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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 55 records · Page 3
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NASA NTRS1977

Electromagnetic plasma wave emissions from the auroral field lines

The most important types of auroral radio emissions are reviewed, both from a historical perspective as well as considering the latest results. Particular emphasis is placed on four types of electromagnetic emissions which are directly associated with the plasma on the auroral field lines. These emissions are (1) auroral hiss, (2) saucers, (3) ELF noise bands, and (4) auroral kilometric radiation. Ray tracing and radio direction finding measurements indicate that both the auroral hiss and auroral kilometric radiation are generated along the auroral field lines relatively close to the earth, at radial distances from about 2.5 to 5 R sub e. For the auroral hiss the favored mechanism appears to be amplified Cerenkov radiation. For the auroral kilometric radiation several mechanisms have been proposed, usually involving the intermediate generation of electrostatic waves by the precipitating electrons.

Gurnett, D. A.↗
NASA NTRS1985

Remote sensing of the magnetic moment of Uranus - Predictions for Voyager

The current understanding of the power transfer mechanisms by which power is supplied to a planet's magnetosphere by the kinetic energy of planetary spin and the energy flux of the impinging solar wind is applied to the case of Uranus, in order to predict the detectability of radio and auroral emissions by the planetary radio astronomy (PRA) and UV spectrometer (UVS) instruments of the Voyager spacecraft. The power available for the two energy transfer phenomena cited is a function of Uranus' magnetic moment, which is presently derived for each power source as a function of the date of first detection of radio emissions by the PRA or auroral emissions by the UVS.

Hill, T. W.↗
NASA NTRS1980

An empirical determination of the production efficiency for auroral 6300-A emission by energetic electrons

Auroral data from the soft particle spectrometer and the red line photometer on board the Isis satellite are presented as a data base for electron energy flux and optical auroral emission and are used to derive production efficiencies for 6300 A emission. Data obtained during a single orbit in which an aurora was observed is corrected by integrating the precipitating flux observed in four energy bands over the loss cone and eliminating the effects of airglow and albedo in the optical emission measurements. A regression analysis of the corrected data from 12 orbits is used to determine 6300 A production coefficients for the 5-60 eV, 60-300 eV, 0.3-1 keV and 1-15 keV electron energy bands. Production coefficients are shown to be in good agreement with theoretical calculations and rocket measurements in conjugate photoelectrons, and applications of the method to other data sets are discussed.

Shepherd, G. G.↗
NASA NTRS2014

Aurora on Ganymede

Jupiter's largest satellite Ganymede is unique because it possesses an internal magnetic field strong enough to create a small magnetosphere around the satellite. The interaction between Ganymede's magentosphere and the Jovian magnetic field and magnetospheric plasma generates classic polar auroral emissions from Ganymede which have been captured in a series of stunning ultraviolet images using the Hubble Space Telescope on six different dates from 1998 to 2012. Analysis of these images (McGrath et al. 2013) has provided a nearly global mapping of the location of Ganymede's auroral ovals, which appear to be remarkably stable in the 14 years spanning the observations. The location of the auroral ovals is compared with several model predictions of the boundary between open and closed magnetic field lines (Koop and Ip 2002; Khurana et al. 2007; Jia et al. 2009), a region where strong field aligned potentials are a likely candidate to accelerate electrons that produce the auroral emissions. The location of the auroral emission is also compared with the polar cap boundary on Ganymede, thought to be produced by charged particle bombardment of the polar region, as delineated by color ratio images acquired by the Galileo mission (Khurana et al. 2007).

McGrath, Melissa A.↗
NASA NTRS1987

Low-altitude ISIS 1 observations of auroral radio emissions and their significance to the cyclotron maser instability

Low-altitude ISIS 1 observations are presented which support the cyclotron maser instability as the generation mechanism of most, but not all, auroral radio emissions. The observations confirm the ability of the mechanism to generate different wave modes and harmonics as the ratio of the plasma frequency to gyrofrequency f(N)/f(H) increases depending on the form of the energetic electron distribution function. The present observations correspond to conditions where f(N)/F(H) is always larger than 0.2 and is typically about 1.0. The observations and instability growth rate calculations indicate that second harmonic radiation in the extraordinary and the Z modes is directly generated at low altitudes. By far the most common signals observed near ISIS 1 perigee, however, are in the fundamental Z and whistler modes. The observations and calculations in the case of the Z mode suggest that the pertinent process involves a maser that does not saturate, i.e., that group velocity effects are important. The whistler mode is the most intense signal observed at low altitudes. Since the cyclotron maser generated whistler mode waves have peak growth rates for downward field-aligned propagation, they should be able to penetrate the ionosphere. This process may provide the explanation of occasional reports, dating back to the International Geophysical Year time period, of ground level detection of radio noise associated with the aurora.

Benson, Robert F.↗
NASA NTRS1993

Artificial auroras in the upper atmosphere. I - Electron beam injections

The Atlas-1 Spacelab payload's Space Experiments with Particle Accelerators generated artificial electron beams for the stimulation of auroral emissions at southern auroral latitudes. Optical measurements were made by the Shuttle Orbiter's onboard TV cameras, as well as by the Atmospheric Emissions Photometric Imager (in both white light and the 427.8 nm N2(+) emission line). Shuttle-based auroral imaging furnished a novel perspective on the artificial auroras; the emissions were traced from 295 km to the 110 km level along the curved magnetic-field lines.

