Browsing by Subject "methods: numerical"

Sort by: Order: Results:

Now showing items 1-20 of 38
  • Martikainen, Julia; Penttilä, Antti; Gritsevich, M.; Videen, Gorden; Muinonen, Karri Olavi (2019)
    We present a new physics-based approach to model the absolute reflectance spectra of asteroid (4) Vesta. The spectral models are derived by utilizing a ray-optics code that simulates light scattering by particles large compared to the wavelength of the incident light. In the light of the spectral data obtained by the Dawn spacecraft, we use howardite powder to model Vesta's surface regolith and its particle size distribution for 10-200 mu m sized particles. Our results show that the modelled spectrum mimics well the observations. The best match was found using a power-law particle size distribution with an index 3.2. This suggests that Vesta's regolith is dominated by howardite particles
  • Keihänen, E.; Kiiveri, K.; Kurki-Suonio, H.; Reinecke, M. (2017)
    We present two novel methods for the estimation of the angular power spectrum of cosmic microwave background (CMB) anisotropies. We assume an absolute CMB experiment with arbitrary asymmetric beams and arbitrary sky coverage. The methods differ from the earlier ones in that the power spectrum is estimated directly from the time-ordered data, without first compressing the data into a sky map, and they take into account the effect of asymmetric beams. In particular, they correct the beam-induced leakage from temperature to polarization. The methods are applicable to a case where part of the sky has been masked out to remove foreground contamination, leaving a pure CMB signal, but incomplete sky coverage. The first method (deconvolution quadratic maximum likelihood) is derived as the optimal quadratic estimator, which simultaneously yields an unbiased spectrum estimate and minimizes its variance. We successfully apply it to multipoles up to l = 200. The second method is derived as a weak-signal approximation from the first one. It yields an unbiased estimate for the full multipole range, but relaxes the requirement of minimal variance. We validate the methods with simulations for the 70 GHz channel of Planck surveyor, and demonstrate that we are able to correct the beam effects in the TT, EE, BB and TE spectra up to multipole l = 1500. Together, the two methods cover the complete multipole range with no gap in between.
  • Martikainen, J.; Muinonen, K.; Penttilä, A.; Cellino, A.; Wang, X. -B. (2021)
    Aims. We perform light curve inversion for 491 asteroids to retrieve phase curve parameters, rotation periods, pole longitudes and latitudes, and convex and triaxial ellipsoid shapes by using the sparse photometric observations from Gaia Data Release 2 and the dense ground-based observations from the DAMIT database. We develop a method for the derivation of reference absolute magnitudes and phase curves from the Gaia data, allowing for comparative studies involving hundreds of asteroids.Methods. For both general convex shapes and ellipsoid shapes, we computed least-squares solutions using either the Levenberg-Marquardt optimization algorithm or the Nelder-Mead downhill simplex method. Virtual observations were generated by adding Gaussian random errors to the observations, and, later on, a Markov chain Monte Carlo method was applied to sample the spin, shape, and scattering parameters. Absolute magnitude and phase curve retrieval was developed for the reference geometry of equatorial illumination and observations based on model magnitudes averaged over rotational phase.Results. The derived photometric slope values showed wide variations within each assumed Tholen class. The computed Gaia G-band absolute magnitudes matched notably well with the V-band absolute magnitudes retrieved from the Jet Propulsion Laboratory Small-Body Database. Finally, the reference phase curves were well fitted with the H, G(1), G(2) phase function. The resulting G(1), G(2) distribution differed, in an intriguing way, from the G(1), G(2) distribution that is based on the phase curves corresponding to light curve brightness maxima.
