W. S. Kurth

University of Iowa, Iowa City, Iowa, United States

Are you W. S. Kurth?

Claim your profile

Publications (647)2116.79 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the excitation and propagation of equatorial magnetosonic waves observed by the Van Allen Probes, and describe evidence for a trapping mechanism for magnetosonic waves in the Earth's plasmasphere. Intense equatorial magnetosonic waves were observed inside the plasmasphere in association with a pronounced proton ring distribution, which provides free energy for wave excitation. Instability analysis along the inbound orbit demonstrates that broad-band magnetosonic waves can be excited over a localized spatial region near the plasmapause. The waves can subsequently propagate into the inner plasmasphere and remain trapped over a limited radial extent, consistent with the predictions of near-perpendicular propagation. By performing a similar analysis on another observed magnetosonic wave event, we demonstrate that magnetosonic waves can also be trapped within local density structures. We suggest that perpendicular wave propagation is important for explaining the presence of magnetosonic waves in the Earth's plasmasphere at locations away from the generation region.
    Geophysical Research Letters. 09/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: On September 26th, 2005, Cassini conducted its only close targeted flyby of Saturn's small, irregularly shaped moon Hyperion. Approximately 6 minutes before the closest approach, the Electron spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) detected a field-aligned electron population originating from the direction of the moon's surface. Plasma wave activity detected by the Radio and Plasma Wave (RPWS) instrument suggests electron beam activity. A dropout in energetic electrons was observed by both CAPS-ELS and the Magnetospheric Imaging Instrument Low Energy Magnetospheric Measurement System (MIMI-LEMMS), indicating that the moon and the spacecraft were magnetically connected when the field-aligned electron population was observed. We show that this constitutes a remote detection of a strongly negative (~ − 200 V) surface potential on Hyperion, consistent with the predicted surface potential in regions near the solar terminator.
    Geophysical Research Letters. 09/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We use an E × B-driven plasmapause test particle (PTP) simulation to provide global contextual information for in situ measurements by the Van Allen Probes (RBSP) during 15–20 January 2013. During 120 h of simulation time beginning on 15 January, geomagnetic activity produced three plumes. The third and largest simulated plume formed during enhanced convection on 17 January, and survived as a rotating, wrapped, residual plume for tens of hours. To validate the simulation, we compare its output with RBSP data. Virtual RBSP satellites recorded 28 virtual plasmapause encounters during 15–19 January. For 26 of 28 (92%) virtual crossings, there were corresponding actual RBSP encounters with plasmapause density gradients. The mean difference in encounter time between model and data is 36 min. The mean model-data difference in radial location is 0:40±0:05 RE. The model-data agreement is better for strong convection than for quiet or weakly disturbed conditions. On 18 January, both RBSP spacecraft crossed a tenuous, detached plasma feature at approximately the same time and nightside location as a wrapped residual plume, predicted by the model to have formed 32 h earlier on 17 January. The agreement between simulation and data indicates that the model-provided global information is adequate to correctly interpret the RBSP density observations.
    Journal of Geophysical Research: Space Physics. 09/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present the first systematic investigation of the polar cap boundary in Saturn's high-latitude magnetosphere through a multi-instrument assessment of various Cassini in situ data sets gathered between 2006 and 2009. We identify 48 polar cap crossings where the polar cap boundary can be clearly observed in the step in upper cut-off of auroral hiss emissions from the plasma wave data, a sudden increase in electron density, an anisotropy of energetic electrons along the magnetic field, and an increase in incidence of higher energy electrons from the low energy electron spectrometer measurements as we move equatorward from the pole. We determine the average level of coincidence of the polar cap boundary identified in the various in situ data sets to be 0.34° ± 0.05° co-latitude. The average location of the boundary in the southern (northern) hemisphere is found to be at 15.6° (13.3°) co-latitude. In both hemispheres we identify a consistent equatorward offset between the poleward edge of the auroral upward-directed field-aligned current region of ~1.5-1.8° co-latitude to the corresponding polar cap boundary. We identify atypical observations in the boundary region, including observations of ~ hourly periodicities in the auroral hiss emissions close to the pole. We suggest that the position of the southern polar cap boundary is somewhat ordered by the southern planetary-period oscillation phase, but that it cannot account for the boundary's full latitudinal variability. We find no clear evidence of any ordering of the northern polar cap boundary location with the northern planetary-period magnetic field oscillation phase.
