Engineering topics

Gurgiolo, C.

Publications and source records attributed to Gurgiolo, C..

At least 19 records

NASA NTRS2011

Observations of Electron Vorticity in the Inner Plasma Sheet

From a limited number of observations it appears that vorticity is a common feature in the inner plasma sheet. With the four Cluster spacecraft and the four PEACE instruments positioned in a tetrahedral configuration, for the first time it is possible to directly estimate the electron fluid vorticity in a space plasma. We show examples of electron fluid vorticity from multiple plasma sheet crossings. These include three time periods when Cluster passed through a reconnection ion diffusion region. Enhancements in vorticity are seen in association with each crossing of the ion diffusion region.

Gurgiolo, C.↗

NASA NTRS2010

Whistler Waves Driven by Anisotropic Strahl Velocity Distributions: Cluster Observations

Observed properties of the strahl using high resolution 3D electron velocity distribution data obtained from the Cluster/PEACE experiment are used to investigate its linear stability. An automated method to isolate the strahl is used to allow its moments to be computed independent of the solar wind core+halo. Results show that the strahl can have a high temperature anisotropy (T(perpindicular)/T(parallell) approximately > 2). This anisotropy is shown to be an important free energy source for the excitation of high frequency whistler waves. The analysis suggests that the resultant whistler waves are strong enough to regulate the electron velocity distributions in the solar wind through pitch-angle scattering

Vinas, A.F.↗

NASA NTRS2008

A New Technique using Electron Velocity Data from the Four Cluster Spacecraft to Explore Magnetofluid Turbulence in the Solar Wind

It is now possible in certain circumstances to use velocity moments computed from the Plasma Electron and Current Experiment (PEACE) on the four Cluster spacecraft to determine a number of turbulence properties of the solar wind, including direct measurements of the vorticity and compressibility. Assuming that the four spacecraft are not co-planar and that there is only a linear variation of the plasma variables across the volume defined by the four satellites, one can estimate the curl of the fluid velocity, i.e., the vorticity. From the vorticity it is possible to explore directly intermittent regions in the solar wind where dissipation is likely to be enhanced. In addition, one can estimate directly the Taylor microscale.

Goldstein, Melvyn L.↗

NASA NTRS2008

Using PEACE Data from the four CLUSTER Spacecraft to Measure Compressibility, Vorticity, and the Taylor Microscale in the Magnetosheath and Plasma Sheet

We present determinations of compressibility and vorticity in the magnetosheath and plasma sheet using moments from the four PEACE thermal electron instruments on CLUSTER. The methodology used assumes a linear variation of the moments throughout the volume defined by the four satellites, which allows spatially independent estimates of the divergence, curl, and gradient. Once the vorticity has been computed, it is possible to estimate directly the Taylor microscale. We have shown previously that the technique works well in the solar wind. Because the background flow speed in the magnetosheath and plasma sheet is usually less than the Alfven speed, the Taylor frozen-in-flow approximation cannot be used. Consequently, this four spacecraft approach is the only viable method for obtaining the wave number properties of the ambient fluctuations. Our results using electron velocity moments will be compared with previous work using magnetometer data from the FGM experiment on Cluster.

Goldstein, Melvyn L.↗

NASA NTRS2003

Gyrophase-bunched Electrons: Cluster Observations and Simulations

The interaction of the solar wind with the Earth's bow shock is responsible for a number of phenomena both upstream and downstream. One of the least understood and studied of these is the generation of gyrophase-bunched electrons. We will describe initial work searching for and analyzing such events in (Plasma Electron And Current Experiment (PEACE) data from CLUSTER. Plasma simulations will complement the data analysis. Recent analyses suggest that gyrophase-bunched electrons are present rather frequently in the upstream region, due either to energization in the reflection off the shock front or, more likely, to phase trapping in locally produced whistler wave fields.

Gurgiolo, C.↗

NASA NTRS1998

Observations of Reflected Ions and Plasma Turbulence for Satellite Potentials Greater than the Ion Ram Energy

During the TSS-1R mission, the behavior of the ions flowing from the forward hemisphere of the Tethered Satellite System (TSS) satellite was examined as the potential of the satellite was changed from below to above 5 V. The ram energy of the ambient atomic oxygen ions is approximately 5 eV. For satellite potentials less than 5 V, no ions were observed on the ram side of the satellite. When the satellite potential was raised greater than 5 V, ions were observed to be flowing from the forward region of the satellite. In the region sampled, the ion flux was a few percent of the ambient with energies of approximately 5 eV. The temperature of the out-flowing ions was observed to be enhanced, relative to the ambient ionosphere. The net current to the probe package became much more noisy for satellite potentials greater than 5 V as compared with satellite potentials less than 5 V, indicating a more disturbed plasma environment.

