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At least 613 records · Page 34
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NASA NTRS1981

The electric field structure of auroral arcs as determined from barium plasma injection experiments

Barium plasma injection experiments have revealed a number of features of electric fields in and near auroral forms extending from a few hundred to many thousands of km in altitude. There is evidence for V-type potential structures over some auroras, but not in others. For some auroral arcs, large E fields are found at ionospheric altitudes outside the arc but the E field inside the arc is near zero. In a few other auroras, most recently one investigated in an experiment conducted from Poker Flat on March 22, 1980, large, rapidly fluctuating E fields were detected by barium plasma near 600 km altitude. These E fields suggest that the motion of auroral rays can be an effect of low-altitude electric fields, or that V-type potential structures may be found at low altitudes.

Wescott, E. M.↗
NASA NTRS1981

Jupiter and Io - A binary magnetosphere

A qualitative assessment is presented of Voyager 1 and 2 data analysis and theoretical interpretation, regarding the Io torus and Jovian aurora, dominant magnetospheric components, plasma waves and radio emissions, with emphasis on the difficulty of accounting for either the Jupiter aurora or Io torus EUV emission luminosities in energetic terms. Jupiter's middle atmosphere is also considered, with attention to observations of corotating ions, their ambiguities and their implications. After a discussion of the question of Jupiter's interaction with the solar wind, as manifested by its magnetic tail, terrestrial magnetospherics are invoked in the construction of a tentative unification of observed phenomena which is within the latitude afforded by the current state of data reduction.

Scarf, F. L.↗
NASA NTRS1982

Global observations of a SAR arc

A description is presented of the first results from observations of a stable auroral red (SAR) arc with the global auroral imaging instrumentation on board Dynamics Explorer (DE) 1. This instrumentation includes three spin-scan imaging photometers and a data control unit. The last visible-wavelength imaging sequence prior to onset of the geomagnetic storm begins at 0517 UT on October 20, 1981. An auroral substorm is observed in progress two orbits later, and as the substorm subsides and the diffuse aurora retreats poleward a narrow band of 630-nm emissions is observed at latitudes previously occupied by the aurora. This band is detected at 630 nm only, and is identified as the first optical observation of the SAR arc with DE 1. The third orbit of observations commences at 0830 UT on October 21 with the arc remaining visible across the nightside of earth. No visual signature of the SAR arc is detected during the sixth orbit.

Craven, J. D.↗
NASA NTRS1983

Eddy intrustion of hot plasma into the polar cap and formation of polar-cap arcs

Under the simple postulate that multiple large scale detachable magnetospheric convection eddies can exist in the vicinity of the convection reversal boundary and in the polar cap, by Kelvin-Helmholtz instability or otherwise, it is shown that a number of seemingly disconnected plasma and electric field observations in the polar cap can be organized into a theory of magnetosheath and plasmasheet plasma intrusion into the polar cap. Current theory of inverted V structures then predicts existence of similar, but weaker, structures at the eddy convection reversal boundaries in the polar cap. A possible consequence is that the polar cap auroras are natural offshoots from discrete oval arcs and evidently are formed by similar processes. The two arc systems can occassionally produce an optical image in the form of the theta aurora.

Chiu, Y. T.↗
NASA NTRS1983

Energetic ion losses near Io's orbit

The interactions between Io and the Jovian particle population have been of considerable interest ever since the discovery that Io modulates Jupiter's decametric radiation. Voyager 1 observations established that a plasma torus around Io dominates the mass and energy budgets of the Jovian magnetosphere. This torus must then play a central role in the particle loss process around Io. The present investigation provides a picture for particle transport and energetic ion losses near Io's orbit which considers both low energy plasma data and energetic ion data. The approximate match between the energy needed to power the Jovian UV aurora and the 'missing' energy of the low energy charged particle (LECP) measured ions is consistent with the concept that significant precipitation of energetic ions into the Jovian atmosphere can maintain the Jovian aurora.

