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At least 1,531 records · Page 85
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NASA NTRS1989

Trends in environmentally induced spacecraft anomalies

The Spacecraft Anomaly Data Base was useful in identifying trends in anomaly occurrence. Trends alone do not provide quantitative testimony to a spacecraft's reliability, but they do indicate areas that command closer study. An in-depth analysis of a specific anomaly can be expensive and difficult without access to the spacecraft. Statistically verified anomaly trends can provide a good reference point to begin anomaly analysis. Many spacecraft experience an increase in anomalies during the period of several days centered on the solar equinox, a period that is also correlated with sun eclipse at geostationary altitude and an increase in major geomagnetic storms. Increase anomaly occurrence can also be seen during the local time interval between midnight and dawn. This local time interval represents a region in Earth's near space that experiences an enhancement in electron plasma density due to a migration from the magnetotail during or following a geomagnetic substorm.

Wilkinson, Daniel C.↗
NASA NTRS1988

Definitive IGRF models for 1945, 1950, 1955, and 1960

Definitive internal geomagnetic field models are obtained for the epochs 1945, 1950, 1955, and 1960, with the solution including local biases for the fixed observatories. In order to derive a priori models for the epochs, the International Geomagnetic Reference Field (12/83) model (based on Magsat data) was projected backward in time using a spline fit to annual secular-variation models. An uncertainty model was developed which included estimates of the effects of crustal and core fields not represented by the model. Differences between biases from year to year are found to be within the predicted bounds.

Langel, R. A.↗
NASA NTRS1989

Energy deposition of corpuscular radiation in the middle atmosphere

Main components of corpuscular radiation contributing to energy deposition (ED in eV/cu cm/s) in the atmosphere (10 to 100 km) are cosmic ray nuclei (CR - galactic and solar) and high energy electrons (HEE), mainly of magnetospheric origin. Galactic CR depending on solar cycle phase and latitude are dominant source of ED by corpuscular radiation below 50 to 60 km. Below 20 km secondaries must be assumed. More accurate treatment need assuming of individual HE solar flare particles, cut off rigidities in geomagnetic field and their changes during magnetospheric disturbances. Electrons E sub e greater than 30 keV of magnetospheric origin penetrating to atmosphere contribute to production rate below 100 km especially on night side. High temporal variability, local time dependence and complicated energy spectra lead to complicated structure of electron ED rate. Electrons of MeV energy found at geostationary orbit, pronouncing relation to solar and geomagnetic activity, cause maximum ED at 40 to 60 km. Monitoring the global distribution of ED by corpuscular radiation in middle atmosphere need continuing low altitude satellite measurements of both HEE and x ray BS from atmosphere as well as measurements of energy spectra and charge composition of HE solar flare particles.

Kudela, K.↗
NASA NTRS1989

SC- and SI-associated ULF and HF-Doppler oscillations during the great magnetic storm of February 1986

Results are presented of an investigation of SC- and SI-associated ULF and HF-Doppler pulsations observed during the great geomagnetic storm of February 1986, which began with a sudden commmencement on February 6 at about 13:12 UT, developed slowly over the next two days, and, after a rapid intensification late on February 8, reached a minimum. It is shown that these ULF and geomagnetic pulsations can be explained by the dynamo-motor mechanism of ionospheric electric fields and by global compressional oscillations in the magnetosphere and ionosphere, respectively.

Yumoto, K.↗
NASA NTRS1990

A two-dimensional model of plasma expansion in the ionosphere

A systematic parameter study is conducted of the motion of artificial plasma clouds across the geomagnetic field at ionospheric altitudes. The study is based on a two-dimensional numerical model. Taken into consideration are the effects solar cycle variations on the cloud evolution, as well as the effects of seasonal and geomagnetic activity conditions, and variations in the ionospheric release conditions. The effects of magnetospheric electric fields and thermospheric winds are also considered. The plasma cloud model and related assumptions are discussed, and numerical results are presented.

Ma, T.-Z.↗
NASA NTRS1991

Geodynamics branch data base for main magnetic field analysis

The data sets used in geomagnetic field modeling at GSFC are described. Data are measured and obtained from a variety of information and sources. For clarity, data sets from different sources are categorized and processed separately. The data base is composed of magnetic observatory data, surface data, high quality aeromagnetic, high quality total intensity marine data, satellite data, and repeat data. These individual data categories are described in detail in a series of notebooks in the Geodynamics Branch, GSFC. This catalog reviews the original data sets, the processing history, and the final data sets available for each individual category of the data base and is to be used as a reference manual for the notebooks. Each data type used in geomagnetic field modeling has varying levels of complexity requiring specialized processing routines for satellite and observatory data and two general routines for processing aeromagnetic, marine, land survey, and repeat data.

