Satellite Magnetometer Research Articles

Spherical earth gravity and magnetic anomaly analysis by equivalent point source inversion

To facilitate geologic interpretation of satellite elevation potential field data, analysis techniques are developed and verified in the spherical domain that are commensurate with conventional flat earth methods of potential field interpretation. A powerful approach to the spherical earth problem relates potential field anomalies to a distribution of equivalent point sources by least squares matrix inversion. Linear transformations of the equivalent source field lead to corresponding geoidal anomalies, pseudo-anomalies, vector anomaly components, spatial derivatives, continuations, and differential magnetic pole reductions. A number of examples using 1°-averaged surface free-air gravity anomalies and POGO satellite magnetometer data for the United States, Mexico and Central America illustrate the capabilities of the method.

Dynamical response of the dayside ionosphere of Venus to the solar wind

Dayside ion composition measurements made by the orbiter ion mass spectrometer and the orbiter electron temperature probe on the Pioneer Venus orbiter are used to infer the dominant processes involved in the dynamic response of the Venus ionosphere to the solar wind. The analysis is confined to the topside ionosphere in the vicinity of the subsolar point, where the ionosphere‐solar wind interaction is expected to be maximized. Height profiles of the ion composition and plasma temperatures in the main body of the topside ionosphere, lying between the ionopause and chemical equilibrium regions, reveal that the ionosphere is in a compressed state. This region of the ionosphere is interpreted in terms of a stationary equilibrium where the compression is derived from the ponderomotive force j × B. The estimated magnitude of this force is confirmed by the magnetic field measurements made by the orbiter magnetometer.

Observations of large scale steady magnetic fields in the dayside Venus ionosphere

Although the dayside ionosphere of Venus is often “field‐free” except for fine scale features, large scale, steady ionospheric magnetic fields with magnitudes sometimes exceeding 100 gammas are occasionally observed by the Pioneer Venus Orbiter magnetometer. These fields are mainly horizontal and can assume any angle in the horizontal plane. The orientation of the field may change along the spacecraft trajectory. The field magnitude in the upper ionosphere usually shows a distinct minimum near ∼200 km altitude, but the altitude profile is otherwise arbitrary. With few exceptions, the observations of these large scale fields occur when periapsis is at solar zenith angles <50°. The occurrence of large scale fields is often coincident with the observation of high solar wind dynamic pressures by the Pioneer Venus Orbiter plasma analyzer closely following the ionosphere encounter. However, the detection of this phenomenon even during some orbits for which the dynamic pressure is not extraordinarily high suggests that other factors, such as hysteresis effects, must also play a role in determining the occurrence frequency of large scale magnetic fields in the dayside Venus ionosphere.

Comparisons of magnetic anomalies of lithospheric origin measured by satellite and airborne magnetometers over western Canada

Crustal magnetic anomaly data from the Orbiting Geophysical Observatories 2, 4, and 6 (Pogo) satellites are compared with upward-continued aeromagnetic data between 50–85°N latitude and 220–260°E longitude. Agreement is good, both in anomaly location and in amplitude, giving confidence that it is possible to proceed with the derivation and interpretation of satellite anomaly maps in all parts of the globe. The data contain a magnetic high over the Alpha ridge suggesting continental composition and a magnetic low over the southern Canada basin and northern Canadian Arctic Islands (Sverdrup basin). The low in the Sverdrup basin corresponds to a region of high heat flow, suggesting a shallow Curie isotherm. A ridge of high field, with two distinct peaks in amplitude, is found over the northern portion of the platform deposits and a relative high is located in the central portion of the Churchill Province. No features are present to indicate a magnetic boundary between Slave and Bear Provinces, but a trend change is evident between Slave and Churchill Provinces. South of 60° latitude a broad magnetic low is located over very thick (40–50 km) crust, interpreted to be a region of low magnetization.

The reduction and analysis of satellite magnetometer data

Analyses of the POGO satellite magnetometer data have demonstrated the basic utility of such measurements to solid earth studies and provided the basis for Magsat, a new satellite specifically designed for such studies. Such data can play a useful role in the compilation, reduction, and analysis of magnetic survey data, especially at regional scales, and are complementary to those obtained in such surveys. However, the reduction and interpretation techniques for satellite magnetometer measurements differ significantly from the techniques routinely applied to conventional data. Although the unequal spacing, in three dimensions, of the satellite data and the variation in the direction and intensity of the main geomagnetic field pose some interpretation problems, these problems can be suitably handled by using modifications of several standard techniques. The variation in altitude of the satellite measurements can also be used to advantage in some analyses.