  • Muinonen, K.; Torppa, J.; Wang, X-B; Cellino, A.; Penttilä, A. (2020)
    Context. We assess statistical inversion of asteroid rotation periods, pole orientations, shapes, and phase curve parameters from photometric lightcurve observations, here sparse data from the ESA Gaia space mission (Data Release 2) or dense and sparse data from ground-based observing programs.Aims. Assuming general convex shapes, we develop inverse methods for characterizing the Bayesian a posteriori probability density of the parameters (unknowns). We consider both random and systematic uncertainties (errors) in the observations, and assign weights to the observations with the help of Bayesian a priori probability densities.Methods. For general convex shapes comprising large numbers of parameters, we developed a Markov-chain Monte Carlo sampler (MCMC) with a novel proposal probability density function based on the simulation of virtual observations giving rise to virtual least-squares solutions. We utilized these least-squares solutions to construct a proposal probability density for MCMC sampling. For inverse methods involving triaxial ellipsoids, we update the uncertainty model for the observations.Results. We demonstrate the utilization of the inverse methods for three asteroids with Gaia photometry from Data Release 2: (21) Lutetia, (26) Proserpina, and (585) Bilkis. First, we validated the convex inverse methods using the combined ground-based and Gaia data for Lutetia, arriving at rotation and shape models in agreement with those derived with the help of Rosetta space mission data. Second, we applied the convex inverse methods to Proserpina and Bilkis, illustrating the potential of the Gaia photometry for setting constraints on asteroid light scattering as a function of the phase angle (the Sun-object-observer angle). Third, with the help of triaxial ellipsoid inversion as applied to Gaia photometry only, we provide additional proof that the absolute Gaia photometry alone can yield meaningful photometric slope parameters. Fourth, for (585) Bilkis, we report, with 1-sigma uncertainties, a refined rotation period of (8.5750559 0.0000026) h, pole longitude of 320.6 degrees +/- 1.2 degrees, pole latitude of - 25.6 degrees +/- 1.7 degrees, and the first shape model and its uncertainties from convex inversion.Conclusions. We conclude that the inverse methods provide realistic uncertainty estimators for the lightcurve inversion problem and that the Gaia photometry can provide an asteroid taxonomy based on the phase curves.
  • Siltala, L.; Granvik, M. (2020)
    Context. The bulk density of an asteroid informs us about its interior structure and composition. To constrain the bulk density, one needs an estimated mass of the asteroid. The mass is estimated by analyzing an asteroid's gravitational interaction with another object, such as another asteroid during a close encounter. An estimate for the mass has typically been obtained with linearized least-squares methods, despite the fact that this family of methods is not able to properly describe non-Gaussian parameter distributions. In addition, the uncertainties reported for asteroid masses in the literature are sometimes inconsistent with each other and are suspected to be unrealistically low.Aims. We aim to present a Markov-chain Monte Carlo (MCMC) algorithm for the asteroid mass estimation problem based on asteroid-asteroid close encounters. We verify that our algorithm works correctly by applying it to synthetic data sets. We use astrometry available through the Minor Planet Center to estimate masses for a select few example cases and compare our results with results reported in the literature.Methods. Our mass-estimation method is based on the robust adaptive Metropolis algorithm that has been implemented into the OpenOrb asteroid orbit computation software. Our method has the built-in capability to analyze multiple perturbing asteroids and test asteroids simultaneously.Results. We find that our mass estimates for the synthetic data sets are fully consistent with the ground truth. The nominal masses for real example cases typically agree with the literature but tend to have greater uncertainties than what is reported in recent literature. Possible reasons for this include different astrometric data sets and weights, different test asteroids, different force models or different algorithms. For (16) Psyche, the target of NASA's Psyche mission, our maximum likelihood mass is approximately 55% of what is reported in the literature. Such a low mass would imply that the bulk density is significantly lower than previously expected and thus disagrees with the theory of (16) Psyche being the metallic core of a protoplanet. We do, however, note that masses reported in recent literature remain within our 3-sigma limits.Results. The new MCMC mass-estimation algorithm performs as expected, but a rigorous comparison with results from a least-squares algorithm with the exact same data set remains to be done. The matters of uncertainties in comparison with other algorithms and correlations of observations also warrant further investigation.
  • Pihajoki, Pauli; Mannerkoski, Matias; Johansson, Peter H. (2019)
    Interpolation of data represented in curvilinear coordinates and possibly having some non-trivial, typically Riemannian or semi-Riemannian geometry is a ubiquitous task in all of physics. In this work, we present a covariant generalization of the barycentric coordinates and the barycentric interpolation method for Riemannian and semi-Riemannian spaces of arbitrary dimension. We show that our new method preserves the linear accuracy property of barycentric interpolation in a coordinate-invariant sense. In addition, we show how the method can be used to interpolate constrained quantities so that the given constraint is automatically respected. We showcase the method with two astrophysics related examples situated in the curved Kerr space-time. The first problem is interpolating a locally constant vector field, in which case curvature effects are expected to be maximally important. The second example is a general relativistic magnetohydrodynamics simulation of a turbulent accretion flow around a black hole, wherein high intrinsic variability is expected to be at least as important as curvature effects.
