J. P. McFadden

University of California, Berkeley, Berkeley, California, United States

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Publications (343)617.54 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We present ARTEMIS observations of electron-wave interactions which extend to quite large distances upstream from the Moon. We first study electron velocity distributions and wave spectra on an event basis. In both the solar wind and terrestrial plasma sheet, we observe strong whistler wave activity on the magnetic field lines connected to the dayside lunar surface. These whistlers are most likely driven by the anisotropy of upward electrons caused by surface absorption. The whistler growth rates computed from the measured electron distributions successfully reproduced the spectral characteristics of the observed ~100 Hz narrowband oscillations and those reaching lower frequencies. Meanwhile, the incoming solar wind strahl beam is occasionally isotropized near the Moon and broadband electrostatic waves are observed simultaneously, suggesting streaming instabilities between the incoming and outgoing beams. Based on the case study, we statistically survey the spatial variations of the characteristic quantities of the upstream electron-wave interactions. Both the electron anisotropy and electromagnetic wave intensity decay with increasing field-line distances but remain higher than the ambient level at 6 lunar radii (~10,000 km) or more. The strahl electron isotropization and electrostatic waves are found mainly at lower altitudes below one lunar radius. The electron anisotropy and whistler intensity exhibit clear anti-correlation with crustal magnetic fields, indicating that the magnetic anomalies suppress the whistler wave growth. The ARTEMIS observations convincingly illustrate that the lunar influence on electrons reaches out to 6 lunar radii or more upstream from the Moon.
    Journal of Geophysical Research: Space Physics 11/2014; · 3.44 Impact Factor
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    ABSTRACT: ARTEMIS observes structures near the Moon that display many properties commonly associated with collisionless shocks, including a discontinuity with downstream compression of magnetic field and density, heating and wave activity, and velocity deflections away from the Moon. The two-probe ARTEMIS measurements show that these features do not exist in the pristine solar wind, and thus must result from lunar influences. Discontinuity analyses indicate mass flux and heating across the boundary, with the normal velocity dropping from super-magnetosonic to sub-magnetosonic across the discontinuity. The shock location with respect to crustal magnetic fields suggests a causal relationship, implying that solar wind protons reflected from crustal fields may produce the observed structures. These observations may indicate some of the smallest shocks in the solar system (in terms of plasma scales), driven by solar wind interaction with magnetic fields on the order of the ion gyro-radius and inertial length.
    Geophysical Research Letters. 10/2014;
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    ABSTRACT: We have performed a detailed analysis of plasma and wave observations in a magnetic flux rope encountered by THEMIS-D at the subsolar magnetopause. The extent of the flux rope was ~ 270 ion skin depths in the outflow direction and it was flanked by two active X-lines producing colliding plasma jets in the flux rope core where ion heating and supra-thermal electrons were observed. The colliding jet region was highly dynamic and characterized by enhanced wave power in a broad frequency range. High frequency waves, including ion acoustic like waves, electron holes, and whistler mode waves were observed in a limited spatial region near the flux rope center and did not appear to be associated with the observed large-scale heating and energization. Low frequency kinetic Alfvén waves (KAWs), on the other hand, were enhanced in the entire flux rope core, suggesting a possible link with the observed ion heating.
    Journal of Geophysical Research: Space Physics 07/2014; · 3.44 Impact Factor
  • J. S. Halekas, A. R. Poppe, J. P. McFadden
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    ABSTRACT: The lunar plasma wake refills from all directions, with processes operating both parallel and perpendicular to the magnetic field. The resulting wake structure depends sensitively on the properties of the flowing plasma, including the form of the ion and electron velocity distributions. In this manuscript, we discuss theoretical approximations for the refilling of the lunar wake along the magnetic field. While an often-used treatment for the parallel refilling assumes cold ions, one can derive solutions for arbitrary ion velocity distributions. Similarly, though the most tractable theory utilizes Maxwellian electrons, one can derive solutions for other types of distributions. We discuss the theoretical framework for various one-dimensional solutions, spanning the full range from cold ion theories to gas-dynamic solutions, and utilizing both Maxwellian and kappa electron distributions. We compare these solutions to ARTEMIS observations of the lunar wake, for time periods with appropriate plasma parameters. We also present cases that reveal the inherent limitations of one-dimensional approximations, including those related to electron anisotropies and those related to perpendicular processes associated with both fluid flow and ion gyro-motion.