    Journal of Geophysical Research: Space Physics. 09/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: First nanodust detection by Cassini near 1 AUNanodust flux near 1 AU is determinedNanodust flux and variability are consistent with STEREO and modelsFirst nanodust detection by Cassini near 1 AUNanodust flux near 1 AU is determinedNanodust flux and variability are consistent with STEREO and models
    Geophysical Research Letters 08/2014; 41(15). · 3.98 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Wave normal distributions of lower-band whistler-mode waves observed outside the plasmapause exhibit two peaks; one near the parallel direction and the other at very oblique angles. We analyze a number of conjunction events between the Van Allen Probes near the equatorial plane and POES satellites at conjugate low altitudes, where lower-band whistler-mode wave amplitudes were inferred from the two-directional POES electron measurements over 30–100 keV, assuming that these waves were quasi-parallel. For conjunction events, the wave amplitudes inferred from the POES electron measurements were found to be overestimated as compared with the Van Allen Probes measurements primarily for oblique waves and quasi-parallel waves with small wave amplitudes (< ~20 pT) measured at low latitudes. This provides plausible experimental evidence of stronger pitch-angle scattering loss caused by oblique waves than by quasi-parallel waves with the same magnetic wave amplitudes, as predicted by numerical calculations.
    Geophysical Research Letters. 08/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The Cassini Radio and Plasma Wave Science (RPWS) instrument can detect dust particles when voltage pulses induced by the dust impacts are observed in the wideband receiver. The size of the voltage pulse is proportional to the mass of the impacting dust particle. For the first time, the dust impacts signals measured by dipole and monopole electric antennas are compared, from which the effective impact area of the spacecraft is estimated to be 4 m2. In the monopole mode, the polarity of the dust impact signal is determined by the spacecraft potential and the location of the impact (on the spacecraft body or the antenna), which can be used to statistically infer the charge state of the spacecraft. It is shown that the differential number density of the dust particles near Saturn can be characterized as a power law dn /dr ∝ rμ, where μ ~–4 and r is the particle size. No peak is observed in the size distribution, contrary to the narrow size distribution found by previous studies. The RPWS cumulative dust density is compared with the Cosmic Dust Analyzer (CDA) High Rate Detector (HRD) measurement. The differences between the two instruments are within the range of uncertainty estimated for RPWS measurement. The RPWS on-board dust recorder and counter data are used to map the dust density and spacecraft charging state within Saturn's magnetosphere.
    Journal of Geophysical Research: Space Physics. 07/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Nanodust grains of a few nanometer in size are produced near the Sun by collisional break-up of larger grains and picked-up by the magnetized solar wind. They have so far been detected at 1 AU by only the two STEREO spacecraft. Here we analyze the spectra measured by the radio and plasma wave instrument onboard Cassini during the cruise phase close to Earth orbit; they exhibit bursty signatures similar to those observed by the same instrument in association to nanodust stream impacts on Cassini near Jupiter. The observed wave level and spectral shape reveal impacts of nanoparticles at about 300 km/s, with an average flux compatible with that observed by the radio and plasma wave instrument onboard STEREO and with the interplanetary flux models.
    Geophysical Research Letters. 06/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Local acceleration driven by whistler-mode chorus waves is fundamentally important for accelerating seed electron populations to highly relativistic energies in the outer radiation belt. In this study, we quantitatively evaluate chorus-driven electron acceleration during the 17 March 2013 storm, when the Van Allen Probes observed very rapid electron acceleration up to several MeV within ~12 hours. A clear radial peak in electron phase space density (PSD) observed near L* ~4 indicates that an internal local acceleration process was operating. We construct the global distribution of chorus wave intensity from the low-altitude electron measurements made by multiple Polar Orbiting Environmental Satellites (POES) satellites over a broad region, which is ultimately used to simulate the radiation belt electron dynamics driven by chorus waves. Our simulation results show remarkable agreement in magnitude, timing, energy dependence, and pitch angle distribution with the observed electron PSD near its peak location. However, radial diffusion and other loss processes may be required to explain the differences between the observation and simulation at other locations away from the PSD peak. Our simulation results, together with previous studies, suggest that local acceleration by chorus waves is a robust and ubiquitous process and plays a critical role in accelerating injected seed electrons with convective energies (~100 keV) to highly relativistic energies (several MeV).