  • Micelotta, Elisabetta R.; Juvela, Mika; Padoan, Paolo; Ristorcelli, Isabelle; Alina, Dana; Malinen, Johanna (2021)
    Context. The all-sky survey from the Planck space telescope has revealed that thermal emission from Galactic dust is polarized on scales ranging from the whole sky down to the inner regions of molecular clouds. Polarized dust emission can therefore be used as a probe for magnetic fields on different scales. In particular, the analysis of the relative orientation between the density structures and the magnetic field projected on the plane of the sky can provide information on the role of magnetic fields in shaping the structure of molecular clouds where star formation takes place.Aims. The orientation of the magnetic field with respect to the density structures has been investigated using different methods. The goal of this paper is to explicitly compare two of these: the Rolling Hough Transform (RHT) and the gradient technique (GRAD).Methods. We generated synthetic surface brightness maps at 353 GHz (850 mu m) via magnetohydrodynamic simulations. We applied RHT and GRAD to two morphologically different regions identified in our maps. Region 1 is dominated by a dense and thick filamentary structure with some branches, while Region 2 includes a thinner filament with denser knots immersed in a more tenuous medium. Both methods derive the relative orientation between the magnetic field and the density structures, to which we applied two statistics, the histogram of relative orientation and the projected Rayleigh statistic, to quantify the variations of the relative orientation as a function of column density.Results. Both methods find areas with significant signal, and these areas are substantially different. In terms of relative orientations, in all our considered cases the predominant orientation of the density structures is perpendicular to the direction of the magnetic field. When the methods are applied to the same selected areas the results are consistent with each other in Region 2 but show some noticeable differences in Region 1. In Region 1, RHT globally finds the relative orientation becoming more perpendicular for increasing column density, while GRAD, applied at the same resolution as RHT, gives the opposite trend. These disparities are caused by the intrinsic differences in the methods and in the structures that they select.Conclusions. Our results indicate that the interpretation of the relative orientation between the magnetic field and density structures should take into account the specificity of the methods used to determine such orientation. The combined use of complementary techniques such as RHT and GRAD provides more complete information, which can be advantageously used to better understand the physical mechanisms operating in magnetized molecular clouds.
  • Wilson, Lynn B.; Chen, Li-Jen; Wang, Shan; Schwartz, Steven J.; Turner, Drew L.; Stevens, Michael L.; Kasper, Justin C.; Osmane, Adnane; Caprioli, Damiano; Bale, Stuart D.; Pulupa, Marc P.; Salem, Chadi S.; Goodrich, Katherine A. (2019)
    Analyses of 15,314 electron velocity distribution functions (VDFs) within +/- 2 hr of 52 interplanetary (IP) shocks observed by the Wind spacecraft near 1 au are introduced. The electron VDFs are fit to the sum of three model functions for the cold dense core, hot tenuous halo, and field-aligned beam/strahl component. The best results were found by modeling the core as either a bi-kappa or a symmetric (or asymmetric) bi-self-similar VDF, while both the halo and beam/strahl components were best fit to bi-kappa VDF. This is the first statistical study to show that the core electron distribution is better fit to a self-similar VDF than a bi-Maxwellian under all conditions. The self-similar distribution deviation from a Maxwellian is a measure of inelasticity in particle scattering from waves and/or turbulence. The ranges of values defined by the lower and upper quartiles for the kappa exponents are k(ec) similar to 5.40-10.2 for the core, k(eh) similar to 3.58-5.34 for the halo, and k(eb) similar to 3.40-5.16 for the beam/strahl. The lower-to-upper quartile range of symmetric bi-self-similar core exponents is s(ec) similar to 2.00-2.04, and those of asymmetric bi-self-similar core exponents are p(ec) similar to 2.20-4.00 for the parallel exponent and q(ec) similar to 2.00-2.46 for the perpendicular exponent. The nuanced details of the fit procedure and description of resulting data product are also presented. The statistics and detailed analysis of the results are presented in Paper II and Paper III of this three-part study.
