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At least 1,495 records · Page 83
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NASA NTRS1982

Pc 1-2 magnetic pulsation spectra and heavy ion effects at synchronous orbit - ATS 6 results

Two types of Pc 1-2 wave events have been identified in ATS 6 flux gate magnetometer data recorded approximately 12 deg off the geomagnetic equator. Left-hand polarized waves observed in the afternoon are typical of IPDP ground events. Highly elliptical and linearly polarized waves occur around local noon. Spectra often show slots near the equatorial helium and oxygen cyclotron frequencies. The linear polarization may be interpreted in terms of propagation at a large wave normal angle or a crossover frequency that is a consequence of propagation in a multicomponent plasma. The spectral slots may be associated with either heavy ion cyclotron resonanace or propagation effects.

Fraser, B. J.↗
NASA NTRS1981

Substorm associated micropulsations at synchronous orbit

The state of the art of observations of substorm associated waves in GEO is reviewed and research directions are indicated. Data were taken from fluxgate magnetometers on board the ATS 1 and 6 spacecraft in GEO. Mixed mode Pc 4 and 5 waves, with the largest amplitude of magnetic pulsations observed at GEO, have been found to display a quasi-sinusoidal waveform with amplitudes from 10-30 gamma and a period of 50-200 sec. The wave spectra confined the excited frequencies to a narrow band, and possible generating mechanisms for the Pc 4 and 5 waves are discussed. Pc 1 and 2 magnetic pulsations are the most common in GEO, with Pc 1 occurring every third day, particularly in the afternoon to dusk sector and during major substorm expansion onset. Pi 2 outbursts are observed in the majority of substorms passing through the midnight sector and have been correlated with changes in field aligned currents.

Mcpherron, R. L.↗
NASA NTRS1982

An analysis of the structure of Saturn's magnetic field using charged particle absorption signatures

A new technique is derived for determining the structure of Saturn's magnetic field. This technique uses the observed positions of charged particle signatures due to the satellites and rings of Saturn to determine the parameters of an axially symmetric, spherical harmonic model of the magnetic field using the method of least squares. Absorption signatures observed along the Pioneer 11, Voyager 1, and Voyager 2 spacecraft trajectories are used to derive values for the orientation of the magnetic symmetry axis relative to Saturn's axis of rotation, the axial displacement of the center of the magnetic dipole from the center of Saturn, and the magnitude of the external field component. Comparing these results with the magnetic field model parameters deduced from analyses of magnetometer data leads to a preference for models that incorporate a northward offset of the dipole center by about 0.05 Saturn radii.

Chenette, D. L.↗
NASA NTRS1982

The deep lunar electrical conductivity profile - Structural and thermal inferences

Simultaneous lunar surface and orbital magnetometer records are reexamined, to ascertain intervals which may be suitable for measuring lunar inductive response in the solar wind and terrestrial magnetosheath. Power spectral estimates of the response tangent to the lunar surface, defined in terms of transfer and gain functions, are obtained for the 0.0001-0.01 Hz frequency range. The maximum consistency of estimates from different time intervals is found when the initial analysis is limited to the tangential direction of maximum incident power, or that direction in which the ratio of signal to background noise is greatest. Spherically symmetric plasma confinement theory is used in the interpretation of transfer function data, by way of forward model calculations, under the assumption of continuous electrical conductivity increase with depth. Results are presented for internal electrical conductivity profile, metallic core radius, and selenotherm limits.

Hood, L. L.↗
NASA NTRS1982

Large-amplitude magnetic variations in quasi-parallel shocks - Correlation lengths measured by ISEE 1 and 2

Wide separations up to more than 1 earth radius between ISEE 1 and 2 during the second half of 1978 have been used to measure the correlation length of magnetic pulsations in quasiparallel shocks. When the two spacecraft were less than a few hundred km apart, magnetic oscillations measured by magnetometers on both spacecraft exhibited virtually identical waveforms, but at distances of several thousand km, the two time series of field variation showed no detailed similarily at all. The correlation coefficients of the pulsations dropped from close to 1.0 for spacecraft separations of less than 100 km to 0.2 for separations of greater than 800 km. A correlation length of several hundred km may be related to the gyroradius of return protons with energy typical of the peaks of diffuse and beam ion distributions.

Greenstadt, E. W.↗
NASA NTRS1982

Electromagnetic induction in the moon

The moon constitutes a nonhydromagnetic, but electrically conducting, target for the solar wind whose response reaches a peak as frequency increases and diminishes with further increase in frequency, suggesting the presence of the magnetic quadrupole moment. Magnetometer measurements of induction using Explorer and Apollo instruments are studied from both the harmonic and transient standpoint, and the resulting determination of internal bulk electrical conductivity is discussed. The closeness of the estimated internal temperature to the Ringwood-Essene solidus at 150-250 km depths suggests a layer of enhanced conductivity in lieu of high temperature. A reduced core radius estimate with a one-sigma upper limit of 360 km is reported. The discussion of lunar electrodynamics presented is restricted to the problem of induction, with only passing reference to flow fields and regional electric fields.

