-
Pecseli, Hans Laszlo
(2024).
Bohm sheaths and pre-sheaths in one, two and three spatial dimensions.
Show summary
The time evolutions of the Bohm sheath and the related pre-sheath are analyzed as an initial value problem. The standard classical fluid model of a collisionless plasma is used with cold ions and Boltzmann distributed electrons. Numerical solutions of the basic equations show that a stationary plasma sheath itself is established within a few ion plasma periods. It is demonstrated analytically that for infinitely extended plasmas in one spatial dimension, the only possible solution is dynamic, giving a steady expansion: no stationary solution exists for this 1 dimensional case. The potential and density drops along the dynamic pre-sheath differ notably from the values found for stationary models suggested in the literature. Collisions give only formal changes to the collisionless results, their substance remains the same in one spatial dimension. Cylindrical and spherical geometries, on the other hand, allow for physically acceptable, fully nonlinear, stationary solutions with analytical results given. These are supported by numerical solutions.
-
Pecseli, Hans Laszlo
(2023).
Kinetically unstable ion velocity distributions formed via charge exchange collisions by E×B-drifts in magnetized plasmas.
Show summary
The stability of ion velocity distributions in magnetized plasmas is studied under conditions where the plasma has an E × B-drift with respect to a neutral background. Charge–exchange collisions can give rise to velocity distributions in the form of a ring or a loss-cone being linearly unstable. Sometimes the distributions have distorted forms and a stability analysis is not straightforward. Numerical simulations offer the most convenient method of a stability analysis in such cases. A particle-in-cell code that includes collisional interactions shows the formation of such velocity distributions and the instabilities associated with them. We observe the saturation of the linear instability by quasi-linear velocity space diffusion. The parameter dependence of the instability conditions is illustrated by examples. The results are relevant for explaining some of the low-frequency oscillations observed in the lower parts of the Earth’s ionosphere (E- and F-regions), where collisions with neutrals are abundant. The results can be important for laboratory experiments as well.
-
Pecseli, Hans Laszlo
(2023).
Minute time-scale oscillations of the magnetopause.
Show summary
It is argued that a simple model based on magnetic image arguments suffices to give a convincing insight into both the basic static as well as some transient dynamic properties of the near-Earth's magnetosphere, particularly accounting for damped oscillations being excited in response to impulsive perturbations. The parameter variations of the frequency are given. Qualitative results can also be obtained for heating due to the compression of the radiation belts. The properties of this simple dynamic model for the solar wind–magnetosphere interaction are discussed and compared to observations. In spite of its simplicity, the model gives convincing results concerning the magnitudes of the near-Earth's magnetic and electric fields. The database contains ground-based results for magnetic field variation in response to shocks in the solar wind. Here, the observations also include data from the two Van Allen satellites.
-
Sato, Hiroatsu; Pecseli, Hans Laszlo; Trulsen, Jan Karsten; Sandholt, Per Even & Farrugia, C
(2023).
Impulse-driven oscillations of near-Earth magnetosphere.
-
Pecseli, Hans Laszlo
(2022).
Cascade conditions in turbulent incompressible fluids.
Show summary
The basic equations for three dimensional incompressible flows are expanded in terms of the eigenfunctions for the curl-operator. Conditions for a dual as well as forward energy cascades in wavenumber space are presented.
-
Pecseli, Hans Laszlo
(2022).
Mesospheric turbulence diagnosed by radar scattering off meteor trails: Summary of work by Vierinen et al.
-
Pecseli, Hans Laszlo
(2021).
On the applicability of Taylor’s hypothesis, including small sampling velocities.
-
Pecseli, Hans
(2018).
Predation in a turbulent environment.
Show summary
Analytical models for the statistical distribution of the gut content of fish larvae in a turbulent ocean environment are compared to data obtained in a field experiment. The proposed model allows the nutrition state and thereby the survival probability of plankton populations to be estimated for given conditions and parameters characterizing their environment, i.e., in terms of prey concentrations and turbulence levels. These parameters were all available in the field data. The species are characterized by their capture range and fields of view together with a characteristic time for digestion of their prey. The analysis allows an estimate for the entire probability density of the gut content of plankton in terms of the number of nauplii. In particular, the analytical results give a basis for an estimate of the average gut content of a given plankton population on the basis of basic informations concerning the environment.