    Journal of Geophysical Research: Space Physics. 06/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the wave observations, associated plasma measurements, and linear theory testing of electromagnetic ion cyclotron (EMIC) wave events observed by the Van Allen Probes on 28 April 2013. The wave events are detected in their generation regions as three individual events in two consecutive orbits of Van Allen Probe-A, while the other spacecraft, B, does not detect any significant EMIC wave activity during this period. Three overlapping H+ populations are observed around the plasmapause when the waves are excited. The difference between the observational EMIC wave growth parameter (Σh) and the theoretical EMIC instability parameter (Sh) is significantly raised, on average, to 0.10 ± 0.01, 0.15 ± 0.02, and 0.07 ± 0.02 during the three wave events, respectively. On Van Allen Probe-B, this difference never exceeds 0. Compared to linear theory (Σh > Sh), the waves are only excited for elevated thresholds.
    Geophysical Research Letters. 06/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: It has been reported from Van Allen Probe observations that plasmaspheric hiss intensification in the outer plasmasphere, associated with a substorm injection on Sept 30 2012, occurred with a peak frequency near 100 Hz, well below the typical plasmaspheric hiss frequency range, extending down to ~20 Hz. We examine this event of unusually low frequency plasmaspheric hiss to understand its generation mechanism. Quantitative analysis is performed by simulating wave ray paths via the HOTRAY ray tracing code with measured plasma density and calculating ray path-integrated wave gain evaluated using the measured energetic electron distribution. We demonstrate that the growth rate due to substorm injected electrons is positive but rather weak, leading to small wave gain (~10 dB) during a single equatorial crossing. Propagation characteristics aided by the sharp density gradient associated with the plasmapause, however, can enable these low frequency waves to undergo cyclic ray paths, which return to the unstable region leading to repeated amplification to yield sufficient net wave gain (>40 dB) to allow waves to grow from the thermal noise.
    Geophysical Research Letters. 06/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present observations of significant dynamics within two UV auroral storms observed on Saturn using the Hubble Space Telescope in April/May 2013. Specifically, we discuss bursts of auroral emission observed at the poleward boundary of a solar wind-induced auroral storm, propagating at ~330% rigid corotation from near ~01 h LT toward ~08 h LT. We suggest these are indicative of ongoing, bursty reconnection of lobe flux in the magnetotail, providing strong evidence that Saturn's auroral storms are caused by large-scale flux closure. We also discuss the later evolution of a similar storm, and show that the emission maps to the trailing region of an energetic neutral atom enhancement. We thus identify the auroral form with the upward field-aligned continuity currents flowing into the associated partial ring current.
    Geophysical Research Letters. 05/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Local acceleration driven by whistler-mode chorus waves largely accounts for the enhancement of radiation belt relativistic electron fluxes, whose favored region is usually considered to be the plasmatrough with magnetic local time approximately from midnight through dawn to noon. On 2 October 2013, the Van Allen Probes recorded a rarely-reported event of intense duskside lower-band chorus waves (with power spectral density up to 10−3 nT2 / Hz) in the low-latitude region outside of L = 5. Such chorus waves are found to be generated by the substorm-injected anisotropic suprathermal electrons, and have a potentially strong acceleration effect on the radiation belt energetic electrons. This event study demonstrates the possibility of broader spatial regions with effective electron acceleration by chorus waves than previously expected. For such intense duskside chorus waves, the occurrence probability, the preferential excitation conditions, the time duration and the accurate contribution to the long term evolution of radiation belt electron fluxes may need further investigations in future.
