Hans Laszlo Pecseli

H. L. Pécseli, Fluctuations in Physical Systems, Cambridge University Press, UK (2000) pp. 190.

H. L. Pécseli, Waves and Oscillations in Plasmas, Taylor & Francis, UK (2012) pp. 553.

H. L. Pécseli, Low Frequency Waves and Turbulence in Magnetized Laboratory Plasmas and in the Ionosphere. Institute of Physics (IOP), UK (2016). pp. 631.

• Kono, Mitsuo & Pecseli, Hans Laszlo (2026). On the Possibility of a Dual Cascade in Three-Dimensional Incompressible Turbulent Flows. Physics. 8(1). doi: 10.3390/physics8010013. Fulltekst i vitenarkiv
• Pecseli, Hans Laszlo; Trulsen, Jan Karsten; Yan, Ning; Nielsen, Anders H.; Rasmussen, Jens Juul & Zhang, Yufei (2026). Small scale turbulence and plasma transport in the EAST tokamak. Plasma Physics and Controlled Fusion. ISSN 0741-3335. 68(3). doi: 10.1088/1361-6587/ae4f20. Fulltekst i vitenarkiv Vis sammendrag

  A movable multi-pin probe in deuterium discharges in the superconducting EAST tokamak was used to recover local plasma parameters and electrostatic fluctuation characteristics along prescribed radial traverses. Several spatial regions are distinguished during the radial probe motion. The present analysis is restricted to a region inside the last closed magnetic flux surface, where locally homogeneous conditions can be argued. The data are analyzed with particular attention to the presence of cm-scale electrostatic plasma fluctuations. The detailed characteristics of the observed structures are described and quantified. Their contribution to anomalous plasma transport across magnetic field lines is demonstrated. Supported by correlation analyses, the primary analysis tool is conditional averaging, where the results are here presented with conditional variances included. Detailed information on properties of plasma blobs propagating across magnetic field lines is recovered. Alternative diagnostic methods are described and given analytic support. These tools can be applied when partial or reduced results suffice, offering also means of cross-checking basic fluctuation parameters. The agreement is good for the present plasma conditions.

• Poblet, Facundo L.; Chau, J.L.; Pecseli, Hans Laszlo; Renkwitz, Toralf; Vierinen, Juha & Avsarkisov, Victor [Vis alle 7 forfattere av denne artikkelen] (2025). Spatial Structure Functions of Horizontal Winds in the Upper Troposphere and Lower Stratosphere Using Taylor's Hypothesis Over MAARSY (69.30°N, 16.04°E). Geophysical Research Letters. ISSN 0094-8276. 52(9). doi: 10.1029/2025GL114927. Fulltekst i vitenarkiv
• Pecseli, Hans Laszlo (2025). Pitfalls in the interpretations of Langmuir probe currents. Physics of Plasmas. ISSN 1070-664X. 32. doi: 10.1063/5.0251766. Fulltekst i vitenarkiv Vis sammendrag

  When analyzing the signals from Langmuir probes by numerical simulations, for instance, the currents are usually interpreted by counting the number of charged particles arriving at the probe surface per unit time. Under stationary homogeneous conditions, this model will give the correct average result, but the individual particle contributions are often interpreted incorrectly. As demonstrated by a specific physically realistic example, the contribution from induced charges will be missing. Collected and passing particles will both contribute to the power spectrum of the probe noise. This noise will set the lower limit to the deterministic signals being detectable. Particles that give the current contribute at the same time also to the noise. The emphasis is here on biased probes, but the basic analysis applies to floating probes as well, or conducting macroscopic particles embedded in plasmas. It is argued that a current can be induced into a probe without any charges reaching its surface.

• Kono, Mitsuo & Pecseli, Hans Laszlo (2024). Dynamic evolutions of Bohm sheaths and pre-sheaths. Physics of Plasmas. ISSN 1070-664X. 31(2). doi: 10.1063/5.0176287. Fulltekst i vitenarkiv Vis sammendrag

  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 homogeneous plasmas in one spatial dimension, the only possible pre-sheath solution is dynamic, giving a steady expansion: no stationary solution exists for this one-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, and 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.

• Vierinen, Juha-Pekka; Poblet, F.L.; Chau, J.L.; Avsarkisov, V.; Pecseli, Hans Laszlo & Tsutsumi, M. [Vis alle 10 forfattere av denne artikkelen] (2024). Dissipation Rates of Mesospheric Stratified Turbulence From Multistatic Meteor-Radar Observations. Geophysical Research Letters. ISSN 0094-8276. 51(11). doi: 10.1029/2023GL105751. Fulltekst i vitenarkiv Vis sammendrag

  Stratified turbulence (ST) has been proposed as a model for the dynamics of the mesosphere-lower thermosphere (MLT) region. This theory postulates that for horizontal mesoscales (∼1–400 km), the kinetic energy of horizontal winds dissipates from large to small scales with an approximately mean constant rate. In this investigation, dissipation rates are quantified using meteor-radar observations conducted in Northern Norway. The observed seasonal variability of dissipation rates exhibits maxima during the summer and winter, and minima near the equinoxes, between 80 and 95 km altitude. The results are compared with model predictions and earlier medium frequency radar, rocket, lidar, and satellite observations of MLT turbulence. The findings suggest that multi-static meteor radar measurements of ST can provide a novel way to continuously monitor turbulent dissipation rates in the MLT region.