Cheng, A. F.↗
NASA NTRS1983

O I (7990 A) emission and radiative entrapment of auroral EUV

The auroral O I 7990-A multiplet intensity distribution has been observed by means of a 2-m grille spectrometer and a Fabry-Perot interferometer during 1977 and 1982, respectively, in Alaska. Grille spectrometer results indicate a nonstatistical distribution in the excited state, consistent with the preferential scattering of the O I 989-A EUV emission and in accord with recent radiative transfer calculations. A comparison of O I 7990-A intensities measured on the nightside with those from dayside magnetospheric cleft aurora shows a cascade contribution to the O I 8446-A line, and ultimately to the O I 1304-A transition, that depends on the altitude of the aurora. This is attributed to the greater radiative enhancement of O I 989-A at low altitude, and the correspondingly enhanced probability for branching from the 3 s-prime to the 3 p level in atomic O.

Christensen, A. B.↗
NASA NTRS1984

Magnetospheric Structures: Uranus and Neptune

Magnetospheric structures that might be encountered at Uranus and Neptune are described. Statistics indicate a sufficiently high probability to warrant consideration of their likely properties in advance of the Voyager encounters. Because the spin axis of Uranus lies nearly in the ecliptic and presently points approximately sunward, Voyager is likely to encounter the unique pole on configuration that has special theoretical significance. Corotation in the magnetospheres of Uranus and Neptune would probably exclude solar wind drive convection as an important driver of global magnetospheric dynamics, as it does at Jupiter and Saturn. The magnetospheres of Uranus and Neptune probably lack sufficient internal sources of plasma to produce significant levels of rotationally driven convection. The reported observation of auroral emission from Uranus has therefore motivated the development of an alternative model in which solar wind motion is coupled directly to the rotation of the ionosphere to establish a dynamo circuit which generates Birkeland currents and polar cap aurora. This model predicts the strength and configuration of the aurora as functions of the magnitude and polarity, respectively, of the planetary magnetic moment.

T W Hill↗
NASA NTRS1985

Global auroral responses to magnetospheric compressions by shocks in the solar wind: Two case studies

The global auroral responses to shocks in the solar wind at Earth were studied. The z-component of the interplanetary magnetic field, Bz, is negative ahead and behind the first shock and positive for the second case. A sudden-commencement geomagnetic storm develops in each case, with maximum D sub st 190 nT. An immediate auroral response is detected at all longitudes around the auroral oval, in which auroral luminosities increase by a factor of 2 to 3 with the first samples after each sudden commencement. The time delay in obtaining the first sample varies with local time from approx. 1 to 18 mins. No other significant variations in the aurora are associated with the immediate response. Beginning approx. 30 mins after each sudden commencement, the aurora becomes active and displays significant variations in its luminosity and spatial distribution. For Bz 0 an intense substorm develops. A sun-aligned transpolar arc forms when Bz 0, appearing first at local midnight as a polar arc and then lengthening sunward from the auroral oval across the polar cap to noon at an average speed of approx. 1 km/sec.

Craven, J. D.↗
NASA NTRS1985

The polar caps

According to the most common definition, the 'polar cap' is the region bounded by the average or statistical auroral oval. Studies of the effects of the interplanetary magnetic field (IMF) on various upper atmospheric phenomena are reviewed. The Antarctic region and the Arctic region represent an area for such investigations. Particular attention is given in this paper to those observations in the highest latitude region which provide some information concerning corresponding changes of the internal structure of the magnetosphere. A definition and working definition of the polar cap are considered along with the IMF and magnetospheric models, the entry of solar energetic electrons, statistical studies regarding the aurora, individual events, polar cap arcs, the cusp aurora, auroral electron precipitation, convection, ionospheric currents and field-aligned currents, the ionosphere, the thermosphere, polar rain, polar wind, and hopping motions of heavy ions.

