E. C. Stone

California Institute of Technology, Pasadena, California, United States

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Publications (684)1202.83 Total impact

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    ABSTRACT: On or about 2012 August 25, the Voyager 1 spacecraft crossed the heliopause into the nearby interstellar plasma. In the nearly three years that the spacecraft has been in interstellar space, three notable particle and field disturbances have been observed, each apparently associated with a shock wave propagating outward from the Sun. Here, we present a detailed analysis of the third and most impressive of these disturbances, with brief comparisons to the two previous events, both of which have been previously reported. The shock responsible for the third event was first detected on 2014 February 17 by the onset of narrowband radio emissions from the approaching shock, followed on 2014 May 13 by the abrupt appearance of intense electron plasma oscillations generated by electrons streaming outward ahead of the shock. Finally, the shock arrived on 2014 August 25, as indicated by a jump in the magnetic field strength and the plasma density. Various disturbances in the intensity and anisotropy of galactic cosmic rays were also observed ahead of the shock, some of which are believed to be caused by the reflection and acceleration of cosmic rays by the magnetic field jump at the shock, and/or by interactions with upstream plasma waves. Comparisons to the two previous weaker events show somewhat similar precursor effects, although differing in certain details. Many of these effects are very similar to those observed in the region called the "foreshock" that occurs upstream of planetary bow shocks, only on a vastly larger spatial scale. © 2015. The American Astronomical Society. All rights reserved..
    The Astrophysical Journal 08/2015; 809(2):121. DOI:10.1088/0004-637X/809/2/121 · 5.99 Impact Factor
  • U. K. Senanayake · V. Florinski · A. C. Cummings · E. C. Stone ·
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    ABSTRACT: When the Voyager 1 spacecraft crossed the termination shock (TS) on 2004 December 16, the energy spectra of anomalous cosmic rays (ACRs) could not have been produced by steady-state diffusive shock acceleration. However, over the next few years, in the declining phase of the solar cycle, the spectra began to evolve into the expected power-law profile. Observations at the shock led to a broad range of alternative theories for ACR acceleration. In spite of that, in this work we show that the observations could be explained by assuming ACRs are accelerated at the TS. In this paper, we propose that the solar cycle had an important effect on the unrolling of the spectra in the heliosheath. To investigate the spectral evolution of ACRs, a magnetohydrodynamic background model with stationary solar-wind inner boundary conditions was used to model the transport of helium and oxygen ions. We used a backward-in-time stochastic integration technique where phase-space trajectories are integrated until the so-called "injection energy" is reached. Our simulation results were compared with Voyager 1 observations using three different diffusion models. It is shown that the spectral evolution of ACRs in the heliosheath at Voyager 1 could be explained by an increase in the source strength and an enhancement in diffusion as a result of a decrease of the turbulent correlation length in the declining phase of the solar cycle. At the same time, drift effects seem to have had a smaller effect on the evolution of the spectra.
    The Astrophysical Journal 05/2015; 804(1). DOI:10.1088/0004-637X/804/1/12 · 5.99 Impact Factor
  • V. Florinski · E. C. Stone · A. C. Cummings · J. A. le Roux ·
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    ABSTRACT: In the preceding paper, we showed that large second-order anisotropies of heliospheric ions measured by the Voyager 1 space probe during the August 2012 boundary crossing event could be explained by a magnetic shear across the heliopause preventing particles streaming along the magnetic field from escaping the inner heliosheath. According to Stone et al., the penetration distance of heliospheric ions into the outer heliosheath had a strong dependence on the particle's Larmor radius. By comparing hydrogen, helium, and oxygen ions with the same energy per nucleon, these authors argued that this effect must be attributed to larger cyclotron radii of heavier species rather than differences in velocity. We propose that gradient drift in a nonuniform magnetic field was the cause of both the large second-order anisotropies and the spatial differentiation based on the ion's rigidity. A latitudinal gradient of magnetic field strength of about 10% per AU between 2012.7 and 2012.9 could have provided drift motion sufficient to match both LECP and CRS Voyager 1 observations. We explain the transient intensity dropout observed prior to the heliocliff using flux tube structures embedded in the heliosheath and magnetically connected to interstellar space. Finally, this paper reports a new indirect measurement of the plasma radial velocity at the heliopause on the basis of the time difference between two cosmic-ray telescopes measuring the same intensity dropout.