• Kono, Mitsuo & Pecseli, Hans Laszlo (2022). Cascade conditions in electron magneto-hydrodynamic turbulence. Physics of Plasmas. ISSN 1070-664X. 29(12). doi: 10.1063/5.0124404. Fulltekst i vitenarkiv
• Sato, Hiroatsu; Pecseli, Hans Laszlo; Trulsen, Jan Karsten; Sandholt, Per Even & Farrugia, Charles J. (2022). Impulse-driven oscillations of the near-Earth's magnetosphere. Annales Geophysicae. ISSN 0992-7689. 40(6), s. 641–663. doi: 10.5194/angeo-40-641-2022. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Guio, Patrick & Pécseli, Hans L (2021). The Impact of Turbulence on the Ionosphere and Magnetosphere. Frontiers in Astronomy and Space Sciences. 7. doi: 10.3389/fspas.2020.573746. Fulltekst i vitenarkiv Vis sammendrag

  An important property associated with turbulence in plasmas and fluids is anomalous transport. Plasma, being a good conductor, can in addition be affected by turbulence causing an anomalous resistivity that can significantly exceed its classical counterpart. While turbulent transport may be adequately described in configuration space, some aspects of the anomalous resistivity are best accounted for in phase space. Kinetic phenomena like electron and ion phase space vortices can thus act as obstacles for the free flow of slow charged particles. Plasma instabilities and large amplitude plasma waves are candidates for contributions to the anomalous resistivity by generating such structures. Langmuir waves can be relevant, but also others, such as upper- as well as lower-hybrid waves in magnetized plasmas. Often these anomalous resistivity effects can be small, but due to the large spatial and temporal scales involved in space plasmas, planetary ionosphere and magnetosphere in particular, even such moderate effects can be important. This mini-review is discussing elements of the description of plasma turbulence with particular attention to wave phenomena that contribute to anomalous resistivity and diffusion. Turbulence effects can have relevance for space weather phenomena as well, where ground based and airborne activities relying on for instance Global Positioning and Global Navigation Satellite Systems are influenced by plasma conditions in geospace.

• Pécseli, Hans L & Trulsen, Jan Karsten (2021). On the applicability of Taylor's hypothesis, including small sampling velocities. Journal of Fluid Mechanics. ISSN 0022-1120. 932. doi: 10.1017/jfm.2021.969. Fulltekst i vitenarkiv Vis sammendrag

  Taylor's hypothesis, or the frozen turbulence approximation, can be used to estimate also the specific energy dissipation rate ϵ by comparing experimental results with the Kolmogorov–Obukhov expression. The hypothesis assumes that a frequency detected by an instrument moving with a constant large velocity <i><b>V</i></b> can be related to a wavenumber by <i><b>ω=kV</i></b>. It is, however, not obvious how large the translational velocity has to be in order to make the hypothesis valid, or at least applicable with some acceptable uncertainty. Using the space–time-varying structure function for homogeneous and isotropic conditions, this question is addressed in the present study with emphasis on small velocities <i><b>V</i></b>. The structure function is obtained using results from numerical solutions of the Navier–Stokes equation. Particular attention is given to the <i><b>V</i></b> variation of the estimated specific energy dissipation, <i><b>ϵ_est</i></b>, compared with the actual value, <i><b>ϵ</i></b>, used in the numerical calculations. In contrast to previous studies, the results emphasize velocities <i><b>V</i></b> less than or comparable to the one-component root-mean-square velocity, <i><b>u_rms</i></b>. We find that <i><b>ϵ</i></b> can be determined to an acceptable accuracy for <i><b>V</i></b>≥<b>0.3</b><i><b>u_rms</i></b>. A simple analytical model is suggested to explain the main features of the observations, both Eulerian and Lagrangian. The model assumes that the observed time variations are solely due to eddies moving past the observer, thus ignoring eddy deformation and intermittency effects. In spite of these simplifications, the analysis accounts for most of the numerical results when also eddy-size-dependent velocities are accounted for.

• Fredriksen, Åshild; Pécseli, Hans L & Trulsen, Jan Karsten (2020). Anomalous transport due to large coherent structures in a magnetized toroidal plasma. Plasma Physics and Controlled Fusion. ISSN 0741-3335. 63(1). doi: 10.1088/1361-6587/abc518. Fulltekst i vitenarkiv Vis sammendrag

  Anomalous transport due to large coherent structures, or coherent events, is studied experimentally in a magnetized toroidal discharge plasma. The present analysis emphasizes anomalous plasma losses as well as transport of electron thermal energy density across magnetic field lines. The experiment was carried out in the Blaamann device at the University of Tromsø. The diagnostics are based on data for potential, density and electron temperature variations as detected by Langmuir probes. Most of the data analysis uses conditional sampling. The flux of thermal energy density can be broken down into several components which are analyzed separately. Experimental means for obtaining the space-time variation of the complete polarization drifts are demonstrated, and the results compared with the fluctuating E × B₀/<i>B</i>²₀-velocities. For the present conditions these polarization drifts contribute only little to the plasma losses, but for other discharge parameters in heavier gases they can be important. Our results show that while the density, the potential and the electric field variations are dominated by large scales, the vorticity of the E × B₀/<i>B</i>²₀-flow as well as the ion polarization drifts are strongly intermittent.