The application of satellite data to potential field studies

Satellite data can play a useful role in the compilation, reduction, and analysis of gravity and magnetic survey data, especially at regional scales. The global coverage and altitude variations afforded by satellite observations offer distinct advantages in the definition of the background terrestrial potential fields and in the analysis of related spatial anomalies. These data are also complementary to those obtained in more detailed surface and airborne surveys and can be combined in many analyses. Satellite magnetometers have been used to map the main geomagnetic field and crustal magnetic anomalies. These two signals are not completely independent, as an accurate definition of the main field is necessary for complete reduction of magnetic survey data. However, presently available mathematical models describing this field are becoming less reliable, because of the lack of recent global measurements. These measurements could be obtained, as they have been previously, through satellite observations. Although satellite data can be used to differentiate only magnetic anomalies that have wavelengths approximately 1 4 to 1 2 as long as the orbital altitude, analysis of the POGO (Polar Orbiting Geophysical Observatory) satellite data has revealed crustal anomalies of these wavelengths. Precision tracking observations of artificial satellites have been analyzed to provide information on the long-wavelength (i.e., greater than 2000 km) component of the earth's gravity field. Combining the satellite-derived gravity data with existing surface measurements provides detailed information on the terrestrial gravitational field and the undulations of the geoid relative to a reference ellipsoid. The long-wavelength gravity anomalies can provide information on the regional field through the identification of distinct broad features within the global field, information that is especially important in areas devoid of surface data. In areas where surface data are available, the two data sets can be used to produce detailed regional anomaly maps.

Dodge satellite observations of Pc 3 and Pc 4 magnetic pulsations and correlated effects in the ground observations

The analysis of the magnetic field observations from Dodge satellite magnetometer data at L = 6.25 RE shows dominant low‐frequency waves (Pc 3 and Pc 4) in three major frequency bands: 40–35, 25–20, and 8–12 mHz. During the observation period of July–August 1968 the 25‐ to 20‐mHz band always showed high spectral density, indicating the most dominant wave frequency. Similar results were obtained in the analysis of ground magnetometer observations from L = 6.9 to 1.5 near the local time of the satellite observations. The simultaneous observations of the waves at low as well as high L values are important in wave propagation models. This pattern of observations and the polarizations of certain wave events which occurred when Dodge was close to the ground observatory are discussed in terms of contemporary theoretical models.

Field-aligned currents in the dayside cusp observed by Triad

The characteristics of field-aligned currents at an altitude of 800 km in the dayside high-latitude region over the northern hemisphere were determined from the Triad satellite magnetometer data recorded at College, Alaska, from January 1973 to October 1974. The field-aligned currents discussed here are located poleward of the large-scale field-aligned currents reported earlier and referred to as 'region 1 field-aligned currents' by the authors (Iijima and Potemra, 1976). These high-latitude field-aligned currents are most often observed in the dayside sector between 0930 and 1430 MLT and are statistically distributed between 78/sup 0/ and 80/sup 0/ invariant latitude during weakly disturbed conditions as indicated by westward electrojet activity (abs. value of AL < 100..gamma..). Although these high-latitude field-aligned currents show complicated variations, they generally flow away from the ionosphere in the forenoon hours (0930--1200 MLT) and into the ionosphere in the afternoon hours (1200--1430 MLT). These flow directions are opposite to the quasi-permanent region 1 field-aligned currents related to the S/sub q//sup p/ currents previously discussed by the authors. The directions and spatial distribution of these field-aligned currents are consistent with the antisolarward equivalent ionospheric current near 1200 MLT deduced from simultaneous ground-based magnetograms at approx.81/sup 0/ invariant latitude. The intensitymore » of these high-latitude field-aligned currents increases as the interplanetary magnetic field increases in the southward direction. These field-aligned currents are located within the region associated with the dayside magnetospheric cusp, and their relationship to geomagnetic activity, especially interplanetary magnetic field variations, suggests that they may play an important role in the coupling between the interplanetary medium and the magnetosphere. (AIP)« less

A global magnetic anomaly map

A subset of POGO satellite magnetometer data has been formed that is suitable for analysis of crustal magnetic anomalies. Using a thirteenth order field model, fit to these data, magnetic residuals have been calculated over the world to latitude limits of plus 50 deg. These residuals averaged over one degree latitude-longitude blocks represent a detailed global magnetic anomaly map derived solely from satellite data. Preliminary analysis of the map indicates that the anomalies are real and of geological origin.

