy and temperature while blobs descend, impact and sink into the transition region. In addition, we explain the mechanisms leading to concurrent upflows while the blobs descend. As a result, there are plenty of shear flows generated with relative velocity difference around 80 km s$^{-1}$ in our simulations. These shear flows are siphon flows set up by multiple blob dynamics and they in turn affect the deformation of the falling blobs. In particular, we show how shear flows can break apart blobs into smaller fragments, within minutes.

Speaker: Mr Xia Fang (Afdeling Plasma-astrofysica)

12:10 Coronal rain events in 3D simulations

Coronal rain phenomena are simulated in 3D setups in a magnetic configuration of a quadrupolar arcade system. A magnetohydrodynamic simulation is presented including three gain-loss terms, anisotropic thermal conduction, optically thin radiative losses, and parametrised heating to construct a realistic arcade configuration from chromospheric to coronal heights. Evaporated plasma from chromospheric and transition region heights eventually causes localised runaway condensation events. Plasma blobs are formed due to thermal instability, evolve dynamically in the heated arcade part and move gradually downwards due to gravitational instability. Unlike earlier 2.5D simulations, no formation of large scale prominence is observed here, but a continuous coronal rain develops which shows clear indications of Rayleigh-Taylor or interchange instability. The denser plasma located above the transition region falls down as the system seeks stability. Linear stability analysis is used in the non-linear regime to provide insight and to give a prediction of the system's evolution. After the plasma blobs descend through interchange, they move along the magnetic field lines in the lower coronal regions, where they are guided by the magnetic dips.

Speaker: Mrs Sofia-Paraskevi Moschou (BISA-IASB & CmPA, KULeuven)

12:35 Narrow-band bursts of coronal radio emission at 2-3 solar radii and their possible generation mechanisms

UTR-2 (Kharkiv, Ukraine) and URAN-2 (Poltava, Ukraine) are world-largest radio telescopes at decametric wavelengths. High sensitivity and time resolution of these telescopes allowed revealing many new features of the coronal radio emission. We discuss narrow-band coronal radio bursts at 23-35 MHz with frequency drift rates ~ 110 kHz. These bursts appear between type III and type II bursts and imply intermediate velocities of the emission sources ~ 10^8 cm/s. We discuss such radio bursts and possible generation mechanisms for them. In particular, our results suggest that kinetic Alfven waves can be responsible for these bursts. Observations and analysis of these bursts provide a promising tool for studying coronal processes and remote coronal diagnostics.

Speaker: Dr Yuriy Voitenko (Belgian Institute for Space Aeronomy)

13:00 → 14:00 Lunch

Sandwishes and drinks

14:00 → 15:15 Session 2: Validation through observations

Model validation thriugh observations

14:00 FIR and dust properties of present-day normal galaxies in the EAGLE simulation

Do the simulated EAGLE galaxies look like observed galaxies? The EAGLE project is a campaign of large-scale cosmological simulations aimed at examining the formation of galaxies (Schaye et al. 2014). One major improvement is the treatment of feedback from massive stars and accreting black holes, which allows winds to develop naturally without predetermined speed or mass loading factors. EAGLE reproduces many observations with an accuracy that is unprecedented for hydrodynamical simulations. In the present work, we perform 3D dust radiative transfer simulations on a sizable set of EAGLE galaxies, with the goal of calculating their observable properties (images and SEDs) from UV to submm wavelengths. We select redshift-zero galaxies in the same stellar mass regime as the Herschel Reference Survey (HRS), a volume-limited sample of “normal” galaxies in the Local Universe. We compare the dust-related properties (e.g. dust mass, gas-to-dust ratio, dust-to-stellar-mass ratio) of the simulated galaxies to the HRS galaxies, and investigate their dust scaling relations.