    Journal of Geophysical Research: Space Physics 06/2014; · 3.44 Impact Factor
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    ABSTRACT: THEMIS observed several magnetopause crossings periodically at the duskside of magnetopause under southward IMF, with significant sunward returning flows inside the magnetopause. The vortex features of the flows and the periodic enhancements in the calculated vorticity normal to the spacecraft plane could be found in the observation. The distortion of the magnetopause, the periodic features of vortex flows, the tailward propagation, and the evaluation of Kelvin-Helmholtz instability (KHI) condition support the evidence of the Kelvin-Helmholtz vortices produced by the velocity shear at the duskside of magnetopause. Based on three-point simultaneous observations of the flow, the vorticity was calculated to be about 0.15 s-1, similar to previous results. The tailward propagation of the vortices along the flank magnetopause was estimated about 292 km/s. The circular induced electric field of several mV/m was deduced perpendicular to the magnetic field when the magnetic field compression occurred at the edge of the vortices.
    Geophysical Research Letters. 06/2014;
  • J. S. Halekas, A. R. Poppe, J. P. McFadden
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    ABSTRACT: ARTEMIS measures the ionized constituents of the exosphere, providing a long-term exospheric dataset covering ~2.5 years, including the LADEE mission.
    02/2014;
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    ABSTRACT: To investigate the physical mechanism responsible for substorm triggering, we performed a superposed-epoch analysis using plasma and magnetic-field data from THEMIS probes. Substorm onset timing was determined based on auroral breakups detected by all-sky imagers at the THEMIS ground-based observatories. We found earthward flows associated with north-south auroral streamers during the substorm growth phase. At around X = -12 Earth radii (RE), the northward magnetic field and its elevation angle decreased markedly approximately 4 min before substorm onset. Moreover, a northward magnetic-field increase associated with pre-onset earthward flows was found at around X = -17 RE. This variation indicates that local dipolarization occurs. Interestingly, in the region earthwards of X = -18 RE, earthward flows in the central plasma sheet (CPS) reduced significantly approximately 3 min before substorm onset, which was followed by a weakening of dawn-/duskward plasma-sheet boundary-layer flows (subject to a 1 min time lag). Subsequently, approximately 1 min before substorm onset, earthward flows in the CPS were enhanced again and at the onset, tailward flows started at around X = -20 RE. Following substorm onset, an increase in the northward magnetic field caused by dipolarization was found in the near-Earth region. Synthesizing these results, we confirm our previous results based on GEOTAIL data, which implied that significant variations start earlier than both current disruption and magnetic reconnection, at approximately 4 min before substorm onset roughly halfway between the two regions of interest; i.e. in the catapult current sheet.
    01/2014; 32(2).
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    ABSTRACT: The Solar Wind Ion Analyzer (SWIA) on the MAVEN mission will measure the solar wind ion flows around Mars, both in the upstream solar wind and in the magneto-sheath and tail regions inside the bow shock. The solar wind flux provides one of the key energy inputs that can drive atmospheric escape from the Martian system, as well as in part controlling the structure of the magnetosphere through which non-thermal ion escape must take place. SWIA measurements contribute to the top level MAVEN goals of characterizing the upper atmosphere and the processes that operate there, and parameterizing the escape of atmospheric gases to extrapolate the total loss to space throughout Mars' history. To accomplish these goals, SWIA utilizes a toroidal energy analyzer with electrostatic deflectors to provide a broad 360∘×90∘ field of view on a 3-axis spacecraft, with a mechanical attenuator to enable a very high dynamic range. SWIA provides high cadence measurements of ion velocity distributions with high energy resolution (14.5 %) and angular resolution (3.75∘×4.5∘ in the sunward direction, 22.5∘×22.5∘ elsewhere), and a broad energy range of 5 eV to 25 keV. Onboard computation of bulk moments and energy spectra enable measurements of the basic properties of the solar wind at 0.25 Hz.
