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Hartl, P, Tauxe L, Constable C.  1993.  Early Oligocene Geomatnetic-Field Behavior From Deep-Sea Drilling Project Site-522. Journal of Geophysical Research-Solid Earth. 98:19649-19665.   10.1029/93jb02019   AbstractWebsite

Hydraulic piston coring operations at Deep Sea Drillng Project site 522 in the South Atlantic retrieved an unusually continuous section of late Eocene to late Oligocene pelagic sediments, which we sampled at 3-4 cm intervals (approximately 3-5 kyr). Natural remanent magnetization demagnetization studies indicate a well-behaved remanence. Various rock magnetic procedures strongly suggest the magnetic carrier is dominated by pseudo-single domain magnetite appropriate for recording relative intensity variations of the paleomagnetic field. Nine zones of unusually low relative paleointensity were identified within the 2 my Chron C12R interval. Seven can be typified by a approximately 20-40 kyr interval of low field intensity accompanied by apparently random, low-amplitude, short-duration directional fluctuations. The other two are of approximately equal duration and intensity but exhibit an orderly progression of directional changes that result in well-defined virtual geomagnetic pole (VGP) paths confined along a preferred meridian of approximately 70-90-degrees-W longitude. We propose that both styles occur when the main dipole term diminishes significantly but that the former result when undimished ''normal'' secular variation is continuous during the period of low axial dipole moment. We propose that the other two lows in relative paleointensity, along with one reversal record, reflect a field structure of low axial dipole moment dominated by a low-degree nonzonal spherical harmonic term. Alternatively, the confined VGP paths could be an artifact of heavy remanence smoothing between nonantipodal, semistable transitional geomagnetic pole positions. Geographical control of VGP paths, particularly along approximately 70-90-degrees-W longitude, has recently been noted for much younger reversals. The site 522 record may indicate that the underlying cause of this phenomenon was present at 32 Ma. We compare our C12R record of paleointensity lows with C12R marine magnetic anomaly ''tiny wiggles''. These data appear to indicate that C12R tiny wiggles resulted from periods of low geomagnetic field intensity that were sometimes accompanied by directional excursions.

Constable, C.  2016.  Earth's electromagnetic environment. Surveys in Geophysics. 37:27-45.   10.1007/s10712-015-9351-1   AbstractWebsite

The natural spectrum of electromagnetic variations surrounding Earth extends across an enormous frequency range and is controlled by diverse physical processes. Electromagnetic (EM) induction studies make use of external field variations with frequencies ranging from the solar cycle which has been used for geomagnetic depth sounding through the 10-10 Hz frequency band widely used for magnetotelluric and audio-magnetotelluric studies. Above 10 Hz, the EM spectrum is dominated by man-made signals. This review emphasizes electromagnetic sources at 1 Hz and higher, describing major differences in physical origin and structure of short- and long-period signals. The essential role of Earth's internal magnetic field in defining the magnetosphere through its interactions with the solar wind and interplanetary magnetic field is briefly outlined. At its lower boundary, the magnetosphere is engaged in two-way interactions with the underlying ionosphere and neutral atmosphere. Extremely low-frequency (3 Hz-3 kHz) electromagnetic signals are generated in the form of sferics, lightning, and whistlers which can extend to frequencies as high as the VLF range (3-30 kHz).The roughly spherical dielectric cavity bounded by the ground and the ionosphere produces the Schumann resonance at around 8 Hz and its harmonics. A transverse resonance also occurs at 1.7-2.0 kHz arising from reflection off the variable height lower boundary of the ionosphere and exhibiting line splitting due to three-dimensional structure. Ground and satellite observations are discussed in the light of their contributions to understanding the global electric circuit and for EM induction studies.

Constable, CG.  1985.  Eastern Australian Geomagnetic-Field Intensity Over the Past 14000 yr. Geophysical Journal of the Royal Astronomical Society. 81:121-130.   10.1111/j.1365-246X.1985.tb01354.x   AbstractWebsite

Two north-eastern Australian volcanic crater lake cores have been used to obtain relative intensity estimates for the geomagnetic field. ARM imparted in a low DC bias field has been used as a normalizing parameter. The intensity fluctuations in the two lakes are in excellent agreement with each other and with south-eastern Australian archaeointensity data over their coeval time spans. This strongly suggests that the same sources are influencing the geomagnetic secular variation throughout eastern Australia at this time. The relative intensity records go back to about 14000yr BP thereby extending currently available recent Australian intensity records by some 7000 yr.