Speaker: Mr Peter Camps (Ghent University)

14:25 Sub-minute quasi-periodic oscillations in solar flares

Sub-minute quasi-periodic pulsations have been reported in the impulsive phase of several solar flares in wavelengths ranging from radio waves to Hard X-Ray. However, their detection remains a challenge as observations with a good signal-to-noise ratio and a high acquisition cadence are required. LYRA, the Large Yield Radiometer onboard PROBA2 has a signal-to-noise ratio and a high acquisition cadence enabling it to observe sub-minute QPPs and can therefore provide new insights on this topic. In this talk, I will present my progresses in the analysis of the QPPs observed by LYRA.

Speaker: Marie Dominique (ROB)

14:50 EPT observations of flux enhancements and depletion of particle in the Radiation Belts after the March 17 event.

Geomagnetic storms can be unpredictable, as long as the response of the radiation belts. Sometimes we observe flux enhancements mainly after Dst events (electrons), or SEP (protons and alpha particles), but there are also another times, which are less commons, when particles almost disappear from certain regions. These flux variations affect and change the configuration of the Van Allen Belts. We present as an example of this situation the event occurred on March 17, 2015. The new instrument EPT (Energetic Particle Telescope), onboard ESA satellite PROBA-V, launched May 7, 2013, has observed this behavior: the particles flux enhancement, a depletion of particles after the increment, and finally a restructuration of the radiation belts. We are trying to understand which mechanism are behind those variation, and why can be so different from an event to another one.

Speaker: Graciela López Rosson (BISA)

15:15 → 15:40 Coffee break

15:40 → 17:35 Session 3: Turbulence and particle acceleration in astrophysical plasmas

Turbulence and particle acceleration

15:40 Turbulent ion scattering by oblique Alfven-cyclotron waves

We perform 2.5D hybrid simulations to study the turbulent heating and acceleration of protons and minor ions in collisionless homogeneous fast solar wind. The focus of this work is wave-particle interactions at the intermediate ion scales, therefore the small-scale electrons are considered as charge-neutralizing isothermal fluid and the electron intertia in our approach is neglected. The dissipation of magnetic field fluctuations in collisionless plasmas can occur via fluid cascade at large-scales, followed by various intermediate (ion) and small-scale eloectron kinetic wave-particle interactions. The efficiency of the ion heating depends on the characteristics of the waves carrying energy at the ion scales, such as polarization, direction of propagation and spectral properties of the fluctuations. Realistic solar wind turbulence includes different types of waves at all scales, starting with fluid regime and going down to the electron scales. Although the spatial and temporal scales of these fluctuaction are separated by few orders of magnitudes, they can still exchange energy as large and small-scales turbulent cascades oparate. As a simplistic attempt to model part of the solar wind turbulence at the ion scales we assume a superposition of left-hand circularly-polarized Alfven ion-cyclotron waves. These waves are frequently observed in situ in the solar wind, and yet their specific role for the energetization of minor ions remains unclear. In this study we perform 2.5D hybrid simulations to test the importance of parallel and obliquely propagating Alfven-cyclotron waves for the anisotropic heating of minor ions in the solar wind. We start with initially isotropic plasma with equal temperatures for the protons and alpha particles and impose an initial turbulent spectrum of Alfven-cyclotron waves to subsequently follow the preferential heating for the minor ions and the onset of temperature anisotropies for both ion species. We compare the efficiency of the parallel and oblique waves in heating the protons and the minor ions. In the course of the nonlinear evolution of the system when an initial parallel wave spectrum is assumed we observe a substaintial anisotropic cascade of the magnetic field power spectrum towards perpendicular wave numbers.