Akasofu, S.-I.↗
NASA NTRS1986

The Io Torus and the Jovian magnetosphere

The IUE monitored the physical conditions in the Jovian magnetospheric system using the in situ Voyager measurements as a basis for comparison. Both the Io plasma torus, observable in emission of S(+), S(++), and S(+3), and the Jovian H2 polar aurorae are accessible to the IUE short wavelength spectrograph. Despite significant short-term variations observed, the electron density and temperature structure of the torus has not changed appreciably in the 7 yr since the Voyager encounters. The total radiated power from the polar aurorae remained relatively constant during this period.

Feldman, P. D.↗
NASA NTRS1986

Continued observations of the H Ly alpha emission from Uranus

Observations of Uranus obtained over four years with the IUE Observatory supports the initial identification of a bright H Ly alpha flux which varies independently of the solar H Ly alpha flux, implying a largely self-excited emission. An average brightness of 1400 Rayleighs is derived, and limits for the possible contribution by reflected solar H Ly alpha emission, estimated to be about 200 Rayleighs, suggest that the remaining self-excited emission is produced by an aurora. Based on comparison with solar wind measurements obtained in the vicinity of Uranus by Voyager 2 and Pioneer 11, no evidence for correlation between the solar wind density and the H Ly alpha brightness is found. The upper limit to H2 emission gives a lower limit to the ratio of H Ly alpha/H2 emissions of about 2.4, suggesting that the precipitating particles may be significantly less energetic on Uranus than those responsible for the aurora on Jupiter. The average power in precipitating particles is estimated to be of the order of 10 to the 12th W.

Clarke, J.↗
NASA NTRS1986

Vibrational excitation in O2(B1Sigma g+)

Excitation processes for O2(b1Sigma g+) in aurorae, in the nightglow, and in laboratory sources are discussed. It is shown that the observed vibrational distribution in aurorae is consistent with the O2(+) + NO charge-transfer mechanism. Arguments are presented for the case the quenching of O2(b) in vibrational levels above v-prime = 1 is rapid, and that therefore the auroral source is much larger than previously supposed. It is suggested that oxygen atoms are an efficient quencher for O2(b) levels above v-prime = 1.

Slanger, Tom G.↗
NASA NTRS1986

Global auroral responses to magnetospheric compressions by shocks in the solar wind - Two case studies

The global auroral responses to shocks in the solar wind at earth were studied. The z-component of the interplanetary magnetic field, Bz, is negative ahead and behind the first shock and positive for the second case. A sudden-commencement geomagnetic storm develops in each case, with maximum D sub st 190 nT. An immediate auroral response is detected at all longitudes around the auroral oval, in which auroral luminosities increase by a factor of 2 to 3 with the first samples after each sudden commencement. The time delay in obtaining the first sample varies with local time from approx. 1 to 19 mins. No other significant variations in the aurora are associated with the immediate response. Beginning approx. 30 mins after each sudden commencement, the aurora becomes active and displays significant variations in its luminosity and spatial distribution. For Bz 0 an intense substorm develops. A sun-aligned transpolar arc forms when Bz 0, appearing first at local midnight as a polar arc and then lengthening sunward from the auroral oval across the polar cap to noon at an average speed of approx. 1 km/sec.

Craven, J. D.↗
NASA NTRS1988

Thermospheric dynamics during November 21-22, 1981 - Dynamics Explorer measurements and thermospheric general circulation model predictions

Time-dependent aurora and magnetospheric convection parameterizations have been derived from solar wind and aurora particle data for November 21-22, 1981, and are used to drive the auroral and magnetospheric convection models that are embedded in the National Center for Atmospheric Research thermospheric general circulation model (TGCM). Neutral wind speeds and transition boundaries between the midlatitude solar-driven circulation and the high-latitude magnetospheric convection-driven circulation are examined on an orbit-by-orbit basis. The results show that TGCM-calculated winds and reversal boundary locations are in generally good agreement with Dynamics Explorer 2 measurements for the orbits studied. This suggests that, at least for this particular period of relatively moderate geomagnetic activity, the TGCM parameterizations on the eveningside of the auroral oval and polar cap are adequate.

