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Gurnett, D. A.

Publications and source records attributed to Gurnett, D. A..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

GEOTAIL and POLAR Observations of Auroral Kilometric Radiation and Terrestrial Low Frequency Bursts and their Relationship to Energetic Particles, Auroras, and Other Substorm Phenomena

Terrestrial low frequency (LF) bursts are plasma wave phenomena that appear to be a part of the low frequency end of the auroral kilometric radiation (AKR) spectrum and are observed during strong substorms, GEOTAIL and POLAR plasma wave observations from within the magnetosphere show that the AKR increases in intensity and its lower frequency limits decrease when LF bursts are observed. The first is expected as it is shows substorm onset and the latter indicates that the AKR source region is expanding to higher altitudes. Images from the POLAR VIS Earth Camera operating in the far-UV range and the POLAR UVI experiment usually feature an auroral brightening and an expansion of the aurora to higher latitudes at the time of the LF bursts. Enhanced fluxes of X-rays from precipitating electrons have also been observed by POLAR PIXIE. High resolution ground Abstract: magnetometer data from the CANOPUS and IMAGE networks show that the LF bursts occur when the expansive phase onset signatures are most intense. The ground magnetometer data and the CANOPUS meridian scanning photometer data sometimes show that during the LF burst events the expansive phase onset starts at unusually low latitudes and moves poleward. Large injections of energetic protons and electrons have also been detected by the GOES and LANL geosynchronous satellites during LF burst events. While most of the auroral brightenings and energetic particle injections associated with the LF bursts occur near local midnight, several have been observed as early as mid-afternoon. From these various measurements, we are achieving a better understanding of the plasma and particle motions during substorms that are associated with the generation and propagation of terrestrial LF bursts

Anderson, R . R.↗
NASA NTRS1999

Cassini and Wind Stereoscopic Observations of Jovian Non-Thermal Radio Emissions

During two intervals in 1999, simultaneous observations of Jupiter's decametric and hectometric radio emissions were made with the Cassini radio and plasma wave instrument (RPWS) and the radio and plasma wave instrument (WAVES) on the Wind spacecraft in Earth orbit. During January, the Jovian longitude difference between the two spacecraft was about 5 deg, whereas for the August-September Earth flyby of Cassini, the angle ranged from 0 deg to about 2.5 deg. With these separations, the instantaneous widths of the walls of the hollow conical radiation beams of some of the decametric arcs were measured suggesting that the typical width is approximately 2 deg. The conical beams seem to move at Io's revolution rate rather than with Jupiter's rotation rate. Additionally, some of the non-arc emissions have very narrow and quite peculiar beamwidths.

Kaiser, Michael L.↗
NASA NTRS1999

Polar Spacecraft Observations of the Turbulent Outer Cusp/Magnetopause Boundary Layer of Earth

The orbit of the Polar spacecraft has been ideally suited for studying the turbulent region of the cusp that is located near or just outside the magnetopause current sheet at 7-9 R(sub E). The wave data obtained in this region show that electromagnetic turbulence is dominant in the frequency range 1-10 Hz. The waves responsible for this turbulence usually propagate perpendicular to the local magnetic field and have an index of refraction that generally falls between the estimated cold plasma theoretical values of the electromagnetic lower hybrid and whistler modes and may be composed of both modes in concert with kinetic Alfven waves and/or fast magnetosonic waves. Fourier spectra of the higher frequency wave data also show the electromagnetic turbulence at frequencies up to and near the electron cyclotron frequency. This higher frequency electromagnetic turbulence is most likely associated with whistler mode waves. The lower hybrid drift and current gradient instabilities are suggested as possible mechanisms for producing the turbulence. The plasma and field environment of this turbulent region is examined and found to be extremely complex. Some of the wave activity is associated with processes occurring locally, such as changes in the DC magnetic field, while others are associated with solar wind and interplanetary magnetic field changes.

