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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 271 records · Page 15
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NASA NTRS1980

Some features of inverted-V events as seen from simulated double layers

Results from a numerical simulation of a double layer show some features similar to those of inverted-V events. The strong heating of thermal and precipitating electrons is observed along with extremely low frequency fluctuations found during inverted-V events. It is suggested that after the acceleration of auroral electrons by the double layer, the precipitating free electrons are heated by the nonlinear effects of the electron beam plasma instability. Fluctuations and pulsations of auroral electron fluxes during auroral events are caused by a relaxation type of oscillation. The finite extent of one dimensional plasma is simulated by solving the Vlasov and Poisson equations as an initial and boundary value problem.

Singh, N.↗
NASA NTRS1991

A search for parallel electric fields by observing secondary electrons and photoelectrons in the low-altitude auroral zone

Model calculations are performed demonstrating the effect of weak parallel electric fields on the differential spectra of the low-energy electrons observed in the inverted-V electron precipitation events in the topside ionosphere. A comparison of the altitude dependence of the observed spectra with the model calculations shows that there can be, on average, no more than a 2-V potential drop between the altitudes of 400 and 900 km, corresponding to a distributed parallel dc electric field of less than 4 microV/m under the inverted-V electron precipitation regions. Statistical results are presented on the spectral dependence of secondary electrons on the inverted-V primary beam parameters.

Fung, Shing F.↗
NASA NTRS1997

Global Auroral Energy Deposition Derived from Polar UVI Images

Quantitative measurement of the transfer of energy and momentum to the ionosphere from the solar wind is one of the main objectives of the ISTP program. Global measurement of auroral energy deposition derived from observations of the longer wavelength LBH band emissions made by the Ultraviolet Imager on the Polar spacecraft is one of the key elements in this satellite and ground-based instrument campaign. These "measurements" are inferred by combining information from consecutive images using different filters and have a time resolution on the average of three minutes and are made continuously over a 5 to 8 hour period during each 18 hour orbit of the Polar spacecraft. The energy deposition in the ionosphere from auroral electron precipitation augments are due to Joule heating associated with field aligned currents. Assuming conjugacy of energy deposition between the two hemispheres the total energy input to the ionosphere through electron precipitation can be determined at high time resolution. Previously, precipitating particle measurements along the tracks of low altitude satellites provided only local measurements and the global energy precipitation could be inferred through models but not directly measured. We use the UVI images for the entire month of January 1997 to estimate the global energy deposition at high time resolution. We also sort the energy deposition into sectors to find possible trends, for example, on the dayside and nightside, or the dawn and dusk sides.

Fillingim, M. O.↗
NASA NTRS2003

FAST/Polar Conjunction Study of Field-Aligned Auroral Acceleration and Corresponding Magnetotail Drivers

The discrete aurora results when energized electrons bombard the Earth's atmosphere at high latitudes. This paper examines the physical processes that can cause field-aligned acceleration of plasma particles in the auroral region. A data and theoretical study has been carried out to examine the acceleration mechanisms that operate in the auroral zone and to identi@ the magnetospheric drivers of these acceleration mechanisms. The observations used in the study were collected by the Fast Auroral Snapshot (FAST) and Polar satellites when the two satellites were in approximate magnetic conjunction in the auroral region. During these events FAST was in the middle of the auroral zone and Polar was above the auroral zone in the near-Earth plasma sheet. Polar data were used to determine the conditions in the magnetotail at the time field-aligned acceleration was measured by FAST in the auroral zone. For each of the magnetotail drivers identified in the data study, the physics of field-aligned acceleration in the auroral region was examined using existing theoretical efforts and/or a long-system particle in cell simulation to model the magnetically connected region between the two satellites. Results from the study indicate that there are three main drivers of auroral acceleration: (1) field-aligned currents that lead to quasistatic parallel potential drops (parallel electric fields), (2) earthward flow of high-energy plasma beams from the magnetotail into the auroral zone that lead to quasistatic parallel potential drops, and (3) large-amplitude Alfven waves that propagate into the auroral region from the magnetotail. The events examined thus far confm the previously established invariant latitudinal dependence of the drivers and show a strong dependence on magnetic activity. Alfven waves tend to occur primarily at the poleward edge of the auroral region during more magnetically active times and are correlated with intense electron precipitation. At lower latitudes away from the poleward edge of the auroral zone is the primary field-aligned current region which results in the classical field- aligned acceleration associated with the auroral zone (electrons earthward and ion beams tailward). During times of high magnetic activity, high-energy ion beams originating from the magnetotail are observed within, and overlapping, the regions of primary and return field-aligned current. Along the field lines where the high-energy magnetotail ion beams are located, field-aligned acceleration can occur in the auroral zone leading to precipitating electrons and upwelling ionospheric ion beams. Field-aligned currents are present during both quiet and active times, while the Alfven waves and magnetotail ion beams were observed only during more magnetically active events.