• Guio, Patrick & Pécseli, Hans L (2020). Weakly nonlinear ion sound waves in gravitational systems. Physical Review E (PRE). ISSN 2470-0045. 101(4). doi: 10.1103/PhysRevE.101.043210. Fulltekst i vitenarkiv Vis sammendrag

  Ion sound waves are studied in a plasma subject to gravitational field giving rise to vertically inhomogeneous steady-state plasma conditions. Such systems are interesting by exhibiting a wave growth that is a result of energy flux conservation for pulses propagating in an inhomogeneous system. The increase of the amplitude of a pulse as it propagates along the density gradient in the direction of decreasing density gives rise to an enhanced interaction between waves and plasma particles that can be modeled by a modified Korteweg–de Vries equation. Analytical results are compared with numerical particle-in-cell simulations of the problem. Our code assumes isothermally Boltzmann distributed electrons resulting in a nonlinear Poisson equation. The ion component is treated as a collection of individual particles interacting through collective electric fields. Deviations from quasineutrality are allowed for.

• Pécseli, Hans L; Trulsen, Jan Karsten; Stiansen, Jan Erik & Sundby, Svein (2020). Feeding of plankton in a turbulent environment: A comparison of analytical and observational results covering also strong turbulence. Fluids. 5(1). doi: 10.3390/fluids5010037. Fulltekst i vitenarkiv Vis sammendrag

  The present studies address feeding of plankton in turbulent environments, discussed by a comparison of analytical results and field data. Various models for predator-prey encounters and capture probabilities are reviewed. Generalized forms for encounter rates and capture probabilities in turbulent environments are proposed. The analysis emphasizes ambush predators, exemplified by cod larvae Gadus morhua L. in the start-feeding phase (stage 7 larvae) collected in shallow waters near Lofoten, Norway. During this campaign, data were obtained at four sites with strongly turbulent conditions induced by tidal currents and long-wave swells, and one site where the turbulence had a lower level in comparison. The guts of the selected cod larvae were examined in order to determine the number of nauplii ingested. Analytically obtained probability densities for the gut content were compared with observations and the results used for estimating the rate of capture of the nauplii. This capture rate was then compared with analytical results using also data for the surroundings, such as measured prey densities and turbulence conditions, as quantified by the specific energy dissipation rate. Different from earlier studies, the presented data include conditions where the turbulence exceeds the level for optimal larval encounter-capture rates.

• Miyake, Yohei; Miloch, Wojciech Jacek; Høymork, Solveig Helene & Pécseli, Hans L (2020). Electron Wing-Like Structures Formed at a Negatively Charged Spacecraft Moving in a Magnetized Plasma. Journal of Geophysical Research (JGR): Space Physics. ISSN 2169-9380. 125. doi: 10.1029/2019JA027379. Fulltekst i vitenarkiv Vis sammendrag

  Spacecraft‐plasma interactions are studied with self‐consistent numerical simulations of magnetized plasmas, where electrons are strongly magnetized whereas ions are weakly magnetized. It is found that for a spacecraft in such a magnetized plasma corresponding to a low Earth orbit, electrons can be reflected from a negatively charged spacecraft and then guided by geomagnetic field lines. The reflected electrons can leave a sharp trail like wings if the spacecraft size is greater than an average electron gyroradius of the environment. Such an electron wing‐like structure is associated with propagating Langmuir waves. This results in nontrivial asymmetric electrostatic potentials close to the spacecraft and even farther than the Debye screening distance. The convective electric field also gives rise to a differential potential of the spacecraft with respect to the plasma, resulting in yet another asymmetry in the plasma dynamics and the potential distribution around the spacecraft. These asymmetries in the plasma dynamics can significantly influence in‐situ measurements of space plasma. The results show a good qualitative agreement with actual measurements by a satellite in the polar regions.

• Pécseli, Hans L; Trulsen, Jan Karsten; Stiansen, Jan Erik; Sundby, Svein & Fossum, Petter (2019). Feeding of plankton in turbulent oceans and lakes. Limnology and Oceanography. ISSN 0024-3590. 64(3), s. 1034–1046. doi: 10.1002/lno.11094. Fulltekst i vitenarkiv Vis sammendrag

  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., prey concentrations and turbulence levels. These parameters are all available in the field data. Other parameters such as the capture range and fields of view together with a characteristic time for digesting prey are assumed to be known. The analysis allows an estimate for the probability density of the gut content of plankton in terms of the number of nauplii in the gut. In particular, the analytical results give a basis for evaluating the average gut content of a given plankton population on the basis of basic information concerning the prey concentration and the turbulence intensity. Also analytical models for the prey capture rates are compared with results based on the field data. The analysis emphasizes the effects of turbulence.