Pickett, J. S.↗
NASA NTRS1999

Polar Spacecraft Observations of the Turbulent Outer Cusp/Magnetopause Boundary Layer of Earth

The orbit of the Polar spacecraft has been ideally suited for studying the turbulent region of the cusp that is located near or just outside the magnetopause current sheet at 7-9 R(sub E). The wave data obtained in this region show that electromagnetic turbulence is dominant in the frequency range 1-10 Hz. The waves responsible for this turbulence usually propagate perpendicular to the local magnetic field and have an index of refraction that generally falls between the estimated cold plasma theoretical values of the electromagnetic lower hybrid and whistler modes and may be composed of both modes in concert with kinetic Alfven waves and/or fast magnetosonic waves. Fourier spectra of the higher frequency wave data also show the electromagnetic turbulence at frequencies up to and near the electron cyclotron frequency. This higher frequency electromagnetic turbulence is most likely associated with whistler mode waves. The lower hybrid drift and current gradient instabilities are suggested as possible mechanisms for producing the turbulence. The plasma and field environment of this turbulent region is examined and found to be extremely complex. Some of the wave activity is associated with processes occurring locally, such as changes in the DC magnetic field, while others are associated with solar wind and interplanetary magnetic field changes.

Pickett, J. S.↗
NASA NTRS1999

Plasma Waves Observed During CUSP Energetic Particle Events and Their Correlation With Polar and Akebono Satellite and Ground Data

We present Polar plasma wave data during cusp energetic particle (CEP) events at 6-9 R(sub E). These data suggest the presence of coherent electrostatic structures that are highly localized and that have typical velocities on the order of hundreds to thousands of kilometers per second along the ambient magnetic field. Some of the wave signatures are solitary waves and some are wave packets. The Polar wave instrument also provides evidence that some of the bursts of electromagnetic waves (with frequencies of a few hundred Hz and just below the electron cyclotron frequency around 800 Hz to 1-2 kHz) that are observed are coher&nt and propagating both up and down the field lines. Electron cyclotron harmonic (ECH) waves are often detected but their duration is usually short (less than 1 second). Low Frequency (less than 1 kHz), broadband, bursty electromagnetic waves are also present. The Polar wave data results are used to obtain a better understanding of the macro/microphysics during a CEP event that takes place on September 11, 1996, by correlating various Polar (approximately 7.0 R(sub E)) and Akebono (approximately 1.4 R(sub E)) data while both spacecraft are in or near the cusp/cleft region and nearly on the same field line, and magnetometer data from the Canadian Intermagnet and Canopus ground stations, which lie near the base of the magnetic footprint passing through Polar. Solar wind and magnetic field data from the interplanetary medium and magnetosheath are provided by the Geotail and IMP-8 satellites, respectively. Some of the cusp waves may be indicators of the reconnection process taking place through the cusp, the result of mixing of magnetosheath with magnetospheric plasma, and the consequence of an anisotropic electron population in a depressed magnetic field. The low frequency electromagnetic waves are still under study to determine their role, if any, in the heating and acceleration of the MeV He ions during CEP events.

Pickett, J. S.↗
NASA NTRS1998

Second Harmonic Hectometric Radio Emission at Jupiter

Galileo has been in orbit around Jupiter since December 1995. The plasma wave instrument on board the spacecraft has occasionally detected a rotationally modulated attenuation band in the hectometric (HOM) emission that most likely is due to scattering of the radiation from density fluctuations along the Io L-shell, as reported earlier. The occurrence of the attenuation band is likely to be dependent on Io activity and the presence of density scattering centers along the Io L-shell as well as the location of the source region. Some of the attenuation bands show clear indications of second harmonic emission. Without polarization measurements, it is difficult to place constraints on the local generation conditions based on the cyclotron maser instability, but the results imply that second harmonic emission could be present in the decametric (DAM) radiation as well. A survey of the data has revealed about 30 examples of second harmonic HOM.

Menietti, J. D.↗
NASA NTRS1998

Second Harmonic Hectometric Radio Emission at Jupiter

Galileo has been in orbit around Jupiter since December 1995. The plasma wave instrument on board the spacecraft has occasionally detected a rotationally modulated attenuation band in the hectometric (HOM) emission that most likely is due to scattering of the radiation from density fluctuations along the Io L-shell, as reported earlier. The occurrence of the attenuation band is likely to be dependent on Io activity and the presence of density scattering centers along the Io-L-shell as well as the location of the source region. Some of the attenuation bands show clear indications of second harmonic emission. Without polarization measurements, it is difficult to place constraints on the local generation conditions based on the cyclotron maser instability, but the results imply that second harmonic emission could be present in the decametric (DAM) radiation as well. A survey of the data has revealed about 30 examples of second harmonic HOM.

