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Donald A. Gurnett

Bio: Donald A. Gurnett is an academic researcher from University of Iowa. The author has contributed to research in topics: Saturn & Waves in plasmas. The author has an hindex of 23, co-authored 43 publications receiving 2517 citations.

Papers
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Journal ArticleDOI
TL;DR: The Cassini radio and plasma wave investigation is designed to study radio emissions, plasma waves, thermal plasma, and dust in the vicinity of Saturn as mentioned in this paper, which is the only spacecraft that can perform radio and plasmas measurements.
Abstract: The Cassini radio and plasma wave investigation is designed to study radio emissions, plasma waves, thermal plasma, and dust in the vicinity of Saturn. Three nearly orthogonal electric field antennas are used to detect electric fields over a frequency range from 1 Hz to 16 MHz, and three orthogonal search coil magnetic antennas are used to detect magnetic fields over a frequency range from 1 Hz to 12 kHz. A Langmuir probe is used to measure the electron density and temperature. Signals from the electric and magnetic antennas are processed by five receiver systems: a high frequency receiver that covers the frequency range from 3.5 kHz to 16 MHz, a medium frequency receiver that covers the frequency range from 24 Hz to 12 kHz, a low frequency receiver that covers the frequency range from 1 Hz to 26 Hz, a five-channel waveform receiver that covers the frequency range from 1 Hz to 2.5 kHz in two bands, 1 Hz to 26 Hz and 3 Hz to 2.5 kHz, and a wideband receiver that has two frequency bands, 60 Hz to 10.5 kHz and 800 Hz to 75 kHz. In addition, a sounder transmitter can be used to stimulate plasma resonances over a frequency range from 3.6 kHz to 115.2 kHz. Fluxes of micron-sized dust particles can be counted and approximate masses of the dust particles can be determined using the same techniques as Voyager. Compared to Voyagers 1 and 2, which are the only spacecraft that have made radio and plasma wave measurements in the vicinity of Saturn, the Cassini radio and plasma wave instrument has several new capabilities. These include (1) greatly improved sensitivity and dynamic range, (2) the ability to perform direction-finding measurements of remotely generated radio emissions and wave normal measurements of plasma waves, (3) both active and passive measurements of plasma resonances in order to give precise measurements of the local electron density, and (4) Langmuir probe measurements of the local electron density and temperature. With these new capabilities, it will be possible to perform a broad range of studies of radio emissions, wave-particle interactions, thermal plasmas and dust in the vicinity of Saturn.

561 citations

Journal ArticleDOI
TL;DR: In this article, the authors measured the electric field at both high and low altitude and found that the large-scale electric field is the same at both altitudes, as expected, but small-scale features with wavelengths less than 100 km which are larger in magnitude at higher altitude.
Abstract: Nearly simultaneous measurements of auroral zone electric fields are obtained by the Dynamics Explorer spacecraft at altitudes below 900 km and above 4,500 km during magnetic conjunctions. The measured electric fields are usually perpendicular to the magnetic field lines. The north-south meridional electric fields are projected to a common altitude by a mapping function which accounts for the convergence of the magnetic field lines. When plotted as a function of invariant latitude, graphs of the projected electric fields measured by both DE-1 and DE-2 show that the large-scale electric field is the same at both altitudes, as expected. Superimposed on the large-scale fields, however, are small-scale features with wavelengths less than 100 km which are larger in magnitude at the higher altitude. Fourier transforms of the electric fields show that the magnitudes depend on wavelength. Outside of the auroral zone the electric field spectrums are nearly identical. But within the auroral zone the high and low altitude electric fields have a ratio which increases with the reciprocal of the wavelength. The small-scale electric field variations are associated with field-aligned currents. These currents are measured with both a plasma instrument and magnetometer on DE-1.

199 citations

Journal ArticleDOI
TL;DR: In this article, it was shown that the first component originates from the southern auroral region, and that the second component originated from the northern auroral regions, which has potentially important implications on how angular momentum is transferred from the interior to the magnetosphere.
Abstract: [1] For many years it has been known that Saturn emits intense radio emissions at kilometer wavelengths and that this radiation is modulated by the rotation of the planet at a rate that varies by up to one percent on a time scale of years. Recent radio observations from the Cassini spacecraft have revealed the appearance of a second component, with a rotation period of about 10.6 hours, significantly less than the period of the previously known component, which is currently about 10.8 hours. In this paper we show that the first component originates from the southern auroral region, and that the second component originates from the northern auroral region. This north-south asymmetry in the rotation period has potentially important implications on how angular momentum is transferred from the interior to the magnetosphere.