-
Pecseli, Hans
(2017).
Solvable models for "blobs" in magnetized plasmas.
-
Pecseli, Hans
(2016).
Low frequency oscillations of the Earth's magnetosphere.
-
Pecseli, Hans
(2016).
Low frequency oscillations of the magnetosphere.
-
Pecseli, Hans
(2016).
Linear electrostatic drift waves (3 lectures),
Nonlinear electrostatic drift waves (3 lectures).
-
Pecseli, Hans
(2015).
Excess statistics with applications to turbulence in marine environments.
Show summary
Studies of excess statistics are standard in signal analysis, addressing some basic questions like average frequencies of level crossings in a random signal, and average time durations between an up and a down crossing. Results like these have applications for many problems, noise in electric circuits, turbulent diffusion, etc. It is here argued that the results can be applied also to estimating the "noise" experienced by aquatic microorganisms in a turbulent environment. This problem has particular interest in relations to predators and prey that rely on signals transmitted through motions in the surrounding water. The joint probability densities needed in the analysis can be obtained from numerical solutions of the Navier-Stokes equation for turbulent conditions, and be given universal scaling laws. Two limiting cases need to be considered: the inertial and viscous sub-ranges of the turbulence. It is possible to give estimates for how often a predator can mistake turbulence induced noise for a signal from prey in given turbulence conditions. Such mistakes can be observed as unmotivated attack reactions. Similarly, prey can mistake noise for presence of a predator, giving rise to seemingly unmotivated escape responses. The results can be verified experimentally in a laboratory, at least in principle.
-
Pecseli, Hans & Trulsen, Jan Karsten
(2015).
Excess statistics with applications to turbulence in marine environments.
-
Pecseli, Hans
(2015).
Drift wave turbulence in laboratory and space plasmas.
-
Pecseli, Hans
(2015).
Excess statistics with applications to turbulence in marine enviroments.
-
Pecseli, Hans
(2015).
Excess statistics with applications to anomalous transport in magnetized plasmas.
-
Pecseli, Hans
(2015).
Nonlinear waves and turbulence.
-
Pecseli, Hans
(2015).
Weakly Nonlinear Waves in Plasmas (4 lectures).
-
Pecseli, Hans
(2015).
Introduction to Electrostatic Drift Waves (4 lectures).
-
Pecseli, Hans
(2015).
Anomalous transport in a magnetized toroidal plasma.
-
Pecseli, Hans
(2014).
Excess statistics with applications to turbulence in marine environments.
-
Børve, Steinar; Sato, Hiroatsu; Pecseli, Hans & Trulsen, Jan Karsten
(2014).
LOW FREQUENCY OSCILLATIONS OF THE MAGNETOSPHERE.
-
Bergsaker, Anne Schad; Fredriksen, Åshild; Pecseli, Hans & Trulsen, Jan Karsten
(2014).
Analytical models for skewness-kurtosis relations in turbulent transport.
-
Pecseli, Hans
(2014).
Predator-prey encounters in turbulent environments: life on small scales in the oceans.
Show summary
Micro-organisms in the oceans, fish larvae and similar (here called "predators"), have only little motion of their own, and it can safely be assumed that they are passively carried along by the local flow velocity, at least to a first approximation. Similarly, it can be argued that their food (micro-zooplancton, for instance, here called "prey") is also passively convected by the same flow. It is then rather evident that such micro-organisms will eventually be starving, unless motions in the water changes the predator-prey separations, and brings, at least from time to time, some prey within reach of the predators. It is then an interesting question how the known laws of turbulence are reflected in the feeding processes of micro-organisms.
Fortunately, the motion of particles in turbulent flows has been investigated in great detail, in part because of the strong environmental importance of the problem. A wealth of data are available for these types of phenomena, and it will be demonstrated how some of these data can be utilized in a simple way, in order to get insight also into the present problem. The existence of seemingly universal scaling laws for the average prey flux to a selected predator can be demonstrated. These laws depend on the radius in a suitably defined "reach of interception", as well as on the intensity of the turbulence. Some of these scaling laws are in contradiction with our intuitive expectations.
In a larger perspective, the results have wider application, for combustion for instance, and more generally for many problems where turbulent mixing has importance.
-
Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten
(2013).
Kinetic plasma instabilities due to charge exchange collisions.
