M. Scholer

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

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Publications (359)689.68 Total impact

  • Shuichi Matsukiyo, Manfred Scholer
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    ABSTRACT: The microstructure of the heliospheric termination shock and the accompanied local acceleration processes of both ions and electrons are investigated by utilizing one-dimensional full particle-in-cell simulations for a variety of parameters. The relative pickup ion density is assumed to be 20-30%. The magnetic field and the shock potential profiles exhibit significant differences, since the former mostly reflects the dynamics of solar wind ions whereas the latter is mainly sustained by the bulk motion of the reflected pickup ions in the extended foot. The discrepancy between the magnetic field profile and the potential profile increases with Alfvén Mach number. Most of the downstream thermal energy is gained by the pickup ions, while some heating of the solar wind ions and electrons occurs through the modified two-stream instability excited in the extended foot. Self-reformation can occur when the relative pickup ion density is 20%, but is blurred when it becomes as large as 30%. Reformation is also suppressed if the local solar wind ion temperature in the extended foot is high, which can either be due to heating by the modified two-stream instability or is already determined by the solar wind temperature far upstream. In all runs presented in this study no evidence for shock surfing acceleration of pickup ions could be found. Non-thermal particle acceleration occurs for oblique shocks. Electron (pickup ion) shock drift acceleration is evidenced when the shock angle is below 80° (60°).
    Journal of Geophysical Research: Space Physics. 04/2014;
  • D. Burgess, M. Scholer
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    ABSTRACT: Shocks in collisionless plasmas require dissipation mechanisms which couple fields and particles at scales much less than the conventional collisional mean free path. For quasi-parallel geometries, where the upstream magnetic field makes a small angle to the shock normal direction, wave-particle coupling produces a broad transition zone with large amplitude, nonlinear magnetic pulsations playing an important role. At high Mach numbers, ion reflection and acceleration are dominant processes which control the structure and dissipation at the shock. Accelerated particles produce a precursor, or foreshock, characterized by low frequency magnetic waves which are convected by the plasma flow into the shock transition zone. The interplay between energetic particles, waves, ion reflection and acceleration leads to a complicated interdependent system. This review discusses the spacecraft observations which have motivated the current view of the high Mach number quasi-parallel shock, and the theories and simulation studies which have led to a better understanding of the microphysics on which the quasi-parallel shock depends.
    Space Science Reviews 03/2013; · 5.52 Impact Factor
  • Shuichi Matsukiyo, Manfred Scholer
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    ABSTRACT: A one-dimensional full particle-in-cell (PIC) code is utilized to investigate energetic electron bursts produced at a nonstationary quasi-perpendicular shock. A number of electrons are intermittently energized by interacting with nonstationary electromagnetic fields in the shock front. Some of the energetic electrons are reflected at the shock and form an upstream non-thermal population. The reflection process is strongly affected by the non-coplanar magnetic field component which is temporarily rather strong in the transition region of a highly nonstationary shock. Oblique whistler waves in the transition region influence the distribution function of the reflected electrons. Waves excited by the modified two-stream instability may pitch angle scatter the electrons and thus blur the loss cone feature of the reflected electrons. Dispersive standing whistler waves are also emitted locally in the foot even when a Mach number exceeds a critical value. These whistler waves may also scatter the electrons to blur the loss cone. Furthermore, the whistler waves produce clumps of the reflected electrons in a phase space. Some electrons are trapped by the ion holes produced downstream as a remnant of a self-reformation process of the shock front and accelerated through a drift mechanism. It is also discussed how physical quantities associated with the reflected electrons observed upstream of the shock can give information about the shock front nonstationarity as well as about local small scale wave activities in the transition region.
    Journal of Geophysical Research 11/2012; 117(A11):11105-. · 3.17 Impact Factor
  • M. Scholer, H. Comisel
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    ABSTRACT: A high Mach number quasi-perpendicular collisionless shock is studied with one-dimensional (1-D) full particle simulations. The Alfven Mach number is M_A=22, the shock normal-magnetic field angle is Θ=85 and the ion and electron beta (particle to magnetic pressure) is 0.5. We have used in the simulations a large value for the ratio of the electron plasma frequency to the gyrofrequency of ω_pe/Ω_ce=20, and a high value of the ion to electron mass ratio, (m_i/m_e=1500). The shock is highly non-stationary but does not exhibit the reformation pattern seen in previous simulations of lower Mach number perpendicular or quasi-perpendicular shocks. The magnetic field profiles flattens and steepens with a time period of 1.4-1.5 inverse ion gyrofrequencies while the ions are specular reflected from the steepened ramp and finally return downstream just at the subsequent steepening of the ramp. The scale of the ramp varies between ~ 10 to ~ 20 electron inertial lengths corresponding to the changes from a steep to a flat profile. By tracing all trajectories of the reflected ions in the simulation box we have determined the absolute reflection rate as well as an average energy gain related to the non-stationarity of the shock ramp. The reflection rate varies between almost zero percent during flat profiles and ~ 100 percent during steep profiles.