Wright, K. H., Jr.↗

NASA NTRS1997

Observations of Reflected Ions and Plasma Turbulence for Satellite Potentials Greater Than the Ion Ram Energy

During the TSS-1R mission, the behavior of the ions flowing from the forward hemisphere of the Tethered Satellite System (TSS) satellite was examined as the potential on the satellite was changed from below to above 5 Volts. The ram energy of the ambient atomic oxygen ions is about 5 eV. For satellite potentials less than 5 V, no ions were observed on the ram side of the satellite. When the satellite potential was raised above 5 V, ions were observed to be flowing from the forward region of the satellite. In the region sampled, the ion flux was a few percent of the ambient with energies of about 5 eV. The temperature of the outflowing ions was observed to be enhanced, relative to the ambient ionosphere, and had a maximum in a plane containing the center of the satellite and normal to the geomagnetic field. The net current to the probe package became much more noisy for satellite potentials above 5 V as compared with satellite potentials below 5 V indicating a more disturbed plasma environment.

Wright, K. H., Jr.↗

NASA NTRS1996

Satellite Particle Collection During Active States of the Tethered Satellite System (TSS)

The reflight of the Tethered Satellite System (TSS-1R) was carried aboard the Space Shuttle Columbia on February 22, 1996. After deploying a day later than planned, the satellite almost reached its full deployed distance before the tether broke. Data was collected for over 5 hours during deployment out to a distance of 19.7 km. Maximum emf attained during deployment was 3700 V while the maximum current achieved was just under 0.5 A. The current collected was factors of 2 to 4 greater than the predictions of the conventional Parker-Murphy theory. The microscopic view of the collection process at the satellite showed exotic behavior with the existence of 100 - 200 eV suprathermal electrons and significant spin phase modulation of the electron fluxes. Although the data set acquired from TSS- 1R was considerably less than planned, the quality of the data allows one of the main goals of the mission to be met--characterizing the system I-V response. A "quick look" assessment of the data has already shown that an understanding of the TSS-1R electrodynamic behavior will require modification of the standard picture of current collection in space plasmas

Wright, K. H., Jr.↗

NASA NTRS1990

The electron signature of parallel electric fields

Dynamics Explorer I High-Altitude Plasma Instrument electron data are presented. The electron distribution functions have characteristics expected of a region of parallel electric fields. The data are consistent with previous test-particle simulations for observations within parallel electric field regions which indicate that typical hole, bump, and loss-cone electron distributions, which contain evidence for parallel potential differences both above and below the point of observation, are not expected to occur in regions containing actual parallel electric fields.

Burch, J. L.↗

NASA NTRS1990

Ion acceleration at the contact surface of Comet P/Halley

Suprathermal ions form a major component of the plasma within the inner coma of Comet P/Halley. A close inspection of these ions shows them to contain at least two separate populations: one population covers the energy range below about 400 eV, and a second population exists in a narrow energy band centered between 3 and 4 keV. Similar distributions are found in two regions upstream of the contact surface. It is postulated that the suprathermal ions observed upstream of the contact surface were created at the contact surface. Sudden movements of the contact surface, in the cometary frame of reference, in response to changes in the magnetic field pressure, or cometary production rate, give rise to an induction electric field able to accelerate the local thermal cometary ions up into the tens of electron volts range. Higher energies can be attained with the reacceleration of these ions in subsequent movements of the contact surface. Besides creating the ion enhancements observed upstream of the contact surface, this acceleration mechanism wil release large amounts of suprathermal ions into the inner coma.

Gurgiolo, C.↗

NASA NTRS1988

The effects of magnetic storm phases on F layer irregularities below the auroral oval

Observations of F-layer irregularity development and intensity were obtained between September and October 1981, primarily over subauroral latitudes in the area of the plasmapause. The results reveal the descent of the auroral irregularity region to include subauroral latitudes in the general area of the plasmapause during the main phases of a series of magnetic storms. Irregularities were found primarily at lower latitudes during the subauroral or plasmapause storm. A model for the subauroral irregularities in recovery phases of magnetic storms is proposed in which energy stored in the ring current is slowly released.

Aarons, J.↗

NASA NTRS1988

Simulation of electron distributions within auroral acceleration regions

Plasma data from the Dynamics Explorer 1 satellite are used to investigate the electron distributions found within regions of parallel potential drops. A computer simulation was developed to predict major features of the observed electron distributions found within these regions. The simulation results are discussed and compared with observations. Good agreement is observed only when the potential distribution along the field line is split into separate potential drops, one above and one below the satellite. The simulated electron distribution exhibits a trapped particle region, the formation of the electron hole, and the evacuation of the loss cone.

