W. S. Kurth

University of Iowa, Iowa City, Iowa, United States

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Publications (677)2385.17 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The rotation period of Saturn’s magnetosphere was found to vary with time, and changing periodicities were identified in magnetic fields, radio emissions, and charged particles. Here we analyze the varying period of Saturn kilometric radiation (SKR) from 2009 to early 2013, i.e. mainly after Saturn equinox of August 2009. A periodicity analysis is first applied to the complete SKR signal, and second to SKR intensities separated by spacecraft latitude and wave polarization, attributed to SKR from the northern and southern hemisphere. Our analyses are done with the tracking filter approach of Gurnett et al. (Gurnett et al. [2009a]. Geophys. Res. Lett. 36, L16102) and by simply tracing the phases of normalized SKR intensity maxima (north and south) with time. It is shown that SKR periods from the northern and southern hemisphere converged during 2009, crossed shortly after equinox, and coalesced in spring 2010. We will show that SKR from both hemispheres not only exhibited similar periods, but also similar phases from March 2010 until February 2011 and from August 2011 until June 2012. The in-between time interval (March to July 2011) shows a slowdown of the southern SKR rotation rate and a slight increase in rotation speed for the northern SKR before rotation rates and phases become equal again in August 2011. We also identify SKR signals where the modulation phase deviation exceeds one rotation each time Cassini completes one orbit, i.e. this is consistent with the characteristic of a rotating signal. However, the main SKR modulation signals from 2009 to 2012 can be viewed as being clock-like with no correction needed for the derived periods. A comparison of SKR periodicities after equinox to the planetary period oscillations of the magnetic field shows major differences, and we compare SKR phases to magnetic field phases to explain the deviations.
    Icarus 07/2015; 254:72-91. DOI:10.1016/j.icarus.2015.03.014 · 2.84 Impact Factor
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    B Cecconi, Philippe Zarka, William S Kurth, DA Gurnett
  • Journal of Geophysical Research: Space Physics 05/2015; DOI:10.1002/2015JA021095 · 3.44 Impact Factor
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    ABSTRACT: The solar system contains solids of all sizes, ranging from km-size bodies to nano-sized particles. Nanograins have been detected in situ in the Earth's atmosphere, near cometary and giant planet environments, and more recently in the solar wind at 1 AU. These latter nano grains are thought to be formed in the inner solar system dust cloud, mainly through collisional break-up of larger grains and are then picked-up and accelerated by the magnetized solar wind because of their large charge-to-mass ratio. In the present paper, we analyze the low frequency bursty noise identified in the Cassini radio and plasma wave data during the spacecraft cruise phase inside Jupiter's orbit. The magnitude, spectral shape and waveform of this broadband noise is consistent with the signature of nano particles impinging at nearby the solar wind speed on the spacecraft surface. Nanoparticles were observed whenever the radio instrument was turned on and able to detect them, at different heliocentric distances between Earth and Jupiter, suggesting their ubiquitous presence in the heliosphere. We analyzed the radial dependence of the nano dust flux with heliospheric distance and found that it is consistent with the dynamics of nano dust originating from the inner heliosphere and picked-up by the solar wind. The contribution of the nano dust produced in asteroid belt appears to be negligible compared to the trapping region in the inner heliosphere. In contrast, further out, nano dust are mainly produced by the volcanism of active moons such as Io and Enceladus.