Schriver, D.↗
NASA NTRS1988

Comment on 'Highly relativistic magnetospheric electrons: A role in coupling to the middle atmosphere?'

It is argued that Baker et al. (1987) may have underestimated the electron precipitation rate in the southern hemisphere in the region extending west to about 90 deg in longitude from the southern extension of the center of the South Atlantic geomagnetic anomaly (SAGA). In a reply, it is noted that Baker et al. restricted their study to the case of uniform precipitation in latitude and longitude throughout the outer-zone area, thus entailing a great simplification of the real-world situation. Comments are made on the effects of the SAGA on electron precipitation.

Sheldon, W. R.↗
NASA NTRS1980

Rocket observations at the northern edge of the eastward electrojet

The paper discusses a Nike-Tomahawk rocket launched north over quiet, late evening auroral arcs in March 1975. A northward magnetic disturbance was observed on the ground under the rocket trajectory; south of the arcs the northward electric field was 60 mV/m, indicating strong westward plasma flow. An eastward electrojet current layer was penetrated in the upward flight, and precipitating electrons were observed over each arc. Using the observed electron flux and a model of the ionosphere, the Hall and Pedersen conductivities were calculated which were used to compute the eastward and northward components of the horizontal ionospheric currents. The joule power decreased abruptly in the auroral arcs, as the precipitating electron power increased; the total dissipated power was the same inside the arcs, between them and southward. North of the aurora the electric field and dissipated power remained low; field-aligned currents carried by the observed electrons were about a factor of 3 lower than those inferred from the magnetic field measurements.

Cahill, L. J., Jr.↗
NASA NTRS1998

Unloading Versus Driven Processes Derived from Auroral Energy Deposition and Polar Cap Size

The intensity of far ultraviolet auroral emissions at all local times during the three substorm phases has been monitored by the Ultraviolet Imager (UVI) on the Polar spacecraft for many substorms. Changes in the energy flux and characteristic energy of the precipitating electrons can be derived from these observations by modeling of the spectral emission processes. The global and local energy deposition is a new parameter that can be used in substorm studies since it provides a measure of energy transfer from the tail to the ionosphere due to precipitating electrons at a time resolution of three minutes. The polar cap area and area of auroral emissions can also be determined at high time resolution during substorms from the UVI images. An example of a substorm that appears to be driven by solar wind dynamic pressure alone will be presented. The polar cap area and other parameters do not indicate a growth phase prior to substorm onset. In another example, the slow growth phase followed by a very rapid increase in energy deposition during the expansion phase will be shown. This substorm was preceded by a southward IMF orientation. In these two examples, the role the solar wind in determining polar cap area is discussed. The time development of the area of auroral emissions is also discussed in relation to substorm phase and energy deposition. If the auroral emissions occur on closed field lines then the area of auroral emissions may provide an indication of changes in the thickness of the plasma sheet during each substorm phase.

Brittnacher, M. J.↗
NASA NTRS2023

Diffuse Auroral Intensities Produced By Whistler Mode and Electron Cyclotron Harmonic Waves

Whistler mode waves and ECH waves as observed by the THEMIS-D and THEMIS-E satellites have been analyzed. It is observed that ECH waves are very weak for pitch-angle diffusion whereas whistler mode waves are more efficient. Bounce-averaged pitch-angle diffusion rates at the edge of atmospheric loss-cone have been calculated for both waves. Further, these are used to obtain electron precipitation flux inside the loss-cone. The electron flux at the edge of the loss-cone is represented by the kappa distribution. Numerical calculation of precipitation flux is performed by varying the fitting parameters (which determine the shape of electron precipitation flux) appearing in the kappa distribution. Precipitation flux is used to obtain volume excitation rates and height-integrated volume excitation rates for the seven excitation states. Calculations have been performed using two standard atmosphere models corresponding to mean and high solar and geomagnetic activities. It has been observed from studies that the excitation rates produced by the ECH wave are several orders of magnitude smaller than the rates produced by whistler mode waves. We show that the scattering efficiency of whistler Mode is higher than that of ECH waves. The oblique whistler mode waves allow efficient scattering of electrons in diffuse aurora. Outcomes are discussed.