181 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used dynamics Explorer 1 measurements to investigate regions of low electron density in the night-side auroral zone and found that electron density profile variations of a factor of 2 or more on spatial scales of tens of kilometers are found, and the electron plasma frequency to electron cyclotron frequency ratios are 0.02-0.4.
Abstract: Dynamics Explorer 1 measurements are used to investigate regions of low electron density in the nightside auroral zone. Sharply defined regions of low electron density are found in auroral zone crossings from the predusk hours until the early morning hours at all radial distances up to at least 4.6 earth radii. Densities in the auroral cavity are shown to fall to values below 0.3/cu cm. Within the auroral cavity, electron-density-profile variations of a factor of 2 or more on spatial scales of tens of kilometers are found, and the electron plasma frequency to electron cyclotron frequency ratios are 0.02-0.4. The results suggest associations between the density depletions in the nightside auroral zone and auroral acceleration processes.

181 citations


Cited by
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Journal ArticleDOI
19 Dec 2013-Nature
TL;DR: High-resolution electron observations obtained during the 9 October storm are reported and chorus scattering explains the temporal evolution of both the energy and angular distribution of the observed relativistic electron flux increase, and detailed modelling demonstrates the remarkable efficiency of wave acceleration in the Earth's outer radiation belt.
Abstract: Recent analysis of satellite data obtained during the 9 October 2012 geomagnetic storm identified the development of peaks in electron phase space density, which are compelling evidence for local electron acceleration in the heart of the outer radiation belt, but are inconsistent with acceleration by inward radial diffusive transport. However, the precise physical mechanism responsible for the acceleration on 9 October was not identified. Previous modelling has indicated that a magnetospheric electromagnetic emission known as chorus could be a potential candidate for local electron acceleration, but a definitive resolution of the importance of chorus for radiation-belt acceleration was not possible because of limitations in the energy range and resolution of previous electron observations and the lack of a dynamic global wave model. Here we report high-resolution electron observations obtained during the 9 October storm and demonstrate, using a two-dimensional simulation performed with a recently developed time-varying data-driven model, that chorus scattering explains the temporal evolution of both the energy and angular distribution of the observed relativistic electron flux increase. Our detailed modelling demonstrates the remarkable efficiency of wave acceleration in the Earth's outer radiation belt, and the results presented have potential application to Jupiter, Saturn and other magnetized astrophysical objects.

665 citations

Journal ArticleDOI
TL;DR: The Cassini radio and plasma wave investigation is designed to study radio emissions, plasma waves, thermal plasma, and dust in the vicinity of Saturn as mentioned in this paper, which is the only spacecraft that can perform radio and plasmas measurements.
Abstract: The Cassini radio and plasma wave investigation is designed to study radio emissions, plasma waves, thermal plasma, and dust in the vicinity of Saturn. Three nearly orthogonal electric field antennas are used to detect electric fields over a frequency range from 1 Hz to 16 MHz, and three orthogonal search coil magnetic antennas are used to detect magnetic fields over a frequency range from 1 Hz to 12 kHz. A Langmuir probe is used to measure the electron density and temperature. Signals from the electric and magnetic antennas are processed by five receiver systems: a high frequency receiver that covers the frequency range from 3.5 kHz to 16 MHz, a medium frequency receiver that covers the frequency range from 24 Hz to 12 kHz, a low frequency receiver that covers the frequency range from 1 Hz to 26 Hz, a five-channel waveform receiver that covers the frequency range from 1 Hz to 2.5 kHz in two bands, 1 Hz to 26 Hz and 3 Hz to 2.5 kHz, and a wideband receiver that has two frequency bands, 60 Hz to 10.5 kHz and 800 Hz to 75 kHz. In addition, a sounder transmitter can be used to stimulate plasma resonances over a frequency range from 3.6 kHz to 115.2 kHz. Fluxes of micron-sized dust particles can be counted and approximate masses of the dust particles can be determined using the same techniques as Voyager. Compared to Voyagers 1 and 2, which are the only spacecraft that have made radio and plasma wave measurements in the vicinity of Saturn, the Cassini radio and plasma wave instrument has several new capabilities. These include (1) greatly improved sensitivity and dynamic range, (2) the ability to perform direction-finding measurements of remotely generated radio emissions and wave normal measurements of plasma waves, (3) both active and passive measurements of plasma resonances in order to give precise measurements of the local electron density, and (4) Langmuir probe measurements of the local electron density and temperature. With these new capabilities, it will be possible to perform a broad range of studies of radio emissions, wave-particle interactions, thermal plasmas and dust in the vicinity of Saturn.