Everett, ME, Constable S, Constable CG.  2003.  Effects of near-surface conductance on global satellite induction responses. Geophysical Journal International. 153:277-286.   10.1046/j.1365-246X.2003.01906.x   AbstractWebsite

A 3-D finite-element simulation of global electromagnetic induction is used to evaluate satellite responses in geomagnetic dipole coordinates for harmonic ring-current excitation of a three-layer mantle overlain by a realistic near-surface conductance distribution. Induced currents are modelled for lithospheric and asthenospheric upper-mantle conductivities in the range sigma= 10(-4) -0.1 S m(-1) . The magnetic scalar intensity B is calculated at a typical satellite altitude of 300 km. At short periods, T = 2 and 12 h, the induction signal owing to the near-surface conductance is large when a resistive upper mantle is present, but drops off with increasing mantle conductivity. At longer periods, T = 2 d, the near-surface induction signal is generally much smaller and nearly independent of upper-mantle conductivity. The near-surface induction signal is very sensitive to the electrical conductivity of the lithospheric mantle, but only moderately sensitive to that of the asthenospheric mantle. Induced currents are confined to the heterogeneous surface shell at periods of less than 2 h, and flow predominantly in the mantle at periods of longer than 2 d. In the intervening period range, induced currents are partitioned between the near-surface and the upper mantle. These results indicate the importance of carrying out a full 3-D analysis in the interpretation of satellite induction observations in the period range from hours to days.

Lowe, DAJ, Parker RL, Purucker ME, Constable CG.  2001.  Estimating the crustal power spectrum from vector Magsat data. Journal of Geophysical Research-Solid Earth. 106:8589-8598.   10.1029/2000jb900437   AbstractWebsite

The Earth's magnetic field can be subdivided into core and crustal components and we seek to characterize the crustal part through its spatial power spectrum, R-1. We process vector Magsat data to isolate the crustal field and then invert power spectral densities of flight-local components along-track for R-1 following O'Brien et al. [1999]. Our model, designated LPPC, is accurate up to approximately spherical harmonic degree 45 (lambda = 900 km): this is the resolution limit of our data and suggests that global crustal anomaly maps constructed from vector Magsat data should not contain features with wavelengths less than 900 km. We find continental power spectra to be greater than oceanic ones and attribute this to the relative thicknesses of continental and oceanic crust.

Panovska, S, Constable CG, Korte M.  2018.  Extending global continuous geomagnetic field reconstructions on timescales beyond human civilization. Geochemistry Geophysics Geosystems. 19:4757-4772.   10.1029/2018gc007966   AbstractWebsite

Study of the late Quaternary geomagnetic field contributes significantly to understanding the origin of millennial-scale paleomagnetic secular variations, the structure of geomagnetic excursions, and the long-term shielding by the geomagnetic field. A compilation of paleomagnetic sediment records and archeomagnetic and lava flow data covering the past 100ka enables reconstruction of the global geomagnetic field on such long-term scales. We use regularized inversion to build the first global, time-dependent, geomagnetic field model spanning the past 100ka, named GGF100k (Global Geomagnetic Field over the past 100 ka). Spatial parametrization of the model is in spherical harmonics and time variations with cubic splines. The model is heavily constrained by more than 100 continuous sediment records covering extended periods of time, which strongly prevail over the limited number of discrete snapshots provided by archeomagnetic and volcanic data. Following an assessment of temporal resolution in each sediment's magnetic record, we have introduced smoothing kernels into the forward modeling when assessing data misfit. This accommodates the smoothing inherent in the remanence acquisition in individual sediment paleomagnetic records, facilitating a closer fit to both high- and low-resolution records in regions where some sediments have variable temporal resolutions. The model has similar spatial resolution but less temporal complexity than current Holocene geomagnetic field models. Using the new reconstruction, we discuss dipole moment variations, the time-averaged field, and paleomagnetic secular variation activity. The new GGF100k model fills the gap in the geomagnetic power spectrum in the frequency range 100-1,000Ma(-1).