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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.↗

NASA NTRS1984

Power dissipation at slow-mode shocks in the distant geomagnetic tail

An estimate is made of the decreases in Poynting flux across slow shocks in the geomagnetic tail detected by the ISEE-3 spacecraft. An electron analyzer and a magnetometer recorded 26 of the events in January-February 1983. Two-dimensional electron velocity distributions parallel to the magnetic field across the shock transition characterized the data. The shocks were of relatively high strength, close to the switch-off limit, and displayed a large upstream Alfven Mach number. The Poynting flux decreased an average of 0.0018-0.0166 ergs/sq cm per sec. The power dissipated from lobe-magnetic energy density to plasma sheet convection across the shocks is estimated to be 5 x 10 to the 18th ergs/sec.

Feldman, W. C.↗

NASA NTRS1984

A comparative study of distant magnetotail structure at Venus and earth

Spacecraft data on the magnetotails of earth and Venus are discussed with regards to interactions between the two planets and the solar wind. Consideration is given to Pioneer Venus Orbiter plasma analyzer, magnetometer and plasma wave observations around Venus and equivalent ISEE-3 data from the terrestrial region. The discussion is constrained to clear lobe and plasmasheet intervals greater than 5 and 30 min for Venus and earth, respectively. The orientation of the Venusian plasma sheet was normal to that of earth's, while both regions displayed distinct mantle, lobe, plasmasheet boundary layer, and plasmasheet regions. Additionally, the plasmasheet magnetic field distributions in distant regions of the tails exhibited characteristics of disconnection from the planets and a return to the solar wind. The lobe field magnitudes were consistent with the interaction of the planets with the solar wind. It is noted that the Venus plasmasheet is hotter and/or denser than the terrestrial magnetosheath.

Slavin, J. A.↗

NASA NTRS1984

Shuttle tethered satellite program

Features and applications of the Shuttle tethered satellite system (TSS) are outlined. The TSS would be mounted on a normal cargo pallet and used to deploy or retrieve payloads weighing up to 500 kg from the Orbiter by reeling in or unreeling a tether line extended along the gravity gradient vertical. Attaching a conducting wire to the tether would permit electrodynamic measurements of space plasma. An initial mission under development is to deploy an instruments package 20 km above the Orbiter with a conducting wire tether, thereby obtaining data on low frequency hydromagnetic waves generated by tether passage through the space plasma. An aerodynamics package could also be lowered to 130 km, where measurements can be made with magnetometers for geoscience data, atmospheric composition, and cosmic dust can be collected. Details of engineering assignments for the TSS divided up between NASA and the Italian National Research Council are discussed.

Nolan, M. B.↗

NASA NTRS1984

Standing hydromagnetic oscillations in the magnetosphere

The mechanisms for generating Alfven waves in the magnetosphere are determined, on the basis of a comprehensive survey of satellite magnetometer data. The mechanisms are classified in terms of the physical properties of standing Alfven waves such as the direction of the major axis of polarization, harmonic mode, and the azimuthal wave number. In an analysis of the statistical organization of the pulsations, it is shown that the most common transverse waves seem to be azimuthally polarized waves involving the fundamental and several harmonics, with radially polarized waves excited at the second harmonic. The former are probably driven externally, while the latter are likely to be excited internally through particle-wave interactions.

Takahashi, K.↗

NASA NTRS1984

Growth and maintenance of large-scale magnetic fields in the dayside Venus ionosphere

Observations from the Pioneer Venus orbiter magnetometer reveal the presence of large-scale magnetic fields in the dayside ionosphere during or after periods of high solar wind dynamic pressure. Various hypotheses have been proposed concerning the spatial and temporal evolution of these field structures. The hypothesis that the field is produced by a diffusion/convection process rather than by currents driven by electric fields resulting from the solar wind interaction is examined. Dynamic pressure variations occur on various time scales at Venus, producing transient and quasi-steady magnetization features. A one-dimensional diffusion/convection calculation is performed, using typical ionopause field and pressure values for a variety of altitudes. The Venus ionopause is considered to be the altitude at which ionospheric thermal pressure is equal to the magnetosheath magnetic pressure, which in turn is well correlated with the normal component of solar wind dynamic pressure. A subsolar model of downward plasma velocity in the 140-290 km range is used. The calculated growth of the ionospheric magnetic field, and the resulting quasi-steady altitude profiles, compare favorably with the observed profiles. The majority of the observed magnetic structures are best explained as the quasi-steady effects of the prevailing solar wind dynamic pressure, which determines the altitude and magnetic field strength of the ionopause boundary.