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    ABSTRACT: Quasi-periodic, short-period injections of relativistic electrons have been observed in both Jupiter’s and Saturn’s magnetospheres, but understanding their origin or significance has been challenging, primarily due to the limited number of in-situ observations of such events by past flyby missions. Here we present the first survey of such injections in an outer planetary magnetosphere using almost nine years of energetic charged particle and magnetic field measurements at Saturn. We focus on events with a characteristic period of about 60 minutes (QP60, where QP stands for quasi-periodic). We find that the majority of QP60, which are very common in the outer magnetosphere, map outside Titan’s orbit. QP60 are also observed over a very wide range of local times and latitudes. A local time asymmetry in their distribution is the most striking feature, with QP60 at dusk being between 5 to 25 times more frequent than at dawn. Field-line tracing and pitch angle distributions suggest that most events at dusk reside on closed field lines. They are distributed either near the magnetopause, or, in the case of the post-dusk (or pre-midnight) sector, up to about 30 inside it, along an area extending parallel to the dawn-dusk direction. QP60 at dawn map either on open field lines and/or near the magnetopause. Both the asymmetries and varying mapping characteristics as a function of local time indicate that generation of QP60 cannot be assigned to a single process. The locations of QP60 seem to trace sites that reconnection is expected to take place. In that respect, the subset of events observed post-dusk and deep inside the magnetopause may be directly or indirectly linked to the Vasyliunas reconnection cycle, while magnetopause reconnection/Kelvin-Helmholtz (KH) instability could be invoked to explain all other events at the duskside. Using similar arguments, injections at the dawnside magnetosphere may result from solar-wind induced storms and/or magnetopause reconnection/KH-instability. Still, we cannot exclude that the apparent collocation of QP60 with expected reconnection sites is coincidental. given also the large uncertainties in field line tracing with the available magnetic field models. The intensity of the QP60 spectrum is strong enough such that if transport processes allow, these injections can be a very important source of energetic electrons for the inner saturnian magnetosphere or the heliosphere. We also observe that electrons in a QP60 can be accelerated at least up to 6 MeV and that the distribution of QP60 appears to trace well the aurora’s local time structure, an observation that may have implications about high-latitude electron acceleration and the connection of these events to auroral dynamics. Despite these new findings, it is still unclear what determines the rather well-defined 65-minute period of the electron bursts and how electrons can rapidly reach several MeV.
    Icarus 04/2015; DOI:10.1016/j.icarus.2015.04.017 · 2.84 Impact Factor
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    ABSTRACT: Plasmaspheric hiss is known to play an important role in controlling the overall structure and dynamics of radiation belt electrons inside the plasmasphere. Using newly available Van Allen Probes wave data, which provide excellent coverage in the entire inner magnetosphere, we evaluate the global distribution of the hiss wave frequency spectrum and wave intensity for different levels of substorm activity. Our statistical results show that observed hiss peak frequencies are generally lower than the commonly adopted value (~550 Hz), which was in frequent use, and that the hiss wave power frequently extends below 100 Hz, particularly at larger L-shells (> ~3) on the dayside during enhanced levels of substorm activity. We also compare electron pitch angle scattering rates caused by hiss using the new statistical frequency spectrum and the previously adopted Gaussian spectrum, and find that the differences are up to a factor of ~5 and are dependent on energy and L-shell. Moreover, the new statistical hiss wave frequency spectrum including wave power below 100 Hz leads to increased pitch angle scattering rates by a factor of ~1.5 for electrons above ~100 keV at L ~ 5, although their effect is negligible at L ≤ 3. Consequently, we suggest that the new realistic hiss wave frequency spectrum should be incorporated into future modeling of radiation belt electron dynamics.
    Journal of Geophysical Research: Space Physics 04/2015; DOI:10.1002/2015JA021048 · 3.44 Impact Factor
  • Journal of Geophysical Research: Space Physics 04/2015; DOI:10.1002/2014JA020941 · 3.44 Impact Factor
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    ABSTRACT: Plasmaspheric hiss is one of the important plasma waves controlling radiation belt dynamics. Its spatiotemporal distribution and generation mechanism are presently the object of active research. We here give the first report on the shock-induced disappearance of plasmaspheric hiss observed by the Van Allen Probes on 8 October 2013. This special event exhibits the dramatic variability of plasmaspheric hiss and provides a good opportunity to test its generation mechanisms. The origination of plasmaspheric hiss from plasmatrough chorus is suggested to be an appropriate prerequisite to explain this event. The shock increased the suprathermal electron fluxes, and then the enhanced Landau damping promptly prevented chorus waves from entering the plasmasphere. Subsequently, the shrinking magnetopause removed the source electrons for chorus, contributing significantly to the several-hours-long disappearance of plasmaspheric hiss.