  • Pöntinen, M.; Granvik, M.; Nucita, A. A.; Conversi, L.; Altieri, B.; Auricchio, N.; Bodendorf, C.; Bonino, D.; Brescia, M.; Capobianco, V.; Carretero, J.; Carry, B.; Castellano, M.; Cledassou, R.; Congedo, G.; Corcione, L.; Cropper, M.; Dusini, S.; Frailis, M.; Franceschi, E.; Fumana, M.; Garilli, B.; Grupp, F.; Hormuth, F.; Israel, H.; Jahnke, K.; Kermiche, S.; Kitching, T.; Kohley, R.; Kubik, B.; Kunz, M.; Laureijs, R.; Lilje, P. B.; Lloro, I.; Maiorano, E.; Marggraf, O.; Massey, R.; Meneghetti, M.; Meylan, G.; Moscardini, L.; Padilla, C.; Paltani, S.; Pasian, F.; Pires, S.; Polenta, G.; Raison, F.; Roncarelli, M.; Rossetti, E.; Saglia, R.; Schneider, P.; Secroun, A.; Serrano, S.; Sirri, G.; Tereno, I.; Toledo-Moreo, R.; Valenziano, L.; Wetzstein, M.; Zoubian, J. (2020)
    Context. The ESA Euclid space telescope could observe up to 150 000 asteroids as a side product of its primary cosmological mission. Asteroids appear as trailed sources, that is streaks, in the images. Owing to the survey area of 15 000 square degrees and the number of sources, automated methods have to be used to find them. Euclid is equipped with a visible camera, VIS (VISual imager), and a near-infrared camera, NISP (Near-Infrared Spectrometer and Photometer), with three filters.Aims. We aim to develop a pipeline to detect fast-moving objects in Euclid images, with both high completeness and high purity.Methods. We tested the StreakDet software to find asteroids from simulated Euclid images. We optimized the parameters of StreakDet to maximize completeness, and developed a post-processing algorithm to improve the purity of the sample of detected sources by removing false-positive detections.Results.StreakDet finds 96.9% of the synthetic asteroid streaks with apparent magnitudes brighter than 23rd magnitude and streak lengths longer than 15 pixels (10 arcsec h(-1)), but this comes at the cost of finding a high number of false positives. The number of false positives can be radically reduced with multi-streak analysis, which utilizes all four dithers obtained by Euclid.Conclusions.StreakDet is a good tool for identifying asteroids in Euclid images, but there is still room for improvement, in particular, for finding short (less than 13 pixels, corresponding to 8 arcsec h(-1)) and/or faint streaks (fainter than the apparent magnitude of 23).
  • Euclid Collaboration; Knabenhans, Mischa; Stadel, Joachim; Marelli, Stefano; Potter, Doug; Teyssier, Romain; Legrand, Laurent; Schneider, Aurel; Sudret, Bruno; Blot, Linda; Awan, Saeeda; Burigana, Carlo; Carvalho, Carla Sofia; Kurki-Suonio, Hannu; Sirri, Gabriele (2019)
    We present a new power spectrum emulator named EuclidEmulator that estimates the nonlinear correction to the linear dark matter power spectrum depending on the six cosmological parameters ωb, ωm, ns, h, w0, and σ8. It is constructed using the uncertainty quantification software UQLab using a spectral decomposition method called polynomial chaos expansion. All steps in its construction have been tested and optimized: the large highresolution N-body simulations carried out with PKDGRAV3 were validated using a simulation from the Euclid Flagship campaign and demonstrated to have converged up to wavenumbers k ≈ 5 h Mpc−1 for redshifts z ≤ 5. The emulator is based on 100 input cosmologies simulated in boxes of (1250 Mpc/h)3 using 20483 particles. We show that by creating mock emulators it is possible to successfully predict and optimize the performance of the final emulator prior to performing any N-body simulations. The absolute accuracy of the final nonlinear power spectrum is as good as one obtained with N-body simulations, conservatively, ∼1 per cent for k 1 h Mpc−1 and z 1. This enables efficient forward modelling in the nonlinear regime, allowing for estimation of cosmological parameters using Markov ChainMonteCarlo methods. EuclidEmulator has been compared to HALOFIT, CosmicEmu, and NGenHalofit, and shown to be more accurate than these other approaches. This work paves a new way for optimal construction of future emulators that also consider other cosmological observables, use higher resolution input simulations, and investigate higher dimensional cosmological parameter spaces.
