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NASA NTRS1984

On the relationship between morning sector irregular magnetic pulsations and field aligned currents

For three magnetically disturbed days in early 1980, data from south polar masses of the Magsat satellite are compared with data from search coil magnetometer, riometer, and photometer instrumentation at Siple, Antarctica. It is found that during each Magsat polar pass in the morning sector, the level of Pi 1 activity correlates well with the intensities of three-dimensional current systems. Fine structure is often observed in the field-aligned currents during periods of intense Pi activity. Among the Birkeland currents are 2-s to 10-s (16-80 km) structured perturbations; these are evident in the transverse components of the field and are thought to indicate filamentary currents. Pi 1 amplitudes are found to be considerably larger when region 2 Birkeland currents are overhead than when they are not. In one case, detailed features are identified in the high-resolution Magsat magnetic field data that may be current fluctuations related to asymmetric Pi 1.

Engebretson, M. J.↗

NASA NTRS1984

The evolution of large-scale magnetic fields in the ionosphere of Venus

Large-scale magnetic fields are often observed in the ionosphere of Venus by the magnetometer on the Pioneer Venus Orbiter, especially near the subsolar point or when the solar wind dynamic pressure is high. An equation for the time evolution of the magnetic field is derived which includes both a term representing the time rate of change of the field due to the convection of magnetic flux by plasma motions, and a magnetic diffusion/dissipation term. The ionospheric plasma velocities required by these equations were obtained by numerically solving the momentum equation. Numerical solutions to the magnetic field equation indicate that large-scale magnetic fields, which are not being actively maintained, decay with time scales ranging from tens of minutes to several hours. The vertical convection of magnetic flux enables magnetic field structures deep within the ionosphere to persist longer than would otherwise be expected. This vertical convection also explains the shape of these structures.

Cravens, T. E.↗

NASA NTRS1984

Large scale temporal and radial gradients in the IMF - Helios 1, 2, ISEE-3, and Pioneer 10, 11

Recent investigations using measurements at 1 AU have discovered three types of long term variation in the interplanetary magnetic field: solar minimum decreases, solar maximum enhancements, and small decreases around solar reversal. In this study the 1972-1982 Helios 1, 2, ISEE-3, and Pioneer 10, 11 observations between 0.3 and 12 AU are examined to further investigate these changes. It was found that all three IMF solar cycle effects are also present in the Helios and Pioneer measurements, confirming that these variations occur throughout the low latitude heliosphere. In addition, the comparison of measurements by identical magnetometers on ISEE-3, Pioneer 10 and Pioneer 11 has revealed a more rapid decrease in IMF intensity than predicted by classical Parker theory. Causes and ramifications of both the long term variations and steeper-than-expected radial gradients in the interplanetary magnetic field are discussed.

Slavin, J. A.↗

NASA NTRS1983

GRM - Observing the terrestrial gravity and magnetic fields in the 1990's

NASA is proposing to launch a new geopotential fields exploration system called the Geopotential Research Mission (GRM). Two spacecraft will be placed in a circular polar orbit at 160 km altitude. Distances between these satellites will vary from 100 to 600 km. Both scalar and vector magnetic fields will be measured by magnetometers mounted on a boom positioned in the forward direction on the lead satellite. Gravity data will be computed from the measured change in distance between the two spacecraft. This quantity, called the range-rate, will be determined from the varying frequency (Doppler shift) between transmitter and receiver on each satellite. Expected accuracies (at the one-sigma level) are: gravity field, 1.0 milliGal, 5 cm geoid height; magnetics, scalar field 2 nT, vector to 20 arcsec, both resolved to less than 100 km. With these more accurate and higher resolution data, it will be possible to investigate the earth's structure from the crust (with the shorter wavelength gravity and magnetic anomalies) through the mantle (from the intermediate wavelength gravity field) and into the core (using the longer wavelength gravity and magnetic fields).

Taylor, P. T.↗

NASA NTRS1984

ISEE 1 and 2 observations of an oscillating outward moving current sheet near midnight

The ISEE 1 and 2 magnetometers are used to examine a current sheet which is moving outward with a velocity of about 17 km/sec, and whose current is flowing into the ionosphere. A wave is traveling along the current sheet, from midnight toward the east, at 400 km/sec. The plasma oscillations normal to the current sheet associated with this wave are sufficient for explaining the amplitude of the electric field oscillations in the plane of the current sheet observed by the University of California's electric field detector. The present oscillating current sheet model explains both magnetic and electric field observations.

Kelly, T. J.↗

NASA NTRS1984

Wave normal directions of chorus near the equatorial source region

OGO 5 triaxial search coil magnetometer data are used to determine the wave propagation directions of postmidnight chorus in the near-equatorial region at L shells of 6 to 7. The methods used to estimate the wave normal directions involved minimum variance, the imaginary part of the cross-spectral matrix, the eigenvector of the Hermitian cross-spectral matrix, and a fitting of dispersion relations for one-wave and two-wave models to the cross-spectral matrix. Wave propagation at all frequencies within chorus tones is found to occur most frequently along the magnetic field with median and average cone angles of 9.1 deg and 12.2 deg, respectively. It is concluded that for waves propagating parallel to B, wave growth is maximum.