    The Astrophysical Journal 04/2015; 803(1). DOI:10.1088/0004-637X/803/1/47 · 5.99 Impact Factor
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    ABSTRACT: Using observations from the High Energy Telescopes (HETs) on the STEREO A and B spacecraft and similar observations from near-Earth spacecraft, we summarize the properties of more than 200 individual > 25 MeV solar proton events, some detected by multiple spacecraft, that occurred from the beginning of the STEREO mission in October 2006 to December 2013, and provide a catalog of these events and their solar sources and associations. Longitudinal dependencies of the electron and proton peak intensities and delays to onset and peak intensity relative to the solar event have been examined for 25 three-spacecraft particle events. Expressed as Gaussians, peak intensities fall off with longitude with σ=47±14∘ for 0.7 – 4 MeV electrons, and σ=43±13∘ for 14 – 24 MeV protons. Several particle events are discussed in more detail, including one on 3 November 2011, in which ∼ 25 MeV protons filled the inner heliosphere within 90 minutes of the solar event, and another on 7 March 2012, in which we demonstrate that the first of two coronal mass ejections that erupted from an active region within ∼ 1 hour was associated with particle acceleration. Comparing the current Solar Cycle 24 with the previous cycle, the first > 25 MeV proton event was detected at Earth in the current solar cycle around one year after smoothed sunspot minimum, compared with a delay of only two months in Cycle 23. Otherwise, solar energetic particle event occurrence rates were reasonably similar during the rising phases of Cycles 23 and 24. However, the rate declined in 2013, reflecting the decline in sunspot number since the peak in the northern-hemisphere sunspot number in November 2011. Observations in late 2013 suggest that the rate may be rising again in association with an increase in the southern sunspot number.
    Solar Physics 08/2014; 289(8):3059-3107. DOI:10.1007/s11207-014-0524-8 · 4.04 Impact Factor
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    ABSTRACT: The SuperTIGER (Super Trans-Iron Galactic Element Recorder) instrument was developed to measure the abundances of galactic cosmic-ray elements from Ne-10 to Zr-40 with individual element resolution and the high statistics needed to test models of cosmic-ray origins. SuperTIGER also makes exploratory measurements of the abundances of elements with 40 <= Z <= 60 and measures the energy spectra of the more abundant elements for Z <= 30 from about 0.8 to 10 GeV/nucleon. This instrument is an enlarged and higher resolution version of the earlier TIGER instrument. It was designed to provide the largest geometric acceptance possible and to reach as high an altitude as possible, flying on a standard long-duration 1.11 million m(3) balloon. SuperTIGER was launched from Williams Field, McMurdo Station, Antarctica, on 2012 December 8, and made about 2.7 revolutions around the South Pole in 55 days of flight, returning data on over 50 x 10(6) cosmic-ray nuclei with Z <= 10, including similar to 1300 with Z > 29 and similar to 60 with Z > 49. Here, we describe the instrument, the methods of charge identification employed, the SuperTIGER balloon flight, and the instrument performance.
    The Astrophysical Journal 06/2014; 788(1):18. DOI:10.1088/0004-637X/788/1/18 · 5.99 Impact Factor
  • E C Stone · A C Cummings · F B McDonald · B C Heikkila · N Lal · W R Webber ·
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    ABSTRACT: On 25 August 2012, Voyager 1 was at 122 astronomical units when the steady intensity of low-energy ions it had observed for the last 6 years suddenly dropped for a third time and soon completely disappeared as the ions streamed away into interstellar space. Although the magnetic field observations indicate that Voyager 1 remained inside the heliosphere, the intensity of cosmic ray nuclei from outside the heliosphere abruptly increased. We report the spectra of galactic cosmic rays down to ~3 × 10(6) electron volts per nucleon, revealing H and He energy spectra with broad peaks from 10 × 10(6) to 40 × 10(6) electron volts per nucleon and an increasing galactic cosmic ray electron intensity down to ~10 × 10(6) electron volts.