    AGU Fall Meeting Abstracts. 12/2011;
  • H. Comisel, M. Scholer
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    ABSTRACT: The electron dynamics through a quasi-perpendicular shock is studied with one-dimensional (1-D) full particle simulations. Assuming the conservation of the phase space density along its characteristic and following the traces of each electron in the simulation box, the actual distribution functions determined in regions of the foot, ramp and overshoot of the shock are compared with the Liouville mapped upstream distributions. The Liouville mapping is significantly improved if the particles still conserve their adiabaticity while crossing the shock. We then mapped the distribution functions for the particles obeying a given deviation of the magnetic momentum from the upstream initial value. The larger the spread of the magnetic momentum the bigger is the difference between the mapped and the actual distribution functions. The Liouville mapping procedure is often used in deriving the cross shock potential during bow shock crossings under the assumption of adiabaticity and energy conservation in the de Hoffmann-Teller frame. We compare the predictions of the cross shock potential obtained from the Liouville mapping and the computed potential in the simulation code and discuss the implication for deriving the cross shock potential from in situ measurements.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: In our study we analyze several individual upstream events observed by Cluster in the foreshock region. The region in front of the Earth's quasi-parallel bow shock is dominated by energetic particles and low-frequency, large-amplitude magnetic waves. The energetic ions are scattered in pitch-angle by these waves while the waves are thought to be excited locally by these energetic ions. The theoretical model of this coupling between waves and energetic particles has been verified in the past and proved to describe the physical process accurately. In our analysis we compare the predictions of the theory at each event with the observations by focusing on the two aspects of the wave-particle coupling: the excitation of waves by energetic particles and the pitch-angle scattering of energetic ions by waves.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: We have performed full particle electromagnetic simulations of a quasi-perpendicular shock. The shock parameters have been chosen to be appropriate for the quasi-perpendicular Earth's bow shock observed by Cluster on 24 January 2001 (Lobzin et al., 2007). We have performed two simulations with different ion to electron mass ratio: run 1 with m i/ m e=1840 and run 2 with m i/ m e=100. In run 1 the growth rate of the modified two-stream instability (MTSI) is large enough to get excited during the reflection and upstream gyration of part of the incident solar wind ions. The waves due to the MTSI are on the whistler mode branch and have downstream directed phase velocities in the shock frame. The Poynting flux (and wave group velocity) far upstream in the foot is also directed in the downstream direction. However, in the density and magnetic field compression region of the overshoot the waves are refracted and the Poynting flux in the shock frame is directed upstream. The MTSI is suppressed in the low mass ratio run 2. The low mass ratio run shows more clearly the non-stationarity of the shock with a larger time scale of the order of an inverse ion gyrofrequency (Ωci): the magnetic field profile flattens and steepens with a period of ~1.5Ωci−1. This non-stationarity is different from reformation seen in previous simulations of perpendicular or quasi-perpendicular shocks. Beginning with a sharp shock ramp the large electric field in the normal direction leads to high reflection rate of solar wind protons. As they propagate upstream, the ion bulk velocity decreases and the magnetic field increases in the foot, which results in a flattening of the magnetic field profile and in a decrease of the normal electric field. Subsequently the reflection rate decreases and the whole shock profile steepens again. Superimposed on this 'breathing' behavior are in the realistic mass ratio case the waves due to the MTSI. The simulations lead us to a re-interpretation of the 24 January 2001 bow shock observations reported by Lobzin et al. (2007). It is suggested that the high frequency waves observed in the magnetic field data are due to the MTSI and are not related to a nonlinear phase standing whistler. Different profiles at the different spacecraft are due to the non-stationary behavior on the larger time scale.