  • Euclid Collaboration; Adam, R.; Kurki-Suonio, H. (2019)
    Galaxy cluster counts in bins of mass and redshift have been shown to be a competitive probe to test cosmological models. This method requires an efficient blind detection of clusters from surveys with a well-known selection function and robust mass estimates, which is particularly challenging at high redshift. The Euclid wide survey will cover 15 000 deg(2) of the sky, avoiding contamination by light from our Galaxy and our solar system in the optical and near-infrared bands, down to magnitude 24 in the H-band. The resulting data will make it possible to detect a large number of galaxy clusters spanning a wide-range of masses up to redshift similar to 2 and possibly higher. This paper presents the final results of the Euclid Cluster Finder Challenge (CFC), fourth in a series of similar challenges. The objective of these challenges was to select the cluster detection algorithms that best meet the requirements of the Euclid mission. The final CFC included six independent detection algorithms, based on different techniques, such as photometric redshift tomography, optimal filtering, hierarchical approach, wavelet and friend-of-friends algorithms. These algorithms were blindly applied to a mock galaxy catalog with representative Euclid-like properties. The relative performance of the algorithms was assessed by matching the resulting detections to known clusters in the simulations down to masses of M-200 similar to 10(13.25) M-circle dot. Several matching procedures were tested, thus making it possible to estimate the associated systematic effects on completeness to 80% completeness for a mean purity of 80% down to masses of 10(14) M-circle dot and up to redshift z = 2. Based on these results, two algorithms were selected to be implemented in the Euclid pipeline, the Adaptive Matched Identifier of Clustered Objects (AMICO) code, based on matched filtering, and the PZWav code, based on an adaptive wavelet approach.
  • Price, Daniel; Pomoell, Jens; Kilpua, Emilia (2020)
    Aims. We present a detailed examination of the magnetic evolution of AR 12473 using time-dependent, data-driven magnetofrictional modelling.Methods. We used maps of the photospheric electric field inverted from vector magnetogram observations, obtained by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO), to drive our fully time-dependent, data-driven magnetofrictional model. Our modelled field was directly compared to extreme ultraviolet observations from the Atmospheric Imaging Assembly, also onboard SDO. Metrics were also computed to provide a quantitative analysis of the evolution of the magnetic field.Results. The flux rope associated with the eruption on 28 December 2015 from AR 12473 was reproduced by the simulation and found to have erupted due to a torus instability.
  • Pihajoki, Pauli; Mannerkoski, Matias; Nattila, Joonas; Johansson, Peter H. (2018)
    Ray tracing is a central tool for constructing mock observations of compact object emission and for comparing physical emission models with observations. We present ARCMANCER, a publicly available general ray-tracing and tensor algebra library, written in C++ and providing a Python interface. ARCMANCER supports Riemannian and semi-Riemannian spaces of any dimension and metric, and has novel features such as support for multiple simultaneous coordinate charts, embedded geometric shapes, local coordinate systems, and automatic parallel propagation. The ARCMANCER interface is extensively documented and user friendly. While these capabilities make the library well suited for a large variety of problems in numerical geometry, the main focus of this paper is in general relativistic polarized radiative transfer. The accuracy of the code is demonstrated in several code tests and in a comparison with GRTRANS, an existing ray-tracing code. We then use the library in several scenarios as a way to showcase the wide applicability of the code. We study a thin variable-geometry accretion disk model and find that polarization carries information of the inner disk opening angle. Next, we study rotating neutron stars and determine that to obtain polarized light curves at better than a similar to 1% level of accuracy, the rotation needs to be taken into account both in the spacetime metric and in the shape of the star. Finally, we investigate the observational signatures of an accreting black hole lensed by an orbiting black hole. We find that these systems exhibit a characteristic asymmetric twin-peak profile both in flux and polarization properties.
