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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 361 records · Page 20
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NASA NTRS1982

Factors controlling degree of correlation between ISEE 1 and ISEE 3 interplanetary magnetic field measurements

Correlation variability between ISEE 1 and 3 IMF measurements is investigated, and factors governing the variability are discussed. About 200 two-hour periods when correlation was good, and 200 when correlation was poor, are examined, and both IMF variance and spacecraft separation distance in the plane perpendicular to the earth-sun line exert substantial control. The scale size of magnetic features is larger when variance is high, and abrupt changes in the correlation coefficient from poor to good or good to poor in adjacent two-hour intervals appear to be governed by the sense of change of IMF variance and vice versa. During periods of low variance, good correlations are most likely to occur when the distance between ISEE 1 and 3 perpendicular to the IMF is less than 20 earth radii.

Crooker, N. U.↗
NASA NTRS1985

Pitch Angle Distributions of Solar Energetic Particles and the Local Scattering Properties of the Interplanetary Magnetic Field

An approximate solution of the Fokker-Planck equation containing pitch angle scattering and adiabatic focusing is discussed. For modest focusing effects the omnidirectional density obeys an ordinary diffusion equation with a modified diffusion coefficient. The anisotropic part of the distribution function is properly normalized and split into an even and an odd part. The even part is determined by the ratio between the scattering mean free path and the focusing length and by the degree of polarization of the magnetic field fluctuations. The odd part is determined by the deviation of the pitch angle scattering from isotropic scattering. The method supplies a powerful tool to obtain the local characteristics of pitch angle scattering. It is insensitive to long lasting solar injections and to moderate radial variations of the mean free path. The method is applied to solar particle events observed on Helio-1 and -2.

Beeck, J.↗
NASA NTRS1984

Unusual auroral features observed on January 10-11, 1983 and their possible relationships to the interplanetary magnetic field

Intense perturbations observed in the auroral oval by the DMSP satellite on Jan. 10, 1983 are analyzed for possible influences by the IMF. The oval shrank in the early afternoon and was accompanied by intense activity in the morning sector. The latter feature was rare and occurred during relative quiescence in the evening sector, which exhibited a smaller width than the morning sector. Simultaneously, the ISEE-3 and IMP-8 spacecraft recorded large negative By values and positive Bz values in the magnetotail, assumed to be caused by activity of the IMF. The asymmetry in the oval is attributed to the negative By component, which has previously been proven to shift the open region to the evening sector. The asymmetry appeared, however, only when the Bz component attained large positive values.

I-Akasofu, S.↗
NASA NTRS1985

Neutral motions in the polar thermosphere for northward Interplanetary Magnetic Field

During the rare occurrence of November 24, 1982, when the north-south B(z) component of the IMF became positive for a period of about 11 hours and reached a steady value of about 25 nT, the DE-2 spacecraft simultaneously measured the ionic and neutral species of the high latitude F-region along the track of the polar-orbiting satellite. Data from two northern winter polar passes of DE-2 illustrate the response of the neutral F-region to ion drag forcing that arises from a configuration of ion convection characteristics of strongly northward IMF. The multicellular ion drift pattern associated with positive B(z) is observed to drive a similar, but less structured and weaker, neutral wind configuration in the winter polar cap. While major features of the ion drift pattern are mimicked by the neutral circulation, smaller scale ion flow structures are not. These results demonstrate that the sunward flow of neutral gas can be established and maintained by ion drag in the central polar cap, for positive B(z).

Killeen, T. L.↗
NASA NTRS1985

Theoretical interpretation of the observed interplanetary magnetic field radial variation in the outer solar system

Observations of the azimuthal component of the IMF are evaluated through the use of an MHD model which shows the effect of magnetic flux tubes opening in the outer solar system. It is demonstrated that the inferred meridional transport of magnetic flux is consistent with predictions by the MHD model. The computed azimuthal and radial magnetic flux deficits are almost identical to the observations. It is suggested that the simplest interpretation of the observations is that meridional flows are created by a direct body force on the plasma. This is consistent with the analytic model of Nerney and Suess (1975), in which such flux deficits in the IMF arise naturally from the meridional gradient in the spiralling field.