Burch, J. L.↗
NASA NTRS2011

Aurora on Ganymede

Jupiter's largest satellite Ganymede is unique because it possesses an internal magnetic field strong enough to create a small magnetosphere around the satellite.The interaction between Ganymede's magnetosphere and the Jovian magnetic field and magnetospheric plasma generates classic polar auroral emissions from Ganymede which have been captured in a series of stunning ultraviolet images using the Hubble Space Telescope on four different dates from 1998 to 2007. Only data from the first set of obsevations in 1998 has been published (Feldman et al. 2000; Eviatar et al. 2001). We have used a common data reduction process on all four sets of images, and combined them to produce a near global map of the location of Ganymede's auroral oval, which appears to be relatively stable in the seven years spanning the observations. We compare the location of this auroral oval with several model predictions of the boundary between open and closed magnetic field lines (Koop and Ip 2002; Khurana et al. 2007; Jia et al. 2009), a region where strong field aligned currents are thought to produce the auroral emissions. We also compare the location of the auroral emission with the polar cap boundary on Ganymede as dlineated by color ratio images acquired by the Galileo mission (Khurana et al. 2007).

McGrath, Melissa A.↗
NASA NTRS1983

Cyclotron maser emission of auroral Z mode radiation

Results are presented suggesting that loss cone driven cyclotron maser emission by upgoing electrons, closely analogous to auroral kilometric radiation (AKR), may be the mechanism behind the observed Z mode radiation. With this hypothesis, the lack of a strong correlation between the Z mode radiation and AKR is not surprising; the ray paths for the X mode and the Z mode are markedly different, with the former directed upward and the latter downward. In addition, it is expected that the generation of the Z mode will be favored only in regions where the ratio of the plasma frequency to the electron cyclotron frequency is greater than or approximately equal to 0.3, that is, where the X mode radiation is suppressed. If the fraction of the radiation generated that crosses the cyclotron layer is large, then the argument in favor of the loss cone driven cyclotron maser as the source of the observed Z mode radiation is a strong one. The spatial growth rates are fairly large in comparison with those for the X mode, and there seems to be little doubt that Z mode radiation should be generated under conditions that differ only slightly from those for the generation of X mode radiation in AKR.

Melrose, D. B.↗
NASA NTRS1992

H I Lyman alpha emission from Saturn (1980-1990)

The characteristics of the H I Ly alpha dayglow and auroral emissions from Saturn are investigated by analyzing observations made aboard the IUE satellite on Saturn over the 1980-1990 decade (covering a complete solar activity cycle). It was found that, in this period, the dayglow emission from Saturn had significant long-term variability, with decreasing disk brightness between 1980 and 1984 and increasing brightness, as well as increased short-term (days to months) variability, between 1987 and 1990. Both the long-term and short-term variability in the Saturn Ly alpha dayglow were strongly correlated with several different solar indices. Neither the dayglow nor the auroral emission exhibited any dependence on magnetic longitude. Attempts to observe auroral activity associated with a major solar flare which produced unprecedented geomagnetic activity in March 1989 were not successful.

Mcgrath, Melissa A.↗
NASA NTRS1989

Observations of non-thermal radiation from planets

Nonthermal radio emissions from earth, Jupiter, Saturn, and Uranus are reviewed. The dominant source of emission at each planet appears to be AKR-like auroral emission in the X-mode. O-mode emissions are substantially responsible. There is a remarkably constant scaling factor relating the total solar wind input power into each planetary system and the AKR-like auroral emissions.

Kaiser, M. L.↗
NASA NTRS1988

Ground-level detection of low- and medium-frequency auroral radio emissions

Natural radio emissions in the frequency range of several hundred kHz were detected at ground level in the auroral region during a ground-level observing program conducted near Fairbanks, Alaska in the spring of 1986 in an attempt to detect auroral cyclotron-maser-generated whistler modes. Most of the strongest emissions were observed in a small time interval around midnight magnetic local time, suggesting a close tie-in with auroral processes. The results support earlier reports of radio noise from the aurora at frequencies of about 100 kHz and indicate the importance of extending the upper frequency of ground-based high-latitude VLF stations to the vicinity of 1 MHz.