Menietti, J. D.↗
NASA NTRS1998

Control of Jovian Radio Emission by Ganymede

Galileo has been in orbit around Jupiter since December 1995. We present the results of a survey of the data for the frequency range 3.2 MHz to 5.6 MHz, the low-frequency decametric (DAM) emissions. While the control of a portion of the radio emission by the moon Io is well-known, we report that a small but significant portion of low-frequency DAM emission is seen to be correlated with the orbital phase of Ganymede. This result is in agreement with other recent results indicating a significant interaction of the magnetosphere of Ganymede with that of Jupiter.

Menietti, J. D.↗
NASA NTRS1998

Chorus Source Locations from VLF Poynting Flux Measurements with the Polar Spacecraft

Previous studies have used indirect evidence to argue that whistler-mode chorus emissions are generated near the magnetic equator. In this paper a spatial survey of wave normals and Poynting vectors computed from three-component electric and magnetic field measurements is used to show that chorus is generated very close to the magnetic equator. One surprising result is that there are almost no chorus emissions propagating toward the magnetic equator, such as might be expected from high-latitude magnetospheric reflections. The absence of a reflected component indicates that the chorus is reabsorbed, probably by Landau damping, before returning to the magnetic equatorial plane.

LeDocq, M. J.↗
NASA NTRS1998

Auroral Kilometric Radiation and the Auroral Electrojet Index for the January 1997 Magnetic Cloud Event

Auroral kilometric radiation (AKR) observations by Polar and Geotail are compared with the auroral electro jet index for the January 1997 magnetic cloud event. These two-spacecraft measurements are complementary in covering the AKR emission cones throughout the event and,together,reasonably represent the auroral electrojet AE index. We point out, however, limitations of both the AKR index and the AE index in providing truly global measurements of substorm activity.

Kurth, W. S.↗
NASA NTRS1998

Relationship of Topside Ionospheric Ion Outflows to Auroral Forms and Precipitation, Plasma Waves, and Convection Observed by Polar

The POLAR satellite often observes upflowing ionospheric ions (UFIs) in and near the aurora] oval on southern perigee (approx. 5000 km altitude) passes. We present the UFI features observed by the thermal ion dynamics experiment (TIDE) and the toroidal imaging mass angle spectrograph (TIMAS) in the dusk-dawn sector under two different geomagnetic activity conditions in order to elicit their relationships with auroral forms, wave emissions, and convection pattern from additional POLAR instruments. During the active interval, the ultraviolet imager (UVI) observed a bright discrete aurora on the duskside after the substorm onset and then observed a small isolated aurora form and diffuse auroras on the dawnside during the recovery phase. The UFIs showed clear conic distributions when the plasma wave instrument (PWI) detected strong broadband wave emissions below approx. 10 kHz, while no significant auroral activities were observed by UVI. At higher latitudes, the low-energy UFI conics gradually changed to the polar wind component with decreasing intensity of the broadband emissions. V-shaped auroral kilometric radiation (AKR) signatures observed above -200 kHz by PWI coincided with the region where the discrete aurora and the UFI beams were detected. The latitude of these features was lower than that of the UFI conics. During the observations of the UFI beams and conics, the lower-frequency fluctuations observed by the electric field instrument were also enhanced, and the convection directions exhibited large fluctuations. It is evident that large electrostatic potential drops produced the precipitating electrons and discrete auroras, the UFI beams, and the AKR, which is also supported by the energetic plasma data from HYDRA. Since the intense broadband emissions were also observed with the UFIs, the ionospheric ions could be energized transversely before or during the parallel acceleration due to the potential drops.

Hirahara, M.↗
NASA NTRS1998

An Unusual Rotationally Modulated Attenuation Band in the Jovian Hectometric Radio Emission Spectrum

A well-defined attenuation band modulated by the rotation of Jupiter has been found in the spectrum of Jovian hectometric radiation using data from the Galileo plasma wave instrument. The center frequency of this band usually occurs in the frequency range from about 1 to 3 MHz and the bandwidth is about 10 to 20 percent. The center frequency varies systematically with the rotation of Jupiter and has two peaks per rotation, the first at a system III longitude of about 50 deg, and the second at about 185 deg. It is now believed that the attenuation occurs as the ray path from a high-latitude cyclotron maser source passes approximately parallel to the magnetic field near the northern or southern edges of the Io L-shell. The peak at 50 deg system 3 longitude is attributed to radiation from a southern hemisphere source and the peak at 185 deg is from a northern hemisphere source. The attenuation is thought to be caused by coherent scattering or shallow angle reflection from field-aligned density irregularities near the Io L-shell. The narrow bandwidth indicates that the density irregularities are confined to a very narrow range of L values (Delta L = 0.2 to 0.4) near the Io L-shell.