Suess, S. T.↗
NASA NTRS1986

Radial and latitudinal gradients in the interplanetary magnetic field - Evidence for meridional flux transport

Magnetic field data obtained by the Pioneer 10 and 11 spacecraft in the heliosphere from 1972-1982 and earth orbiting satellite data are examined in terms of radial and latitudinal gradients in the field components and magnitude. The data reveal that higher than expected gradients are observed in the magnetic field and time variations affect the field throughout the low-latitude heliosphere. It is determined that the high radial gradient is caused by meridional flux transport with low-latitude field lines moving to higher heliographic latitudes. High pressure near the solar equator and pressure due to heating in compressive solar wind interaction regions and the large field magnitudes that occur in these regions are investigated as mechanisms that produce the meridional flux. A solar cycle variation in the level of flux transport is analyzed.

Thomas, B. T.↗
NASA NTRS1986

The interplanetary magnetic field during solar cycle 21 ISEE-3/ICE observations

Temporal variations in the IMF during solar cycle 21 are investigated using magnetic field observations collected by the vector helium magnetometer on the ISEE-3/ICE spacecraft. Analysis of the observations reveal that the IMF magnitude, which had declined to 4.7 nT in 1976, peaked in late 1982 (two years after solar maximum) at 9.0 nT and rapidly decreased during 1983-1984 to an intensity of 6.2 nT in early 1985. The IMF intensities are compared with the auroral AE index; the observed peak in strength during 1981-1983 is related to a 50 percent increase in substorm activity levels. A decrease in Parker spiral angle, revealing the existence of high-speed streams is detected in the declining phase of the solar cycle. Variations in the intensity of the IMF correlate with Mt. Wilson magnetograph measurements of full disk magnetic flux. Source regions for the evolution of solar wind and the IMF are proposed.

Slavin, J. A.↗
NASA NTRS1986

Macroscopic perturbations of the Interplanetary Magnetic Field (IMF) by P/Halley as seen by the Giotto magnetometer

Giotto magnetic field data were used to analyze the macroscopic field structure in the vicinity of P/Halley. During the Giotto flyby at comet P/Halley the IMF showed a quite stable away polarity. Draping of magnetic field lines is clearly observed along the outbound leg of the trajectory. Inside the magnetic pile-up region the field reverses its polarity several times. A symmetry of oppositely magnetized sheets with respect to the nucleus is found and explained in terms of convected IMF features.

Raeder, J.↗
NASA NTRS1986

An interplanetary magnetic field dependent model of the ionospheric convection electric field

An IMF-dependent model of the magnetospheric electric field at ionospheric altitudes has been developed based on published observations, qualitative models, and a limited understanding of the electric field source. The empirical inputs are discussed, and the model is presented in an ionospheric convection situation where corotation is an important ingredient. This leads to a description of sunward ionospheric plasma transport in the polar cap for northward IMF orientations. The validity of the model is discussed, and areas in which more empirical results are required are specified.

Sojka, J. J.↗
NASA NTRS1986

Interplanetary magnetic field control of the Venus magnetosheath field and bow shock location

Six Venus years of Pioneer Venus Orbiter (PVO) data are analyzed in a coordinate system which isolates magnetic effects. The field draping pattern features two lobes easily identifiable even in the near planet region. The magnetosheath magnetic magnitude has a hemispherical asymmetry controlled by the IMF orientation in a manner suggesting preferrential ion pickup in one hemisphere. This result complements recent findings that the bow shock position is responsive to IMF direction. Examination of the ionopause position to evaluate ionospheric effects on the overall asymmetry of thfe Venus-solar wind interaction produces negative but not conclusive results.

Phillips, J. L.↗
NASA NTRS1987

Large scale response of the magnetosphere to a southward turning of the interplanetary magnetic field

A set of coordinated measurements obtained on March 4, 1979 in the solar wind, in the magnetotail, at geostationary orbit, in the auroral ionosphere, and on the ground has been used to conduct a detailed study of the overall magnetospheric response to a southward turning of the IMF. The change of the IMF direction is found to be associated with a delay of less than about 5 min for increasing eastward and westward electrojets, and precedes by about 10 min the beginning of a taillike deformation of the B field geometry at geostationary orbit near midnight. After the IMF reversal and before the main substorm onset, the general motion of the westward electrojet is southward. At 6.6 R(E) in the nightside, the equatorially trapped high-energy particles adiabatically respond to the local change of the B field by moving earthward. Short-lived substorms occur soon after the IMF reversal.