    Space Science Reviews 11/2013; · 5.52 Impact Factor
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    ABSTRACT: For the unique case of magnetic field parallel to the solar wind flow, a column of reflected protons can accumulate upstream from the Moon. We investigate observations from the ARTEMIS probes for an extended period with this geometry. During this time, P2 observes strong wave turbulence in two frequency bands above and below the ion cyclotron frequency near the Moon, not seen by P1 farther from the Moon. The lower frequency oscillations prove consistent with kinetic magnetosonic waves resonantly generated by reflected protons, and test particle calculations confirm that a significant column of reflected protons lies upstream when the waves occur. The reflected protons perturb a large volume of plasma around the Moon, extending upstream as well as into the wake. The waves observed near the Moon during this time period have many similarities to those found in the terrestrial foreshock and at comets, suggesting the potential for comparative studies.
    Geophysical Research Letters 09/2013; 40(17):4544-4548. · 3.98 Impact Factor
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    ABSTRACT: We report observations by the dual-probe ARTEMIS mission of Moon-related electron and ion signatures obtained above the dayside lunar surface in the terrestrial magnetotail lobes. While the Moon is often thought of as a passive absorber, recent observations from Kaguya, Chandrayaan, Chang'E, and ARTEMIS indicate that plasma of lunar origin can have significant effects on the near-lunar environment. We now present new observations from ARTEMIS showing that lunar plasma can play a dominant role in the low-density environment of the terrestrial magnetotail. Two-point observations reveal that the density of plasma of lunar origin is higher than that of the ambient lobe plasma even several hundreds of kilometers above the Moon's dayside. Meanwhile, the distributions of incoming electrons exhibit modifications correlated with Moon-related populations, suggesting direct or indirect interactions of the lobe electrons with plasma of lunar origin. We also observe high-energy photoelectron emission from the dayside lunar surface, supporting the existence of large positive potentials on the lunar surface. Pickup ions with nonzero parallel-velocity components provide further evidence for positive surface potentials of tens of volts or more. ARTEMIS data reveal not only the existence of the positive surface potentials much larger than those predicted from a current-balance model based on Maxwellian plasmas, but also their significant implications for the dynamics of both the dominant Moon-originating ions and the tenuous ambient plasma populations in the tail lobe.
    Journal of Geophysical Research: Space Physics 01/2013; · 3.44 Impact Factor
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    ABSTRACT: We present a method for deriving constraints on the structure and composition of the lunar atmosphere by using pickup ion measurements from ARTEMIS, mapping observed fluxes from the spacecraft location to derive production rates at the source region, and fitting to a parameterized neutral atmosphere model. We apply this technique to ~12 min of high-resolution burst data collected by ARTEMIS P2 above the sunlit lunar surface, in the dawnside terrestrial magnetosheath. During this time period, ARTEMIS observed multiple velocity components, requiring the presence of multiple species and/or source regions. We use species at or near masses 12, 16, 24, 28, and 40 to derive a best-fit model that proves consistent with most known abundances and limits on neutral densities as well as predictions thereof. However, we find indications of large neutral abundances at mass ~16, exceeding optical limits on oxygen by a factor of ~20, possibly indicating either "seeding" of the Moon by terrestrial oxygen during its magnetotail passage or significant contributions by OH or CH4. We also derive new upper limits on the abundance of OH and Al in the atmosphere.