-
Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten
(2013).
Numerical simulations of kinetic plasma instabilities induced by charge exchange collisions.
-
Pecseli, Hans
(2013).
Simple models for electrostatic drift waves and their relevance for the Earth’s ionosphere.
Show summary
A simple model for electrostatic drift waves in magnetized plasmas is outlined. All the model needs is the ion continuity equation, the ExB ion drift velocity perpendicular to magnetic field lines, concluded by some reasonable assumptions concerning a quasi-stationary low frequency electron motion. This very simple model indicates that electrostatic waves can propagate along the plasma surface, but can not explain why they are spontaneously excited. There is a source of free energy in the plasma inhomogeneity (or, if you like, the diamagnetic current). An instability is excited, for instance, due to deviations of the electron dynamics from the simplified model. We can for instance have resistive drift waves or current driven drift waves. The seminar is concluded by illustrations of how a standard ionosphere can be disturbed to give conditions for excitation of electrostatic drift waves. A very simple (yet novel) idea for a battery mechanism is described, whereby magnetic field aligned currents can be generated at plasma density gradients in the direction perpendicular to magnetic field lines.
-
Pecseli, Hans
(2013).
Unstable Ring-Shaped Ion Distributions Functions Induced by Charge-Exchange Collisions.
-
Pecseli, Hans
(2013).
Dynamics of Collisional Plasmas in Crossed Electric and Magnetic Fields.
-
Pecseli, Hans; Miloch, Wojciech Jacek & Trulsen, Jan Karsten
(2013).
Kinetically unstable velocity distributions formed by charge exchange collisions with drifting ions.
-
Pecseli, Hans; Trulsen, Jan Karsten & Fiksen, Øyvind
(2013).
Predator-prey encounter and capture rates in turbulent environments.
-
Sato, Hiroatsu; Pecseli, Hans; Børve, Steinar & Trulsen, Jan Karsten
(2012).
Dynamic models for magnetospheric oscillations on the minute scale.
-
Pecseli, Hans
(2012).
Predator-prey encounters in turbulent environments - life on small scales in the oceans.
-
Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten
(2012).
Numerical simulations of collisional partially ionized plasmas.
Show summary
The stability of plasma can be related to the shape of its velocity distributions [1]. For instance, for ionospheric plasmas it is argued that ion velocity distributions can deviate from Maxwellian giving rise to plasma instabilities [2, 3]. In order to study the possibility for losscone instabilities in collisional plasmas with Hall currents, we have modified our particle-in-cell code to include collisional effects in a magentized plasma. We have the option of adding a DCelectric field in the direction perpendicular to the magnetic field. We present the results from the self-consistent three dimensional simulations of the plasma velocity distributions under such conditions. In this study we investigate the role of collisions with the neutral component for the evolution and stability of the velocity distributions. In the most general case, we include elastic electron-neutral collisions, as well as charge exchange and elastic ion-neutral collisions. The results will be important for a detailed understanding of low frequency ionospheric plasmas, for instance in the E-region.
References
[1] O. Penrose, Phys. Fluids 3, 258 (1960).
[2] J.-P. St.-Maurice, E. Winkler, and A. M. Hamza, J. Geophys. Res. 99, 19527 (1994)
[3] J.-P. St.-Maurice and R. W. Schunk, Planet. Space Sci. 25, 243 (1977
-
Rekaa, Vegard Lundby; Pecseli, Hans & Trulsen, Jan Karsten
(2012).
Numerical studies of a plasma diode with external forcings.
Show summary
With reference to laboratory studies we analyze the dynamics of a plasma diode under external forcing. Assuming a strong axial magnetic field, the problem is analyzed in
one spatial dimension by a Particle in Cell code. The cathode is assumed to be operated in electron rich conditions, supplying an abundance of electrons. We compare different forcing schemes with the results obtained by solving the van der Pol equation. In one method of forcing we apply an oscillation in addition to the DC end plate bias, and consider both amplitude and frequency variations. An alternative method of perturbation consists of modelling an absorbing grid at some internal position. Also in this case we can have a constant frequency with varying amplitude, or alternatively an oscillation with chirped frequency but constant amplitude. We find that the overall features of the forced van der Pol equation are recovered, but the details in the plasma response require more attention to the harmonic responses, requiring extensions of the model equation. The analysis is extended by introducing collisional effects, where we emphasize
charge exchange collisions of ions, since these processes usually have the largest cross sections. Charge exchange collisions give significant modifications of the diode performance when the effects are compared with the influence of elastic electron-neutral and ion-neutral collisions with the same collisional mean free paths.