Sonett, C. P.↗
NASA NTRS1982

Whistler mode turbulence in the disturbed solar wind

The magnetic field fluctuations with frequencies lying between the ion and electron cyclotron frequenices are enhanced downstream of interplanetary shocks in fast streams. Although spectra synthesized from ISEE 3 magnetometer and plasma wave instrument data can be described by one power law below 1 Hz, and another one above, measurement behind four shocks of the spectral index above the ion cyclotron frequency showed it to be twice the figure below, while no clear relationship is apparent in the weaker fast stream events. Although the data base is limited, the ratios of the average wave magnetic amplitude to electric field amplitudes confirm that the waves are whistler mode emissions, as suggested by their frequency range. It is indirectly deduced that the whistler waves are generated in such a way as to propagate at large angles to the local interplanetary field.

Coroniti, F. V.↗
NASA NTRS1982

Multiple-spacecraft and correlated riometer study of magnetospheric substorm phenomena

Double-spacecraft observations presented suggest that growth phase phenomena are widespread in the outer magnetosphere and can occur simultaneously at nightside locations 4-5 hours apart in local time. Evidence is also adduced from multiple-spacecraft data and comprehensive magnetometer and riometer data against the concept that growth phase features are not generally due to substorm activity at other locations in the auroral or polar regions. A close relationship is noted between putative energy input to the magnetosphere and the sensitive response of the magnetic field and energetic particles at pre- and near-midnight geostationary orbit locations. This relationship is shown explicitly for the cases of three event periods. Data are also presented which suggest that growth phase features at synchronous orbit can be strictly controlled by IMF orientations.

Baker, D. N.↗
NASA NTRS1982

Energetic protons accelerated at corotating shocks - Pioneer 10 and 11 observations from 1 to 6 AU

Vector helium magnetometer and proton telescope data are used to examine the relationship between low energy proton increases and corotating interaction regions (CIRs). A general correlation is noted between the maximum CIR field intensity and the maximum proton count rate. The minimum proton flux, located between the two proton maxima, appears to be correlated with the maximum field strength of the CIR. The evidence presented strongly supports shock acceleration as the primary source of the 1-MeV protons. A schematic figure incorporating many of the features deduced in the present study is given to illustrate the relationship between energetic protons, forward and reverse shocks, and the interplanetary magnetic field structure. The predictions of various theories and mechanisms for interplanetary nucleon acceleration are discussed in light of the experimental results presented.

Tsurutani, B. T.↗
NASA NTRS1982

Measurement of the potential drop across the earth's collisionless bow shock

The normal component of the dc electric field measured on ISEE-1 ordinarily exhibits an enhancement of a few mV/m over both upstream and downstream values at the earth's bow shock. Using the measured relative velocity between the shock and the spacecraft (from the ISEE-1/2 time delay in the magnetometer data), it is possible to transform the observed E enhancement to a potential drop (delta phi). For a subcritical shock the potential drop is found to be very close to the measured change of particle kinetic energy (delta phi, approximately 280 V on day 330, 1977), whereas for a supercritical shock the potential drop is only a fraction of the measured change of kinetic energy (delta phi, approximately 140 V on day 324, 1977).

Formisano, V.↗
NASA NTRS1982

Space telescope - Meeting the pointing control challenge with today's technology

The pointing control system of the Space Telescope, which provides target-to-target maneuvering capability and precision pointing on the target star (with 0.007-arcsec stability and 0.01-arcsec accuracy), is described. Spacecraft attitude control is undertaken by onboard computer processing of attitude and rate sensor data that generates reaction wheel torque commands. The Space Telescope Operations Control Center communicates with the Space Telescope via the synchronous altitude tracking and data relay satellite system, and determines vehicle attitude more precisely by means of sun sensors, magnetometers and fixed-head star trackers. Such disturbance torques as those of gravity gradients and aerodynamics act on the Space Telescope, causing the speeds of the four reaction wheels to increase. In order to prevent the wheels from reaching a speed-saturated condition, a momentum control system is provided for the management of reaction wheel speed buildup. Attention is given to development testing and control hardware investigations and improvements.

Dougherty, H.↗
NASA NTRS1982

Development of a magnetic acquisition and attitude control system for spinning rocket payloads

An attitude control system capable of pointing the roll principle axis of a spinning payload to within one degree of the magnetic field vector was flown successfully in 1981-82 aboard three Black Brant X payloads and one Terrier Malemute payload, and additional flights with the system have been requested. Using a filtered linear combination of signals provided by a single-axis rate gyro and a two-axis magnetometer, and control actuation from two cold-gas thrusters, the system can be assembled from standard rocket components and fitted to any payload size.