Gurnett, D. A.↗
NASA NTRS1998

Radio Emissions Observed by Galileo near Io

The Galileo spacecraft observed spin modulation of radio wave emissions near Io in the frequency range from about 600 kHz to about 1.2 MHz. Assuming transverse EM radiation, we have used the modulation of the high-frequency sweep-frequency receiver signals of the electric dipole antenna over many spins to estimate the plane through the source. The emission has a range of frequencies close to the local upper hybrid frequency of the plasma near Io. We conclude that the emission may be associated with either the plasma torus or magnetic flux tubes in the wake of Io (the Alfven current system). We postulate this emission may be associated with a free-energy source such as density gradients, energetic plasma beams and/or an electron distribution with a temperature anisotropy. All of these free-energy sources are observed or expected in the torus near Io. The observations are the first in the hectometric frequency range that have a source associated with Io or in the Io torus.

Menietti, J. D.↗
NASA NTRS1998

Auroral Kilometric Radiation Integrated Power Flux as a Proxy for A E

We propose to use the integrated intensity of auroral kilometric radiation from Polar measurements as a proxy for the auroral electrojet index A(sub E) in support of studies of the response of the magnetosphere and the geomagnetic tail to changes in magnetic activity. In addition to providing event timing information to understand the effects of perturbations and substorms, the resulting space-based auroral kilometric radiation index would be useful as an input to space weather efforts as an auroral activity metric.

Kurth, W. S.↗
NASA NTRS1998

Polar Plasma Wave Observations in the Auroral Region and Polar Cap

Auroral kilometric radiation (AKR), sometimes associated with auroral myriametric radiation (AMR), has been observed by the plasma wave instrument on board Polar on almost every northern hemisphere pass. High spectral resolution plots of the AKR obtained by the wide-band receiver of the plasma wave instrument on board the spacecraft often show discrete, negative-slope striations each extending over a period of several seconds. A preliminary survey of over 4000 spectrograms (each for 48 seconds of data) indicates that the striations are seen in the northern hemisphere near apogee about 5% of the time. The frequency range is 40 kHz less than f less than 100 kHz, but a few observations of signatures have been made at higher frequency (f less than 225 khz. The frequency drift rates R, are similar ranging from -9.0 kHz/sec less than R less than -1.0 kHz/sec. No data is currently available for perigee (southern hemisphere) passes. The paucity of positive-slope features may be due to the location of the satellite at altitudes well above the AKR source region. Past studies have suggested these features are due to AKR wave growth stimulated by the propagation of electromagnetic ion cyclotron waves travelling up (-R) or down (+R) the field line, through the source region. High-resolution waveform data from both Polar and FAST show the presence of solitary waves in the auroral region which may also be a source of these striations. AMR is seen as diffuse emission associated with, but at lower frequency than the lower AKR. Direction finding of these emissions is not conclusive, but for one case, they have a source region distinct from the magnetic field line containing the AKR source, but possibly associated with the auroral cavity density gradient.

Menietti, J. D.↗
NASA NTRS1997

Micron-Sized Dust Particles Detected in the Outer Solar System by the Voyager 1 and 2 Plasma Wave Instruments

During the Voyager 1 and 2 flybys of the outer planets it has been demonstrated that the plasma wave instrument can detect small dust particles striking the spacecraft. In this paper, we examine the Voyager plasma wave data for dust impacts in the interplanetary medium at heliocentric radial distances ranging from 6 to 60 astronomical units (AU). The results show that a small but persistent level of dust impacts exists out to at least 30 to 50 AU. The average number density of these particles is about 2 x 10(exp -8)/cu m, and the average mass of the impacting particles is believed to be a few times 10(exp -11) g, which corresponds to particle diameters in the micron range. Possible sources of these particles are planets, moons, asteroids, comets, and the interstellar medium. Of these, comets appear to be the most likely source. ne number densities are only weakly dependent on ecliptic latitude, which indicates that the particles probably do not originate from planets, moons, or asteroids. Comparisons with interstellar dust fluxes measured in the inner regions of the solar system by the Ulysses spacecraft indicate that the particles are not of interstellar origin.

Gurnett, D. A.↗