    Annales Geophysicae 01/2011; · 1.52 Impact Factor
  • Source
    Shuichi Matsukiyo, Manfred Scholer
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    ABSTRACT: Microstructure of the heliospheric termination shock is investigatedPIC simulations on quasi-perpendicular shocks including pickup ionsLarge shock potential in the extended foot due to reflected pickup ions
    Journal of Geophysical Research 01/2011; 116. · 3.17 Impact Factor
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    ABSTRACT: We analyze simultaneous multipoint observations of several diffuse ion events observed upstream of Earth's quasi-parallel bow shock. Each upstream ion event is treated individually in order to determine the similarities and differences of the physical process of energetic ion scattering as a function of interplanetary magnetic field and plasma conditions. The data were provided by the CIS and the FGM instruments onboard Cluster SC1 and SC3 at times of large (i.e., 1-1.5 Re) interspacecraft separation distance. The diffuse ion partial density gradients were determined at different distances from the quasi-parallel bow shock along the magnetic field lines and the e-folding distances (and the diffusion coefficients) were calculated. Our results show that during times of quiet interplanetary magnetic field (IMF) conditions (i.e., when the IMF presents a substantial directional stability) the field aligned beam (FAB) intensity is high which has an impact on the diffuse ion scattering process. We demonstrate that high intensity FAB generated waves are convected deep in the foreshock region and scatter especially the lower energy (i.e., 10-18 keV) diffuse ions efficiently which results in unusually low e-folding distance (and diffusion coefficient). These new results reveal the complexity of the energetic ion scattering process in the foreschock region and help to understand the process of ion acceleration at Earth's bow shock.
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: We present simultaneous multipoint observations of diffuse ions in front of the Earth's quasi-parallel bow shock. For the analysis we use data provided by the Cluster CIS-HIA particle instrument and data from FGM magnetic field instrument. Several individual diffuse ion events during various solar wind conditions are presented and analysed. The diffusion coefficients at each analysed upstream ion event present unique characteristics especially at lower diffuse ion energies (around 10 keV). We analyse in detail the reasons for the observed differences in the value of the diffusion coefficient; results are also compared with predictions of the theory and the reason for the eventual difference is explained.
    05/2010;
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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.
    Annales Geophysicae 03/2010; · 1.52 Impact Factor
  • M. Scholer, H. Comisel
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    ABSTRACT: Based on Cluster magnetic field and particle data obtained during a quasi-perpendicular bow shock crossing on 24 January 2001 Lobzin et al. (GRL 34, 2007) have concluded that the shock is nonstationary and reforming on the time scale of the ion gyroperiod. The argument for shock reformation is mainly based on the different magnetic field profiles observed by different spacecraft when traversing the bow shock. We have performed one-dimensional full-particle simulations for parameters appropriate to the 24 January 2001 shock crossing with the physical ion to electron mass ratio (it should be noted that the actual ion beta in the solar wind is ~ 0.6 as compared to 2.0 given by Lobzin et al.). The simulation exhibits large amplitude small wavelength waves in the foot, ramp, and overshoot simultaneously with vortices in the ion phase space of the incoming ions indicating the excitation of the modified two stream instability between incoming ions and electrons. These vortices lead to nonstationarity on a considerably smaller time scale than the ion gyroperiod. We have flown two closely spaced artificial satellite from upstream through the foot and ramp to downstream and have recorded the magnetic field profile during shock traversal. Due to the slight inherent motion of the ramp the two spacecraft measure, after low-pass filtering the data, different magnetic profiles. This is very similar to what has been observed after filtering the data obtained from the actual bow shock crossing. It is concluded that during the 24 January 2001 crossing the bow shock is actually not reforming, but nonstationary on a time scale much smaller than the ion gyroperiod. This nonstationarity can not be seen in the filtered spacecraft data. The difference of the large scale magnetic field profiles measured aboard different Cluster spacecraft is probably due to a combination of small back and forth motion of the bow shock and small temporal changes of the shock structure, but is not necessarily prove for shock reformation.
    AGU Fall Meeting Abstracts. 12/2009;
  • H. Comisel, M. Scholer
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    ABSTRACT: The Liouville mapping technique was applied in order to determine the evolution of the electron distribution function through high Mach number quasi-perpendicular shocks. The method is based on following a given collection of particles from the far upstream region to the foot, ramp, and overshoot of the shock. For this purpose, a one-dimensional particle in cell code was modified in order to keep track of the orbit of a large number of selected particles during the simulation run. Assuming that the distribution is constant along a particle trajectory we then map the upstream distribution to any point in the foot, ramp or downstream of the shock (exact Liouville mapping). These distributions are compared with the actual distribution obtained at a certain position relative the shock during the simulation. Furthermore, we have applied a second Liouville mapping procedure assuming that particle trajectories can be derived from the assumption of adiabaticity and energy conservation in the de Hoffmann-Teller frame (adibatic Liouville mapping). This method is often used in order to derive the cross shock potential from the observed evolution of the electron distribution function during bow shock crossings. We compare the prediction of the exact Liouville mapping, of the adiabatic Liouville mapping and the actual local distribution function and discuss the implication for deriving the cross shock potential from in situ electron measurements.