-
Sato, Hiroatsu; Børve, Steinar; Pécseli, Hans & Trulsen, Jan Karsten
(2012).
Minute-scale period oscillations of the magnetosphere.
Show summary
Oscillations with periods on the order of 5–10 min have been observed by instrumented spacecrafts in the Earth’s magnetosphere. These oscillations often follow sudden impacts related to coronal mass ejections. It is demonstrated that a simple model is capable of explaining the basic properties of these oscillations and give scaling laws for their basic characteristics in terms of the basic parameters of the problem. The period of the oscillations and their anharmonic nature, in particular, are accounted for. The model has no free adjustable numerical parameters. We use measurable quantities as inputs (such as Solar wind momentum density), and our results can be seen as an effort to predict some dynamic properties of magnetospheres on the basis of measurable steady state characteristics. A simple test of the model is found by comparing its prediction of the Earth-Magnetopause distance with observed values. The general results agree with observations. The analysis is supported by numerical simulations solving the Magneto-Hydro-Dynamic (MHD) equations in two spatial dimensions, where we let a solar wind interact with a magnetic dipole representing a magnetized Earth. Two tilt-angles of the magnetic dipole axis were considered. We observed the formation of a magnetosheath, with the magnetopause at a distance corresponding well to the analytical results. Sudden pulses in the model solar wind sets the model magnetosphere into damped oscillatory motions and quantitatively good agreement with the analytical results is found. The models seem to be robust, and give good qualitative agreement with the numerical simulations for a range of parameters.
-
Miloch, Wojciech Jacek; Yaroshenko, V.; Vladimirov, Sergei V.; Pecseli, Hans & Trulsen, Jan Karsten
(2011).
Spacecraft charging in space and ionospheric plasma; numerical simulations.
-
Pecseli, Hans
(2011).
Experimental studies of intermittency and turbulent transport in linear and toroidal magnetized plasmas.
-
Pecseli, Hans & Trulsen, Jan Karsten
(2011).
Encounter rates and transit time distributions for surfacesmoving in turbulent flows.
Show summary
Reaction rates as relevant for many chemical processes as well as a number of biological applications depend on encounter rates of reacting elements and the probability of a reaction occurring given an encounter. Turbulent mixing can enhance the encounter rate, but it can also have adverse effects by reducing the reaction probability. We model the probability for a reaction by letting it depend on the time available, i.e. the time spent by of reacting particles within some given interaction volume. We study the problems by numerical simulations where we analyze the statistics of the motion of passively convected point particles in turbulent flows. The database used is obtained by direct numerical solution of the Navier-Stokes equation. We estimate the encounter rates and also the probability distribution of the transit times of particles through reference volumes with given forms and sizes. A selected position within the reference volume is moving self consistently with the local flow velocity, thus determining the motion of the entire surface. The transit time is defined as the interval between entrance and exit times of surrounding particles convected through the volume by the turbulent motions. Scale sizes in the inertial as well as in the viscous subranges of the turbulence are considered. Simple, and seemingly universal, scaling laws are obtained for the encounter rate as well as the probability density of the transit times in terms of the basic properties of the turbulent flow and the geometry. In the present formulation, the results of the analysis are relevant for chemical reactions, but also for understanding details of the feeding rate of micro-organisms in turbulent waters, for instance.
-
Rekaa, Vegard Lundby; Pecseli, Hans & Trulsen, Jan Karsten
(2011).
Numerical simulations of anomalous resistivity in the ion acoustic and Buneman regime.
-
Pecseli, Hans; Trulsen, Jan Karsten & Fiksen, Øyvind
(2009).
Predators in turbulent waters: an analytical model and a numerical test.
-
Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten
(2009).
Numerical studies of ion focusing in streaming plasmas; two and three dimensions.
-
Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten
(2009).
Interactions of two objects in flowing plasmas studied by numerical simulations.
-
Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten
(2009).
Ion focusing behind finite size objects in flowing plasmas; numerical studies in two and three dimensions.