• Fredriksen, Åshild; Pecseli, Hans & Trulsen, Jan Karsten (2018). A solvable model for the basic properties of a simple magnetized plasma torus. Plasma Physics and Controlled Fusion. ISSN 0741-3335. 60:085021(8). doi: 10.1088/1361-6587/aacd2a. Fulltekst i vitenarkiv Vis sammendrag

  A simple magnetized plasma torus is modeled by using a 'top-hat' density variation. The benefit of this simplification is an exactly solvable model, allowing for a few additions, such as an externally maintained parabolic steady state potential variation imposed on the plasma. The combined effects of the resulting plasma rotation and the plasma polarization due to magnetic field gradient particle drifts can be described. The stabilizing and confining effects of plasma rotation are explicitly demonstrated. In a special high plasma density limit the results are confirmed by a more general model, indicating that the results of the top-hat model can be used with confidence in more general cases. A small vertical magnetic field component can be included for a plasma with neutral collisions and its influence on the electron dynamics studied. The effects of ion–neutral collisions are also included. The rotation and polarization of the plasma has different effects on the time variations of the plasma density and potential. As a reference we use data from the Blaamann device at the University of Tromsø obtained by a movable multi-probe, measuring variations in density, floating potential and an electric field component. The fluctuations in the plasma are characterized by auto-correlations and by cross-correlations between the signal from a fixed reference probe and data. The model accounts adequately for the phase variations of the signals for varying spatial multi-probe positions.

• Kono, Mitsuo & Pecseli, Hans (2017). Stability of electron wave spectra in weakly magnetized plasmas. Journal of Plasma Physics. ISSN 0022-3778. 83(6). doi: 10.1017/S0022377817000873. Fulltekst i vitenarkiv
• Kono, Mitsuo & Pecseli, Hans (2016). Parametric decay of wide band Langmuir wave-spectra. Journal of Plasma Physics. ISSN 0022-3778. 82(6). doi: 10.1017/S0022377816001070. Fulltekst i vitenarkiv
• Pecseli, Hans & Trulsen, Jan Karsten (2016). Plankton’s perception of signals in a turbulent environment. Advances in Physics: X. 1(1), s. 20–34. doi: 10.1080/23746149.2015.1136567. Fulltekst i vitenarkiv
• Garcia, Odd Erik; Kube, Ralph; Theodorsen, Audun & Pecseli, Hans (2016). Stochastic modelling of intermittent fluctuations in the scrape-off layer: Correlations, distributions, level crossings, and moment estimation. Physics of Plasmas. ISSN 1070-664X. 23(5), s. 1–14. doi: 10.1063/1.4951016. Fulltekst i vitenarkiv Vis sammendrag

  A stochastic model is presented for intermittent fluctuations in the scrape-off layer of magnetically confined plasmas. The fluctuations in the plasma density are modeled by a super-position of uncorrelated pulses with fixed shape and duration, describing radial motion of blob-like structures. In the case of an exponential pulse shape and exponentially distributed pulse amplitudes, predictions are given for the lowest order moments, probability density function, auto-correlation function, level crossings, and average times for periods spent above and below a given threshold level. Also, the mean squared errors on estimators of sample mean and variance for realizations of the process by finite time series are obtained. These results are discussed in the context of single-point measurements of fluctuations in the scrape-off layer, broad density profiles, and implications for plasma–wall interactions due to the transient transport events in fusion grade plasmas. The results may also have wide applications for modelling fluctuations in other magnetized plasmas such as basic laboratory experiments and ionospheric irregularities.

• Pecseli, Hans; Sortland, Daniel Selnes & Garcia, Odd Erik (2016). A solvable blob-model for magnetized plasmas. Plasma Physics and Controlled Fusion. ISSN 0741-3335. 58(10), s. 1–8. doi: 10.1088/0741-3335/58/10/104002. Fulltekst i vitenarkiv Vis sammendrag

  A simple analytically solvable model for blobs in magnetized plasmas is proposed. The model gives results for a scaling of the blob velocity with the amplitude of the density perturbation. Limiting cases are considered: one where the plasma motion is strictly perpendicular to an externally imposed toroidal magnetic field, and one where the electrons can move along magnetic field lines to compensate partly the collective electric fields. For these limiting cases, the model predicts scaling laws for the dependence of the blob velocities and accelerations with varying cross section, plasma density and temperature. Also the scaling with the dominant ion mass is derived. The analysis is completed by including the effects of collisions between ions and neutrals.