Sauvaud, J. A.↗
NASA NTRS1987

Thermospheric neutral wind signatures dependent on the east-west component of the interplanetary magnetic field for Northern and Southern Hemispheres as measured from Dynamics Explorer-2

DE-2 neutral-wind measurements obtained between November and January 1981-1982 and 1982-1983 were analyzed to investigate the effect of the sign of the IMF By component on neutral thermosphere dynamics at F-region altitudes. Maps of the mean neutral circulation for the northern (winter) and southern (summer) polar regions were derived for By positive and negative using polar geomagnetic coordinates. The mean neutral winds demonstrate clear By-dependent neutral wind signatures in both hemispheres.

Thayer, J. P.↗
NASA NTRS1987

Response of the auroral oval precipitation and magnetospheric convection to changes in the interplanetary magnetic field

High-latitude observations on January 18, 1984 during the first GISMOS campaign are examined. An intense substorm occurred after a long period of quiescent geomagnetic activity when the IMF decreased gradually over a 2-hour period from +5 to -8 nT. This present study investigates how the auroral precipitation boundary, the energy flux, and the convection respond to this change from quiet to active conditions.

De La Beaujardiere, O.↗
NASA NTRS1988

Interplanetary magnetic field draping about fast coronal mass ejecta in the outer heliosphere

The possibility that the IMF becomes draped around coronal mass ejections (CMEs) propagating rapidly through the quiescent solar wind into the outer heliosphere is investigated theoretically. The results are presented in diagrams and graphs and discussed in detail. It is found that large sunward-directed structures analogous to the Venus and cometary magnetotails should form when the CME velocity exceeds the solar-wind velocity by more than the local Alfven speed; such structures could hang up swept-up IMF flux for as long as several days. Pioneer 11 magnetic-field measurements at 6.9-9.4 AU from three 20-d periods in 1978 are examined and shown to contain some features consistent with CME draping.

Mccomas, D. J.↗
NASA NTRS1987

A magnetohydrodynamic simulation of reconnection in the magnetotail during intervals with southward interplanetary magnetic field

Results are reported from a simulation of the interaction between the solar wind and the earth magnetosphere, using a time-dependent three-dimensional MHD model. The calculation was performed for several orientations of the IMF between dawnward pointing and southward. When the IMF has a dawnward component, the plasma sheet rotates northward on the dawnside of the tail and toward the south on the duskside. As the southward component becomes larger, the plasma sheet becomes thinner and develops a wavy cross section because of patchy or localized tail reconnection. The field-aligned currents (FACs) associated with this localized reconnection have a filamentary layered structure. When projected onto the polar cap the filamentary FACs are located in the same region as the tail region 1 currents. At lower latitudes strong region 2 sense currents that originate in the plasma sheet are found. FACs are found on field lines that map to the polar cap even for southward IMF. These currents have many of the properties of the observed polar-cusp currents. The polar-cusp FACs evolve from the polar-cap NB(z) FACs as the IMF is rotated from northward to southward.

Walker, R. J.↗
NASA NTRS1988

The solar origin of long-term variations of the interplanetary magnetic field strength

Using simple models for the coronal field structure, the spacecraft observations of the photospheric field during sunspot cycle 21 were extrapolated for the purpose of modeling quantitatively the long-term behavior of the IMF during the sunspot cycle 21. Results were compared with the measurements of the radial component of the IMF at earth. The results indicate that the solar source of the IMF can be represented to a first approximation by the dipole component of the photospheric field, whose axis is nearly perpendicular to the ecliptic plane around sunspot minimum, but tilts more strongly toward it around sunspot maximum. It was also found that the average radial IMF strength varies with heliographic latitude; around sunspot minimum, the radial IMF is expected to be roughly twice as strong above the sun's poles as near the ecliptic plane. The average strength of the photospheric field above latitude 55 deg is about 10 G around sunspot minimum.

Wang, Y.-M.↗
NASA NTRS1988

Coherent mesoscale convection patterns during northward interplanetary magnetic field

Coincident DE 2 satellite plasma-drift and particle measurements and all-sky imaging photometer data were used to construct two-dimensional convection patterns in a study of polar ionospheric convection in the presence of subvisual-intensity soft-particle excited 6300-A sun-aligned polar cap arcs. Results show a highly anisotropic temporally stable convection with greater order than has previously been suspected.

Carlson, H. C.↗