    Journal of Geophysical Research 01/2013; 118(1):81-88. · 3.17 Impact Factor
  • S. Lee, K. Shiokawa, J. P. McFadden, K. Seki
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    ABSTRACT: We investigate pitch‐angle distributions and spectral shapes of auroral electrons simultaneously observed during three conjunction events by the FAST satellite at altitudes of 2500–3500 km and by the THEMIS satellite in the plasma sheet at 7–13 RE. All three events were on the lower latitude of the auroral oval. Conjunction event 1 occurred at ~1914:45 UT on 10 April 2008. Electron spectra at energies of 0.1–3 keV are correlated well between the two satellites, while the precipitating electrons above 3 keV are missing at FAST. Event 2 occurred at 1255:26 UT on 26 April 2008. Electron spectra above 3 keV are correlated well between the two satellites. An additional broad spectral peak at energies of 0.1–0.5 keV was observed by FAST. Event 3 occurred at 0058:04 UT on 25 December 2008. Precipitating electrons of 0.5–5 keV obtained by FAST are correlated well with those of THEMIS‐C, while a monoenergetic peak at 0.1–0.2 keV was observed only by FAST. For the three conjunction events, we conclude that high‐energy precipitating auroral electrons observed by FAST directly come from the equatorial plasma sheet, while low‐energy precipitating electrons may come from middle altitudes as a result of acceleration by static potential differences. For missing high‐energy (>3 keV) electrons of event 1, we speculate that the pitch‐angle scattering by waves occurs only at a limited energy range.
    Journal of Geophysical Research 01/2013; 118(1):132-145. · 3.17 Impact Factor
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    ABSTRACT: We reexamine a long-period (10-18 min) poloidal pulsation observed by THEMIS-A in the outer dawn magnetosphere from 10:00 to 12:30 UT on 7 November 2007. The interval was originally reported by Korotova et al. (2009). Although the nonlinear compressional plasma and magnetic field perturbations observed by THEMIS-A during this interval agree well with model predictions for the linear perturbations associated with antisymmetric waves generated by the ballooning-mirror mode instability, the phase relationships between these perturbations indicates a complex frequency rather than the purely imaginary frequency that theory predicts for the outer dawnside magnetosphere. Variations in the radial plasma velocity confirm that a phase-locked north/south oscillation in the equatorial line of nodes associated with the ballooning-mirror mode waves doubles the frequency of the compressional component of the magnetic field during these pulsations. The same velocity and magnetic field perturbations exclude explanations for the frequency doubling in terms of spatial gradients sweeping back and forth across the spacecraft or drift-bounce resonances. Azimuthal electric fields associated with the pulsations generate field-aligned anisotropies in the pitch angle distributions that become more prominent with increasing ion energy due to the presence of drift-shell splitting and radial flux gradients that steepen with increasing energy. Although there was no evidence for drift-bounce interactions during this event, the role of such events in ion energization in other events and at other locations remains to be evaluated.
    Journal of Geophysical Research 11/2012; 117(A11):11215-. · 3.17 Impact Factor
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    ABSTRACT: We report observations by the twin-probe mission ARTEMIS of pick-up ions of lunar origin obtained during times when the Moon was within the terrestrial magnetotail lobes. These ions were detected as two separate focused beams above the dayside lunar surface. Analysis of these beams has shown that they possess both field-aligned and field-perpendicular velocities, implying the presence of electric fields both parallel and perpendicular to the magnetotail lobe magnetic field. We suggest that the sources of these two electric fields are (a) the near-surface electric field due to the lunar photoelectron sheath and (b) the electric field generated by the magnetotail lobe convection velocity. We use the energy and pitch angle spectra to constrain the source locations and compositions of these ions, and conclude that exospheric ionization of the neutral exosphere is the dominant lunar pick-up ion production mechanism in the tail lobes.
    Geophysical Research Letters 09/2012; 39(17):17104-. · 3.98 Impact Factor
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    ABSTRACT: A moderately intense substorm on 1 March 2008, from 0830 to 1000 UT, observed by THEMIS probes and the Ground Based Observatory (GBO) is examined to investigate the global evolution of substorm phenomena. During this interval, all five THEMIS probes are closely aligned along the tail axis near midnight covering a radial range from ˜9 Re to ˜18 Re. After the substorm onset, plasma sheet expansions take place successively at multiple locations in the magnetotail as measured by different probes. The positions of the plasma sheet expansions have a tailward leap progression with an average velocity of ˜36 km/s. There are two types of dipolarization detected in this substorm. The first type is the dipolarization front which is associated with the bursty bulk flow (BBF). While the second type, which we call ‘global dipolarization’, is associated with plasma sheet expansions. In the substorm studied, there are four intensifications as shown in the THEMIS AE index. We can detect the effects of localized and short-lived magnetic energy release processes occurring in the magnetotail corresponding to each of the four AE intensifications. Furthermore, the inner four probes can detect the global dipolarization signatures ˜4-15 min earlier than plasma sheet expansions, while the outermost probe (P1) cannot detect this before the plasma sheet expansion. These two phenomena are caused by the same process (magnetic energy release process) but the effects detected by probes locally appear delayed. The observations in this case are not sufficient to distinguish between the two competing substorm models.