    Journal of Geophysical Research: Space Physics. 05/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The recent launching of Van Allen probes provides an unprecedent opportunity to investigate variations of the radiation belt relativistic electrons. During the March 17–19 2013 storm, the Van Allen probes simultaneously detected strong chorus waves and substantial increases in fluxes of relativistic (2–4.5 MeV) electrons around L = 4.5. Chorus waves occurred within the lower-band 0.1–0.5fce (the electron equatorial gyrofrequency), with a peak spectral density ~10−4 nT2/Hz. Correspondingly, relativistic electron fluxes increased by a factor of 102–103 during the recovery phase compared to the main phase levels. By means of a Gaussian fit to the observed chorus spectra, the drift and bounce-averaged diffusion coefficients are calculated and then used to solve a 2D Fokker-Planck diffusion equation. Numerical simulations demonstrate that the lower-band chorus waves indeed produce such huge enhancements in relativistic electron fluxes within 15 hours, fitting well with the observation.
    Journal of Geophysical Research: Space Physics. 05/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Measurements made by the Galileo Energetic Particles Detector (EPD) and the plasma wave/radio instrument (PWS) are analyzed to establish relationships between dynamic processes observed independently in the distant and the inner jovian disk (at 80–120 Jovian radii (RJ) and 10–25 RJ, respectively). It is first shown that global magnetospheric disturbances identified from the radio emissions (the ‘energetic events’ [Louarn et al., 1998]) are well correlated with reconnection/reconfiguration events observed at the outer edge of the disk [Kronberg et al., 2005, Vogt et al., 2010]). Then, considering all Galileo perijoves, it is also demonstrated that the ‘energetic events’ occurring as Galileo was at less than ~25 RJ are systematically associated with new injections of energetic particles [Mauk et al., 1999] seen from ~10 to 25 RJ. This demonstrates that major disturbances commonly affect the whole magnetodisc, from 10 to 80–120 RJ. Overall, their phenomenology involves simultaneous auroral activation, formation of new sources of radio emission (nKOM) and particle injections in the Io torus, magnetic reconfigurations and radial flow bursts in the distant disk, over time scale of a few hours.
    Journal of Geophysical Research: Space Physics. 05/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: In recent years we have benefitted greatly from the first in-orbit multi-wavelength images of Saturn's polar atmosphere from the Cassini spacecraft. Specifically, images obtained from the Cassini UltraViolet Imaging Spectrograph (UVIS) provide an excellent view of the planet's auroral emissions, which in turn give an account of the large-scale magnetosphere-ionosphere coupling and dynamics within the system. However, obtaining near-simultaneous views of the auroral regions with in situ measurements of magnetic field and plasma populations at high-latitudes is more difficult to routinely achieve. Here we present an unusual case, during Revolution 99 in January 2009, where UVIS observes the entire northern UV auroral oval during a two-hour interval while Cassini traverses the magnetic flux tubes connecting to the auroral regions near 21 LT, sampling the related magnetic field, particle, and radio and plasma wave signatures. The motion of the auroral oval evident from the UVIS images requires a careful interpretation of the associated latitudinally “oscillating” magnetic field and auroral field-aligned current signatures whereas previous interpretations have assumed a static current system. Concurrent observations of the auroral hiss (typically generated in regions of downward directed field-aligned current) support this revised interpretation of an oscillating current system. The nature of the motion of the auroral oval evident in the UVIS image sequence, and the simultaneous measured motion of the field-aligned currents (and related plasma boundary) in this interval, is shown to be related to the northern hemisphere magnetosphere oscillation phase. This is in agreement with previous observations of the auroral oval oscillatory motion.
    Journal of Geophysical Research: Space Physics. 04/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Enceladus’ southern plume is one of the major discoveries of the Cassini mission. The water neutrals and ice crystals ejected by the cryovolcanic activity populate Saturn's E-ring and the neutral torus, and they interact with the plasma environment of Saturn's magnetosphere. The plasma neutrality inside Enceladus’ plume has been shown by the Langmuir probe measurement to be modified by the presence of the dust particles. We present an independent method of determining the electron density inside the plume. Sometimes after dust impacts, plasma oscillations (ringing) were detected by the Cassini Radio and Plasma Wave Science (RPWS) instrument. The frequencies of these oscillations have been shown to be consistent with the local plasma frequency, thus providing a measurement of the local electron density.