561 citations

Journal ArticleDOI
TL;DR: A review of the present status of existing models for particle acceleration during impulsive solar flares, was inspired by a week-long workshop held in the Fall of 1993 at NASA Goddard Space Flight Center as mentioned in this paper.
Abstract: This paper, a review of the present status of existing models for particle acceleration during impulsive solar flares, was inspired by a week-long workshop held in the Fall of 1993 at NASA Goddard Space Flight Center. Recent observations from Yohkoh and the Compton Gamma Ray Observatory, and a reanalysis of older observations from the Solar Maximum Mission, have led to important new results concerning the location, timing, and efficiency of particle acceleration in flares. These are summarized in the first part of the review. Particle acceleration processes are then discussed, with particular emphasis on new developments in stochastic acceleration by magnetohydrodynamic waves and direct electric field acceleration by both sub- and super-Dreicer electric fields. Finally, issues that arise when these mechanisms are incorporated into the large-scale flare structure are considered. Stochastic and super-Dreicer acceleration may occur either in a single large coronal reconnection site or at multiple “fragmented” energy release sites. Sub-Dreicer acceleration requires a highly filamented coronal current pattern. A particular issue that needs to be confronted by all theories is the apparent need for large magnetic field strengths in the flare energy release region.

496 citations

Journal ArticleDOI
TL;DR: A comprehensive review of dispersive Alfven waves in space and laboratory plasmas is presented in this article, where the authors show how the inclusion of ion gyroradius, parallel electron inertia, and finite frequency effects modify the Alfven wave properties.
Abstract: This paper presents a comprehensive review of dispersive Alfven waves in space and laboratory plasmas. We start with linear properties of Alfven waves and show how the inclusion of ion gyroradius, parallel electron inertia, and finite frequency effects modify the Alfven wave properties. Detailed discussions of inertial and kinetic Alfven waves and their polarizations as well as their relations to drift Alfven waves are presented. Up to date observations of waves and field parameters deduced from the measurements by Freja, Fast, and other spacecraft are summarized. We also present laboratory measurements of dispersive Alfven waves, that are of most interest to auroral physics. Electron acceleration by Alfven waves and possible connections of dispersive Alfven waves with ionospheric-magnetospheric resonator and global field-line resonances are also reviewed. Theoretical efforts are directed on studies of Alfven resonance cones, generation of dispersive Alfven waves, as well their nonlinear interactions with the background plasma and self-interaction. Such topics as the dispersive Alfven wave ponderomotive force, density cavitation, wave modulation/filamentation, and Alfven wave self-focusing are reviewed. The nonlinear dispersive Alfven wave studies also include the formation of vortices and their dynamics as well as chaos in Alfven wave turbulence. Finally, we present a rigorous evaluation of theoretical and experimental investigations and point out applications and future perspectives of auroral Alfven wave physics.

478 citations

Journal ArticleDOI
TL;DR: The Cassini Plasma Spectrometer (CAPS) as discussed by the authors is a three-dimensional mass-resolved measurements of the full variety of plasma phenomena found in Saturn's magnetosphere.
Abstract: The Cassini Plasma Spectrometer (CAPS) will make comprehensive three-dimensional mass-resolved measurements of the full variety of plasma phenomena found in Saturn’s magnetosphere. Our fundamental scientific goals are to understand the nature of saturnian plasmas primarily their sources of ionization, and the means by which they are accelerated, transported, and lost. In so doing the CAPS investigation will contribute to understanding Saturn’s magnetosphere and its complex interactions with Titan, the icy satellites and rings, Saturn’s ionosphere and aurora, and the solar wind. Our design approach meets these goals by emphasizing two complementary types of measurements: high-time resolution velocity distributions of electrons and all major ion species; and lower-time resolution, high-mass resolution spectra of all ion species. The CAPS instrument is made up of three sensors: the Electron Spectrometer (ELS), the Ion Beam Spectrometer (IBS), and the Ion Mass Spectrometer (IMS). The ELS measures the velocity distribution of electrons from 0.6 eV to 28,250 keV, a range that permits coverage of thermal electrons found at Titan and near the ring plane as well as more energetic trapped electrons and auroral particles. The IBS measures ion velocity distributions with very high angular and energy resolution from 1 eV to 49,800 keV. It is specially designed

477 citations