Arvind K. Tripathi↗
NASA NTRS1972

Electric field and plasma observations in the magnetosphere

Satellite-borne electric field measurements using the double probe technique have now provided a comprehensive survey of convection electric fields at low altitudes in the magnetosphere. The most prominent features of the convection electric fields are reversals located at high magnetic latitudes, with generally anti-sunward convection poleward and sunward convection equatorward of the electric field reversal location. On the day side of the magnetosphere the electric field reversal is observed to coincide with the equatorward boundary of the polar cusp. In the local afternoon and evening regions inverted V electron precipitation bands occur at or near the electric field reversal and in regions usually characterized by large fluctuations in the electric field. In the local midnight region strong convection electric fields have also been observed deep within the magnetosphere, near the equatorward boundary of the plasma sheet. Recent measurements of electric fields near the inverted V electron precipitation bands suggests that these events are associated with large electrostatic potential gradients along the geomagnetic field.

Gurnett, D. A.↗
NASA NTRS1973

Observed relationships between electric fields and auroral particle precipitation.

Simultaneous electric field and plasma observations with the low-altitude polar-orbiting satellite Injun 5 have provided a comprehensive survey of convection electric fields and their association with magnetospheric plasma phenomena. The most prominent features of the convection electric fields are reversals located at high magnetic latitudes, with generally antisunward convection poleward and sunward convection equatorward of the electric field reversal location. The electric field reversal is interpreted as the boundary between open and closed magnetic field lines. During local day the electric field reversal is observed to coincide with the equatorward boundary of the polar cusp. The plasma flow in the dayside polar cusp region is dominantly E-W, away from the stagnation point, the convection velocities typically being about 1 km/sec. At local evening, 'inverted V' electron precipitation bands are observed near or at the position of the electric field reversal. In the local late-evening sector the electric field reversal becomes less distinct, and often no single well-defined electric field reversal can be identified. In all cases the inverted V electron precipitation events are closely associated with large, typically greater than 30 mV/m, irregular electric field fluctuations with time scales of a few seconds or less.

Gurnett, D. A.↗
NASA NTRS1988

Electrodynamic patterns in the polar region during periods of extreme magnetic quiescence

Using data from the DE 2 satellite, the electrodynamic patterns in the polar region during periods of extreme magnetic and auroral quiescence are analyzed for the existence of region-1 and region-2 field-aligned current systems, convection patterns, and electron-precipitation boundaries. The large-scale region-1 and region-2 currents were found to be usually absent during quiet periods, except in the cusp region. Field-aligned currents continue to be present, but as small-scale structures distributed through the auroral oval and over the entire polar cap. Convection patterns were always found in the summer hemisphere, but not always in the winter hemisphere, while structured electron precipitation was found to extend over the entire polar region independent of season.

Hoffman, R. A.↗
NASA NTRS2000

Behavior of the Aurora During 10-12 May, 1999 When The Solar Wind Nearly Disappeared

The aurora was still active with occasional pseudobreakup events when the solar wind density diminished to unusually small densities (0.2 cc -1) during May 10-12, 1999. The aurora was observed at high magnetic latitudes indicating that the electron precipitation source moved northward as the geomagnetic activity decreased. The events we have studied indicate that the solar wind density alone is not the primary parameter that controls the auroral activity. The weak auroral activity was observed with 150 nT magnetic bays and when the interplanetary magnetic field (IMF) Bz was small and positive resulting in small _ parameter. A new auroral feature was observed on May 11, 1999, between 0900-2000 UT. The electron precipitation was energetic, uniform, and covered the region commonly identified as the polar cap. This precipitation lasted for more than 10 hours and was stable over time scales of tens of minutes. On May 12, as the solar wind began to recover, a prolonged period of dayside activity occurred and was followed by a typical aurora at 0500 UT.

Parks, G.↗
NASA NTRS1992

The effects of magnetic storm phases on F-layer irregularities

The effects of magnetic storm phases on F-layer irregularities from auroral to equatorial latitudes in a nearly constant western longitude zone are presented by considering scintillation, spread F, and low-energy (less than 12 eV) electron precipitation data for eight magnetic storms that occurred during the high solar flux period September-November 1981. In the equatorial region, F-layer irregularities can be inhibited during the main phase. In the high latitude region, F-layer irregularities are found to be generated at the auroral latitudes during the main phase. During the recovery phase, when there are conditions of low magnetic activity, low auroral irregularities, and low-energy electron precipitation crossing the subauroral regions, strong F-layer irregularities are found to be generated in the subauroral regions.