Goldstein, B. E.↗

NASA NTRS1984

Rocket observations of electron density in the nighttime E region using Faraday rotation

A rocket radio propagation experiment is described in which the electron density profile of the nighttime E region is obtained with an accuracy of 100 per cu cm. The factors limiting the accuracy of the experiment are found to be related to atmospheric and receiver noise and to the use of a magnetometer to determine the spin rate of the rocket. The Fourier analysis used for the frequency measurement may also contribute error under conditions of steep electron density gradients. The accuracy being achieved appears to be adequate for present applications of the experiment.

Smith, L. G.↗

NASA NTRS1984

A reexamination of rotational and tangential discontinuities in the solar wind

An analysis is made of interplanetary tangential and rotational solar wind discontinuities (TD and RD) and comparisons are made between the features of RDs and TDs. An ISEE 3 field and positive ion data set from 1978 includes high time resolution magnetometer data and is used for the comparisons, as are data from a positive ion analyzer. The field magnitude of RDs remains constant as the field rotates, while that of a TD passes through a local minimum. First and second adiabatic invariants for protons and He abundances are usually also conserved for RDs but not for TDs. The velocity change for an RD across a discontinuity is smaller than that predicted by MHD theory. Finally, plasma conditions at a discontinuity more closely resemble RDs than TDs.

Neugebauer, M.↗

NASA NTRS1984

Geopotential Research Mission (GRM)

Two spacecraft, one following the other with a separation of a few hundred kilometers and orbiting at an altitude of 160 kilometers in a 90-degree polar orbit, are used to detect the minute variations in the earth's gravitational field. One spacecraft, equipped with magnetometers, will measure the vector and scalar components of the earth's magnetic field. The scientific data obtained will provide an order-of-magnitude increase beyond the present knowledge of the earth's gravitational and magnetic fields.

Keating, T.↗

NASA NTRS1984

Microstructure of magnetic reconnection in earth's magnetotail

The structure of heated electron reconnection events associated with magnetic substorm events in the earth's magnetotail is examined using IMP 8 spacecraft and ground-based magnetometer, plasma analyzer and spectroscopic data. Plasma, magnetic field and energetic particle data for five events are presented. Reconnection is shown to occur in two phases: preheating and heating. In preheating, lasting about 5 min, a strong tailward plasma flow appears and ends with electron heating. A 1-2 min heating phase starts with electron heating and ends with plasma sheet drop out and/or decay of the electron temperature to pre-event levels. The heating pulse is always connected with a Bx reversal at 30 earth radii tailward, where the reconnection occurs.

Bieber, J. W.↗

NASA NTRS1984

A comparison of specularly reflected gyrating ion orbits with observed shock foot thicknesses

ISEE 1 and 2 flux gate magnetometer data has been used to measure the length or thickness of the 'foot' of 60 quasi-perpendicular supercritical bow shocks along the shock normal. These bow shock crossings are contrasted with 10 quasi-perpendicular subscritical shocks which do not exhibit the precursor foot. The extent of the foot structure is compared to the orbit of an ion having solar wind velocity and undergoing simple specular reflection about the shock normal. Good quantitative agreement between the measured value of the foot and the predicted value of about 0.7 ion gyroradii is evident. The foot thickness normalized to this predicted value is compared to the ratio of fast Mach number to critical fast Mach number. There is no dependence of the normalized foot thickness on Mach number other than at the transition point between subcritical and supercritical regimes. No dependence of the normalized foot thickness on upstream ion Beta is noted.

Livesey, W. A.↗

NASA NTRS1984

Variation of the auroral Birkeland and current pattern associated with the north-south component of the IMF

In this study, the data set acquired from the vector magnetometer on board MAGSAT during the period October 1979 to June 1980 is examined. Auroral zone crossings were sorted by hourly averaged values of the Interplanetary Magnetic Field. The result is a catalog of sorted data files, from which color-coded magnetic local time invariant latitude plots have been generated that image the auroral Birkeland current pattern, over each prescribed parameter range. These images are used to statistically examine the dynamics of the current pattern. Results indicate that during the periods of B(z) less than 0, the region 1 and 2 current system expands toward lower latitudes accompanied by an expansion of the auroral zones. When B(z) is greater than 0, the region 1 and 2 currents continue to flow with greatly reduced amplitude in the presence of extensive small scale structure.

Bythrow, P. F.↗

NASA NTRS1984

Models of auroral-zone conductances

The magnetosphere-ionosphere system is strongly coupled, with magnetospheric Birkeland currents feeding ionospheric Pedersen and Hall currents. Central to any computer simulation of this system is a detailed, valid conductivity model. An accurate conductivity model is also vital in order to infer Birkeland currents and electric field patterns from inversions of magnetometer chain data. Several recent attempts at constructing conductivity models are presented and their strengths and weaknesses discussed. Incoherent scatter radar measurements can determine height profiles of electron content, from which Pedersen and Hall conductances may be calculated. These yield excellent spatial and good temporal resolution; however, they are limited in field of view. A global pattern requires either 24 hours of data or a chain of stations. Synoptic empirical models (quantized by indices such as Kp or AE) typically are limited by their large bin size (1 deg invariant latitude x 1 hour MLT), and cannot reproduce arcs. Estimating conductivity globally from Dynamics Explorer auroral images is promising, and can yield reasonable time scales (of about 10 minutes); however, this procedure is still only now being tested.