    Science 06/2013; 341(6142). DOI:10.1126/science.1236408 · 33.61 Impact Factor
  • L F Burlaga · N F Ness · E C Stone ·
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    ABSTRACT: Magnetic fields measured by Voyager 1 (V1) show that the spacecraft crossed the boundary of an unexpected region five times between days 210 and ~238 in 2012. The magnetic field strength B increased across this boundary from ≈0.2 nT to ≈0.4 nT, and B remained near 0.4 nT until at least day 270, 2012. The strong magnetic fields were associated with unusually low counting rates of >0.5 MeV/nuc particles. The direction of B did not change significantly across any of the 5 boundary crossings; it was very uniform and very close to the spiral magnetic field direction, which was observed throughout the heliosheath. The observations indicate that V1 entered a region of the heliosheath ("the heliosheath depletion region"), rather than the interstellar medium.
    Science 06/2013; 341(6142). DOI:10.1126/science.1235451 · 33.61 Impact Factor
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    ABSTRACT: We report new measurements of the elemental energy spectra and composition of galactic cosmic rays during the 2009-2010 solar minimum period using observations from the Cosmic Ray Isotope Spectrometer (CRIS) onboard the Advanced Composition Explorer. This period of time exhibited record-setting cosmic-ray intensities and very low levels of solar activity. Results are given for particles with nuclear charge 5 ≤ Z ≤ 28 in the energy range ~50-550 MeV nucleon–1. Several recent improvements have been made to the earlier CRIS data analysis, and therefore updates of our previous observations for the 1997-1998 solar minimum and 2001-2003 solar maximum are also given here. For most species, the reported intensities changed by less than ~7%, and the relative abundances changed by less than ~4%. Compared with the 1997-1998 solar minimum relative abundances, the 2009-2010 abundances differ by less than 2σ, with a trend of fewer secondary species observed in the more recent time period. The new 2009-2010 data are also compared with results of a simple "leaky-box" galactic transport model combined with a spherically symmetric solar modulation model. We demonstrate that this model is able to give reasonable fits to the energy spectra and the secondary-to-primary ratios B/C and (Sc+Ti+V)/Fe. These results are also shown to be comparable to a GALPROP numerical model that includes the effects of diffusive reacceleration in the interstellar medium.
    The Astrophysical Journal 06/2013; 770(2):117. DOI:10.1088/0004-637X/770/2/117 · 5.99 Impact Factor
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    ABSTRACT: The Low Energy Telescopes (LETs) onboard the twin STEREO spacecraft have been measuring the anisotropies of energetic particles since before the beginning of solar cycle 24. Large unidirectional anisotropies often appear at the onset of magnetically well-connected solar energetic particle (SEP) events, suggesting beamed particles with relatively little scattering. Also, long-lasting bidirectional flows are seen during the decay phase of several SEP events. Some of these instances appear to be within interplanetary coronal mass ejections (ICMEs), as indicated by characteristics such as magnetic field rotations or bidirectional suprathermal electrons. We present preliminary findings from a survey of LET proton anisotropy observations, which illustrate that bidirectional flows appear more likely to come from directions far from the nominal Parker spiral direction than do unidirectional beams, consistent with previous studies. Individual cases that show unusual intensity depletions perpendicular to the magnetic field or pitch angle distributions otherwise indicative of magnetic mirroring are presented in more detail.
    06/2013; 1539(1):227-230. DOI:10.1063/1.4811029
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    ABSTRACT: The small flotilla of spacecraft: Pioneers 10 and 11 and Voyagers 1 and 2 (V1 and V2) that have traveled from 1 AU to the distant heliosphere continue the quest of Victor Hess to understand the nature of this radiation that comes to us from beyond the confines of our solar system. At this time V1 and V2 are traveling deeper into the heliosheath and approaching its outer boundary - the heliopause. In the heliosheath the intensity of 2.5 - 60 MeV GCR electrons has risen significantly above detector background levels and provides an important new diagnostic tool for exploring cosmic ray transport in this previously unexplored region of space. Over the past seven months the intensity of GCR ions and electrons at V1 have remained constant after a steady, 5.5 year exponential increase whose rate varied with particle species. Is this an indication that V1 is approaching the heliopause?.