    Journal of Geophysical Research: Space Physics. 04/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: [1] Drastic variations of Earth's outer radiation belt electrons ultimately result from various competing source, loss, and transport processes, to which wave-particle interactions are critically important. Using 15 spacecraft including NASA's Van Allen Probes, THEMIS, and SAMPEX missions and NOAA's GOES and POES constellations, we investigated the evolution of the outer belt during the strong geomagnetic storm of 30 September – 03 October 2012. This storm's main phase dropout exhibited enhanced losses to the atmosphere at L* < 4, where the phase space density (PSD) of multi-MeV electrons dropped by over an order of magnitude in <4 hours. Based on POES observations of precipitating >1 MeV electrons and energetic protons, SAMPEX >1 MeV electrons, and ground observations of band-limited Pc1-2 wave activity, we show that this sudden loss was consistent with pitch-angle scattering by EMIC waves in the dusk MLT sector at 3 < L* < 4. At 4 < L* < 5, local acceleration was also active during the main and early recovery phases, when growing peaks in electron PSD were observed by both Van Allen Probes and THEMIS. This acceleration corresponded to the period when IMF Bz was southward, the AE index was >300nT, and energetic electron injections and whistler-mode chorus waves were observed throughout the inner magnetosphere for >12 hrs. After this period, Bz turned northward, and injections, chorus activity, and enhancements in PSD ceased. Overall, the outer belt was depleted by this storm. From the unprecedented level of observations available, we show direct evidence of the competitive nature of different wave-particle interactions controlling relativistic electron fluxes in the outer radiation belt.
    Journal of Geophysical Research: Space Physics. 03/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: quantify the resonant scattering effects of the unusual low-frequency dawnside plasmaspheric hiss observed on 30 September 2012 by the Van Allen Probes. In contrast to normal (~100-2000 Hz) hiss emissions, this unusual hiss event contained most of its wave power at ~20-200 Hz. Compared to the scattering by normal hiss, the unusual hiss scattering speeds up the loss of ~50-200 keV electrons and produces more pronounced pancake distributions of ~50-100 keV electrons. It is demonstrated that such unusual low-frequency hiss, even with a duration of a couple of hours, plays a particularly important role in the decay and loss process of energetic electrons, resulting in shorter electron lifetimes for ~50-400 keV electrons than normal hiss, and should be carefully incorporated into global modeling of radiation belt electron dynamics during periods of intense injections.
    02/2014; 41(6).
  • [Show abstract] [Hide abstract]
    ABSTRACT: increases, or "bursts," of 17-26 keV electron fluxes in conjunction with chorus wave bursts were observed following a plasma injection on 13 January 2013. The pitch angle distributions changed during the burst events, evolving from sinN(α) to distributions that formed maxima at α = 75-80°, while fluxes at 90° and <60° remained nearly unchanged. The observations occurred outside of the plasmasphere in the postmidnight region and were observed by both Van Allen Probes. Density, cyclotron frequency, and pitch angle of the peak flux were used to estimate resonant electron energy. The result of ~15-35 keV is consistent with the energies of the electrons showing the flux enhancements and corresponds to electrons in and above the steep flux gradient that signals the presence of an Alfvén boundary in the plasma. The cause of the quasiperiodic nature (on the order of a few minutes) of the bursts is not understood at this time.
    02/2014; 41(6).

Publication Stats

7k Citations
2,116.79 Total Impact Points

Institutions

  • 1976–2014
    • University of Iowa
      • Department of Physics and Astronomy
      Iowa City, Iowa, United States
  • 2011
    • Swedish Institute of Space Physics
      Kiruna, Norrbotten, Sweden
  • 2009–2011
    • Austrian Academy of Sciences
      • Institut für Weltraumforschung
      Vienna, Vienna, Austria
  • 2008
    • Imperial College London
      • Department of Physics
      Londinium, England, United Kingdom
  • 2005–2007
    • NASA
      Washington, West Virginia, United States
  • 2006
    • IST Austria
      Klosterneuberg, Lower Austria, Austria
    • Los Alamos National Laboratory
      • Space Science and Applications Group
      Los Alamos, NM, United States
  • 2003
    • Hampton University
      Hampton, Virginia, United States
    • Middle Tennessee State University
      Murfreesboro, Tennessee, United States
  • 2001
    • Johns Hopkins University
      • Applied Physics Laboratory
      Baltimore, MD, United States