Speaker: Dr Yana Maneva (CmPA at KU Leuven)

16:05 On the (multi)fractal stucture of turbulence from in-situ data in the terrestrial magnetosheath

In-situ measurements of magnetic field fluctuations in the terrestrial magnetosheath revealed differences between the quasi-perpendicular and quasi-parallel shock geometry. In the quasi-parallel case the fluctuations are strongly intermittent as suggested by the scale dependent non-Gaussian wings of the probability distribution functions (PDFs) and the scale dependent behaviour of their fourth order moment. Further insight on the structure of fluctuations and their scale behavior is provided by the multirfractal analysis. The Rank Ordered Multifractal Analysis (ROMA) invented by Wu and Chang (Phys Rev E, 2008) is capable of disentangling the ranges of fluctuations that exhibit the same fractal (self-similar) dimension in terms of the local invariant property based on the scaled amplitude of fluctuations. It can thereby identify the different regimes of fluctuations (e.g., persistency, antipersistency) and different physical regimes (e.g., kinetic/MHD). A ROMA analysis of the intermittent turbulence in the magnetosheath describing the multiscale and dissipation behaviour over ranges of scales, in the coarse-grained sense, will be presented and provides a coplex view of developping turbulence in the terrestrial magnetosheath.

Speaker: Dr Marius Echim (IASB)

16:30 LYRA detections of Aurora events

The Large Yield RAdiometer (LYRA) is an ultraviolet irradiance radiometer on-board ESA's PROBA2 micro-satellite. Since it's launch in 2009 it observes the Sun in four different passbands, chosen for their relevance to solar physics, aeronomy and space weather. Flying on an altitude of 735km, LYRA proved to be an excellent flare monitor and is involved in the analysis the atmospheric composition of the Earth. One of the most peculiar and intriguing results of LYRA is the detection of short, strong, bursts that do not directly correlate with solar coronal events, nor with pointing of the instrument to Earth's upper atmosphere, but correlate well with high K$_{p}$ index on Earth's surface. As LYRA has the ability to observe in four different UV bandpasses, the comparison between the filters that allow the detection of this activity versus those that do not, reveals very interesting results as to the nature of those detections.

Speaker: Dr Athanassios Katsiyannis (Royal Observatory of Belgium)

16:55 News and practical announcements

Friday, 5 June

10:00 → 12:40 Session 4: MHD/Kinetic models

MHD and kinetic models

10:00 Welcome

10:10 Kinetic models for the solar wind and the inner magnetosphere

Recent progress of kinetic models for space plasmas will be reported. Concerning the solar wind, exospheric model based on kappa distributions for the electrons are coupled with coronal conditions deduced from spacecraft observations and from MHD models. Dynamical simulations are also developed for particles trapped in the inner magnetosphere and the results are compared with satellite observations and especially with the Energetic Particle Telescope (EPT) flux measurements.

Speaker: Viviane Pierrard (Belgian Institute for Space Aeronomy)

10:35 Coupling fluid and kinetic models for astrophysical simulations

Based on the recent works published in 2014: - Daldorff, L. K., Tóth, G., Gombosi, T. I., Lapenta, G., Amaya, J., Markidis, S., & Brackbill, J. U. (2014). Two-way coupling of a global Hall magnetohydrodynamics model with a local implicit particle-in-cell model. Journal of Computational Physics, 268, 236-254. - Ashour‐Abdalla, M., Lapenta, G., Walker, R. J., El‐Alaoui, M., & Liang, H. (2015). Multiscale Study of Electron Energization during Unsteady Reconnection Events. Journal of Geophysical Research: Space Physics. we report on the developments within CHRM of the methodology to couple fluid and kinetic codes. New developments in 2014-15 include the extension to 3D and the consideration of solar and astrophysics problems, besides magnetospheric problems developed in previous years. We report also on the development of a new Lagrangian particle-based code for the fluid component that can more easily be coupled with the kinetic approach. With this approach both models, fluid and kinetic, will use computational particles whose properties will depend on the level of physical description. The methodology and selected applications will be described.