    AGU Fall Meeting Abstracts. 12/2009;
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    ABSTRACT: Because of the apparent difficulty in accelerating previously unaccelerated pickup ions locally at the solar wind termination shock, a model for anomalous cosmic-ray protons is considered in which the initial acceleration of pickup ions to ~10-20 MeV occurs in the inner heliosphere. These accelerated pickup ions are assumed to be accelerated either by propagating interplanetary shocks or by some other, unspecified process. Voyager 2 Low-Energy Charged Particle (LECP) observations at low energies are used to normalize their spectra. The ions are then transported to the termination shock, where they are further accelerated to anomalous cosmic-ray energies. We use a well-established transport model to simulate this process. In this model, the two-dimensional cosmic-ray transport equation is solved using the assumed (and normalized) source of energetic particles (~0.01-20 MeV) located at 10 AU. We show that the computed spectra at higher energies (100 MeV) are consistent with observed anomalous cosmic-ray fluxes and suggest that interplanetary shocks, especially those in the inner heliosphere, may play an important role in the initial acceleration of pickup ions leading to anomalous cosmic rays.
    The Astrophysical Journal 01/2009; 486(1):471. · 6.73 Impact Factor
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    ABSTRACT: The High-Energy Suprathermal Time-of-Flight sensor (HSTOF) of the Charge, Element, and Isotope Analysis System (CELIAS) on the Solar and Heliospheric Observatory (SOHO) near the Lagrangian point L1 is capable of identifying energetic hydrogen atoms (EHAs) between 55 and 80 keV. Between 1996 February 13 and 1997 August 31, near solar minimum, there were 285 "quiet" days when the interplanetary charged-particle flux was low. During these quiet times, HSTOF scanned the apex of the heliosphere once and the antiapex twice. The flux level and time profile, and hence the arrival direction, of the EHAs accumulated during these quiet times are best interpreted as fluxes of EHAs coming from the heliosheath.
    The Astrophysical Journal 01/2009; 503(2):916. · 6.73 Impact Factor
  • M. Scholer, H. Comisel
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    ABSTRACT: Based on Cluster magnetic field and particle data obtained during a quasi-perpendicular bow shock crossing on 24 January 2001 Lobzin et al. (GRL 34, 2007) have concluded that the shock is highly nonstationary. We have performed one-dimensional full particle simulations for parameters appropriate to the 24 January 2001 shock crossing with a realistic ion to electron mass ratio. The shock exhibits large amplitude whistler waves with downstream directed phase velocities and with a frequency close to 2Hz. The Poynting flux is directed into the upstream direction. Such high frequency fluctuations have also been observed in the Cluster magnetic field data. However, in the simulations we do not obtain lower frequency waves characteristic of a nonlinear whistler train embedded in the shock front which eventually results in shock reformation, i.e., the shock is steady and non-reforming. When putting an artificial spacecraft into the simulation which moves through the shock with a constant velocity we obtain magnetic field profiles similar to the observed ones with multiple peaks in the shock transition region. However these peaks are not related to a nonlinear whistler but are due to slight back and forth motions of the ramp/overshoot. It is concluded that either (1) the reformation process during this crossing is due to a two-dimensional effect not adequately described by the 1-D simulation or (2) this shock was not reforming and the different profiles are due to small scale motions of the shock relative to the spacecraft.
    AGU Fall Meeting Abstracts. 12/2008;
  • A. Kis, M. Scholer, E. Lucek, B. Klecker
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    ABSTRACT: Cluster magnetic field and energetic particle data have been used in order to determine the relation between the power in the low frequency magnetic field fluctuations and the energetic ion density upstream of the quasi-parallel bow shock. In particular, we have determined the distance dependence of (1) the power in the frequency range where the waves are resonant with field-aligned beam (FAB) particles, (2) of the power in transverse fluctuations in the frequency range where the waves are resonant with diffuse energetic ions and (3) of the power in compressive fluctuations in the frequency range resonant with the diffuse ions. The power in the transverse fluctuations is about constant as a function of distance along the magnetic field from the spacecraft location to the magnetic field - bow shock intersection point, whereas the power in compressive fluctuations increases exponentially with decreasing distance. The power in fluctuations resonant with FAB particles is constant as a function of distance from the bow shock. It is suggested that the power in transverse fluctuations over a wide frequency range is due to FAB instabilities in the region closer to the foreshock boundary. These waves are convected by the solar wind into the quasi-parallel regime where they are responsible for scattering of diffuse ions. As these transverse fluctuations are convected into a regime with increasing diffuse ion density they are converted partly into compressive fluctuations.