Roble, R. G.↗
NASA NTRS1989

Rocket measurements within a polar cap arc - Plasma, particle, and electric circuit parameters

Results are presented from the Polar Ionospheric Irregularities Experiment (PIIE), conducted from Sondrestrom, Greenland, on March 15, 1985, designed for an investigation of processes which lead to the generation of small-scale (less than 1 km) ionospheric irregularities within polar-cap F-layer auroras. An instrumented rocket was launched into a polar cap F layer aurora to measure energetic electron flux, plasma, and electric circuit parameters of a sun-aligned arc, coordinated with simultaneous measurements from the Sondrestrom incoherent scatter radar and the AFGL Airborne Ionospheric Observatory. Results indicated the existence of two different generation mechanisms on the dawnside and duskside of the arc. On the duskside, parameters are suggestive of an interchange process, while on the dawnside, fluctuation parameters are consistent with a velocity shear instability.

Weber, E. J.↗
NASA NTRS1990

Multipoint observations of a small substorm

Results are presented of multipoint observations of a small substorm which occurred at about 0110 UT on April 25, 1985, carried out by AMPTE CCE, AMPTE IRM, DMSP F6, and DMSP F7, as well as by ground auroral stations and midlatitude stations. These data yield information on the latitudinal extent of the polar cap and provide visual identification of substorm aurorae, magnetic perturbations produced directly beneath aurorae, and the situ magnetic field. In addition, they provide magnetic-particle observations of the disruption of the cross-tail current sheet and observations concerning the spatial expansion of the current disruption region. Evidence is presented that the current sheet disruption observed by CCE in the neutral sheet was located on field lines which mapped to the westward traveling surge observed directly overhead of the ground station at Syowa.

Lopez, R. E.↗
NASA NTRS1992

Aeronomical modeling of the outer planets

The purpose is twofold: to report the programmatic issues of the project as to accomplishments and anticipated work statement for the next six months and to report the technical findings of the project. From the programmatic point of view the study was a clear success. Four talks were given on the work: (1) a talk on electron Bremsstrahlung X ray production at the annual American Astronomical Society, Division of Planetary Studies meeting in October of 1990 in Charlottesville, Virginia; (2) a poster on proton precipitation at the May, 1991 American Geophysical Union in Baltimore, Maryland; (3) an invited paper on multispectral time dependent modeling at the International Union of Geodosy and Geomagnetism in Vienna in August of 1991; and (4) a paper at the annual Division of Planetary Studies of the American Astronomical Society held in Palo Alto in November of 1991 entitled 'Jovian Bremsstrahlung X Rays: A Ulysses Prediction' which predicted the Jovian auroral X ray flux that should be measured by the Ulysses Gamma Ray Burst experiment during the Ulysses spacecraft's closest encounter with Jupiter in February 1992. The Ulysses' prediction paper generated much interest from people studying the Jovian aurora and from experimenters on the Ulysses spacecraft. As a result of this interest two things happened: (1) a paper of the same title was submitted and accepted for publication in the January issue of the Geophysical Research Letters; and (2) a massive observing campaign was organized to provide supporting ultraviolet and infrared observations at the time of the Ulysses encounter. This observational interest allowed personnel at the Southwest Research Institute (SwRI) (Alan Stern, PI; Hunter Waite Co-I) to obtain director's discretionary time on the Hubble Space Telescope to support the Ulysses encounter by obtaining ultraviolet observations. Once the multiple wavelength observations are obtained in February, the auroral model developed under this project will be used to analyze the data with hopes of providing new insight into the physical processes responsible for the Jovian aurora.

Waite, J. Hunter, Jr.↗
NASA NTRS1992

Formation of auroral arcs via magnetosphere-ionosphere coupling

The aurora results from electrons that first are energized at very high altitudes by the solar wind-earth interactions and then travel down magnetic field lines until they hit the atmosphere. The basic physical processes are described that lead to the energization of these electrons and thus to the aurora.

Lyons, L. R.↗