    Journal of Geophysical Research 07/2012; 117(A7):7219-. · 3.17 Impact Factor
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    ABSTRACT: ARTEMIS observes pickup ions around the Moon, at distances of up to 20,000 km from the surface. The observed ions form a plume with a narrow spatial and angular extent, generally seen in a single energy/angle bin of the ESA instrument. Though ARTEMIS has no mass resolution capability, we can utilize the analytically describable characteristics of pickup ion trajectories to constrain the possible ion masses that can reach the spacecraft at the observation location in the correct energy/angle bin. We find that most of the observations are consistent with a mass range of ˜20-45 amu, with a smaller fraction consistent with higher masses, and very few consistent with masses below ˜15 amu. With the assumption that the highest fluxes of pickup ions come from near the surface, the observations favor mass ranges of ˜20-24 and ˜36-40 amu. Although many of the observations have properties consistent with a surface or near-surface release of ions, some do not, suggesting that at least some of the observed ions have an exospheric source. Of all the proposed sources for ions and neutrals about the Moon, the pickup ion flux measured by ARTEMIS correlates best with the solar wind proton flux, indicating that sputtering plays a key role in either directly producing ions from the surface, or producing neutrals that subsequently become ionized.
    Journal of Geophysical Research 06/2012; 117(E6):6006-. · 3.17 Impact Factor
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    ABSTRACT: The two ARTEMIS probes observe significant precursor activity upstream from the Moon, when magnetically connected to the dayside lunar surface. The most common signature consists of high levels of whistler wave activity near half of the electron cyclotron frequency. This precursor activity extends to distances of many thousands of km, in both the solar wind and terrestrial magnetosphere. In the magnetosphere, electrons reflect from a combination of magnetic and electrostatic fields above the lunar surface, forming loss cone distributions. In the solar wind they generally form conics, as a result of reflection from an obstacle moving with respect to the plasma frame (just as at a shock). The anisotropy associated with these reflected electrons provides the free energy source for the whistlers, with cyclotron resonance conditions met between the reflected source population and Moonward-propagating waves. These waves can in turn affect incoming plasma, and we observe significant perpendicular electron heating and plasma density depletions in some cases. In the magnetosphere, we also observe broadband electrostatic modes driven by beams of secondary electrons and/or photoelectrons accelerated outward from the surface. We also occasionally see waves near the ion cyclotron frequency in the magnetosphere. These lower frequency waves, which may result from the presence of ions of lunar origin, modulate the whistlers described above, as well as the electrons. Taken together, our observations suggest that the presence of the Moon leads to the formation of an upstream region analogous in many ways to the terrestrial electron foreshock.
    Journal of Geophysical Research 05/2012; 117(A5):5101-. · 3.17 Impact Factor
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    ABSTRACT: We use THEMIS and GOES observations to investigate the plasma sheet evolution on 28 February 2008 between 6:50 and 7:50 UT, when there developed strong magnetic field oscillations with period of 100 s. Using multi-spacecraft analysis of the plasma sheet observations and an empirical plasma sheet model, we determine both the large-scale evolution of the plasma sheet and the properties of the oscillations. We found that the oscillations exhibited signatures of kinetic ballooning/interchange instability fingers that developed in a bent current sheet. The interchange oscillations had a sausage structure, propagated duskward at a velocity of about 100 km/s, and were associated with periodical radial electron flows. We suggest that the observed negative gradient of the ZGSM magnetic field component (dBZ/dX) was a free energy source for the kinetic ballooning/interchange instability. Tens of minutes later a fast elongation of ballooning/interchange fingers was detected between 6 and 16 RE downtail with the legnth-to-width ratio exceeding 20. The finger elongation ended with signatures of reconnection in an embedded current sheet near the bending point. These observations suggest a complex interplay between the midtail and near-Earth plasma sheet dynamics, involving localized fluctuations both in cross-tail and radial directions before current sheet reconnection.
