G. Paschmann

Max Planck Institute for Extraterrestrial Physics, Arching, Bavaria, Germany

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Publications (329)779.15 Total impact

  • A. Blagau · G. Paschmann · B. Klecker · O. Marghitu
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    ABSTRACT: Rotational discontinuities (RDs) are governed by two relations: the Walen relation predicting that the plasma velocity observed in the deHoffmann-Teller frame equals the local Alfven velocity and another relation that connects the variation in plasma mass density, rho, to variations in the pressure anisotropy factor, alpha, defined as alpha equivalent to (p(parallel to) - p(perpendicular to))mu(0)/B-2, so that rho(1-alpha) is constant. While the Walen relation has become a standard tool for classifying magnetopause crossings as RDs, the rho(1-alpha) = const. condition has never been directly verified at the same time, largely due to problems with determining rho when no ion composition measurements were available. In fact, to overcome the lack of composition information, the validity of the relation has often been assumed and the Walen relation reformulated so that variations in rho are replaced by variations in alpha. In this paper we exploit the availability of high-time-resolution composition measurements on the Cluster spacecraft to directly test the rho(1-alpha)=const. condition for three magnetopause crossings, identified as RDs from the application of the Walen relation to measurements of plasma ions and magnetic field by the CIS (Cluster Ion Spectrometry) and FGM (flux-gate magnetometer) instruments, respectively. We find that the relation is not fulfilled in either case. In one event, with a fairly large content of oxygen ions, the Walen test improved when the contribution from these ions was taken into account. Through comparisons of the measured ion densities with simultaneously measured total electron densities by the Waves of HIgh frequency and Sounder for Probing of Electron density by Relaxation (WHISPER) instrument, we were able to exclude the possibility that ion populations hidden to the CIS instrument because of their very low energies could have changed rho to match the rho(1-alpha) = const. condition. We also excluded the possibility that energetic ions above the CIS energy range could have sufficiently changed the true alpha. It thus appears that the rho(1-alpha) = const. condition, for reasons not presently understood, is not valid for the kind of RD-like structures we observe.
    No preview · Article · Jan 2015 · Annales Geophysicae
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    ABSTRACT: [1] We surveyed 79 magnetopause reconnection exhausts detected by the THEMIS spacecraft to investigate how the amount and anisotropy of electron bulk heating produced by reconnection depend on the inflow boundary conditions. We find that the amount of heating, ΔTe, is correlated with the asymmetric Alfvén speed, VAL,asym, based on the reconnecting magnetic field and the plasma density measured in both the high-density magnetosheath and low-density magnetospheric inflow regions. Best fit to the data produces the empirical relation ΔTe = 0.017 miVAL,asym2, indicating that the amount of heating is proportional to the inflowing magnetic energy per proton-electron pair, with ~1.7% of the energy being converted into electron heating. This finding, generalized to symmetric reconnection, could account for the lack of electron heating in typical solar wind exhausts at 1 AU, as well as strong heating to keV energies common in magnetotail exhausts. We also find that the guide field suppresses perpendicular heating.
    Full-text · Article · Sep 2013
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    G. Paschmann · S. Haaland · B. Sonnerup · T. Knetter
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    ABSTRACT: We examine the Alfvénicity of a set of 188 solar wind directional discontinuities (DDs) identified in the Cluster data from 2003 by Knetter (2005), with the objective of separating rotational discontinuities (RDs) from tangential ones (TDs). The DDs occurred over the full range of solar wind velocities and magnetic shear angles. By performing the Walén test in the de Hoffmann-Teller (HT) frame, we show that 77 of the 127 crossings for which a good HT frame was found had plasma flow speeds exceeding 80% of the Alfvén speed at an average angular deviation of 7.7°; 33 cases had speeds exceeding 90% of the Alfvén speed at an average angle of 6.4°. We show that the angular deviation between flow velocity (in the HT frame) and the Alfvén velocity can be obtained from a reduced form of the Walén correlation coefficient. The corresponding results from the Walén test expressed in terms of jumps in flow speed and corresponding jumps in Alfvén speed are similar: 66 of the same 127 cases had velocity jumps exceeding 80% with average angular deviation of 5.8°, and 22 exceeding 90% of the jump in Alfvén speed, with average angular deviation 6.2°. We conclude that a substantial fraction of the 127 events can be identified as RDs. We present further evidence for coupling across the DDs by showing that, for most of the 127 crossings, the HT frame velocities, evaluated separately on the two sides of the DD, are nearly the same - a result required for RDs but not for TDs. We also show that the degree of Alfvénicity is nearly the same for the DDs and fluctuations in which the DDs are embedded. Whatever process causes deviations from ideal Alfvénicity appears to operate equally for the DDs as for the surrounding fluctuations. Finally, our study has established a unique relation between the strahl electron pitch angle and the sign of the Walén slope, implying antisunward propagation in the plasma frame for all 127 cases.
