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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Two types of magnetospheric ELF chorus and their substorm dependences

The distribution of extremely low frequency (10-1500 Hz) magnetospheric chorus to all local times and latitudes is investigated in order to determine dependence on substorms, and to evaluate the conditions under which chorus is generated. The analysis carefully separates space and time effects by an investigation of data obtained by the OGO 5 search coil magnetometer. A study of spatial dependencies shows that chorus occurs in two magnetic regions: equatorial chorus is located near the equator, and high-latitude chorus is located above 15 degrees. An analysis of chorus in each of the regions illustrates that equatorial chorus is definitely related to substorm, whereas high-latitude chorus often occurs within magnetically quiet intervals.

Tsurutani, B. T.↗
NASA NTRS1977

Vertical lifting of ionization during geomagnetic storms from satellite measurements of ion composition

Magnetic-storm phenomena at low latitudes are discussed based on ion-composition /O(+), H(+), He(+)/ and electron- and ion-temperature measurements from the OGO-4 and Isis-2 satellites. For the moderately severe storms considered, the effects of changes in the neutral composition and in the neutral and plasma temperatures are discussed, and it is shown that these changes would not produce the observed O(+) increase during storms at low latitudes. It is suggested that the observed increase in O(+) in the topside region is a manifestation of the vertical lifting of ionization of the F-layer. The argument in favor of vertical lifting is further substantiated by the observed changes in the F-region critical frequency and the height parameters.

Goel, M. K.↗
NASA NTRS1977

Annual variation in temperature and composition of the thermosphere and upper mesosphere

A three-dimensional circulation model, including UV (O2 dissociation) and EUV sources, is used to study the wind field and the effects of temperature and composition on annual thermospheric variations. The results are compared to those of OGO-6 and AE-C. Within an 800-1200 K temperature range, summer to winter temperature variation is studied as a function of solar activity. It is found that the model correctly predicts H, He, O, N2, O2, and Ar measurements. It is suggested that a small winter maximum in mesospheric temperature is caused by large-scale circulation induced by EUV heating. This effect, however, is masked by the energy released in O2 dissociation. The annual temperature amplitude and the winter oxygen bulge are noted to increase with increasing solar activity, whereas the winter helium bulge is noted to decrease with enhanced exospheric return flow. It is felt that the dependence of the F2 region winter anomaly on solar activity may be significantly affected by the solar activity effect in atomic oxygen.

Mayr, H. G.↗
NASA NTRS1977

Comparison of drag and mass spectrometer measurements during small geomagnetic disturbances

During small geomagnetic disturbances, ESRO 4 and OGO 6 gas analyzer measurements at high altitudes suggest that helium and atomic oxygen concentrations in the lower thermosphere decrease, whereas satellite drag measurements indicate that density increases. This discrepancy is explained by the corresponding temperature increases maximizing at high latitudes. ESRO 4 data suggest that at altitudes where atomic oxygen or helium predominates, the temperature increase compensates for the decrease in lower thermospheric concentrations. This yields a net density increase with geomagnetic disturbances. The Explorer 39 drag satellite measurements verify this conclusion. It is felt that the composition variations associated with minor disturbances indicate the upwelling of the polar atmosphere, circulation towards the equator, and subsidence in the equatorial region. ESRO measurements show that at low latitudes the increases in helium concentrations with geomagnetic disturbances are chiefly caused by the circulation from high latitudes and the subsidence at lower latitudes.

Keating, G. M.↗
NASA NTRS1977

High altitude observations of Birkeland currents

Several models of field-aligned currents (Birkeland currents) in the magnetosphere are discussed, and high altitude observations of these currents, carried out with the aid of highly eccentric earth-orbiting spacecraft of the OGO and IMP series, are reviewed. The essential roles of Birkeland currents are identified: they relieve charge imbalances, transmit stresses, and lead to particle acceleration anomalous resistivity.

Russell, C. T.↗
NASA NTRS1978

Plasma fluctuations in the solar wind

Using Ogo 5 plasma and magnetic field data, the power spectra of solar wind fluctuations are calculated over the 10 to the -3 to 10 to the -1 Hz range. It is found that the spectra calculated from flux measurements equal the density power spectra times the square of the average solar wind speed. The power spectrum relative density is of the same order of magnitude as the power spectrum of speed fluctuations relative to the Alfven speed. In cases where density and field fluctuations are inconsistent with magnetosonic waves, static inhomogeneities with a balance between electron thermal and magnetic pressures is assumed. It is also felt that a resonant proton cyclotron instability driven by the solar wind's proton thermal anisotropy may cause the observed power enhancements near 1 Hz.