    02/2013; 1516:85-88. DOI:10.1063/1.4792545
  • Edward C. Stone ·
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    ABSTRACT: Launched in 1977 on a journey to the giant outer planets and beyond, Voyager 1 and 2 have explored the spatial and dynamical properties of the heliosphere that modulates the inward flow of galactic cosmic rays and is the source of anomalous cosmic rays. The two spacecraft are in the heliosheath beyond the termination shock where the supersonic solar wind has slowed as it approaches the boundary of the heliosphere. The shock crossing was 10 AU closer at Voyager 2 in the south than at Voyager 1 in the north, indicating a local interstellar magnetic field pressing inward more strongly on the southern hemisphere. The expected source of anomalous cosmic rays was not observed at the shock, and their intensity has increased deeper in the heliosheath, indicating the source is elsewhere on the shock or in the heliosheath. Voyager 1, now at 121 AU at 35 degrees north, has been in a quasi-stagnation region since 2010 where there is no outward motion of the wind, the magnetic field is enhanced, and the galactic cosmic ray intensity is increasing. In contrast, the heliosheath flow at Voyager 2 at 99 AU and 30 degrees south is faster and increasingly deflected in a non-radial direction as it turns to flow tailward. These observations will be placed in the context of current models of the interaction of the solar and interstellar winds.
    02/2013; 1516:79-84. DOI:10.1063/1.4792544
  • Alan C. Cummings · Edward C. Stone ·
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    ABSTRACT: Anomalous cosmic rays (ACRs) first started showing up in observations 40 years ago. Within a few years a paradigm was developed to explain their origin: they begin their life as interstellar neutral atoms that drift into the heliosphere, become singly ionized by chargeexchange with a solar wind ion or by photoionization, are picked up by the expanding solar wind, and accelerated to the observed energies by diffusive shock acceleration at the solar wind termination shock. This paradigm became widely accepted and withstood the tests of further observations until 16 December 2004, when Voyager 1 crossed the termination shock and didn't find their source. In August 2007, Voyager 2 crossed the termination shock and also did not find the source location of ACRs. Clearly, the source location was not at the termination shock where the two Voyagers crossed. Alternative models have been proposed with acceleration elsewhere on the shock or by other acceleration processes in the heliosheath. We discuss the latest observations of ACRs from the Voyager spacecraft and hopefully shed more light on this ongoing puzzle.
    02/2013; 1516:97-101. DOI:10.1063/1.4792547
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    ABSTRACT: At the Voyager 1 spacecraft in the outer heliosphere, after a series of complex intensity changes starting at about May 8th, the intensities of both anomalous cosmic rays (ACR) and galactic cosmic rays (GCR) changed suddenly and decisively on August 25th (121.7 AU from the Sun). The ACR started the intensity decrease with an initial e-folding rate of intensity decrease of ~1 day. Within a matter of a few days, the intensity of 1.9-2.7 MeV protons and helium nuclei had decreased to less than 0.1 of their previous value and after a few weeks, corresponding to the outward movement of V1 by ~0.1 AU, these intensities had decreased by factors of at least 300-500 and are now lower than most estimates of the GCR spectrum for these lower energies and also at higher energies. The decrease was accompanied by large rigidity dependent anisotropies in addition to the extraordinary rapidity of the intensity changes. Also on August 25th the GCR protons, helium and heavier nuclei as well as electrons increased suddenly with the intensities of electrons reaching levels ~30-50% higher than observed just one day earlier. This increase for GCR occurred over ~1 day for the lowest rigidity electrons, and several days for the higher rigidity nuclei of rigidity ~0.5-1.0 GV. After reaching these higher levels the intensities of the GCR of all energies from 2 to 400 MeV have remained essentially constant with intensity levels and spectra that may represent the local GCR. These intensity changes will be presented in more detail in this, and future articles, as this story unfolds.
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    ABSTRACT: The solar particle event observed at STEREO Ahead on 18 August 2010 displayed a rich variety of behavior in the particle anisotropies. Sectored rates measured by the Low Energy Telescope (LET) on STEREO showed very large bidirectional anisotropies in 4 – 6 MeV protons for the first ∼ 17 hours of the event while inside a magnetic cloud, with intensities along the field direction several hundred to nearly 1000 times greater than those perpendicular to the field. At the trailing end of the cloud, the protons became isotropic and their spectrum hardened slightly, while the He/H abundance ratio plunged by a factor of approximately four for about four hours. Associated with the arrival of a shock on 20 August was a series of brief (< 10 minute duration) intensity increases (commonly called “shock spikes”) with relatively narrow angular distributions (∼ 45∘ FWHM), followed by an abrupt decrease in particle intensities at the shock itself and a reversal of the proton flow to a direction toward the Sun and away from the receding shock. We discuss the STEREO/LET observations of this interesting event in the context of other observations reported in the literature.