    Preview · Article · May 2013 · Annales Geophysicae
  • Götz Paschmann · Marit Øieroset · Tai Phan
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    ABSTRACT: This paper gives an overview of the insights into the magnetic reconnection process obtained by in-situ measurements across current sheets found in planetary magnetospheres and the solar wind. Emphasis is placed on results that might be of interest to the study of reconnection in regions where no in-situ observations are available. These results include the role of symmetric versus asymmetric boundary conditions, the identification of the onset conditions, the reconnection rates, and the spatial and temporal scales. Special attention is paid to observations in the so-called diffusion region surrounding the reconnection sites, where ions and eventually also electrons become demagnetized and reconnection is initiated.
    No preview · Article · Feb 2013 · Space Science Reviews
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    ABSTRACT: We have performed a statistical study of THEMIS spacecraft crossings of the asymmetric dayside magnetopause to test the prediction that the diamagnetic drift of the X-line due to a plasma pressure gradient across the magnetopause can suppress magnetic reconnection. The study includes crossings both when reconnection exhausts were present and when they were absent in the current sheet. When we restrict the survey to the subsolar region (10 < MLT < 14), we find that for low Δβ (the difference of plasma β on the two sides of the current sheet) the majority of reconnection events occurred over a large range of magnetic shears, whereas when Δβ was high reconnection events occurred only for high shears. Furthermore, nonreconnection events occurred primarily in the Δβ-shear regime in which reconnection is predicted to be suppressed, in good agreement with theory. The Δβ-shear condition should have general consequences for the occurrence of reconnection in space and laboratory plasmas.
    Full-text · Article · Jan 2013 · Geophysical Research Letters
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    J Vogt · S Haaland · G Paschmann
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    ABSTRACT: Recent multi-spacecraft studies of solar wind dis-continuity crossings using the timing (boundary plane tri-angulation) method gave boundary parameter estimates that are significantly different from those of the well-established single-spacecraft minimum variance analysis (MVA) tech-nique. A large survey of directional discontinuities in Cluster data turned out to be particularly inconsistent in the sense that multi-point timing analyses did not identify any rota-tional discontinuities (RDs) whereas the MVA results of the individual spacecraft suggested that RDs form the majority of events. To make multi-spacecraft studies of discontinu-ity crossings more conclusive, the present report addresses the accuracy of the timing approach to boundary parameter estimation. Our error analysis is based on the reciprocal vec-tor formalism and takes into account uncertainties both in crossing times and in the spacecraft positions. A rigorous error estimation scheme is presented for the general case of correlated crossing time errors and arbitrary spacecraft con-figurations. Crossing time error covariances are determined through cross correlation analyses of the residuals. The prin-cipal influence of the spacecraft array geometry on the accu-racy of the timing method is illustrated using error formulas for the simplified case of mutually uncorrelated and identical errors at different spacecraft. The full error analysis proce-dure is demonstrated for a solar wind discontinuity as ob-served by the Cluster FGM instrument.