Neugebauer, M.↗
NASA NTRS1978

Use of the Jacchia /1977/ empirical model in a dynamical model of upper-atmospheric helium

This brief report describes an investigation of the effects of using the Jacchia (1977) model in place of the Jacchia (1971) model for specifying the background gas pressure in the theoretical model of thermospheric helium described by Straus, et al. (1977). Changes in the global transport of helium caused by a small increase in the latitudes at which the background gas pressure extrema occur lead to much better agreement of the model predictions with data taken by the mass spectrometers on board the Esro 4 and OGO 6 satellites.

Straus, J. M.↗
NASA NTRS1978

Ion acoustic stability analysis of the earth's bow shock

A marginal stability criterion for ion acoustic waves is applied to the analysis of five thin bow shock crossings observed by OGO-5; the observations began with the satellite in the magnetosheath. The assumption of marginally stable ion acoustic wave generation in the shock ramp provides the basis for determining the thickness of the magnetic gradients for subcritical, quasi-perpendicular shock features.

Greenstadt, E. W.↗
NASA NTRS1978

The signature of H/+/ in the equatorial anomaly - An empirical model

Ion composition data from the OGO-6 satellite are used to develop an empirical model of the H(+) signature in the equatorial anomaly. The empirical model is an attempt to represent the generally repeatable details of the satellite data by a parameterized numerical model accounting for such variables as the altitude, local time and longitude of the observations. Although the resulting model is in general agreement with past findings on the variability of the equatorial anomaly phenomenon, there appears to be a complexity of short-term or narrowly localized variations which cannot be adequately understood on the basis of data from a single satellite.

Taylor, H. A., Jr.↗
NASA NTRS1978

On large poleward-directed electric fields at sub-auroral latitudes

Intense poleward-directed electric fields were observed by OGO-6 in the premagnetic midnight F-region, close to the Harang discontinuity and at latitudes of less than 60 deg invariant. These results support the recent measurements of Smiddy et al. (1977). Both sets of data are consistent with a compression of equipotentials on the low latitude edge of the premidnight convection cell, caused by an impediment of the substorm-related southwestward expansion of the convection pattern.

Maynard, N. C.↗
NASA NTRS1978

Dynamical interpretation of observed plasmasphere deformations

Density measurements made by OGO-5 during the period from March 1968 to May 1969 were used to locate enhanced light ion abundances in the midst of ion-depleted regions in the plasmasphere. Such abundances were found to be more frequent on the night side. As a possible mechanism for the observed light ion distribution, convection electric fields and subsequent thinning and corotation of plasma tails are considered. Attention is given to wave-particle interactions, especially as influenced by a magnetic field (both during plasmaspheric magnetic storms, and magnetospheric substorms).

Chen, A. J.↗
NASA NTRS1978

CIRA 1972, recent atmospheric models, and improvements in progress

The paper reviews current progress in the development of atmospheric models. Consideration is given to CIRA 1972 (COSPAR International Reference Atmosphere), the OGO 6 model (1974), the ESRO 4 model (1977), the global Doppler-temperature model of Thuillier et al. (1977), the Jacchia 1977 model (based on a synthesis of temperature, mass-spectrometer, and total-density data from many sources), and the MSIS model (1977), based on incoherent-scatter temperatures and mass-spectrometer data from various sources. New models of the disturbed thermosphere, developed by the author, are compared with recent mass-spectrometer measurements at 170 km.

Jacchia, L. G.↗
NASA NTRS1977

Multiple-satellite studies of magnetospheric substorms: Plasma sheet recovery and the poleward leap of auroral-zone activity

Particle observations from pairs of satellites (Ogo 5, Vela 4A and 5B, Imp 3) during the recovery of plasma sheet thickness late in substorms were examined. Six of the nine events occurred within about 5 min in locations near the estimated position of the neutral sheet, but over wide ranges of east-west and radial separations. The time of occurrence and spatial extent of the recovery were related to the onset (defined by ground Pi 2 pulsations) and approximate location (estimated from ground mid-latitude magnetic signatures) of substorm expansions. It was found that the plasma sheet recovery occurred 10 - 30 min after the last in a series of Pi bursts, which were interpreted to indicate that the recovery was not due directly to a late, high latitude substorm expansion. The recovery was also observed to occur after the substorm current wedge had moved into the evening sector and to extend far to the east of the center of the last preceding substorm expansion.