Gurgiolo, C.↗

NASA NTRS1988

A model of auroral potential structures based on Dynamics Explorer plasma data

Dynamics Explorer 1 hot plasma data are used to investigate three major features of the mid-altitude auroral electron distribution -- the hole, the bump (at large pitch angles), and the loss cone. The results of a computer simulation show that these features can be approximately reproduced by a model involving an initial plasma sheet electron distribution and two potential drop regions that are widely separated in altitude. The observed distributions could not be reproduced, even qualitatively, by any of a wide range of distributed potential drops involving parallel electric fields at the point of observation.

Burch, J. L.↗

NASA NTRS1987

A possible energy source to power stable auroral red arcs - Precipitating electrons

Results of coincident measurements by ground-based photometers and the low-altitude plasma instrument on board the Dynamics Explorer 2 satellite are presented that demonstrate the association of precipitating low-energy electrons with stable auroral red (SAR) arcs. A search of available data has yielded 23 instances of DE 2 overflights during the presence of SAR arcs being monitored by the photometers. For each case, downward fluxes of electrons are found to be enhanced along field lines penetrating the arcs in relation to regions both north and south of the features. Modeling of the atmospheric response to these influxes indicates that these electrons can represent a major source of the energy required to establish temperature profiles within the ionospheric electron gas that are sufficient to produce the recorded 6300-A emission rates. The sensitivity of these results to uncertainties of the assumed spacecraft potential and thermospheric composition has been investigated and found to be important, but does not alter the conclusion that precipitating electrons are a fundamental link in the production of SAR arcs.

Slater, D. W.↗

NASA NTRS1986

DE-1 observations of hole electron distribution functions and the cyclotron maser resonance

The hole electron distribution functions observed by the DE-1 satellite within inverted-V events at altitudes of between 9000 km and 15,000 km are examined as a possible free energy source for exciting Z-mode radiation through cyclotron maser resonance. In the DE-1 observations the hole distribution function had center velocities varying between 8000 km/s and 20,000 km/s, with the radii varying between 2000 km/s and 10,000 km/s. The observed distribution function is fitted by an exponential function around the center of the hole, and is used to calculate growth rates of Z-mode radiation. Growth rates as high as 0.001 of the electron gyrofrequency are obtained. It is also shown that the observed hole distribution functions can excite Z-mode radiation at wave frequencies slightly above the gyrofrequency, and wave propagation angles slightly below 90 deg in the source region. The results suggest that the hole distribution function could provide additional amplification for Z-mode waves in the auroral zone.

Lin, C. S.↗

NASA NTRS1986

A possible SAR arc energization source - Precipitating electrons

Coincident measurements by ground-based photometers and satellite-borne electron sensors have shown the association of precipitating electrons and Stable Auroral Red Arcs at midlatitudes. Modeling of these events has suggested that, within the constraints imposed by uncertainties of the electron energy spectrum, the electron influx carries sufficient energy to establish ionospheric temperatures required to power the arcs.

Slater, D. W.↗

NASA NTRS1985

A component of nongyrotropic (phase-bunched) electrons upstream from the earth's bow shock

The count rates of 1.5- and 6-keV electrons measured on ISEE 1 and ISEE 2 are at times strongly spin modulated despite the fact that the axes of the detectors lie along the spin axis of the spacecraft. Spatial gradients in the guiding center density are ruled out as a cause of the spin modulation. The characteristics of the spin modulation can be explained if the electron velocity distribution is nongyrotropic. The procedure is to show that the direction of the plasma flow velocity during times of spin modulation does not lie along the direction of the local magnetic field when both are projected onto the ecliptic plane. While a gyrotropic distribution will cause spin modulation, the count rate maximum will correspond to the direction of the local magnetic field. Gyrophase-bunched electrons are seen within a few tens of kilometers of the shock and as far upstream as 30,000 km from the shock, suggesting that a wave is associated with their propagation. The magnetosheath electrons in the same energy range are also nongyrotropic at times.

Anderson, K. A.↗

NASA NTRS1985

Polar plasmas as observed by Dynamics Explorers 1 and 2

Plasma measurements from the Dynamics Explorer 1 and 2 satellites were used to characterize the polar cap environment. Analysis of numerous polar-cap passes indicate that, in general, three major regimes of plasma exist: (1) polar rain--electrons with magnetosheath-like energy spectra but much lower densities, most intense near the cusp and weakening toward the central polar cap; (2) polar wind--low energy upward flowing ions with both field-aligned and conical distributions; and (3) acceleration events--sporadic events consistent with the acceleration of electrons and positive ions by parallel electric fields. (1) to (3) were observed at high altitudes by Dynamics Explorer 1, while (1) and (3) were also observed at low altitudes by Dynamics Explorer 2. The plasma parameters associated with these plasma regimes are presented and discussed in terms of source and acceleration mechanisms.

Barfield, J. N.↗