• Guio, Patrick & Pecseli, Hans (2016). Weakly nonlinear ion waves in striated electron temperatures. Physical Review E (PRE). ISSN 2470-0045. 93(4). doi: 10.1103/PhysRevE.93.043204. Fulltekst i vitenarkiv
• Bergsaker, Anne Schad; Fredriksen, Åshild; Pecseli, Hans & Trulsen, Jan Karsten (2015). Models for the probability densities of the turbulent plasma flux in magnetized plasmas. Physica Scripta. ISSN 0031-8949. 90(10). doi: 10.1088/0031-8949/90/10/108005. Fulltekst i vitenarkiv
• Pecseli, Hans (2015). Low frequency electrostatic waves propagating in plasmas with parameters varying along magnetic field lines. Plasma Sources Science & Technology. ISSN 0963-0252. 25(1). doi: 10.1088/0963-0252/25/1/015010. Fulltekst i vitenarkiv
• Garcia, Odd Erik; Leer, Egil; Pecseli, Hans & Trulsen, Jan Karsten (2015). Magnetic field-aligned plasma currents in gravitational fields. Annales Geophysicae. ISSN 0992-7689. 33(3), s. 257–266. doi: 10.5194/angeo-33-257-2015. Fulltekst i vitenarkiv Vis sammendrag

  Analytical models are presented for currents along vertical magnetic field lines due to slow bulk electron motion in plasmas subject to a gravitational force. It is demonstrated that a general feature of this problem is a singularity in the plasma pressure force that develops at some finite altitude when a plasma that is initially in static equilibrium is set into slow motion. Classical fluid models thus do not allow general steady-state solutions for field-aligned currents. General solutions have to be non-stationary, varying on time scales of many periods of a plasma equivalent to the Brunt–Väisälä frequency. Except for very special choices of parameters, a steady-state solution exists only in an average sense. The conditions at large altitudes turn out to be extremely sensitive to even small changes in parameters at low altitudes. Low frequency fluctuations detected at large altitudes in the polar regions need not be caused by local low frequency instabilities, but merely reflect small fluctuations in conditions at low altitudes.

• Pecseli, Hans (2015). Spectral properties of electrostatic drift wave turbulence in the laboratory and the ionosphere. Annales Geophysicae. ISSN 0992-7689. 33(7), s. 875–900. doi: 10.5194/angeo-33-875-2015. Fulltekst i vitenarkiv
• Bergsaker, Anne Schad; Fredriksen, Åshild; Pecseli, Hans & Trulsen, Jan Karsten (2014). Analytical models for skewness-kurtosis relations in turbulent transport, 41st EPS Conference on Plasma Physics. European Physical Society. ISSN 9782914771900. Fulltekst i vitenarkiv
• Pecseli, Hans (2014). Modulational stability of electron plasma wave spectra. Journal of Plasma Physics. ISSN 0022-3778. 80(5), s. 745–769. doi: 10.1017/S002237781400035X. Fulltekst i vitenarkiv
• Pecseli, Hans; Trulsen, Jan Karsten & Fiksen, Øyvind (2014). Predator-prey encounter and capture rates in turbulent environments. Limnology and Oceanography. ISSN 0024-3590. 4, s. 85–105. doi: 10.1215/21573689-2768717. Fulltekst i vitenarkiv
• Børve, Steinar; Sato, Hiroatsu; Pecseli, Hans & Trulsen, Jan Karsten (2014). Low frequency oscillations of the magnetosphere, 2014 XXXIst URSI General Assembly and Scientific Symposium (URSI GASS 2014). Curran Associates, Inc.. ISSN 9781467352246. s. 3299–3301. doi: 10.1109/URSIGASS.2014.6929936. Fulltekst i vitenarkiv
• Garcia, Odd Erik & Pecseli, Hans (2013). Models for electrostatic drift waves with density variations along magnetic field lines. Geophysical Research Letters. ISSN 0094-8276. 40(21), s. 5565–5569. doi: 10.1002/2013GL057802. Fulltekst i vitenarkiv Vis sammendrag

  Drift waves with vertical magnetic fields in gravitational ionospheres are considered where the unperturbed plasma density is enhanced in a magnetic flux tube. The gravitational field gives rise to an overall decrease of plasma density for increasing altitude. Simple models predict that drift waves with finite vertical wave vector components can increase in amplitude merely due to a conservation of energy density flux of the waves. Field-aligned currents are some of the mechanisms that can give rise to fluctuations that are truly unstable. We suggest a self-consistent generator or “battery” mechanism that in the polar ionospheres can give rise to magnetic field-aligned currents even in the absence of electron precipitation. The free energy here is supplied by steady state electric fields imposed in the direction perpendicular to the magnetic field in the collisional lower parts of the ionosphere or by neutral winds that have similar effects.

• Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten (2013). Unstable ring-shaped ion distribution functions induced by charge–exchange collisions. Plasma Physics and Controlled Fusion. ISSN 0741-3335. 55(12). doi: 10.1088/0741-3335/55/12/124006. Fulltekst i vitenarkiv
• Rekaa, Vegard Lundby; Pecseli, Hans & Trulsen, Jan Karsten (2013). Self-similar space-time evolution of an initial density discontinuity. Physics of Plasmas. ISSN 1070-664X. 20(7). doi: 10.1063/1.4816953. Fulltekst i vitenarkiv Vis sammendrag