Balan, N.↗
NASA NTRS2000

Behavior of the Aurora During the 10-12 May, 1999 When the Solar Wind Nearly Disappeared

We show that die aurora was still active with occasional pseudo breakup events when die solar wind density diminished to unusually small densities during May 10-12, 1999. The aurora was observed at high magnetic latitudes indicating that the electron precipitation source moved northward as die soar wind and geomagnetic activities decreased. These features are well known and seen at other times when the geomagnetic activities are weak. Intense auroral events accompanied other periods of low solar wind density (for example, February 10, 1997 and May 5,1998) indicating that the solar wind density alone is not die primary parameter that controls die auroral activity. However, a new auroral feature was observed on May, 11, 1999 during approx. 09-20 LIT hours. The electron precipitation was energetic, uniform and covered the polar latitudes commonly identified as die polar cap region. This precipitation lasted form ore that 10 hours and was stable over time scales of tens of minutes. On May 12, as the solar wind began to recover, a prolonged period of dayside activity occurred and was subsequently followed by a "typical" aurora at approx. 05 UT. All of the auroral activities occurred accompanying weak auroral electrojet activity (approx. 150 nT maximum).

Parks, G. K.↗
NASA NTRS1986

The ionosphere of Uranus - A myriad of possibilities

A one-dimensional model has been used to study the effects of exospheric temperature, methane and water influx, ionospheric outflow, and electron precipitation on the composition and structure of the ionosphere of Uranus. Peak ion concentrations range from 1000 to 1 million per cu cm with a wide variation in peak altitude, which depends strongly on the exospheric temperature. In all the cases considered, H(+) is the major ion in the topside ionosphere. At altitudes near or below the peak, H3(+) and CH5(+) can dominate, depending on the magnitude of CH4 and H2O influx. Atomic hydrogen column depths above the methane absorbing layer exceed 10 to the 17th per sq cm and can produce large (400 R) emissions of resonantly scattered Lyman-alpha. In the sunlit polar cap, electron precipitation with energy fluxes of 0.6 to 1.0 erg/sq cm s results in direct production of Lyman-alpha emissions that exceed 1 kR.

Chandler, M. O.↗
NASA NTRS1972

Systematics in auroral energy spectra.

Observation of a systematic increase in the mean energy of auroral electrons as the total precipitated energy increases. The change in 4278-A N2(+) intensity from about 50 R to about 10 kR correlates better with the change in mean energy of the precipitating electrons than with the change in number flux. Nightside aurora that exhibit this property superimpose on a wide region of soft (less than or about equal to 1 keV) precipitation that extends from the equatorial boundary of aurora to well poleward of normal visible aurora. On the dayside, the soft 'cusp' precipitation generates the statistical oval, although there may be larger energy inputs equatorward of the oval.

Eather, R. H.↗
NASA NTRS1994

Electrodynamic parameters in the nighttime sector during auroral substorms

The characteristics of the large-scale electrodynamic parameters, field-aligned currents (FACs), electric fields, and electron precipitation, which are associated with auroral substorm events in the nighttime sector, have been obtained through a unique analysis which places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images. A generic bulge-type auroral emission region has been deduced from auroral images taken by the Dynamics Explorer 1 (DE 1) satellite during a number of isolated substorms, and the form has been divided into six sectors, based on the peculiar emission characteristics in each sector: west of bulge, surge horn, surge, middle surge, eastern bulge, and east of bulge. By comparing the location of passes of the Dynamics Explorer 2 (DE 2) satellite to the simultaneously obtained auroral images, each pass is placed onto the generic aurora. The organization of DE 2 data in this way has systematically clarified peculiar characteristics in the electrodynamic parameters. An upward net current mainly appears in the surge, with little net current in the surge horn and the west of bulge. The downward net current is distributed over wide longitudinal regions from the eastern bulge to the east of bulge. Near the poleward boundary of the expanding auroral bulge, a pair of oppositely directed FAC sheets is observed, with the downward FAC on the poleward side. This downward FAC and most of the upward FAC in the surge and the middle surge are assoc iated with narrow, intense antisunwqard convection, corresponding to an equatorward directed spikelike electric field. This pair of currents decreases in amplitude and latitudinal width toward dusk in the surge and the west of bulge, and the region 1 and 2 FACs become embedded in the sunward convection region. The upward FAC region associated with the spikelike field on the poleward edge of the bulge coincides well with intense electron precipitation and aurora appearing in this western and poleward protion of the bulge. The convection reversal is sharp in the west of bulge and surge horn sectors, and near the high-latitude boundary of the upward region 1, with a near stagnation region often extending over a large interval of latitude. In the eastern bulge and east of bulge sectors, the region 1 and 2 FACs are located in the sunward convection region, while a spikelike electric field occasionally appears poleward of the aurora but usually not associated with a pair of FAC sheets. In the eastern bulge, magnetic field data show complicated FAC distributions which correspond to current segments and filamentary currents.

Fujii, R.↗