    Preview · Article · Dec 2011 · Annales Geophysicae
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    ABSTRACT: We report in-situ measurements by three THEMIS spacecraft showing the evolution of reconnection in a solar wind current sheet as the current sheet transited from the solar wind across the bow shock and close to the magnetopause on July 11, 2008. The observations suggest that the solar wind reconnection exhaust within the current sheet was disrupted by its interaction with the bow shock, while the subsequent compression of the current sheet against the magnetopause significantly reduced both the current sheet thickness and the plasma and initiated reconnection at a new X-line located within the magnetosheath. Furthermore, electrons were heated at the center of the magnetosheath exhaust, in contrast to the previously reported absence of electron heating in solar wind exhausts, but consistent with electron heating occasionally observed in association with magnetopause reconnection. This suggests that the level of electron heating in reconnection exhausts depends strongly on the boundary conditions.
    Full-text · Article · Sep 2011 · Geophysical Research Letters
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    ABSTRACT: We examine local structures of three directional discontinuities (DDs) observed by Cluster in the solar wind, using reconstruction based on the ideal 2-D MHD equations in a steady state. In this novel application of the technique, our goals are the following: (1) to explain why the minimum variance analysis of the magnetic field (MVAB) often fails to meaningfully predict the vector normal to a DD and (2) to use the reconstructed field maps as an aid in interpreting the differences in the magnetic field profiles recorded by the four Cluster spacecraft. From the maps, we learn that the failure of MVAB as a predictor of the normal direction is due to internal structure such as magnetic islands (flux ropes) within the DDs and also that we can partly understand the differences in the fields observed by the four spacecraft. We find fairly good agreement between the normal directions determined from the four-point timing approach and from MVAB, provided the constraint < B(n)> = 0 is imposed on MVAB. Because of the island structures, the DDs cannot be readily identified as either tangential or rotational discontinuities, although the approximately Alfvenic flows on both sides favor the latter interpretation.
    Full-text · Article · Apr 2011 · Journal of Geophysical Research Atmospheres
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    ABSTRACT: We address the conditions for the onset of magnetic reconnection based on a survey of 197 reconnection events in solar wind current sheets observed by the Wind spacecraft. We report the first observational evidence for the dependence of the occurrence of reconnection on a combination of the magnetic field shear angle, {theta}, across the current sheet and the difference in the plasma {beta} values on the two sides of the current sheet, {Delta}{beta}. For low {Delta}{beta}, reconnection occurred for both low and high magnetic shears, whereas only large magnetic shear events were observed for large {Delta}{beta}: Events with shears as low as 11{sup 0} were observed for {Delta}{beta} < 0.1, but for {Delta}{beta} > 1.5 only events with {theta} > 100{sup 0} were detected. Our observations are in quantitative agreement with a theoretical prediction that reconnection is suppressed in high {beta} plasmas at low magnetic shears due to super-Alfvenic drift of the X-line caused by plasma pressure gradients across the current sheet. The magnetic shear-{Delta}{beta} dependence could account for the high occurrence rate of reconnection observed in current sheets embedded within interplanetary coronal mass ejections, compared to those in the ambient solar wind. It would also suggest that reconnection could occur at a substantially higher rate in solar wind current sheets closer to the Sun than at 1 AU and thus may play an important role in the generation and heating of the solar wind.
    Full-text · Article · Aug 2010 · The Astrophysical Journal Letters
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    B. U. Ö. Sonnerup · S. E. Haaland · Paschmann G
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    ABSTRACT: A theoretical model is proposed to account for some of the behavior of arc-polarized magnetic structures seen in the solar wind. To this end, an exact analytical solution is developed that describes infinite plane wave trains of arbitrary amplitude in a plasma governed by ideal Hall MHD. The main focus is on intermediate-mode wave trains, which display double-branched magnetic hodogram signatures similar to those seen in the solar wind. The theoretically derived hodograms have field rotation in the ion-polarized sense at a slightly depressed field magnitude on one branch and an electron-polarized rotation at a slightly enhanced field magnitude on the other branch. The two branches are joined at the two "turning points", at which the normal flow is exactly Alfvénic. The behavior is accounted for in terms of the opposite dispersive properties of ion and electron whistlers. The hodograms derived from the theory are shown to compare favorably with those of one event, observed by the Cluster spacecraft near the ecliptic plane, and one event at high heliographic latitude observed by the Ulysses spacecraft. However, these two observed structures comprise only a single full wave period, approximately from one turning point to the other and then back again. The theory can be used to predict propagation direction (away from, or towards, the sun) from magnetic data alone, provided the sign of the magnetic field component along the wave normal can be reliably determined. Under the same condition, it also predicts whether the ion-polarized branch should precede or follow the electron-polarized branch. Both behaviors are seen in the solar wind. The major shortcoming of the theory is that it fails to reproduce the observed saw-tooth like time series for the magnetic field, in which the field rotation is rapid in the ion sense and slow in the electron sense. Instead, the theory gives about the same rotation rates. Possible explanations for this discrepancy are discussed. Also discussed is the fact that the magnetic field measurements by Cluster, while giving high quality determinations of normal direction and normal field component for each of the four spacecraft, indicate a reversal of the normal field component and the predicted propagation sense during the event, as well as a wide spread in the four normal vector orientations.