  The space-time evolution of an initial step-like plasma density variation is studied. We give particular attention to formulate the problem in a way that opens for the possibility of realizing the conditions experimentally. After a short transient time interval of the order of the electron plasma period, the solution is self-similar as illustrated by a video where the space-time evolution is reduced to be a function of the ratio x/t. Solutions of this form are usually found for problems without characteristic length and time scales, in our case the quasi-neutral limit. By introducing ion collisions with neutrals into the numerical analysis, we introduce a length scale, the collisional mean free path. We study the breakdown of the self-similarity of the solution as the mean free path is made shorter than the system length. Analytical results are presented for charge exchange collisions, demonstrating a short time collisionless evolution with an ensuing long time diffusive relaxation of the initial perturbation. For large times, we find a diffusion equation as the limiting analytical form for a charge-exchange collisional plasma, with a diffusion coefficient defined as the square of the ion sound speed divided by the (constant) ion collision frequency. The ion-neutral collision frequency acts as a parameter that allows a collisionless result to be obtained in one limit, while the solution of a diffusion equation is recovered in the opposite limit of large collision frequencies. © 2013 AIP Publishing LLC.

• Miloch, Wojciech Jacek; Yaroshenko, V.; Vladimirov, Sergei V.; Pecseli, Hans & Trulsen, Jan Karsten (2012). Spacecraft charging in flowing plasmas; numerical simulations. Journal of Physics: Conference Series (JPCS). ISSN 1742-6588. 370. doi: 10.1088/1742-6596/370/1/012004. Fulltekst i vitenarkiv
• Sato, Hiroatsu; Pecseli, Hans & Trulsen, Jan Karsten (2012). Fluctuations in the direction of propagation of intermittent low-frequency ionospheric waves. Journal of Geophysical Research (JGR): Space Physics. ISSN 2169-9380. 117. doi: 10.1029/2011JA017242. Fulltekst i vitenarkiv
• Pecseli, Hans; Trulsen, Jan Karsten & Fiksen, Øyvind (2012). Predator-prey encounter and capture rates for plankton in turbulent environments. Progress in Oceanography. ISSN 0079-6611. 101(1), s. 14–32. doi: 10.1016/j.pocean.2011.12.001. Fulltekst i vitenarkiv
• Fattorini, L.; Fredriksen, Åshild; Pecseli, Hans; Riccardi, C. & Trulsen, Jan Karsten (2012). Turbulent transport in a toroidal magnetized plasma. Plasma Physics and Controlled Fusion. ISSN 0741-3335. 54(8). doi: 10.1088/0741-3335/54/8/085017. Fulltekst i vitenarkiv
• Rekaa, Vegard Lundby; Pecseli, Hans & Trulsen, Jan Karsten (2012). Numerical studies of a plasma diode with external forcing. Physics of Plasmas. ISSN 1070-664X. 19(8). doi: 10.1063/1.4747620. Fulltekst i vitenarkiv Vis sammendrag

  With reference to laboratory Q-machine 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 need 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 and give significant modifications of the diode performance. In particular we find a reduction in oscillator frequency, although a linear scaling of the oscillation time with the system length remains also in this case. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing.

• Daldorff, Lars Kristen Selberg; Pecseli, Hans; Trulsen, Jan Karsten; Ulriksen, Margit Iren; Eliasson, B. & Stenflo, Lennart (2011). Nonlinear beam generated plasma waves as a source for enhanced plasma and ion acoustic lines. Physics of Plasmas. ISSN 1070-664X. 18(5). doi: 10.1063/1.3582084. Fulltekst i vitenarkiv
• Sato, Hiroatsu; Pecseli, Hans & Trulsen, Jan Karsten (2011). Fluctuations in the Direction of Propagation of Low Frequency Ionospheric Waves. I Ouwehand, L (Red.), Proceedings of the 20th ESA Symposium on European Rocket and Balloon Programmes and Related Research. ESA Communications. ISSN 9789290922643. s. 625–628. Fulltekst i vitenarkiv
• Pecseli, Hans & Trulsen, Jan Karsten (2011). Encounter rates and transit time distributions for surfaces moving in turbulent flows. Journal of Physics: Conference Series (JPCS). ISSN 1742-6588. 318. doi: 10.1088/1742-6596/318/5/052034. Fulltekst i vitenarkiv
• Børve, Steinar; Sato, Hiroatsu; Pecseli, Hans & Trulsen, Jan Karsten (2011). Minute-scale period oscillations of the magnetosphere. Annales Geophysicae. ISSN 0992-7689. doi: 10.5194/angeo-29-663-2011. Fulltekst i vitenarkiv Vis sammendrag

  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 these oscillations and give a scaling law 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. The results agree well 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. We consider two tilt-angles of the magnetic dipole axis. We find 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 achieved.