Langel, Robert A.↗
NASA NTRS1990

Solar cycle variations in F-region Te in the vicinity of the midlatitude trough based on AE-C measurements at solar minimum and DE-2 measurements at solar maximum

Magnetospheric energy deposited in the plasmasphere produces large enhancements in the electron temperature in the nightside ionosphere at the foot of the geomagnetic L shell that traverses the plasmapause. This temperature peak, which is associated with the midlatitude trough in electron density, often has a great enough amplitude to produce 630 nm emission known as a Sar-arc. The Atmosphere Explorer-C measurements made at solar minimum and the Dynamics Explorer-2 measurements made at solar maximum are used to illustrate how this signature of F-region electron heating changes with solar activity. Global empirical models of the electron temperature and density have not been able to resolve these features thus far because of their large movements with geomagnetic activity and because of the large bin sizes used in the models. It is not yet clear how this major feature of the F-region temperature structure could be included easily in the IRI model.

Brace, Larry H.↗
NASA NTRS1990

An estimate of the maximum speed of the solar wind, 1938-1989

In an effort to estimate the highest flow velocity that the solar wind has exhibited at earth during the past 50 years, geomagnetic storms that occurred from 1938 to 1989 were surveyed, and the storms that were preceded by a major proton flare were selected. For each identified flare-storm pair, the average speed ('transit speed') of the associated interplanetary shock from the interval between the flare onset and the sudden commencement of the geomagnetic storm was calculated. In each case, the maximum solar wind flow speed was inferred from an empirical relationship (derived for a sample of recent events) between the shock transit speed and the peak flow velocity of the associated transient stream, obtaining a distribution of maximum solar wind speeds, which presumably corresponds to a sample of the most energetic events of this 50-yr period. Results show no evidence for bulk flow velocities greater than the about 2000 km/sec value deduced by Zastenker et al. (1978) and Grunwaldt (1975) for the August 4, 1972 event.

Cliver, E. W.↗
NASA NTRS1990

On the consequences of strong stable stratification at the top of earth's outer core

The consequences of strong stable stratification at the top of the earth's fluid outer core are considered, concentrating on the generation of the geomagnetic secular variation. It is assumed that the core near the core-mantle boundary is both strongly stably stratified and free of Lorentz forces: it is found that this set of assumptions severely limits the class of possible motions, none of which is compatible with the geomagnetic secular variation. Relaxing either assumption is adequate: tangentially geostrophic flows are consistent with the secular variation if the assumption that the core is strongly stably stratified is relaxed (while retaining the assumption that Lorentz forces are negligible); purely toroidal flows may explain the secular variation if Lorentz forces are included.

Bloxham, Jeremy↗
NASA NTRS1991

Quasi-perpendicular/quasi-parallel divisions of earth's bow shock

Computer-drawn diagrams of the boundaries between quasi-perpendicular and quasi-parallel areas of earth's bow shock are displayed for a few selected cone angles of static interplanetary magnetic field (IMF). The effect on the boundary of variable IMF in the foreshock is also discussed and shown for one nominal case. The boundaries demand caution in applying them to the realistic, dynamic conditions of the solar wind and in interpreting the effects of small cone angles on the distributions of structures at the shock. However, the calculated, first-order boundaries are helpful in defining areas of the shock where contributions from active structures inherent in quasi-parallel geometry may be distinguishable from those derived secondarily from upstream reflected ion dynamics. The boundaries are also compatible with known behavior of daytime ULF geomagnetic waves and pulsations according to models postulating that cone angle-controlled, time-dependent ULF activity around the subsolar point of the bow shock provides the source of geomagnetic excitation.

Greenstadt, E. W.↗
NASA NTRS1991

Fluid flow near the surface of earth's outer core

This review examines the recent attempts at extracting information on the pattern of fluid flow near the surface of the outer core from the geomagnetic secular variation. Maps of the fluid flow at the core surface are important as they may provide some insight into the process of the geodynamo and may place useful constraints on geodynamo models. In contrast to the case of mantle convection, only very small lateral variations in core density are necessary to drive the flow; these density variations are, by several orders of magnitude, too small to be imaged seismically; therefore, the geomagnetic secular variation is utilized to infer the flow. As substantial differences exist between maps developed by different researchers, the possible underlying reasons for these differences are examined with particular attention given to the inherent problems of nonuniqueness.

Bloxham, Jeremy↗
NASA NTRS1991

Vorticity and divergence in the high-latitude upper thermosphere

Measurements made from the Dynamics Explorer-2 satellite in November 1981 through January 1982 and November 1982 through January 1983 have been analyzed to determine the divergence and the vertical component of vorticity of the high-latitude neutral wind field in the upper thermosphere for quiet (Kp not greater than 3) and active (Kp between 3+ and 6) geomagnetic conditions and for both Northern (winter) and Southern (summer) Hemispheres. This analysis provides the first experimental determination of the large-scale vorticity and divergence patterns in the polar thermosphere and provides insight into the relative strengths of the different sources of momentum and energy responsible for driving the winds. The principal findings from this work include the following: the mean neutral wind pattern is dominated by rotational flow rather than by divergent flow, with a typical vorticity:divergence ratio of about 2:1 for active conditions and about 4:1 for quiet conditions. Comparison of the divergence and vorticity patterns for quiet and active conditions indicates that the divergent component of the neutral flow intensifies more significantly with increasing geomagnetic activity than does the rotational component.