  • Ford, E. Darragh; Laigle, C.; Gozaliasl, G.; Pichon, C.; Devriendt, J.; Slyz, A.; Arnouts, S.; Dubois, Y.; Finoguenov, A.; Griffiths, R.; Kraljic, K.; Pan, H.; Peirani, S.; Sarron, F. (2019)
    Cosmic filaments are the channel through which galaxy groups assemble their mass. Cosmic connectivity, namely the number of filaments connected to a given group, is therefore expected to be an important ingredient in shaping group properties. The local connectivity is measured in COSMOS around X-ray-detected groups between redshift 0.5 and 1.2. To this end, large-scale filaments are extracted using the accurate photometric redshifts of the COSMOS2015 catalogue in two-dimensional slices of thickness 120 comoving Mpc centred on the group's redshift. The link between connectivity, group mass, and the properties of the brightest group galaxy (BGG) is investigated. The same measurement is carried out on mocks extracted from the light-cone of the hydrodynamical simulation HORIZON-AGN in order to control systematics. More massive groups are on average more connected. At fixed group mass in low-mass groups, BGG mass is slightly enhanced at high connectivity, while in high-mass groups BGG mass is lower at higher connectivity. Groups with a star-forming BGG have on average a lower connectivity at given mass. From the analysis of the HORIZON-AGN simulation, we postulate that different connectivities trace different paths of group mass assembly: at high group mass, groups with higher connectivity are more likely to have grown through a recent major merger, which might be in turn the reason for the quenching of the BGG. Future large-field photometric surveys, such as Euclid and LSST, will be able to confirm and extend these results by probing a wider mass range and a larger variety of environment.
  • Gordon, K. D.; Baes, M.; Bianchi, S.; Camps, P.; Juvela, M.; Kuiper, R.; Lunttila, T.; Misselt, K. A.; Natale, G.; Robitaille, T.; Steinacker, J. (2017)
    Context. The radiative transport of photons through arbitrary three-dimensional (3D) structures of dust is a challenging problem due to the anisotropic scattering of dust grains and strong coupling between different spatial regions. The radiative transfer problem in 3D is solved using Monte Carlo or Ray Tracing techniques as no full analytic solution exists for the true 3D structures. Aims. We provide the first 3D dust radiative transfer benchmark composed of a slab of dust with uniform density externally illuminated by a star. This simple 3D benchmark is explicitly formulated to provide tests of the different components of the radiative transfer problem including dust absorption, scattering, and emission. Methods. The details of the external star, the slab itself, and the dust properties are provided. This benchmark includes models with a range of dust optical depths fully probing cases that are optically thin at all wavelengths to optically thick at most wavelengths. The dust properties adopted are characteristic of the diffuse Milky Way interstellar medium. This benchmark includes solutions for the full dust emission including single photon (stochastic) heating as well as two simplifying approximations: One where all grains are considered in equilibrium with the radiation field and one where the emission is from a single effective grain with size-distribution-averaged properties. A total of six Monte Carlo codes and one Ray Tracing code provide solutions to this benchmark. Results. The solution to this benchmark is given as global spectral energy distributions (SEDs) and images at select diagnostic wavelengths from the ultraviolet through the infrared. Comparison of the results revealed that the global SEDs are consistent on average to a few percent for all but the scattered stellar flux at very high optical depths. The image results are consistent within 10%, again except for the stellar scattered flux at very high optical depths. The lack of agreement between different codes of the scattered flux at high optical depths is quantified for the first time. Convergence tests using one of the Monte Carlo codes illustrate the sensitivity of the solutions to various model parameters. Conclusions. We provide the first 3D dust radiative transfer benchmark and validate the accuracy of this benchmark through comparisons between multiple independent codes and detailed convergence tests.
  • Patton, David R.; Wilson, Kieran D.; Metrow, Colin J.; Ellison, Sara L.; Torrey, Paul; Brown, Westley; Hani, Maan H.; McAlpine, Stuart; Moreno, Jorge; Woo, Joanna (2020)
    We use the IllustrisTNG cosmological hydrodynamical simulations to investigate how the specific star formation rates (sSFRs) of massive galaxies (M-* > 10(10) M-circle dot) depend on the distance to their closest companions. We estimate sSFR enhancements by comparing with control samples that are matched in redshift, stellar mass, local density, and isolation, and we restrict our analysis to pairs with stellar mass ratios of 0.1 to 10. At small separations (similar to 15 kpc), the mean sSFR is enhanced by a factor of 2.0 +/- 0.1 in the flagship (110.7Mpc)(3) simulation (TNG100-1). Statistically significant enhancements extend out to 3D separations of 280 kpc in the (302.6Mpc)(3) simulation (TNG300-1). We find similar trends in the EAGLE and Illustris simulations, although their sSFR enhancements are lower than those in TNG100-1 by about a factor of two. Enhancements in IllustrisTNG galaxies are seen throughout the redshift range explored (0
  • Markkanen, Johannes; Agarwal, Jessica; Väisänen, Timo; Penttilä, Antti; Muinonen, Karri (2018)
    We show that the scattering phase functions of the coma and the nucleus of the comet 67P/Churyumov-Gerasimenko measured by the Rosetta/Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) instrument can be reproduced by a particle model involving clustered, densely packed submicrometer-sized grains composed of organic material and larger micrometer-sized silicate grains. The simulated and measured coma phase functions suggest that near the nucleus scattering is dominated by large particles, and the size distribution of dust particles varies with time and/or local coma environment. Further, we show that the measured nucleus phase function is consistent with the coma phase function by modeling a nucleus-sized object consisting of the same particles that explain the coma phase functions.