    03/2015; DOI:10.1002/2015GL063906
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    ABSTRACT: With data from Van Allen Probes, we investigate EMIC wave excitation using simultaneously observed ion distributions. Strong He-band waves occurred while the spacecraft was moving through an enhanced density region. We extract from Helium, Oxygen, Proton, and Electron (HOPE) Mass Spectrometer measurement the velocity distributions of warm heavy ions as well as anisotropic energetic protons that drive wave growth through the ion cyclotron instability. Fitting the measured ion fluxes to multiple sinm-type distribution functions, we find that the observed ions make up about 15% of the total ions, but about 85% of them are still missing. By making legitimate estimates of the unseen cold (below ~2 eV) ion composition from cutoff frequencies suggested by the observed wave spectrum, a series of linear instability analyses and hybrid simulations are carried out. The simulated waves generally vary as predicted by linear theory. They are more sensitive to the cold O+ concentration than the cold He+ concentration. Increasing the cold O+ concentration weakens the He-band waves but enhances the O-band waves. Finally, the exact cold ion composition is suggested to be in a range when the simulated wave spectrum best matches the observed one.
    Journal of Geophysical Research: Space Physics 03/2015; DOI:10.1002/2014JA020717 · 3.44 Impact Factor
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    ABSTRACT: We present the first systematic study of Langmuir wave amplitudes in Saturn's foreshock using the Cassini RPWS/WBR measurements. We analyzed all foreshock crossings from June 2004 to December 2009 using an automatic method to identify Langmuir waves. Using this method, almost 3×105 waveform intervals of typical duration of about a minute were selected. For each selected waveform interval the position of the satellite inside the foreshock was calculated using an adaptive bow shock model, which was parametrized by the observed magnetic field and plasma data. We determined the wave amplitudes for all waveform intervals, and we found that the probability density function amplitudes follows a log-normal distribution with a power-law tail. A non-linear fit for this tail gives a power-law exponent of −1.37±0.01. The distribution of amplitudes as a function of the depth in the foreshock shows the onset of the waves near the upstream boundary with its maximum slightly shifted inside the foreshock (~1 RS). The amplitudes then fall off with increasing depth in the downstream region. Our results are in agreement with previous observations and roughly follow the generally accepted stochastic growth theory mechanism for the foreshock region, with an exception at the highest observed amplitudes. The estimated energy density ratio W for largest amplitudes does not exceed 10−2, suggesting that modulational instability is not relevantfor a large majority of waves. The decay instability can be important for the stronger electrostatic waves in Saturn's foreshock, as was previously reported for multiple solar system planets.
    Journal of Geophysical Research: Space Physics 03/2015; DOI:10.1002/2014JA020560 · 3.44 Impact Factor
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    ABSTRACT: The Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) mission of opportunity working in tandem with the Van Allen Probes was designed to study the loss of radiation belt electrons to the ionosphere and upper atmosphere. BARREL is also sensitive to X-rays from other sources. During the second BARREL campaign the Sun produced an X-class flare followed by a solar energetic particle event (SEP) associated with the same active region. Two days later on 9 January 2014 the shock generated by the coronal mass ejection (CME) originating from the active region hit the Earth while BARREL was in a close conjunction with the Van Allen Probes. Time History Events and Macroscale Interactions during Substorms (THEMIS) observed the impact of the ICME-shock near the magnetopause, and the Geostationary Operational Environmental Satellite (GOES) satellites were on either side of the BARREL/Van Allen Probe array. The solar interplanetary magnetic field was not ideally oriented to cause a significant geomagnetic storm, but compression from the shock impact led to the loss of radiation belt electrons. We propose that an azimuthal electric field impulse generated by magnetopause compression caused inward electron transport and minimal loss. This process also drove chorus waves, which were responsible for most of the precipitation observed outside the plasmapause. Observations of hiss inside the plasmapause explains the absence of loss at this location. ULF waves were found to be correlated withthe structure of the precipitation. We demonstrate how BARREL can monitor precipitation following a ICME-shock impact at Earth in a cradle-to-grave view; from flare, to SEP, to electron precipitation.
