E. C. Stone

California Institute of Technology, Pasadena, California, United States

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Publications (677)1171.29 Total impact

  • [Show abstract] [Hide abstract]
    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 · 6.28 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 · 31.48 Impact Factor
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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 · 31.48 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 · 6.28 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; 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; 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; 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; 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 · 3.81 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.
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    ABSTRACT: With the twin STEREO spacecraft, significantly separated from L1-based satellites such as ACE, simultaneous multi-point measurements of solar energetic particle (SEP) events can be made for H-Fe ions from a few hundred keV/nuc to over 100 MeV/nuc and for electrons from tens to hundreds of keV. These observations allow studies of the longitudinal characteristics of SEP events to advance beyond statistical analysis of single point measurements. Although there have been few large SEP events thus far in cycle 24, there have been a number of smaller events that have been detected by more than one spacecraft. The composition of these SEP events, as indicated by the H/He and Fe/O abundance ratios, shows a dependence on longitudinal distance from the solar source in some events, at times with ratios varying by an order of magnitude. However, these variations are not organized by either the speed or width of the associated coronal mass ejections.
    AIP Conference Proceedings 01/2012; 1436:103. DOI:10.1063/1.4723596
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    ABSTRACT: Voyager 1 has entered regions of different propagation conditions for energetic cosmic rays in the outer heliosheath beginning at a distance of about 111 AU from the Sun. This conclusion is based on the fact that the low energy 6-14 MeV galactic electron intensity suddenly increased by ~20% over a time period \leg 10 days and the electron radial intensity gradient abruptly decreased from ~19%/AU to ~8%/AU at 2009.7 at a radial distance of 111.2 AU. A sudden radial gradient change was also observed at this time for >200 MeV protons. The gradients were constant during the time period before and after the electron increase. At about 2011.2 at a distance of 116.6 AU a second abrupt intensity increase was observed, this time for both electrons and protons. The increase for electrons was ~25% and occurred over a time period ~15 days or less. For >200 MeV protons the increase at this time was ~5% (unusually large) and occurred over a longer time period ~50 days. Between about 2011.2 and 2011.6, radial intensity gradients ~18%/AU and 3%/AU were observed for electrons and protons, respectively. These gradients were very similar to those observed for these particles before the 1st sudden increase at 2009.7. These large positive gradients observed after 2011.2 indicate that V1, although it has entered a different propagation region, is still within the overall heliospheric modulating region at least up to a time ~2011.6 (118.0 AU). In this paper we will discuss these events in more detail and consider possibilities for their explanation that have recently been suggested.
    Geophysical Research Letters 12/2011; 39(6). DOI:10.1029/2012GL051171 · 4.46 Impact Factor
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    ABSTRACT: Characteristics of solar energetic particle (SEP) events observed at 1 AU from the Sun are known to depend on the magnetic connection of the observer to the acceleration region as well as on whether the acceleration is associated with a shock ("gradual" SEP events) or with flare reconnection ("impulsive" events). Much of our present knowledge of the dependence of SEP event characteristics on heliographic longitude has been the result of single-spacecraft statistical studies that identified systematic correlations of various event properties (e.g., rise time, peak intensity, composition) with the location of the observer relative to the magnetic field line having the best connection to the solar source region. Several studies combining SEP observations made near Earth with data from spacecraft at other heliographic longitudes (Helios and various planetary missions) or latitudes (Ulysses) have also contributed. The two STEREO spacecraft, in conjunction with ACE and Wind operating near Earth, have enabled the first systematic study of the longitudinal dependence of SEP characteristics in individual events at 1 AU. The years 2007 through 2010, during which longitudinal separations of the STEREOs from Earth increased from 0o to nearly 90o, were characterized by a very low level of solar activity. These conditions allowed unusually sensitive measurements of a number of impulsive SEP events and led to the realization that particles accelerated in these events are frequently detectable over a range of longitudes much greater than expected from simple models in which flare-accelerated particles escape into the heliosphere along small clusters of open field lines involved in a reconnection event. Particle fluences, however, were found to have a strong dependence on separation from the well-connected longitude. Since early 2011, as solar activity has been on the increase, a number of gradual SEP events have been observed over the >180o range of heliographic longitudes now spanned by the STEREOs and ACE, enabling multipoint studies of this type of event as well. The interpretation of the SEP data, which are sensitive to both acceleration and transport effects, is benefitting significantly from the 3D observations of coronal activity and CMEs that are now being provided by STEREO, SDO, and SOHO. We will present recent observations of the longitudinal dependence of SEP event characteristics and discuss how they are beginning to modify our understanding of these events. This work was supported by NASA at Caltech (under grants NNX08AI11G, NNX10AQ68G, and NNX10AQ68G, and through UC Berkeley under contract NAS5-03131), JPL, APL (under grant NNX10AT75G), and LMSAL (under grants NNX07AN13G and NNX08AB23G).
    AGU Fall Meeting Abstracts; 12/2011
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    ABSTRACT: We report on observations of protons in the energy range 14 - 60 MeV and electrons in the range 3 - 7 MeV made by the High Energy Telescopes on the Ahead and Behind STEREO spacecraft. These observations were made from December, 2009 through May, 2011. During this time the position of the Ahead spacecraft varied in heliographic longitude from 64 - 94 degrees west of Earth, while the STEREO Behind spacecraft varied from 67 - 93 degrees east of Earth, i.e. the separation between the two spacecraft varied from 131 to 187 degrees. Approximately 19 solar proton events were observed at Ahead and approximately 24 such events were observed at Behind. Eight of these events were seen at both STEREO spacecraft and three events were associated with solar flares which were directly behind the sun for one of the two spacecraft. Previous observations of ~125 proton events (protons 20 - 80 MeV) reported by Van Hollebeke et al. (1975) included only four events from flares more than 120 degrees from the observer. These two sets of observations appear to be inconsistent. We consider two possible explanations: (1) recent unusually quiet conditions at the Sun and in the solar wind are affecting the particle distributions in longitude or (2) difficulty associating events with active regions behind the solar limb adversely affected the earlier results.

Publication Stats

5k Citations
1,171.29 Total Impact Points


  • 1978–2012
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, California, United States
  • 2010
    • University of California, Berkeley
      • Space Sciences Laboratory
      Berkeley, California, United States
  • 1981–2009
    • Washington University in St. Louis
      • Department of Physics
      San Luis, Missouri, United States
  • 2005
    • Johns Hopkins University
      • Applied Physics Laboratory
      Baltimore, MD, United States
  • 2003
    • University of Maryland, College Park
      • Institute for Physical Science and Technology
      College Park, MD, United States
  • 1993
    • NASA
      • Goddard Space Flight Centre
      Вашингтон, West Virginia, United States
  • 1991
    • University of Minnesota Duluth
      • Department of Physics
      Duluth, Minnesota, United States
  • 1986
    • University of New Hampshire
      Дарем, New Hampshire, United States
  • 1976
    • Pasadena City College
      Pasadena, Texas, United States