    Preview · Article · Jun 2010 · Annales Geophysicae
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    ABSTRACT: We have performed a survey of the occurrence of reconnection at the dayside magnetopause observed by the THEMIS spacecraft. Reconnection signatures were absent in more than 50% of the high magnetic shear magnetopause crossings where reconnection might be expected. The controlling factors of whether reconnection occurs or not appear to be a combination of the magnetic shear across the magnetopause and the plasma beta in the adjacent magnetosheath. This beta and shear dependence of reconnection was also found in solar wind reconnection events observed by WIND and ACE, suggesting that this is a universal onset condition for reconnection.
    No preview · Article · May 2010
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    A. Blagau · B. Klecker · G. Paschmann · S. Haaland · O. Marghitu · M. Scholer
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    ABSTRACT: For a four-point mission like Cluster, the differences in position and time when the satellites detect the magnetopause or any other discontinuity, can be used to infer the discontinuity local orientation, thickness and motion. This timing technique, commonly assuming a planar geometry, offers an independent check for various single-spacecraft techniques. In the present paper we propose an extension of the timing method, capable of determining in a self-consistent way the macroscopic parameters of a two-dimensional, non-planar discontinuity. Such a configuration can be produced by a local bulge or indentation in the magnetopause, or by a large amplitude wave traveling on this surface, and is recognized in Cluster data when the single spacecraft techniques provide different individual normals contained roughly in the same plane. The model we adopted for the magnetopause assumes a layer of constant thickness of either cylindrical or parabolic shape, which has one or two degrees of freedom for the motion in the plane of the individual normals. The method was further improved by incorporating in a self-consistent way the requirement of minimum magnetic field variance along the magnetopause normal. An additional assumption, required in a previously proposed non-planar technique, i.e. that the non-planarity has negligible effects on the minimum variance analysis, is thus avoided. We applied the method to a magnetopause transition for which the various planar techniques provided inconsistent results. By contrast, the solutions obtained from the different implementations of the new 2-D method were consistent and stable, indicating a convex shape for the magnetopause. These solutions perform better than the planar solutions from the normal magnetic field variance perspective. The magnetopause dynamics and the presence of a non-zero normal magnetic field component in the analyzed event are discussed.
    Preview · Article · Mar 2010 · Annales Geophysicae
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    ABSTRACT: The Electron Drift Instrument (EDI) contribution to the Cluster Active Archive (CAA) is described. Presented are descriptions of the EDI instrument, the various CAA/EDI data products, the CAA ingestion schedule and the current EDI status. An example of a science application is given for one of the main EDI data products available in the CAA.
    No preview · Article · Jan 2010
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    ABSTRACT: The analysis of boundaries in space plasmas has received a lot of attention since the beginning of space age. A most popular approach to boundary parameter estimation from single-spacecraft measurements is minimum variance analysis (MVA). The physical model underlying the MVA technique suggests that it should work well for locally planar discontinuities in magnetic field data. The MVA technique, however, was recently challenged by a comprehensive study of solar wind discontinuities based on data from ESA's Cluster mission [Knetter et al., JGR 2004]. Such multi-spacecraft missions offer an independent avenue to boundary parameter estimation through the so-called timing or triangulation method. In many cases, discontinuity normal vectors obtained with the timing approach turn out to be inconsistent with the MVA normals. At the International Space Science Institute in Bern a team has formed to look at the problem in detail. This presentation will give a brief account of the team effort and show selected results. We are going to focus on the problem of lag time estimation, and on the accuracy of timing normals in terms of configurational parameters of the spacecraft array.