Pytte, T.↗
NASA NTRS1978

The problem of low energy particle measurements in the magnetosphere

The accurate measurement of low energy (less than 100 eV) particle properties in the magnetosphere has been difficult, partly because of the low density of such particles, but more particularly because of spacecraft interference effects. Some early examples of how these phenomena have affected particle measurements on an OGO spacecraft are presented. Data obtained with the UCSD particle detectors on ATS-6 are then presented showing how some of these difficulties have been partially overcome. Future measurements of low energy particles in the magnetosphere can be improved by: (1) improving the low energy resolution of detectors; (2) building electrostatically clean spacecraft; (3) controlling spacecraft potential; and (4) using auxiliary measurements, particularly wave data.

Whipple, E. C., Jr.↗
NASA NTRS1979

Tables of thermospheric temperature, density and composition derived from satellite and ground based measurements. Volume 1: Ap=4

The tables contain the neutral temperature, neutral densities for N2, O2, O, Ar, He and H, mean molecular weight, and total mass density as predicted by the Mass Spectrometer and Incoherent Scatter empirical thermosphere model for selected altitudes, latitudes, local times, days and other geophysical conditions. The model is based on a least squares fit to density data from mass spectrometers on five satellites and temperature data from four incoherent scatter stations, providing coverage for most of solar sunspot cycle 20. Included in the model data base are longitudinally average N3, He, and O densities from the OGO-6 mass spectrometer longitudinally average N2, He, O and Ar densities from the AEROS-A (NATE) mass spectrometer the N2, He, O, and Ar densities from the San Marco 3 mass spectrometer the N2 densities from the AE-B mass spectrometer and the N2, He, O, and Ar densities from the AE-C (OSS, NACE, NATE) mass spectrometers. The O2 and H densities are inferred using ion mass spectrometer data from AE-C (BIMS). Neutral exospheric temperature data are included from Arecibo, St. Santin, Millstone Hill and Jicamarca.

Hedin, A. E.↗
NASA NTRS1978

Multiple-satellite studies of magnetospheric substorms - Plasma sheet recovery and the poleward leap of auroral zone activity

Particle observations from pairs of satellites (OGO 5 and Vela 4A and 5A) during 28 plasma sheet thickening events indicate that thickening of the nighttime plasma sheet during substorms occurs in two main stages. The early stage involves single or multiple expansions of the near-earth plasma sheet at the onset of substorm expansions (Pi 2 bursts) on the ground, while the later stage of plasma sheet recovery starts near the time of maximum auroral zone bay activity. This stage is characterized by a large-scale thickening toward higher latitudes that occurs over a broad azimuthal scale and at heights that range from the ionosphere to beyond the Vela orbit. A detailed analysis of two-satellite observations during eight plasma sheet recoveries is presented, and events that occurred within 5 min in widely separated locations at small distances from the tail's midplane as well as events that occurred concurrently in the Vela orbit and at high latitudes in the near-earth region are revealed.

Pytte, T.↗
NASA NTRS1978

The energy balance of the nighttime thermosphere

Energy balance considerations for the thermosphere are affected by the uncertainty of the solar EUV flux. The problem is simplified by investigating only the nighttime thermosphere. Vapor trail measurements provide wind data at the sunset and sunrise terminators and near the polar cap boundary. Temperature and density profiles were obtained from incoherent scatter and Ogo 6 measurements. Based on the available data the energy losses are estimated to be 8 times 10 to the 10th W due to downward conduction at 120 km and 3 times 10 to the 10th W due to infrared radiation. The energy carried by the measured winds from the polar cap is found to be 4.6 times 10 to the 10th W and that transported across the mid- and low-latitude terminators, 7.6 to 11 times 10 to the 10th W. The greatest uncertainty in this analysis was in the estimates of the differences in the densities and temperatures in the lower thermosphere at the dawn and dusk terminators. Because of the large rotational velocity of the earth, small differences in these parameters correspond to large amounts of energy transported to the night hemisphere.

Glenar, D. A.↗