Thayer, J. P.↗
NASA NTRS1991

Reconnection voltage as a function of IMF clock angle

Magnetic reconnection between the IMF and the geomagnetic field is thought to play a major role in the transfer of solar wind momentum and energy to the magnetosphere. Both analytic modeling and analysis of geophysical data have shown that this coupling process should be a sensitive function of the clock angle of the IMF. Results are presented from a three-dimensional, MHD, global numerical simulation code for the reconnection voltage between the closed geomagnetic field and the IMF as a function of the IMF clock angle. These results are consistent with a sin(theta/2) functional behavior.

Fedder, J. A.↗
NASA NTRS1991

An initial analysis of the data from the Polar Orbiting Geophysical (POGS) Satellite

The Polar Orbiting Geophysical Satellite (POGS) was launched in 1990 to measure the geomagnetic field. POGS data from selected magnetically quiet days was chosen, quality checked and deleted where thought to be erroneous. A time and position correction was applied. The resulting data was fit to a degree 13 spherical harmonic model. Evaluation of the quality of the data indicates that it is sufficient for definition of the low degree (approximately less than 8) portion of the geomagnetic field. Further correction of the data time and position may improve this quality.

Langel, R. A.↗
NASA NTRS1991

Atomic oxygen flux and fluence calculation for Long Duration Exposure Facility (LDEF)

The LDEF mission was to study the effects of the space environment on various materials over an extended period of time. One of the important factors for materials degradation in low earth orbit is the atomic oxygen fluxes and fluences experienced by the materials. These fluxes and fluences are a function of orbital parameters, solar and geomagnetic activity, and material surface orientation. Calculations of atomic oxygen fluences and fluxes for the LDEF mission are summarized. Included are descriptions of LDEF orbital parameters, solar and geomagnetic data, computer code FLUXAV, which was used to perform calculations of fluxes and fluences, along with a discussion of the calculated fluxes and fluences.

Bourassa, Roger J.↗
NASA NTRS1992

Prediction of LDEF ionizing radiation environment

The Long Duration Exposure Facility (LDEF) spacecraft flew in a 28.5 deg inclination circular orbit with an altitude in the range from 172 to 258.5 nautical miles. For this orbital altitude and inclination two components contribute most of the penetrating charge particle radiation encountered - the galactic cosmic rays and the geomagnetically trapped Van Allen protons. Where shielding is less than 1.0 g/sq cm geomagnetically trapped electrons make a significant contribution. The 'Vette' models together with the associated magnetic filed models were used to obtain the trapped electron and proton fluences. The mission proton doses were obtained from the fluence using the Burrell proton dose program. For the electron and bremsstrahlung dose we used the Marshall Space Flight Center (MSFC) electron dose program. The predicted doses were in general agreement with those measured with on-board thermoluminescent detector (TLD) dosimeters. The NRL package of programs, Cosmic Ray Effects on MicroElectronics (CREME), was used to calculate the linear energy transfer (LET) spectrum due to galactic cosmic rays (GCR) and trapped protons for comparison with LDEF measurements.

Watts, John W.↗
NASA NTRS1992

Low-energy particle layer outside of the plasma sheet boundary

The ISEE spacecraft in the geomagnetic tail frequently crossed the high-latitude boundary of the plasma sheet. On a number of these crossings on the morningside (between 15 RE and 22 RE) the ISEE instruments detected an enhanced population of low-energy electrons and ions immediately adjacent to the plasma sheet boundary layer (PSBL). The electrons in this low-energy layer (LEL) have energies less than a few hundred eV, and they are aligned along the magnetic field direction propagating in the tailward direction. The ions have energies less than 100 eV and are also streaming along the magnetic field direction but in the earthward direction. These particles are clearly distinguished from the bulk of the particles in the plasma sheet and the PSBL. These observations may help clarify where the various particle features in the geomagnetic tail map to in the ionosphere. It is suggested that the LEL maps to the soft (less than 1 keV) electron precipitation region poleward of the plasma sheet boundary.

Parks, G. K.↗
NASA NTRS1993

Revised prediction of LDEF exposure to trapped protons

The Long Duration Exposure Facility (LDEF) spacecraft flew in a 28.5 deg inclination circular orbit with an altitude in the range from 319.4 to 478.7 km. For this orbital altitude and inclination, two components contribute most of the penetrating charge particle radiation encountered - the galactic cosmic rays and the geomagnetically trapped Van Allen protons. Where shielding is less than 1.0 g/sq cm geomagnetically trapped electrons make a significant contribution. The 'Vette' models together with the associated magnetic field models and the solar conditions were used to obtain the trapped electron and proton omnidirectional fluences reported previously. Results for directional proton spectra using the MSFC anisotropy model for solar minimum and 463 km altitude (representative for the LDEF mission) were also reported. The directional trapped proton flux as a function of mission time is presented considering altitude and solar activity variation during the mission. These additional results represent an extension of previous calculations to provide a more definitive description of the LDEF trapped proton exposure.

Watts, John W.↗