    No preview · Article · Jan 2010
  • S. Haaland · M. Foerster · G. Paschmann · R. B. Torbert · H. Vaith
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    ABSTRACT: Recent observations have shown that the ionospheric response to processes in the magnetosphere can be very dissimilar in the northern and southern hemispheres. In this paper we present a statistical study of ionospheric convection patterns obtained from 7 years of electric field observations from the Cluster mission. The results show some prominent asymmetries between the two hemispheres, but most of the differences can probably be attributed to ionospheric conductivities. The results also demonstrate that magnetospheric convection is not simply the result of processes in the magnetospheric boundaries and the magnetotail, but that it is modified and partly controlled by ionospheric effects.
    No preview · Article · Dec 2009
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    ABSTRACT: We have performed a survey of the occurrence of reconnection at the dayside magnetopause in 2008. Reconnection signatures were absent in more than 50% of the high magnetic shear magnetopause crossings where reconnection might be expected. The key controlling factor of whether reconnection occurs or not appears to be the plasma beta in the magnetosheath adjacent to the magnetopause, with the beta being substantially larger for non-reconnection events. The average magnetosheath beta was significantly larger in 2008 than in non-solar minimum years. We attribute some of the unusually high magnetosheath beta in 2008 to the low magnetic field strength in the solar wind during this solar minimum. Our finding suggests a lower rate of solar wind mass and energy transfer into the magnetosphere via reconnection during this particular solar minimum.
    No preview · Article · Dec 2009
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    ABSTRACT: The recent discovery of reconnection in solar wind current sheets has provided an unprecedented opportunity to study the structure and dynamics of reconnection without the complex boundary conditions often found in the Earth's magnetosphere. Studies of solar wind reconnection exhausts have revealed clear evidence for component reconnection with magnetic shear as low as 15 degrees, for reconnection X-lines extending tens of thousands of ion skin depths and remaining quasi-steady over thousands of ion gyroperiods. In this presentation we discuss the results of a survey of Wind, ACE, and THEMIS data which indicate that the occurrence of reconnection in solar wind current sheets depends on both the plasma beta and the magnetic shear. At low beta reconnection occurs for a large range of magnetic shear whereas at high beta, reconnection exhausts are detected only for large magnetic shear. The dependence on plasma beta is also evident from the finding that reconnection, present in the convecting solar wind current sheets, often ceases downstream of the bow shock where the plasma beta is enhanced.
    No preview · Article · Dec 2008
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    Götz Paschmann
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    ABSTRACT: 1] The paper presents a brief review of recent in-situ observations of reconnection in space, with emphasis on results pertaining to the question of anti-parallel versus component reconnection, the implied spatial and temporal scales, the location of the reconnection sites, particle acceleration, reconnection rates, the dependence on plasma b, and the properties of the diffusion region.
    Preview · Article · Oct 2008 · Geophysical Research Letters
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    ABSTRACT: In this study, we investigate statistical, systematic variations of the high-latitude convection cell structure during northward IMF. Using 1-min-averages of Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for six and a half years (February 2001 till July 2007), and mapping the spatially distributed measurements to a common reference plane at ionospheric level in a magnetic latitude/MLT grid, we obtained regular drift patterns according to the various IMF conditions. We focus on the particular conditions during northward IMF, where lobe cells at magnetic latitudes >80° with opposite (sunward) convection over the central polar cap are a permanent feature in addition to the main convection cells at lower latitudes. They are due to reconnection processes at the magnetopause boundary poleward of the cusp regions. Mapped EDI data have a particular good coverage within the central part of the polar cap, so that these patterns and their dependence on various solar wind conditions are well verified in a statistical sense. On average, 4-cell convection pattern are shown as regular structures during periods of nearly northward IMF with the tendency of a small shift toward negative clock angles. The positions of these high-latitude convection foci are within 79° to 85° magnetic latitude and 09:00 15:00 MLT. The MLT positions are approximately symmetric ±2 h about 11:30 MLT, i.e. slightly offset from midday toward prenoon hours, while the maximum (minimum) potential of the high-latitude cells is at higher magnetic latitudes near their maximum potential difference at ≍-10° to -15° clock angle for the North (South) Hemisphere. With increasing clock angle distances from ≍IMFBz+, a gradual transition occurs from the 4-cell pattern via a 3-cell to the common 2-cell convection pattern, in the course of which one of the medium-scale high-latitude dayside cells diminishes and disappears while the other intensifies and merges with the opposite main cell of the same polarity to form the large "round-shaped" convection cell when approaching a well-known IMFBy-dominated configuration. Opposite scenarios with interchanged roles of the respective cells occur for the opposite turning of the clock angle and at the Southern Hemisphere. The high-latitude dayside cells become more pronounced with increasing magnitude of the IMF vector.