Benson, Robert F.↗
NASA NTRS1997

Compensation for Spherical Geometric and Absorption Effects on Lower Thermospheric Emission Intensities Derived from High Earth Orbit Images

Remote sensing of the atmosphere from high earth orbit is very attractive due to the large field of view obtained and a true global perspective. This viewpoint is complicated by earth curvature effects so that slant path enhancement and absorption effects, small from low earth orbit, become dominant even at small nadir view angles. The effect is further complicated by the large range of local times and solar zenith angles in a single image leading to a modulation of the image intensity by a significant portion of the diurnal height variation of the absorbing layer. The latter effect is significant in particular for mesospheric, stratospheric and auroral emissions due to their depth in the atmosphere. As a particular case, the emissions from atomic oxygen (130.4 and 135.6 nm) and molecular nitrogen (two LBH bands, LBHS from 140 to 160 nm and LBHL from 160 to 180 nm) as viewed from the Ultraviolet Imager (UVI) are examined. The LBH emissions are of particular interest since LBHS has significant 02 absorption while LBHL does not, In the case of auroral emissions this differential absorption, well examined in the nadir, gives information about the height of the emission and therefore the energy of the precipitating particles. Using simulations of the viewing geometry and images from the UVI we examine these effects and obtain correction factors to adjust to the nadir case with a significant improvement of the derived characteristic energy. There is a surprisingly large effect on the images from the 02 diurnal layer height changes. An empirical compensation to the nadir case is explored based on the local nadir and local zenith angles for each portion of the image. These compensations are demonstrated as applied to the above emissions in both auroral and dayglow images and compared to models. The extension of these findings to other instruments, emissions and spectral regions is examined.

Swift, W.↗
NASA NTRS1995

Ultraviolet observations of the Saturnian north aurora and polar haze distribution with the HST-FOC

Near simultaneous observations of the Saturnian H2 north ultraviolet aurora and the polar haze were made at 153 nm and 210 nm respectively with the Faint Object Camera on board the Hubble Space Telescope. The auroral observations cover a complete rotation of the planet and, when co-added, reveal the presence of an auroral emission near 80 deg N with a peak brightness of about 150 kR of total H2 emission. The maximum optical depth of the polar haze layer is found to be located approximately 5 deg equatorward of the auroral emission zone. The haze particles are presumably formed by hydrocarbon aerosols initiated by H2+ auroral production. In this case, the observed haze optical depth requires an efficiency of aerosol formation of about 6 percent, indicating that auroral production of hydrocarbon aerosols is a viable source of high-latitude haze.

Gerard, J. C.↗
NASA NTRS1982

Rapid fluctuations of random nature in auroral optical emissions

Rapid intensity fluctuations in the optical emission, N2/+/ (4278 A), H-beta (4861 A), and OI (5577 A) have been observed with rocket-borne photometers during a passage through a homogeneous auroral arc. The measurements indicate that relatively weak (2-5%) intensity fluctuations of random nature, with dominant frequencies in the 2-20 Hz range, do exist in the optical emissions of quiet-form auroras. These variations have well-defined averaged spectral characteristics and apparently differ from the strong quasi-periodic type of variations seen during pulsing auroras. It is argued that the observed fluctuations, which are usually composed of weak short-lived microbursts, reflect the temporal and spatial microstructure which apparently exists always within auroral arcs.

Haldoupis, C. I.↗
NASA NTRS1992

The role of proton precipitation in Jovian aurora: Theory and observation

It was proposed that the Jovian auroral emissions observed by Voyager spacecraft could be explained by energetic protons precipitating into the upper atmosphere of Jupiter. Such precipitation of energetic protons results in Doppler-shifted Lyman alpha emission that can be quantitatively analyzed to determine the energy flux and energy distribution of the incoming particle beam. Modeling of the expected emission from a reasonably chosen Voyager energetic proton spectrum can be used in conjunction with International Ultraviolet Explorer (IUE) observations, which show a relative lack of red-shifted Lyman alpha emission, to set upper limits on the amount of proton precipitation taking place in the Jovian aurora. Such calculations indicate that less than 10 percent of the ultraviolet auroral emissions at Jupiter can be explained by proton precipitation.

Waite, J. H., Jr.↗
NASA NTRS1997

POLAR-UVI and other Coordinated Observations of a Traveling Convection Vortex Event Observed on 24 July 1996

Coordinated analysis of data from the POLAR UVI instrument, ground magnetometers, incoherent scatter radar, solar wind monitors IMP-8 and WIND, and DMSP satellite is focused on a traveling convection vortex (TCV) event on 24 July 1966. Starting at 10:48 UT, ground magnetometers in Greenland and eastern Canada measure pulsations consistent with the passing overhead of a series of alternating TCV field-aligned current pairs. Sondrestrom incoherent scatter radar measures strong modulation of the strength and direction of ionospheric plasma flow, The magnetometer pulsations grow in magnitude over the next hour, peaking in intensity at 11:39 UT, at which time the UVI instrument measures a localized intensification of auroral emissions over central and western Greenland. Subsequent images show the intensification grow in strength and propagate westward (tailward) until approximately 11:58 UT at which time the emissions fade. These observations are consistent with the westward passage of two pairs of moderately intense TCVs over central Greenland followed by a third very intense TCV pair. The intensification of auroral emissions at 11:39 UT is associated with the trailing vortex of the third TCV pair, thought to be the result of an upward field-aligned current. Measurements of the solar wind suggest that a pressure change may be responsible for triggering the first two pairs of TCVS, and that a subsequent sudden change in orientation of the IMF may have produced the intensification of the third TCV pair and the associated aurora] brightening. DMSP particle data indicate that the TCVs occur on field lines which map to the boundary plasma sheet or outer edge of the low latitude boundary layer.

Clauer, C. R.↗