Aeromagnetics and the Transition Between the Colorado Plateau and Basin Range Provinces

An aeromagnetic survey of central Utah has given improved resolution of a regional anomaly previously recognized from satellite magnetometer data. The anomaly consists of a gradient from a high over the Colorado Plateau to a low over the Basin Range province. Magnetic profiles are fit by a model in which the Curie isotherm deepens eastward in a belt up to 80 km wide, whose western side is about 50 km east of the Basin Range physiographic margin. The magnetic profiles could also be fit be a change of average crustal susceptibility instead of Curie depth. In either case we conclude that the major lateral change between Basin Range and Colorado Plateau crustal geophysical parameters occurs not at the Fenneman physiographic boundary but more than 50 km eastward of it. This suggestion is found to be consistent with data from seismic refraction, geomagnetic variation, geochemistry, and geologic structures.

The quasi-stationary coronal magnetic field and electron density as determined from a Faraday rotation experiment

Pioneer VI was launched into a circumsolar orbit on December 16, 1965, and was occulted by the sun in the latter half of November, 1968. During the occultation period, the 2292-MHz S-band telemetry carrier underwent Faraday rotation due to the interaction of this signal with the plasma and magnetic field in the solar corona. The NASA/JPL 210-ft diameter antenna of the Deep Space Network near Barstow, California, was used for the measurement. The antenna feed was modified for automatic polarization tracking for this experiment. The measurement results are interpreted with a theoretical model of the solar corona. This model consists of a modified Allen-Baumbach electron density and a coronal magnetic field calculated both from Mount Wilson magnetograph observations using a source surface model and field extrapolations from the Explorer 33 satellite magnetometer. The observations and the calculated rotation show general agreement with respect to magnitude, sense, and timing, suggesting the source-surface model and field extrapolations from 1 AU are a valid technique to obtain the magnetic field in the corona from 4 to 12 solar radii. Variations present can easily be ascribed to density enhancements known to be present in the corona. Longitudinal variations of the density in the corona cannot be obtained from coronagraph observations, and thus a purely radial variation was assumed. An improved fit to the Faraday rotation data is obtained with an equatorial electron density $$N = 10^8 \left( {\frac{{6000}}{{R^{10} }} + \frac{{0.002}}{{R^2 }}} \right)...{\text{ cm}}^{{\text{ - 3}}} {\text{ (4 < }}R < 12){\text{ }}...$$ where R is in solar radii. The work of W. V. T. Rusch and J. E. Ohlson was supported in part by research sponsored by the Joint Services Electronics Program through the Air Force Office of Scientific Research under Grant AF-AFOSR 69-1622A at the University of Southern California. The work done by K. H. Schatten was in part supported by the National Academy of Science on a National Research Council postdoctoral fellowship. The work of J. M. Wilcox was supported in part by the Office of Naval Research under Contract Nonr 3656(26), by the National Aeronautics and Space Administration under Grant NGR 05-003-230, and by the National Science Foundation under Grant GA-1319 at the University of California at Berkeley.

Relationship of geomagnetic fluctuations to other magnetospheric phenomena

We present an analysis of geomagnetic fluctuations as observed at the surface of the earth during the onsets of the main phases of some magnetic storms. We compare the global morphology of these fluctuations with the position of the ring current and plasmapause formed in the magnetosphere as inferred by satellite magnetometer and particle measurements. It is found that the ring current is formed on the innermost disturbed L shells of the magnetosphere. The plasmapause in the cold magnetospheric plasma is at or near the outermost L shell of the relatively undisturbed magnetosphere. When observed, the SAR arc appears on the innermost disturbed L shells of the magnetosphere.