    Journal of Geophysical Research: Space Physics 03/2015; DOI:10.1002/2014JA020873 · 3.44 Impact Factor
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    ABSTRACT: Interactions between the solar wind and planetary magnetospheres provide important diagnostic information about the magnetospheric dynamics. The lack of monitoring of upstream solar wind conditions at the outer planets, however, restrains the overall scientific output. Here we apply a new method, using Cassini nanodust stream measurements, to derive the interplanetary magnetic field structure during the 2013 Saturn aurora campaign. Due to the complex dynamical interactions with the interplanetary magnetic field, a fraction of fast nanodust particles emerging from the Saturnian system is sent back into the magnetosphere and can be detected by a spacecraft located within. The time-dependent directionality caused by the variable interplanetary magnetic field enable these particles to probe the solar wind structure remotely. Information about the arrival time of solar wind compression regions (coupled with the heliospheric current sheet crossings) as well as the field direction associated with the solar wind sector structure can be inferred. Here we present a tentative identification of the interplanetary magnetic field sector structure based on Cassini nanodust and radio emission measurements during the 2013 Saturn aurora campaign. Our results show that, the interplanetary magnetic field near Saturn during 2013-080 to 176 was consistent with a two-sector structure. The intensifications of aurora and the radio emission on 2013-095, 112 and 140 coincide with the IMF sector boundaries, indicating that the encounter of the compressed solar wind is the main cause of the observed activities.
    Icarus 03/2015; DOI:10.1016/j.icarus.2015.02.022 · 2.84 Impact Factor
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    ABSTRACT: Most theoretical wave models require the power in the wave magnetic field in order to determine the effect of chorus waves on radiation belt electrons. However, researchers typically use the cold plasma dispersion relation to approximate the magneticwave power when only electric field data are available. In this study, the validity of using the the cold plasma dispersion relation in this context is tested using EMFISIS observations of both the electric and magnetic spectral intensities in the chorus wave band (0.1-0.9 fce). Results from this study indicate that the calculated wave intensity is least accurate during periods of enhanced wave activity. For observed wave intensities >10¯3 nT2, using the cold plasma dispersion relation results in an underestimate of the wave intensity by a factor of 2 or greater, 56% of the time over the full chorus wave band, 60% of the time for lower band chorus, and 59% of the time for upper band chorus. Hence during active periods, empirical chorus wave models that are reliant on the cold plasma dispersion relation will underestimate chorus wave intensities to a significant degree, thus causing questionable calculation of wave-particle resonance effects on MeVelectrons.
    Journal of Geophysical Research: Space Physics 02/2015; 120(2). DOI:10.1002/2014JA020808 · 3.44 Impact Factor
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    ABSTRACT: We study the formation process of an oxygen torus during the 12-15 November 2012 magnetic storm, using the magnetic field and plasma wave data obtained by Van Allen Probes. We estimate the local plasma mass density (ρL) and the local electron number density (neL) from the resonant frequencies of standing Alfvén waves and the upper hybrid resonance band. The average ion mass (M) can be calculated by M∼ρL/neL under the assumption of quasi-neutrality of plasma. During the storm recovery phase, both Probe-A and Probe-B observe the oxygen torus at L=3.0-4.0 and L=3.7-4.5, respectively, on the morning side. The oxygen torus has M=4.5-8 amu and extends around the plasmapause that is identified at L∼3.2-3.9. We find that during the initial phase, M is 4-7 amu throughout the plasma tough and remains at ∼1 amu in the plasmasphere, implying that ionospheric O+ ions are supplied into the inner magnetosphere already in the initial phase of the magnetic storm. Numerical calculation under a decrease of the convection electric field reveals that some of thermal O+ ions distributed throughout the plasma trough are trapped within the expanded plasmasphere, whereas some of them drift around the plasmapause on the dawnside. This creates the oxygen torus spreading near the plasmapause, which is consistent with the Van Allen Probes observations. We conclude that the oxygen torus identified in this study favors the formation scenario of supplying O+ in the inner magnetosphere during the initial phase and subsequent drift during the recovery phase.