Olney, D. J.↗
NASA NTRS1982

Stationarity of magnetohydrodynamic fluctuations in the solar wind

Solar wind research and studies of charged particle propagation often assume that the interplanetary magnetic field represents a stationary random process. The extent to which ensemble averages of the solar wind magnetic fields follow the asymptotic behavior predicted by the ergodic theorem was investigated. Several time periods, including a span of nearly two years, are analyzed. Data intervals which span many solar rotations satisfy the conditions of weak stationarity if the effects of solar rotation are included in the asymptotic analysis. Shorter intervals which include a small integral number of interplanetary sectors also satisfy weak stationarity. The results are illustrated using magnetometer data from the ISEE-3, Voyager and IMP spacecraft.

Matthaeus, W. H.↗
NASA NTRS1982

Compression of the Venusian ionosphere on May 10, 1979, by the interplanetary shock generated by the solar eruption of May 8, 1979

Previous solar optical and solar wind conditions observations are summarized, and Venus ionopause data taken with the Helios 2 and Pioneer Venus Orbiter spacecraft are presented for orbits 150-161. Those orbits encountered the results of an interplanetary shock wave initiated by a solar eruption and coronal transient of May 8, 1979. A significant compression of the dayside Venus ionosphere was detected two days later. The solar observations had been made in H-alpha at 10 solar radii, at 0.3 AU, and at 0.7 AU, using plasma and magnetometer monitoring of the shock wave.

Dryer, M.↗
NASA NTRS1982

A cross spectral analysis of high-latitude Pc 5 pulsations in the morning sector

An analysis of the spatial phase and amplitude structures of the ground Pc 5 pulsations in the morning sector was carried out using data from the North American IMS magnetometer network. Signals in the X, Y, and Z components were found to propagate from noon to the nightside with an apparent phase velocity of 13 km/sec. The phase was determined to vary with latitude, with total latitudinal phase differences of 200 and 90 deg, respectively, for the X and Y components. Corresponding latitudinal phase differences in the vertical components were 550 deg for the Alaska chain, and 360 deg for the Fort Churchill chain. Phase variation in the Fort Churchill chain was attributed to the ionospheric Hall current related to the field line resonance and the earth induction current. The earth induction current in the shallower regions near Fort Yukon is a necessary factor to explain the large variation in the Alaska chain.

Saka, O.↗
NASA NTRS1982

Pioneer Venus observations of plasma and field structure in the near wake of Venus

Ionospheric plasma density depletions or 'holes' are observed by the Pioneer Venus orbiter in association with radial magnetic fields in the near wake of Venus. This report presents examples of the collected observations of these unexpected features of the Venus nightside ionosphere obtained by the Langmuir probe, magnetometer, ion mass spectrometer, retarding potential analyzer, plasma analyzer, and electric field experiments. The connection between plasma density depletions and temperature changes, changes in ion composition, plasma wave emissions, and magnetic fields with a substantial radial component is illustrated. Mechanisms that may be responsible for the formation and maintenance of holes are suggested.

Luhmann, J. G.↗
NASA NTRS1982

Disappearing ionospheres on the nightside of Venus

Instruments on the Pioneer Venus Orbiter have detected a substantial ionosphere on the nightside of Venus during most orbits. However, during some orbits the nightside ionosphere seems to have almost disappeared, existing only as irregular patches of low-density plasma. The solar wind dynamic pressure on these occasions is greater than average. Data from several instruments (Langmuir probe, ion mass spectrometer, retarding potential analyzer, magnetometer, and plasma analyzer) have been correlated for a number of orbits during which the nightside ionosphere had disappeared. The magnetic field tends to be coherent, horizontal, and larger than usual, and the electron and ion temperatures are much larger than they usually are on the nightside. Mechanisms are suggested which might explain the reasons for the disappearance of the ionosphere when the solar wind dynamic pressure is large.

Cravens, T. E.↗
NASA NTRS1982

Magnetic field compression at the dayside magnetopause

The degree of compression sustained by the solar wind field as it convects to the magnetopause has been determined empirically with magnetometer data from ISEE 3 in the solar wind and ISEE 1 in the magnetosheath. At the stagnation point, the strength B(SH) of the magnetosheath field may be expressed as B(SH) = square root of 4 X B(SW)' X B(ST), which implies that B(SH) is equal to the geometric mean of the stagnation field strength B(ST) and the shocked solar wind field, approximated by 4 X B(SW)'. For the usual spiral wind pattern of the solar wind field, B(SH) on the duskside of the magnetopause is about 20 percent stronger than it is on the downside. No dependence of B(SH) on the sign of the north-south component of the solar wind field is apparent.

Crooker, N. U.↗