    Solar Physics 11/2012; 281(1). DOI:10.1007/s11207-012-0018-5 · 4.04 Impact Factor
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    ABSTRACT: We have examined features in the structure of the heliosheath using the fine scale time variations of termination shock particles (TSP) between ~0.5 - 20 MeV and electrons between 2.5-14 MeV measured by the CRS instrument as the V2 spacecraft crossed the heliospheric termination shock in 2007. The very disturbed heliosheath at V2 is particularly noteworthy for strong periodic intensity variations of the TSP just after V2 crossed the termination shock (2007.66) reaching a maximum between 2007.75 and 2008.0. A series of 42/21 day periodicities was observed at V2 along with spectral changes of low energy TSP and the acceleration of 6-14 MeV electrons. Evidence is presented for the acceleration of TSP and electrons at the times of the 42/21 day periodicities just after V2 crossed the HTS. Spectra for TSP between 2-20 MeV and electrons between 2.5-14 MeV are derived for three time periods including the time of the HTS crossing. The energy spectra of TSP and electrons at these times of intensity peaks are very similar above ~3 MeV, with exponents of a power law spectrum between -3.0 and -3.6. The ratio of TSP intensities to electron intensities at the same energy is ~500. The electron intensity peaks and minima are generally out of phase with those of nuclei by ~1/2 of a 42 day cycle. These charge dependent intensity differences and the large periodic intensity changes could provide new clues as to a possible acceleration mechanism.
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    ABSTRACT: The Low Energy Telescopes (LETs) onboard the twin STEREO spacecraft have been measuring the anisotropies of energetic particles for elements (or element groups) from protons through iron at energies of 4 to up to 12 MeV/nucleon since launch in October 2006, shortly before the beginning of solar cycle 24. At the onset of magnetically well-connected solar energetic particle (SEP) events observed with this instrument, large unidirectional anisotropies (generally from the sunward direction) often appear for about an hour or two, suggesting beamed particles with relatively little scattering. In addition, bidirectional flows lasting for 10 or more hours are seen during the decay phase of several SEP events. Some of these instances are accompanied by characteristics of interplanetary coronal mass ejections (ICMEs) such as magnetic field rotations or bidirectional suprathermal electrons. The event observed at STEREO-Ahead on 18 August 2010 was particularly noteworthy, in that very large bidirectional anisotropies in 4--6 MeV protons persisted for about 17 hours while inside a magnetic cloud and exhibited intensities along the field direction nearly 1000 times greater than those perpendicular to the field. We present a survey of LET observations throughout the STEREO mission to date, and search for correlations between energetic particle anisotropies and plasma parameters to determine the likelihood and characteristics of large anisotropies appearing inside of ICMEs. We compare our results with similar surveys in the literature, made using other instruments in previous solar cycles.
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    ABSTRACT: Since the original discovery of heavy nuclei in the cosmic radiation more than 60 years ago (Freier et al., Phys. Rev. 74, 213, 1948), the composition of this nuclear component has been providing important clues to the origin of the cosmic rays. Over the past two solar minima, the Cosmic Ray Isotope Spectrometer (CRIS) on NASA's Advanced Composition Explorer (ACE) mission has been measuring the relative abundances and energy spectra of essentially all stable and long-lived nuclides (both elements and isotopes) from He to beyond Ni in the energy range between ˜50 and ˜500 MeV/nuc. We report precise determinations of nuclidic composition obtained from this 14-year data set. In addition, we discuss the implications of these data for the composition of cosmic-ray source material and for the time scales associated with the acceleration and transport of cosmic-rays in the Galaxy.