    Journal of Geophysical Research: Space Physics 02/2015; 120(2). DOI:10.1002/2014JA020593 · 3.44 Impact Factor
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    ABSTRACT: During 18 February to 2 March 2014, the Van Allen Probes encountered multiple geomagnetic storms and simultaneously observed intensified chorus and hiss waves. During this period, there were substantial enhancements in fluxes of energetic (53.8 − 108.3 keV) and relativistic (2 − 3.6 MeV) electrons. Chorus waves were excited at locations L = 4 − 6.2 after the fluxes of energetic were greatly enhanced, with a lower frequency band and wave amplitudes ∼ 20 − 100 pT. Strong hiss waves occurred primarily in the main phases or below the location L = 4 in the recovery phases. Relativistic electron fluxes decreased in the main phases due to the adiabatic (e.g., the magnetopause shadowing) or non-adiabatic (hiss-induced scattering) processes. In the recovery phases, relativistic electron fluxes either increased in the presence of enhanced chorus, or remained unchanged in the absence of strong chorus or hiss. The observed relativistic electron phase space density peaked around L∗ = 4.5, characteristic of local acceleration. This multiple-storm period reveals a typical picture that chorus waves are excited by the energetic electrons at first and then produce efficient acceleration of relativistic electrons. This further demonstrates that the interplay between both competing mechanisms of chorus-driven acceleration and hiss-driven scattering often occurs in the outer radiation belts.
    Journal of Geophysical Research: Space Physics 02/2015; 120(2). DOI:10.1002/2014JA020781 · 3.44 Impact Factor
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    ABSTRACT: Exohiss waves are whistler mode hiss observed in the plasmatrough region. We present a case study of exohiss waves and the corresponding background plasma distributions observed by the Van Allen Probes in the dayside low-latitude region. The analysis of wave Poynting fluxes, suprathermal electron fluxes and cold electron densities supports the scenario that exohiss leaks from the plasmasphere into the plasmatrough. Quasilinear calculations further reveal that exohiss can potentially cause the resonant scattering loss of radiation belt electrons ~<MeV on a comparable timescale to that associated with the storm-time plasmaspheric hiss. These results clearly illustrate that exohiss may need to be taken into account in future radiation belt models.
    02/2015; 42(4). DOI:10.1002/2014GL062964
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    ABSTRACT: The twin Van Allen Probe spacecraft, launched in August 2012, carry identical scientific payloads. The Electric and Magnetic Fields Instrument Suite and Integrated Science (EMFISIS) suite includes a plasma wave instrument (Waves) that measures three magnetic and three electric components of plasma waves in the frequency range of 10 Hz to 12 kHz using triaxial search coils and the Electric Fields and Waves (EFW) triaxial electric field sensors. The Waves instrument also measures a single electric field component of waves in the frequency range of 10 to 500 kHz. A primary objective of the higher frequency measurements is the determination of the electron density ne at the spacecraft, primarily inferred from the upper hybrid resonance frequency fuh. Considerable work has gone into developing a process and tools for identifying and digitizing the upper hybrid resonance frequency in order to infer the electron density as an essential parameter for interpreting not only the plasma wave data from the mission, but also as input to various magnetospheric models. Good progress has been made in developing algorithms to identify fuh and create a data set of electron densities. However, it is often difficult to interpret the plasma wave spectra during active times to identify fuh and accurately determine ne. In some cases there is not a clear signature of the upper hybrid band and the low-frequency cutoff of the continuum radiation is used. We describe the expected accuracy of ne and issues in the interpretation of the electrostatic wave spectrum.