Speaker: Prof. Giovanni Lapenta (KULeuven)

11:00 Slurm: a Lagrangian Particle-in-Cell method for magnetohydrodynamics

We present a new Lagrangian, multi-dimensional, fluid PiC code for MHD space simulations, based on the FLIP-MHD method designed by Brackbill. In contrast with standard kinetic PiC codes, Slurm uses particles as carriers of all the conserved quantities, while advancing the macroscopic properties of the plasma on a Lagrangian grid. The communication between particles and grid is allowed through bilinear interpolation: at each cycle, the grid properties are initialized from the particles, advanced by solving the system of Lagrangian MHD equations and interpolated back to the particle domain. Then, the particles are advanced on the phase space by solving the equations of motion in finite difference form. Slurm aims at solving the two main problems affecting FLIP-MHD, namely the arising of the ringing instability and the high computational cost due to the need of solving a Poisson's equation for the magnetic field scalar potential. The ringing instability is efficiently suppressed by allowing the particles to evolve their volume throughout the computation. This is done by implementing the same strategy adopted by Bardenhagen et al. (2004) in the GIMP method for computational mechanics. The second problem implies to find a suitable strategy for maintaining the div(B)=0 constrain. Possible practical applications of the new method are presented, along with some results obtained from test cases and simulations of CME-driven shocks.

Speaker: Mr Fabio Bacchini (KU Leuven)

11:25 Energetics of the electrons during rapid island coalescence in asymmetric conditions

Results from fully kinetic simulations of magnetic reconnection with asymmetric profiles of the magnetic field and density across the current sheet are presented. Two different initial layers are simulated. The first layer represents the continuous hyperbolic profiles observed at the dayside of the magnetopause. The second layer instead represents the same asymmetries under the presence of a tangential discontinuity. These simulations were performed with the aim of giving further support to the recently launched Nasa mission MMS (Magnetospheric Multiscale Mission), initially devoted to study this region of the magnetosphere. We particularly focus on the energetics of the electrons acquired in the second layer after the formation and coalescence of the magnetic islands originated from multiple reconnection points. Important results are observed within the magnetic islands and in the reconnection regions. Thanks to the direct comparison with the situation at the first layer, we are able to identify three different processes occurring during the rapid island merging. The first type shows the typical evolution of asymmetric reconnection, with the predominant swelling towards the lower magnetic field side and a peculiar anisotropic particles inflows. The second type recalls the first one with no signature of ongoing reconnection. Finally, the third type deals with the reconnection events occurring between the internal magnetic islands of two merging plasmoids.

Speaker: Mr Emanuele Cazzola (KULeuven)

11:50 Simulations of the radiation belt particles

We discuss the dynamics of the radiation belts by launching electrons and protons in the inner magnetosphere and investigate the trajectory of the particles. A hot issue today concerning radiation belts is the question if there are relativistic electrons in the inner belt. The observations of relativistic electrons in the inner belt and the South Atlantic Anomaly are possibly caused by proton contamination. We discuss the distinct properties of the gyromotion trajectories of electrons with energies from 0.5 to 20 MeV and protons with energies from 9.5 to 300 MeV. We discuss adiabatic invariants, gyrofrequency, gyroradius, bouncefrequency, driftfrequency, and pitch angle distributions of the electrons and the protons at spacecraft orbits, like LEO and GEO.

Speaker: Mr Kris Borremans (Belgian Institute for Space Aeronomy)

12:15 3D radiative transfer of solar filaments

We study 3D radiative transfer of solar filaments. When observed on the solar disk, filaments are in absorption and can be seen as dark stripes against the bright solar surface. When seen above the solar limb (also called solar prominences) they are observed in emission and can be recognized as bright phenomena against the dark background of space. In this study, we attempt to create a continuum radiative transfer code, based on the Monte Carlo algorithm, to simulate the interaction of the solar photons with solar filaments. For this, we modify the existing SKIRT code. SKIRT, or ‘Stellar Kinematics including Radiative Transfer’ is mostly used for simulating the radiative transfer in dusty galaxies. We extend the code by including the wavelenth-dependent absorption and emission of photons in the optically thick H-α and Ly-α wavelengths. The ultimate aim is to compare observations of prominence oscillations with the numercial simulations of those.