• Guio, Patrick & Pecseli, Hans (2010). Collisionless Plasma Shocks in Striated Electron Temperatures. Physical Review Letters. ISSN 0031-9007. 104(8). doi: 10.1103/PhysRevLett.104.085002. Fulltekst i vitenarkiv
• Pecseli, Hans; Trulsen, Jan Karsten & Fiksen, Øyvind (2010). Predator-prey encounter rates in turbulent water: Analytical models and numerical tests. Progress in Oceanography. ISSN 0079-6611. 85(3-4), s. 171–179. doi: 10.1016/j.pocean.2010.01.002. Fulltekst i vitenarkiv
• Miloch, Wojciech Jacek; Rekaa, Vegard Lundby; Pecseli, Hans & Trulsen, Jan Karsten (2010). Ion acoustic double layers forming behind irradiated solid objects in streaming plasmas. Journal of Plasma Physics. ISSN 0022-3778. 76, s. 429–439. doi: 10.1017/S002237780999064X. Fulltekst i vitenarkiv
• Krane, Bård; Pecseli, Hans; Sato, Hiroatsu; Trulsen, Jan Karsten & Wernik, A. W. (2010). Low-frequency electrostatic waves in the ionospheric E region. Plasma Sources Science & Technology. ISSN 0963-0252. 19(3). doi: 10.1088/0963-0252/19/3/034007. Fulltekst i vitenarkiv
• Pecseli, Hans & Trulsen, Jan Karsten (2010). Transit times in turbulent flows. Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. ISSN 1539-3755. 81(4). doi: 10.1103/PhysRevE.81.046310. Fulltekst i vitenarkiv
• Jørgensen, H. E.; Mikkelsen, T. & Pecseli, Hans (2010). Concentration Fluctuations in Smoke Plumes Released Near the Ground. Boundary-Layer Meteorology. ISSN 0006-8314. 137(3), s. 345–372. doi: 10.1007/s10546-010-9532-x. Fulltekst i vitenarkiv
• Miloch, Wojciech Jacek; Vladimirov, Sergei V.; Pecseli, Hans & Trulsen, Jan Karsten (2009). Charging of insulating and conducting dust grains by flowing plasma and photoemission. New Journal of Physics. 11. doi: 10.1088/1367-2630/11/4/043005. Fulltekst i vitenarkiv
• Miloch, Wojciech Jacek; Vladimirov, Sergei V.; Pecseli, Hans & Trulsen, Jan Karsten (2009). Charging of Dust in the Presence of a Directed Photon Flux: Numerical Simulations. Journal of Plasma and Fusion Research Series. 8, s. 265–268. Fulltekst i vitenarkiv

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• Pecseli, Hans Laszlo (2025). Introduction to the Theory of Incoherent Scattering of Radar Waves from Plasmas. Springer Nature. ISBN 9783031826511. 107 s. Fulltekst i vitenarkiv
• Pecseli, Hans (2016). Low Frequency Waves and Turbulence in Magnetized Laboratory Plasmas and in the Ionosphere. IOP Publishing. ISBN 9780750312523. 631 s. Fulltekst i vitenarkiv
• Pecseli, Hans (2012). Waves and Oscillations in Plasmas. Taylor & Francis. ISBN 9781439878484. 553 s. Fulltekst i vitenarkiv

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• Jin, Yaqi; Spicher, Andres; Pecseli, Hans Laszlo; Clausen, Lasse; Miloch, Wojciech Jacek & Jacobsen, Knut Stanley [Vis alle 9 forfattere av denne artikkelen] (2024). In-situ observations of plasma blob inside the nightside auroral region and features of small-scale irregularities. Fulltekst i vitenarkiv
• Pecseli, Hans Laszlo (2024). Bohm sheaths and pre-sheaths in one, two and three spatial dimensions. Fulltekst i vitenarkiv Vis sammendrag

  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 (2024). Forward and inverse cascade conditions in turbulent incompressible fluids. Fulltekst i vitenarkiv Vis sammendrag

  For three dimensional turbulent incompressible flows the attention is usually given to the forward cascade, where energy is transported through the power spectrum from large to small scales. For two dimensional flows it is readily demonstrated that such a model is incomplete. It turns out, however, that an inverse cascade can be found even in three dimensional flows. The problem is addressed and illustrated by expanding the basic equations for three dimensional incompressible flows in terms of the eigenfunctions for the curl-operator. Conditions for dual energy cascades in wavenumber space can then be presented. Applications of the analysis can be found also in electron MHD plasma models accounting for whistler turbulence.

• Pecseli, Hans Laszlo (2024). Analysis of data from the EAST tokamak. Fulltekst i vitenarkiv
• Pecseli, Hans Laszlo (2023). Kinetically unstable ion velocity distributions formed via charge exchange collisions by E×B-drifts in magnetized plasmas. Fulltekst i vitenarkiv Vis sammendrag

  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. Fulltekst i vitenarkiv Vis sammendrag

  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. Fulltekst i vitenarkiv
• Pecseli, Hans Laszlo (2022). Cascade conditions in turbulent incompressible fluids. Fulltekst i vitenarkiv Vis sammendrag

  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. Fulltekst i vitenarkiv
• Pecseli, Hans Laszlo (2021). On the applicability of Taylor’s hypothesis, including small sampling velocities. Fulltekst i vitenarkiv
• Pecseli, Hans (2018). Predation in a turbulent environment. Fulltekst i vitenarkiv Vis sammendrag