Phillips, J. L.↗

NASA NTRS1984

Evolution of the earth's distant magnetotail - ISEE 3 electron plasma results

ISEE 3 data form the basis for a study of the evolution of the electron plasma in the magnetotail lobe and plasma sheet regions as a function of radial distance from the earth. The data were plasma electron and magnetometer measurements taken when the ISEE 3 was tailward of the earth during three different orbits. About 91,000 electron spectra were obtained in terms of the magnetosheath, lobe, plasma sheet and radial downstream distances. The lobe density peaked at 60 earth radii, while the sheet density tapered very slowly downstream. At large downstream distances (about 180 radii), the plasma bulk flow velocity increased significantly, surpassing the solar wind velocity. The bimodal electron flow angle near the earth became a single tailward flow at 180 radii.

Zwickl, R. D.↗

NASA NTRS1984

Cross-polar cap potential drop and the energy coupling function

A numerical model is formulated for correlations between the cross polar cap potential, the solar wind, and the AI index and compared with satellite data. The conductivity model was devised from empirically defined relationships between the Hall and Pedersen conductivities and the measured magnitude of the horizontal magnetic disturbance near College, AK. An instantaneous conductivity distribution was generated from data gathered at, e.g., six IMS meridian chain stations. A strong correlation appeared between an energy coupling function (ECF) and the polar cap potential drop. The ECF was derived from the chain station data, implying that potential drop estimates can be made from ground-based magnetometer data.

Ahn, B.-H.↗

NASA NTRS1985

Variable field-to-normal angles in the shock foreshock boundary observed by ISEE 1 and 2

Saturated ULF waves in the foreshock, with amplitudes comparable to the magnitude of the average field, are convected by the solar wind to the quasi-parallel shock where the average field-normal angle is less than, or about, 45 deg. Several examples from ISEE 1 and 2 magnetometer data show waves that defined local, instantaneous field-normal angles very different periodically from the average. Local geometric conditions at the nominally quasi-parallel shock varied from nearly parallel to nearly perpendicular, at the periods of typical upstream waves. Clear magnetic shock transitions occurred under temporarily quasi-perpendicular geometry.

Greenstadt, E. W.↗

NASA NTRS1985

High time resolution plasma wave and magnetic field observations of the Jovian bow shock

High time resolution (60 ms) Voyager magnetometer and plasma wave measurements of a strong (fast Mach number 16), quasi-perpendicular Jovian bow shock reveal an abrupt change in the plasma wave spectrum at the leading edge of the shock foot. Upstream electron plasma waves terminate at the leading edge, and are replaced by a lower-frequency broadband spectrum of ion-acoustic-like waves, which terminates at the main shock ramp. The clear association with the foot region of the lower frequency component suggests that it is generated by reflected ions. If the upstream plasma waves are generated by an escaping electron heat flux, their termination at the leading edge suggests that electrons are heated by the low-frequency waves in the shock foot.

Moses, S. L.↗

NASA NTRS1985

New results on the Pioneer Venus Orbiter February 10-11, 1982 events - A solar wind disturbance not a comet

The characteristics of a series of disturbances observed on February 10-11, 1982 by the plasma analyzer and the magnetometer on the Pioneer Venus Orbiter, obtained in the solar wind upstream of Venus are studied. It is concluded that the events were associated with the propagation of a solar wind disturbance of coronal origin and not with an encounter with a comet or other local outgassing object (Russel, et al., 1983). The plasma analyzer spectra clearly show the presence of increases in He(2) and they argue strongly against Russell, et al.'s interpretatation. The peak in magnetic field magnitude on February 11 is identified as the magnetic signature of a thin, trailing 'filament' at the end of a series of solar-initiated events. Corresponding increases in He(2) and magnetic field fluctuations were observed near earth by ISEE-3 about a day later, and this strongly supports the concept of a solar origin.

Intriligator, D. S.↗

NASA NTRS1985

Energetic particle characteristics of magnetotail flux ropes

During the recent ISEE-3 Geotail Mission three events have been identified from the magnetometer data which are consistent with a spacecraft crossing of a magnetotail flux rope. Energetic electron and proton observations obtained by the Max-Planck-Institut/University of Maryland sensor system during two of the possible flux rope events are presented. During one event remote sensing of the flux rope with energetic protons reveals that the flux rope is crossed by the spacecraft from south to north. This allows determination of the bandedness of the magnetic field twist and of the flux rope velocity relative to the spacecraft. A minimal flux rope radius of 3 earth radii is derived. Energetic proton intensity is highest just inside of the flux rope and decreases towards the core. Energetic electrons are streaming tailward near the outer boundary, indicating openness of the field lines, and are isotropic through the inner part of the flux rope.

Scholer, M.↗