The oblique shock of the proton flare of 7 July 1966

The proton flare of 7 July 1966 initiated a shock in the solar wind which arrived at the Earth on the 8th, and was observed by three satellite magnetometers on Explorer 33, Imp 3, and Vela 3A, all outside the magnetosphere. The positions of the three spacecraft and the times of shock observation combine to give a kinematically determined local shock normal pointing 65–70° below the ecliptic plane at solar ecliptic longitude 165°. The sense of obliquity of the shock is consistent with the position of the flare in the heliographic Northwest quadrant. The high degree of obliquity to the ecliptic suggests a tongue shaped shock possibly caused by the horizontal configuration and/or high latitude of the flare

Gross Estimates of the Conductivity, Dielectric Constant, and Magnetic Permeability Distributions in the Moon

Sources of direct information on the electrical parameters of the moon include earth‐based astatic radar, earth‐satellite bistatic radar, lunar‐lander astatic radar, earth‐based radiothermic measurements, lunar orbital magnetic field measurements, and lunar‐lander magnetic attraction experiments. These pieces of information, when fitted to a reasonable geological model of the moon, permit estimation of the depth profiles of electrical conductivity, dielectric constant, and magnetic permeability. At lunar surface dc conductivities as low as 10‐12 mho/m may occur, and conductivities as high as 10 mhos/m appear possible for the deep lunar interior. It is demonstrated that interpretation of the Explorer 35 lunar satellite magnetometer data is at present based on such inadequate theory that the above range of values of conductivity could be quite consistent with the orbital magnetic field data. Anomalous ‘dc’ dielectric constants as high as 1010 could exist in the lunar interior if pore water and metallic minerals are present in juxtaposition. Magnetic permeabilities as large as 1.7 can be present in the lunar surface rocks. Typical depth profiles of conductivity and dielectic constant that fall within the extremes noted above are presented.

Energetic electrons at 6.6REduring the January 13-14, 1967, geomagnetic storm

The trapped, energetic (Eϵ > 0.4 Mev to > 3.0 Mev) electron fluxes at synchronous altitude are examined during the January 13–14, 1967, geomagnetic storm. Except for a short period after the sudden commencement (sc) at 1202 UT, day 13 (0202 local time), the measured electrons were observed to be stably trapped for about the first twelve hours after the sc. However, the values of the electron fluxes in the local morning and at local noon were quite different than those generally observed during quiet periods. About two hours after local noon, at 2324 UT, day 13, the electron flux dropped sharply by two orders of magnitude in response to a sudden decrease in the magnetic field observed at the satellite. Recovery of the electron flux occurred in a few minutes. About 40 minutes later, 0007 UT, day 14, the electron fluxes were essentially wiped out at the time of an observed reversal in the magnetic field at the satellite. During two separate approximately 10-minute periods after this 0007 UT, day 14, wipeout, the observed particle fluxes indicate that the satellite encountered a region of space with characteristics very different from the normal magnetospheric trapping region. The data during these times are consistent with the interpretation from the magnetic field observations that the satellite was outside the magnetopause. Correlation of the electron data during the storm is made with the satellite magnetometer data and with the magnetometer data from several ground stations.

Magnetopause structure and attitude from Explorer 12 observations

It is shown how satellite magnetometer data at a magnetopause penetration can be used to determine the vector normal to the magnetopause current layer and the magnetic-field component along this normal. According to theory such a component is a measure of the amount of field reconnection at magnetopause. Results from 22 Explorer 12 boundary penetrations are presented indicating normal-field components of less than 5 γ in two-thirds of the cases. Measured field variations within the current layer are presented to demonstrate the existence of two fundamentally different types of boundary structure, the rotational and the tangential discontinuity. The former of these permits a nonzero normal field component, whereas the latter does not. The rotational discontinuity seems to occur predominantly during magnetic storms and two of these cases, involving substantial normal-field components, provide compelling evidence that field reconnection takes place during the storm main phase. Finally, the calculated normal vector is compared with the normal to the surface of the Mead-Beard magnetosphere model.

Approximate calculation of the current distribution when a conducting fluid flows along a channel within a magnetic field

The observed magnetic field configuration in the Venus magnetosheath contains information about the solar wind mass-loading processes occurring as a result of the extension of the neutral atmosphere into the magnetosheath. In this paper, magnetic field signatures of various mass-loading processes are discussed and experimental results from the Pioneer Venus Orbiter magnetometer experiment are examined for evidence of these signatures. The data suggest that the −VXB acceleration process, stochastic pickup of ionospheric ions, and JXB force “scavenging” at the ionopause all occur at various times.