  • Battarbee, Markus; Dalla, Silvia; Marsh, Mike S. (2018)
    Understanding the transport of solar energetic particles (SEPs) from acceleration sites at the Sun into interplanetary space and to the Earth is an important question for forecasting space weather. The interplanetary magnetic field (IMF), with two distinct polarities and a complex structure, governs energetic particle transport and drifts. We analyze for the first time the effect of a wavy heliospheric current sheet (HCS) on the propagation of SEPs. We inject protons close to the Sun and propagate them by integrating fully 3D trajectories within the inner heliosphere in the presence of weak scattering. We model the HCS position using fits based on neutral lines of magnetic field source surface maps (SSMs). We map 1 au proton crossings, which show efficient transport in longitude via HCS, depending on the location of the injection region with respect to the HCS. For HCS tilt angles around 30 degrees-40 degrees, we find significant qualitative differences between A+ and A- configurations of the IMF, with stronger fluences along the HCS in the former case but with a distribution of particles across a wider range of longitudes and latitudes in the latter. We show how a wavy current sheet leads to longitudinally periodic enhancements in particle fluence. We show that for an A+ IMF configuration, a wavy HCS allows for more proton deceleration than a flat HCS. We find that A- IMF configurations result in larger average fluences than A+ IMF configurations, due to a radial drift component at the current sheet.
  • Rantala, Antti; Pihajoki, Pauli; Mannerkoski, Matias; Johansson, Peter H.; Naab, Thorsten (2020)
    We present the novel algorithmically regularized integration method MSTAR for high-accuracy (vertical bar Delta E/E vertical bar greater than or similar to 10(-14)) integrations of N-body systems using minimum spanning tree coordinates. The twofold parallelization of the O(N-part(2)) force loops and the substep divisions of the extrapolation method allow for a parallel scaling up to N-CPU = 0.2 x N-part. The efficient parallel scaling of MSTAR makes the accurate integration of much larger particle numbers possible compared to the traditional algorithmic regularization chain (AR-CHAIN) methods, e.g. N-part = 5000 particles on 400 CPUs for 1 Gyr in a few weeks of wall-clock time. We present applications of MSTAR on few particle systems, studying the Kozai mechanism and N-body systems like star clusters with up to N-part = 10(4) particles. Combined with a tree or fast multipole-based integrator, the high performance of MSTAR removes a major computational bottleneck in simulations with regularized subsystems. It will enable the next-generation galactic-scale simulations with up to 109 stellar particles (e.g. m(star) = 100 M-circle dot) for an M-star = 10(11) M-circle dot galaxy), including accurate collisional dynamics in the vicinity of nuclear supermassive black holes.
  • Shevchenko, Vasilij G.; Belskaya, Irina N.; Mikhalchenko, Olga I.; Muinonen, Karri; Penttilä, Antti; Gritsevich, Maria; Shkuratov, Yuriy G.; Slyusarev, Ivan G.; Videen, Gorden (2019)
    The values of the phase integral q were determined for asteroids using a numerical integration of the brightness phase functions over a wide phase-angle range and the relations between q and the G parameter of the HG function and q and the G(1), G(2) parameters of the HG(1)G(2) function. The phase-integral values for asteroids of different geometric albedo range from 0.34 to 0.54 with an average value of 0.44. These values can be used for the determination of the Bond albedo of asteroids. Estimates for the phase-integral values using the G(1) and G(2) parameters are in very good agreement with the available observational data. We recommend using the HG(1)G(2) function for the determination of the phase integral. Comparison of the phase integrals of asteroids and planetary satellites shows that asteroids have systematically lower values of q.