Reiff, P. H.↗

NASA NTRS1984

Scale lengths in quasi-parallel shocks

A review was carried out of ISEE and Voyager spacecraft magnetometer data to determine if quasi-parallel bow shocks are really broad, disordered regions. The key parameter was the deceleration scale (thickness, Lp) across which random energy would need to increase and a localized electrostatic field (E) would be present. Lp would define the breadth of the shock and be associated with a plasma deceleration. The ISEE 1 satellite collected data on the electron density, bulk speed, magnetic intensity, and electron temperature in November 1977 during five traverses of the bow shock. Similar data were gathered from an interplanetary shock wave in 1981. The evidence supported the concept of a plasma deceleration across a thin layer (Lp) in both types of shocks. The layers were about 50 times (interplanetary) and 20 times (earth) thinner than surrounding magnetic fluctuation regions. It is asserted that the regions of deceleration, although much thinner, are the actual shocks and not the entire regions of magnetic fluctuations.

Scudder, J. D.↗

NASA NTRS1984

Implications of lunar palaeomagnetism for the origin of the Moon

Three issues relating to the origin of the Moon are investigated: the early formation of a fluid iron core, the nature of primeval heat sources in the Moon and the existence of a primeval satellite systems. The remanent magnetization of the Apollo samples was interpreted as evidence for an internally generated lunar magnetic field. The three independent methods of determining paleointensities (the Thellier, ARM (2) and IRM methods (3) are now in general agreement that the field was about 1 G 3.9 b yr ago declining exponentially to .02 G 3.2 b yr ago. Paleomagnetic directions of crustal strata have been determined from the Apollo 15 and 16 subsatellite magnetometer observations. The question whether these are randomly directed such as would be expected from local magnetization processes or are proof of the existence of an early core dynamo field is one of the key issues of lunar science. Although the presence of a lunar core was long ago suggested and there are now various different, although individually not conclusive arguments, the fit of the paleomagnetic data to the dipole hypothesis is strong evidence for the existence of a molten lunar iron core and implies a powerful heat source present in the earliest history of the Moon.

Runcorn, S. K.↗

NASA NTRS1984

On the use of a sunward-libration-point orbiting spacecraft as an IMF monitor for magnetospheric studies

Magnetospheric studies often require knowledge of the orientation of the IMF. In order to test the accuracy of using magnetometer data from a spacecraft orbiting the sunward libration point for this purpose, the angle between the IMF at ISEE 3, when it was positioned around the libration point, and at ISEE 1, orbiting Earth, has been calculated for a data set of two-hour periods covering four months. For each period, a ten-minute average of ISEE 1 data is compared with ten-minute averages of ISEE 3 data at successively lagged intervals. At the lag time equal to the time required for the solar wind to convect from ISEE 3 to ISEE 1, the median angle between the IMF orientation at the two spacecraft is 20 deg, and 80% of the cases have angles less than 38 deg. The results for the angles projected on the y-z plane are essentially the same.

Kelly, T. J.↗

NASA NTRS1984

Rome/Goddard Space Flight Center (GSFC) magnetic field experiment: A summary of results

A triaxial flux gate magnetometer (E3) was developed and operated to accurately measure the vector magnetic field onboard Helios 1 and 2. Observations of large-scale variations and sector structure confirm theoretical descriptions and compuations. Limits on the configuration of a magnetic cloud are set, and the source of at least one magnetic cloud is identified as a coronal mass ejection. Two types of interplanetary flow pattern are identified: a corotating stream and interaction region related to the boundary, and a transient flow associated with a shock wave. Discontinuities in the direction and/or strength of the magnetic field are explained. Alfven waves and magnetic turbulence are characterized by simultaneous Helios 1 and 2 observations.

Mariani, F.↗

NASA NTRS1985

A star camera aspect system suitable for use in balloon experiments

A balloon-borne experiment containing a star camera aspect system was designed, built, and flown. This system was designed to provide offset corrections to the magnetometer and inclinometer readings used to control an azimuth and elevation pointed experiment. The camera is controlled by a microprocessor, including commendable exposure and noise rejection threshold, as well as formatting the data for telemetry to the ground. As a background program, the microprocessor runs the aspect program to analyze a fraction of the pictures taken so that aspect information and offset corrections are available to the experiment in near real time. The analysis consists of pattern recognition of the star field with a star catalog in ROM memory and a least squares calculation. The performance of this system in ground based tests is described. It is part of the NASA/GSFC High Energy Gamma-Ray Balloon Instrument (2).

Hunter, S. D.↗