    No preview · Article · Sep 2008 · Annales Geophysicae
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    ABSTRACT: We investigate an event with oscillations of electron counts at 500 eV, with a period of ∼3 s measured by the electron drift instrument (EDI) on Cluster. These oscillations are detected at the overshoot of a quasi-perpendicular bow shock and in its downstream region. During this event, the Alfvén Mach number is supercritical, and the proton β is >1. The correlation of the electron counts between three pitch angle ranges (∼10°, ∼90°, and ∼170°) is in-phase. However, the correlation between the electron counts and the magnitude of the magnetic fields is antiphase. As EDI has a higher time resolution than that of conventional particle instruments, the correlation at a period of ∼3 s is examined for the first time. The oscillations are inferred as mirror mode waves with oblique propagation angles to the magnetic field. The oscillation of electron counts in the parallel direction might indicate the oscillation of the parallel electric potential. We have estimated this electric potential by Liouville's mapping, taking into account the shape of the electron distribution function as well as using the linear hot plasma dispersion relation. We have found that this kind of parallel electric potential is related to heating of particles with a comparable magnitude as the cross-shock potential. The elliptical polarization of these waves indicates a possible interaction with gyrating particles so that the waves would be related to the particle dynamics. Finally, in-phase correlation between parallel and perpendicular counts is explained in terms of Liouville's mapping.
    Full-text · Article · Aug 2008 · Journal of Geophysical Research: Space Physics

Publication Stats

15k Citations
779.15 Total Impact Points


  • 2-2015
    • Max Planck Institute for Extraterrestrial Physics
      Arching, Bavaria, Germany
  • 2008
    • University of Bergen
      • Department of Physics and Technology
      Bergen, Hordaland, Norway
  • 2006
    • University of Oslo
      Kristiania (historical), Oslo, Norway
  • 1999-2005
    • International Space Science Institute
      Berna, Bern, Switzerland
  • 1980-2002
    • Max Planck Institute of Physics
      München, Bavaria, Germany
  • 1996
    • Dartmouth College
      • Thayer School of Engineering
      Hanover, New Hampshire, United States
  • 1992
    • Technische Universität Braunschweig
      Brunswyck, Lower Saxony, Germany
  • 1976-1990
    • Max Planck Institute for Astrophysics
      Arching, Bavaria, Germany
    • Imperial College London
      Londinium, England, United Kingdom
  • 1987-1989
    • University of Crete
      • Department of Physics
      Retimo, Crete, Greece
  • 1988
    • University of California, San Diego
      San Diego, California, United States
  • 1986
    • University of Washington Seattle
      Seattle, Washington, United States
    • University of California, Berkeley
      Berkeley, California, United States
  • 1978-1986
    • University of California, Los Angeles
      Los Ángeles, California, United States
  • 1985
    • University of Iowa
      • Department of Physics and Astronomy
      Iowa City, IA, United States
  • 1980-1985
    • Los Alamos National Laboratory
      Лос-Аламос, California, United States
  • 1983
    • Palo Alto Research Center
      Palo Alto, California, United States
  • 1982
    • Ruhr-Universität Bochum
      Bochum, North Rhine-Westphalia, Germany