    Journal of Geophysical Research: Space Physics 02/2015; 120(2). DOI:10.1002/2014JA020857 · 3.44 Impact Factor
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    ABSTRACT: We report, for the first time, an auroral undulation event on 1 May 2013 observed by an all-sky imager (ASI) at Athabasca (L = 4.6), Canada, for which in situ field and particle measurements in the conjugate magnetosphere were available from a Van Allen Probes spacecraft. The ASI observed a train of auroral undulation structures emerging spontaneously in the pre-midnight subauroral ionosphere, during the growth phase of a substorm. The undulations had an azimuthal wavelength of ~180 km and propagated westward at a speed of 3–4 km s−1. The successive passage over an observing point yielded quasi-periodic oscillations in diffuse auroral emissions with a period of ~40 s. The azimuthal wave number m of the auroral luminosity oscillations was found to be m ~ −103. During the event the spacecraft – being on tailward stretched field lines ~0.5 RE outside the plasmapause that mapped into the ionosphere conjugate to the auroral undulations – encountered intense poloidal ULF oscillations in the magnetic and electric fields. We identify the field oscillations to be the second harmonic mode along the magnetic field line through comparisons of the observed wave properties with theoretical predictions. The field oscillations were accompanied by oscillations in proton and electron fluxes. Most interestingly, both field and particle oscillations at the spacecraft had one-to-one association with the auroral luminosity oscillations around its footprint. Our findings strongly suggest that this auroral undulation event is closely linked to the generation of second harmonic poloidal waves.
    Journal of Geophysical Research: Space Physics 02/2015; 120(3). DOI:10.1002/2014JA020863 · 3.44 Impact Factor
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    ABSTRACT: A new 3D diffusion code is used to investigate the inward intrusion and slow decay of energetic radiation belt electrons (>0.5 MeV) observed by the Van Allen Probes during a 10-day quiet period in March 2013. During the inward transport the peak differential electron fluxes decreased by approximately an order of magnitude at various energies. Our 3D radiation belt simulation including radial diffusion and pitch angle and energy diffusion by plasmaspheric hiss and Electromagnetic Ion Cyclotron (EMIC) waves reproduces the essential features of the observed electron flux evolution. The decay timescales and the pitch angle distributions in our simulation are consistent with the Van Allen Probes observations over multiple energy channels. Our study suggests that the quiet-time energetic electron dynamics are effectively controlled by inward radial diffusion and pitch angle scattering due to a combination of plasmaspheric hiss and EMIC waves in the Earth's radiation belts.
    02/2015; 42(4). DOI:10.1002/2014GL062977
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    ABSTRACT: Recent ray tracing suggests that plasmaspheric hiss can originate from chorus observed outside of the plasmapause. Although a few individual events have been reported to support this mechanism, the number of reported conjugate events is still very limited. Using coordinated observations between THEMIS and Van Allen Probes, we report on an interesting event, where chorus was observed at a large L-shell (~9.8), different from previously reported events at L < 6, but still exhibited a remarkable correlation with hiss observed in the outer plasmasphere (L ~ 5.5). Ray tracing indicates that a subset of chorus can propagate into the observed location of hiss on a timescale of ~ 5-6 s, in excellent agreement with the observed time lag between chorus and hiss. This provides quantitative support that chorus from large L-shells, where it was previously considered unable to propagate into the plasmasphere, can in fact be the source of hiss.
    01/2015; 42(2). DOI:10.1002/2014GL062832

Publication Stats

9k Citations
2,385.17 Total Impact Points


  • 1976–2015
    • University of Iowa
      • Department of Physics and Astronomy
      Iowa City, Iowa, United States
  • 2011
    • Austrian Academy of Sciences
      • Institut für Weltraumforschung
      Vienna, Vienna, Austria
  • 2001–2010
    • Imperial College London
      • Department of Physics
      Londinium, England, United Kingdom
  • 2009
    • Charles University in Prague
      • Faculty of Mathematics and Physics
      Praha, Praha, Czech Republic
  • 2006–2008
    • University of Cologne
      • Institute of Geophysics and Meteorology
      Köln, North Rhine-Westphalia, Germany
    • IST Austria
      Klosterneuberg, Lower Austria, Austria
  • 2005
    • University of Oslo
      • Department of Physics
      Kristiania (historical), Oslo, Norway
    • Université de Versailles Saint-Quentin
      Versailles, Île-de-France, France
  • 1993–2003
    • Johns Hopkins University
      • Applied Physics Laboratory
      Baltimore, Maryland, United States
  • 1988
    • NASA
      Вашингтон, West Virginia, United States
  • 1987
    • TRW Automotive
      Ливония, Michigan, United States
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, California, United States