Speaker: Jo Raes (KULeuven)

13:00 → 14:00 Lunch

Surprise

14:00 → 15:30 Session 5: Radiation and dynamics

Radiation and dynamics

14:00 An update on the Eagle cosmological simulations

I will present an overview of some of the Eagle projects concentrating on submitted or published papers, and on the paper led by Charm PhD student James Trayford on the galaxy colour-magnitude diagram.

Speaker: Prof. tom theuns (Institute for Computational Cosmology, Durham University)

14:25 How the first stars shaped the faintest gas-rich dwarf galaxies

In the ΛCDM model, cosmic structure forms in a hierarchical fashion. According to this paradigm, even low-mass dwarf galaxies grow via smooth accretion and mergers. Given the low masses of dwarf galaxies and their even smaller progenitors, the UV background is expected to have a significant influence on their gas content and, consequently, their star formation histories. Generally, cosmological simulations predict that most dwarf systems with circular velocities below ~30 km/s do not form significant amounts of stars or contain gas and are "dark" galaxies (Sawala et al. 2013, 2014; Hopkins et al. 2014; Shen et al. 2014). This is in contradiction with the recent discovery of low-mass yet gas- rich dwarf galaxies, such as Leo P (Skillman et al. 2013) and Pisces A (Tollerud et al. 2014). Moreover, Tollerud et al. (2014) point out that most isolated dark-matter halos down to circular velocities of ~15 km/s contain neutral gas, in contradiction with the predictions of current simulations. Based on a suite of simulations of the formation and evolution of dwarf galaxies we show that, if the first peak of star formation can be sufficiently reduced, e.g. by inclusion of Pop III stars in the simulations, the resulting dwarf galaxies have severely suppressed star-formation rates while holding on to their gas reservoirs. Moreover, we show that, although instrumental in shaping these galaxies, only a few Pop III stars are expected to still reside within the simulated dwarfs at z=0, in agreement with observed extremely metal-poor stars in dwarf galaxies. We show that there is a marked difference between a galaxy's "total" star-formation history and the one read from the stars in the center of the galaxy, since most of the oldest stars have been ejected from the galaxy in merger events. To determine the observational properties of the simulations, we mimic the ways observers would determine them as best as possible. When doing this, our feedback model leads to the formation of realistic low-mass, gas-rich dwarfs with a broad range of star formation histories and which adhere to the observed scaling relations, such as the baryonic Tully-Fisher relation.

Speaker: Mr Robbert Verbeke (UGent)

14:50 Implementing mesh-free methods in the simulation code SWIFT

Mesh-free hydrodynamical methods as introduced by *Hopkins (2014)* provide a promising symbiosis of particle based and mesh based Lagrangian hydrodynamical methods. They couple the accuracy of finite volume methods to the computational strengths of smoothed particle hydrodynamics (SPH). To this end, the fluid is discretized as a set of particles, as in SPH. Volumes for the particles are calculated using a kernel-weighted loop over neighbouring particles and are used to convert conserved particle quantities to primitive quantities, as in a finite volume approach. These primitive quantities then serve as an input for a set of Riemann problems at the abstract interfaces between particles, the solution of which ultimately allows us to calculate fluxes of conserved quantities that are exchanged between particles. As in all SPH-like schemes, mesh-free methods spend a large fraction of their computational time in loops over neighbouring particles. Hopkins' GIZMO is based on Gadget-3, which is known to scale poorly above 512 cores, due to bad scaling of the tree algorithm employed for the neighbour loops. The simulation code SWIFT developed at Durham University scales a lot better by replacing the tree algorithm by a hierarchical cell structure, which is comparable to an adaptive mesh refinement (AMR) grid. SWIFT and GIZMO therefore seem to be an ideal match. In my talk, I want to discuss the implementation of a mesh-free method in the simulation code SWIFT. I will explain the method in some more detail and report on some of the first results obtained using the method. I will compare these results to results obtained using standard SPH and to results obtained using the moving mesh code Shadowfax, both on accuracy and efficiency.

Speaker: Mr Bert Vandenbroucke (Universiteit Gent)

15:15 Final discussion