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    ABSTRACT: We summarize measurements made of the elemental and isotopic abundances of galactic cosmic ray nuclei by the Cosmic Ray Isotope Spectrometer (CRIS) on the NASA Advanced Composition Explorer (ACE) satellite over a period of 14 years in space. We have measured the isotopic abundances of Ga (Z=31) and Ge (Z=32) for the first time and have obtained greatly improved measurements of the Cu (Z=29) and Zn (Z=30) isotopes. We have also measured the elemental abundances of nuclei up to Sr (Z=38). A total of ˜700 nuclei heavier than Ni (Z=28) have been collected with energies in the range of ˜150 to 600 MeV/nucleon. Our earlier published work on isotopes with Z<28 has shown abundances consistent with an OB association origin of a substantial fraction of galactic cosmic rays. This is based primarily on the enhanced ^22Ne/^20Ne and ^58Fe/^56Fe ratios relative to solar system abundances. ^22Ne and ^58Fe are copiously produced in Wolf-Rayet stars, which are found primarily in OB associations. The elemental abundances of Z>29 nuclei provide completely independent evidence that also points to an OB association origin. The isotopic abundances of Cu, Zn, Ga, and Ge are consistent with either an OB association or normal interstellar medium origin. This research was supported by NASA under grant NNX11AC49G.
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    ABSTRACT: Super-TIGER is a large area (5.4 m^2) balloon-borne instrument designed to measure ultra-heavy cosmic-ray nuclei (Z = 30 and above) with individual-element resolution and high statistical precision. These measurements will provide sensitive tests of the emerging model of cosmic-ray origins in OB associations and models of the mechanism for selection of nuclei for acceleration. Furthermore, Super-TIGER will measure the energy spectra of the more abundant elements in the interval 10 <=Z <= 28 at energies 0.8 < E < 10 GeV/nucleon to test the hypothesis that microquasars or other sources could superpose spectral features. Super-TIGER, which builds on the heritage of the smaller TIGER, is expected to launch from Antarctica in December 2012. The particle charge and energy will be measured with a combination of plastic scintillators, acrylic and silica-aerogel Cherenkov detectors, and a scintillating fiber hodoscope. The design, expected performance and current status of the instrument along with the scientific implications of the Super-TIGER measurements will be presented.
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    ABSTRACT: The recovery of cosmic ray Carbon nuclei of energy ~20-125 MeV/nuc in solar cycle #23 from 2004 to 2010 has been followed at three locations, near the Earth using ACE data and at V2 between 74-92 AU and also at V1 beyond the heliospheric termination shock at between 91-113 AU. To describe the observed intensity changes and to predict the absolute intensities measured at all three locations we have used a simple spherically symmetric (no drift) two-zone heliospheric transport model with specific values for the diffusion coefficient in both the inner and outer zones. The diffusion coefficient in the outer zone is determined to be ~5-10 times smaller than that in the inner zone out to 90 AU. For both V1 and V2 the calculated C nuclei intensities agree within an average of \pm 10% with the observed intensities. Because of this agreement between V1 and V2 observations and predictions there is no need to invoke an asymmetrical squashed heliosphere or other effects to explain the V2 intensities relative to V1 as is the case for He nuclei. The combination of the diffusion parameters used in this model and the interstellar spectrum give an unusually low overall solar modulation parameter \phi = 250 MV to describe the Carbon intensities observed at the Earth in 2009. At all times both the observed and calculated spectra are very closely ~ E1.0 as would be expected in the adiabatic energy loss regime of solar modulation.

Publication Stats

6k Citations
1,202.83 Total Impact Points


  • 1978-2014
    • California Institute of Technology
      • • Jet Propulsion Laboratory
      • • Division of Physics, Mathematics, and Astronomy
      • • Department of Physics
      Pasadena, California, United States
  • 2010
    • University of California, Berkeley
      • Space Sciences Laboratory
      Berkeley, California, United States
  • 1983-2009
    • Washington University in St. Louis
      • Department of Physics
      San Luis, Missouri, United States
  • 2005
    • Johns Hopkins University
      • Applied Physics Laboratory
      Baltimore, MD, United States
  • 1986
    • NASA
      • Goddard Space Flight Centre
      Вашингтон, West Virginia, United States
    • University of New Hampshire
      Дарем, New Hampshire, United States
  • 1976-1984
    • Pasadena City College
      Pasadena, Texas, United States