    AGU Fall Meeting Abstracts. 12/2008;
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    ABSTRACT: Collisionless shocks can be found in many astrophysical settings but the Earth's environment is easiest to access. The foreshock region of the Earth's bow shock is populated by a variety of waves and particle populations allowing us to study wave–particle interactions. In the parallel and quasi-parallel regime short large amplitude magnetic structures (SLAMS) and their properties have been investigated. At the perpendicular and quasi-perpendicular shocks, advances have been made in ion reflection, thermalization and ion acceleration using Cluster data. Interplanetary disturbances cause temporal and spatial variations at the Earth's bow shock that in turn trigger a variety of processes. Magnetic reconnection in the tail region is one of them. In reconnection, heating and acceleration occur mainly in the slow-mode shocks, which bound the reconnection wedge. The Cluster mission, related simulation and theoretical considerations provided many new insights into the above-mentioned topics.
    Journal of Atmospheric and Solar-Terrestrial Physics 01/2008; · 1.42 Impact Factor
  • S. Matsukiyo, M. Scholer
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    ABSTRACT: A number of one-dimensional full particle simulations of high Mach number quasi-perpendicular shocks are performed to investigate roles of modified two-stream instability (a microscopic process) in a self-reformation process (a macroscopic process). Spatial and temporal scales of self-reformation processes significantly increase with amplitude of small scale waves generated by modified two-stream instatility in the foot. Furthermore, an influence of the increase of the scales on maximum ion energy attained during one reformation cycle is not negligible. Based on the simulation results a mechanism for the above mentioned scale variation of the self-reformation process and the resulting variation of the maximum attainable ion energy in the foot region is proposed. This mechanism is based on scattering of some of the reflected ions by the waves produced by the modified two-stream instability in the foot and subsequently reflected at the ramp and/or downstream.
    AGU Fall Meeting Abstracts. 12/2007;
  • M. Scholer, H. Comisel, S. Matsukiyo
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    ABSTRACT: We have investigated quasi-perpendicular collisionless shocks by one-dimensional full particle simulations in order to determine the instabilities in the foot region in various parameter regimes. Due to specularly reflected ions in the foot there is free energy in the relative streaming of incoming and reflected ions, of incoming electrons and reflected ions, and of incoming ions and incoming electrons. The main instabilities expected are the Buneman instabilty (BI) and the modified two-stream instability (MTSI). In addition, phase standing whistler waves may be radiated upstream from the ramp and stand in the foot. The problem with identifying instabilities occuring in the foot region of collisionless shocks in PIC simulations is that low ratios of the electron plasma to electron gyrofrequency suppress the BI, whereas low ion to electron mass ratios suppress the MTSI. Having both ratios at the physical value is presently computationally not feasable. By making an appropriate compromise we can investigate both instabilities as well as the phase standing whistler waves at the same time and we determine their influence on the shock reformation process at different Mach numbers and shock normal - magnetic field angles. In Particular we will investigate the consequences for self-reformation at very high Mach numbers (MA>20).
    AGU Fall Meeting Abstracts. 12/2007;

Publication Stats

6k Citations
689.68 Total Impact Points

Institutions

  • 1971–2014
    • Max Planck Institute for Extraterrestrial Physics
      Arching, Bavaria, Germany
  • 1999–2011
    • Kyushu University
      Hukuoka, Fukuoka, Japan
  • 2008
    • University of New Hampshire
      • Space Science Center
      Durham, New Hampshire, United States
  • 2007
    • Queen Mary, University of London
      Londinium, England, United Kingdom
  • 2006
    • Austrian Academy of Sciences
      • Institut für Weltraumforschung
      Mondsee, Upper Austria, Austria
    • University of London
      Londinium, England, United Kingdom
  • 1996
    • Max Planck Institute of Physics
      München, Bavaria, Germany
  • 1992
    • Nagoya University
      • Solar-Terrestrial Environment Laboratory
      Nagoya, Aichi, Japan
  • 1983–1985
    • Max Planck Institute for Astrophysics
      Arching, Bavaria, Germany
  • 1981
    • University of Maryland, College Park
      • Department of Physics
      College Park, MD, United States
  • 1979
    • The University of Arizona
      • Department of Physics
      Tucson, AZ, United States