    Journal of Geophysical Research 04/2012; · 3.17 Impact Factor
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    ABSTRACT: The THEMIS mission includes three closely separated probes that provide the opportunity to analyze the small and meso-scale dynamics of the cross-tail current sheet in the near-Earth magnetosphere (10Re). In this study, we focus on dipolarization events which occurred when two of these satellites were: (1) separated only along the Z direction (i.e., at the same location in the XYGSM) plane, and: (2) on separate sides of the neutral sheet. Following these criteria, our search resulted in 25 dipolarization events. Based on these, we demonstrate that dipolarizations systematically correspond to a thickening of the current sheet rather than any other phenomena (e.g., flapping). We also show that the current density in the sheet systematically decreases after onset. Most of the events show an increase of the Laplace force and of the magnetic tension after the dipolarization onset. We find that the total energy density (total pressure) increases after 70% of the dipolarization events. However when excluding the Z-component of the magnetic field (which is cancelled by the dominant curvature terms) from the pressure, we find that the pressure decreases after 64% of events, and the remaining pressure increases occur closest to the neutral sheet. Average values are used in the time ranges [-10 -5] and [+10 +15] minutes around onset. We discuss the importance of both the spacecraft location relative to the neutral sheet and time scales chosen when calculating the pressure changes. These directly impact interpretations relative to dipolarization models.
    Journal of Geophysical Research 04/2012; 117(A9):12753-. · 3.17 Impact Factor
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    ABSTRACT: Based on conjugate ground and THEMIS satellite observations, we show electron spectra and wave characteristics near the magnetic equatorial plane during a pulsating aurora event on the high latitude side of the auroral oval. The pulsating aurora was observed by a 30-Hz sampled all-sky imager (ASI) at Gillam (56.4°N, 265.4°E), Canada, at ˜0840-0910 UT on 8 January 2008. The auroral intensity pulsation at the possible THEMIS D (THD) footprints had frequency peaks at ˜0.1-0.2 Hz. The footprint of THD was in the poleward part of the proton aurora observed by a meridian-scanning photometer. After auroral pulsation began at ˜0842 UT, both THD and THEMIS E which was near THD in the mid-tail at 11.6-11.8 RE, observed enhanced field-aligned electron fluxes at energies of 1-10 keV. However, the amplitudes of whistler mode waves and electrostatic cyclotron harmonics (ECH) waves observed by THD with the highest sampling rate of 8 kHz were not significant, showing a marked contrast to the recent report of clear correlation between whistler mode waves and auroral pulsations observed at 5-9 RE. We suggest that the observed field-aligned electrons, which are probably caused by Fermi-type acceleration associated with earthward plasma flow in the mid-tail plasma sheet, are modulated by some wave processes to cause pulsating auroras.
    Journal of Geophysical Research 03/2012; 117(A3):3219-. · 3.17 Impact Factor

Publication Stats

6k Citations
617.54 Total Impact Points

Institutions

  • 1–2014
    • University of California, Berkeley
      • Space Sciences Laboratory
      Berkeley, California, United States
  • 2001–2009
    • University of Colorado at Boulder
      • • Department of Astrophysical and Planetary Sciences
      • • Laboratory for Atmospheric and Space Physics (LASP)
      Boulder, CO, United States
  • 2–2008
    • University of California, Los Angeles
      • Department of Atmospheric and Oceanic Sciences (AOS)
      Los Angeles, California, United States
  • 1997
    • CSU Mentor
      Long Beach, California, United States