  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. Fulltekst i vitenarkiv
• Pecseli, Hans (2016). Low frequency oscillations of the magnetosphere. Fulltekst i vitenarkiv
• Pecseli, Hans (2016). Low frequency oscillations of the Earth's magnetosphere. Fulltekst i vitenarkiv
• Pecseli, Hans (2016). Linear electrostatic drift waves (3 lectures), Nonlinear electrostatic drift waves (3 lectures). Fulltekst i vitenarkiv
• Pecseli, Hans & Trulsen, Jan Karsten (2015). Excess statistics with applications to turbulence in marine environments. Fulltekst i vitenarkiv
• Pecseli, Hans (2015). Nonlinear waves and turbulence. Fulltekst i vitenarkiv
• Pecseli, Hans (2015). Anomalous transport in a magnetized toroidal plasma. Fulltekst i vitenarkiv
• Pecseli, Hans (2015). Introduction to Electrostatic Drift Waves (4 lectures). Fulltekst i vitenarkiv
• Pecseli, Hans (2015). Weakly Nonlinear Waves in Plasmas (4 lectures). Fulltekst i vitenarkiv
• Pecseli, Hans (2015). Excess statistics with applications to turbulence in marine environments. Fulltekst i vitenarkiv Vis sammendrag

  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 (2015). Excess statistics with applications to anomalous transport in magnetized plasmas. Fulltekst i vitenarkiv
• Pecseli, Hans (2015). Excess statistics with applications to turbulence in marine enviroments. Fulltekst i vitenarkiv
• Pecseli, Hans (2015). Drift wave turbulence in laboratory and space plasmas. Fulltekst i vitenarkiv
• Børve, Steinar; Sato, Hiroatsu; Pecseli, Hans & Trulsen, Jan Karsten (2014). LOW FREQUENCY OSCILLATIONS OF THE MAGNETOSPHERE. Fulltekst i vitenarkiv
• Bergsaker, Anne Schad; Fredriksen, Åshild; Pecseli, Hans & Trulsen, Jan Karsten (2014). Analytical models for skewness-kurtosis relations in turbulent transport. Fulltekst i vitenarkiv
• Pecseli, Hans (2014). Excess statistics with applications to turbulence in marine environments. Fulltekst i vitenarkiv
• Pecseli, Hans (2014). Predator-prey encounters in turbulent environments: life on small scales in the oceans. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Pecseli, Hans (2013). Simple models for electrostatic drift waves and their relevance for the Earth’s ionosphere. Fulltekst i vitenarkiv Vis sammendrag

  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. Fulltekst i vitenarkiv
• Pecseli, Hans (2013). Dynamics of Collisional Plasmas in Crossed Electric and Magnetic Fields. Fulltekst i vitenarkiv
• Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten (2013). Kinetic plasma instabilities due to charge exchange collisions. Fulltekst i vitenarkiv
• Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten (2013). Numerical simulations of kinetic plasma instabilities induced by charge exchange collisions. Fulltekst i vitenarkiv
• Pecseli, Hans; Trulsen, Jan Karsten & Fiksen, Øyvind (2013). Predator-prey encounter and capture rates in turbulent environments. Fulltekst i vitenarkiv
• Pecseli, Hans; Miloch, Wojciech Jacek & Trulsen, Jan Karsten (2013). Kinetically unstable velocity distributions formed by charge exchange collisions with drifting ions. Fulltekst i vitenarkiv
• Sato, Hiroatsu; Pecseli, Hans; Børve, Steinar & Trulsen, Jan Karsten (2012). Dynamic models for magnetospheric oscillations on the minute scale. Fulltekst i vitenarkiv
• Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten (2012). Numerical simulations of collisional partially ionized plasmas. Fulltekst i vitenarkiv Vis sammendrag

  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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Pecseli, Hans (2012). Predator-prey encounters in turbulent environments - life on small scales in the oceans. Fulltekst i vitenarkiv
• Sato, Hiroatsu; Børve, Steinar; Pécseli, Hans & Trulsen, Jan Karsten (2012). Minute-scale period oscillations of the magnetosphere. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Pecseli, Hans (2011). Experimental studies of intermittency and turbulent transport in linear and toroidal magnetized plasmas. Fulltekst i vitenarkiv
• Miloch, Wojciech Jacek; Yaroshenko, V.; Vladimirov, Sergei V.; Pecseli, Hans & Trulsen, Jan Karsten (2011). Spacecraft charging in space and ionospheric plasma; numerical simulations. Fulltekst i vitenarkiv
• Rekaa, Vegard Lundby; Pecseli, Hans & Trulsen, Jan Karsten (2011). Numerical simulations of anomalous resistivity in the ion acoustic and Buneman regime. Fulltekst i vitenarkiv
• Pecseli, Hans & Trulsen, Jan Karsten (2011). Encounter rates and transit time distributions for surfacesmoving in turbulent flows. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten (2009). Interactions of two objects in flowing plasmas studied by numerical simulations. Fulltekst i vitenarkiv
• Miloch, Wojciech Jacek; Pecseli, Hans & Trulsen, Jan Karsten (2009). Numerical studies of ion focusing in streaming plasmas; two and three dimensions. Fulltekst i vitenarkiv
• Pecseli, Hans; Trulsen, Jan Karsten & Fiksen, Øyvind (2009). Predators in turbulent waters: an analytical model and a numerical test. Fulltekst i vitenarkiv

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