La position stratigraphique déterminée paléomagnétiquement de sédiments carbonifères de Minudie Point, Nouvelle Ecosse : à propos de l'horizon repère magnétique du Carbonifère

1977 ◽  
Vol 14 (5) ◽  
pp. 1116-1127 ◽  
Author(s):  
J. L. Roy
Keyword(s):  
Magnetic Field ◽  
Eastern Canada ◽  
Geological Evidence ◽  
Upper Carboniferous ◽  
Tectonic History ◽  
Global Correlation ◽  
Stratigraphic Position ◽  
Normal Polarity ◽  
History Of ◽  
The Magnetic Field

Chemical, thermal, and alternating field cleaning treatments indicate that the Minudie Point sedimentary beds acquired their initial magnetization slowly and progressively during a lengthy process that lasted from deposition to lithification. The magnetization carried by the pigment is separated vectorially from the magnetization acquired during the early phases and its direction is shown to be a reliable indicator of the direction of the magnetic field during lithification. The agreement between the directions of the magnetizations acquired at the beginning (175°, +15°; α95 = 6°; pole 122° E, 36° N) and at the end (172°, +20°; α95 = 5°; pole 125° E, 33° N) of the process indicates that the direction (irrespective of the sign) of the magnetic field remained virtually unchanged during the magnetization process. Slicing of specimens shows that certain apparently aberrant directions are in reality the directions of the resultant vectors of two magnetizations of opposite polarities.The recording of a normal polarity during the early phase of magnetization places these sediments below (and perhaps immediately below) the magnetic horizon marker at the base of the Permo – Upper Carboniferous interval of reversed polarity. This indicates that these beds which on geological evidence could be of Pictou or Riversdale age belong to the Riversdale Group. This result contributes to the pin-pointing of the stratigraphic position of the Carboniferous magnetic horizon marker which can be most useful for the study of the tectonic history of the ancient Carboniferous basin of eastern Canada. The study shows that this magnetic horizon marker is a valuable stratigraphic tool which can be used most effectively for global correlation of Carboniferous units.

Inland, Coastal, and Offshore Magnetotelluric Measurements in Eastern Canada

1971 ◽  
Vol 8 (2) ◽  
pp. 204-216 ◽  
Author(s):  
S. P. Srivastava ◽  
A. White
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
East Coast ◽  
Vertical Component ◽  
Eastern Canada ◽  
Spectral Techniques ◽  
Relative Attenuation ◽  
Field Recordings ◽  
Power Spectral ◽  
The Magnetic Field

For six weeks during the summer of 1966 simultaneous magnetic and electric field recordings were made on the east coast of Canada at Fredericton, N.B., Halifax, N.S., and Sable Island. The data from these stations have been analyzed using power spectral techniques. Comparison of the simultaneous recordings from Halifax and Sable Island with those from Fredericton and Agincourt indicate some enhancement in the intensity of the vertical component of the magnetic field for periods less than 40 min at Halifax and attenuation in its intensity for periods less than 3 h at Sable Island. The enhancement at Halifax has been interpreted in terms of the "coast effect" while the effect of the island and of differences in the subsurface conductivity under the continent and under the ocean have been shown to be possible causes of the relative attenuation in the Z variations at Sable Island.

scholarly journals Multipoint spacecraft observations of long-lasting poloidal Pc4 pulsations in the dayside magnetosphere on 1–2 May 2014

2016 ◽  
Vol 34 (11) ◽  
pp. 985-998 ◽  
Author(s):  
Galina Korotova ◽  
David Sibeck ◽  
Mark Engebretson ◽  
John Wygant ◽  
Scott Thaller ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Time History ◽  
Satellite System ◽  
Local Times ◽  
Spectral Structure ◽  
Nodal Structure ◽  
Van Allen Probes ◽  
History Of ◽  
Solar Wind Parameters ◽  
The Magnetic Field

Abstract. We use magnetic field and plasma observations from the Van Allen Probes, Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Geostationary Operational Environmental Satellite system (GOES) spacecraft to study the spatial and temporal characteristics of long-lasting poloidal Pc4 pulsations in the dayside magnetosphere. The pulsations were observed after the main phase of a moderate storm during low geomagnetic activity. The pulsations occurred during various interplanetary conditions and the solar wind parameters do not seem to control the occurrence of the pulsations. The most striking feature of the Pc4 magnetic field pulsations was their occurrence at similar locations during three of four successive orbits. We used this information to study the latitudinal nodal structure of the pulsations and demonstrated that the latitudinal extent of the magnetic field pulsations did not exceed 2 Earth radii (RE). A phase shift between the azimuthal and radial components of the electric and magnetic fields was observed from ZSM  =  0.30 RE to ZSM  =  −0.16 RE. We used magnetic and electric field data from Van Allen Probes to determine the structure of ULF waves. We showed that the Pc4 magnetic field pulsations were radially polarized and are the second-mode harmonic waves. We suggest that the spacecraft were near a magnetic field null during the second orbit when they failed to observe the magnetic field pulsations at the local times where pulsations were observed on previous and successive orbits. We investigated the spectral structure of the Pc4 pulsations. Each spacecraft observed a decrease of the dominant period as it moved to a smaller L shell (stronger magnetic field strength). We demonstrated that higher frequencies occurred at times and locations where Alfvén velocities were greater, i.e., on Orbit 1. There is some evidence that the periods of the pulsations increased during the plasmasphere refilling following the storm.

scholarly journals Prominence Oscillations and the Influence of the Distant Photosphere

1998 ◽  
Vol 167 ◽  
pp. 147-150
Author(s):  
N.A.J. Schutgens ◽  
M. Kuperus ◽  
G.H.J. van den Oord
Keyword(s):  
Magnetic Field ◽  
Boundary Condition ◽  
The Other ◽  
Forced Oscillations ◽  
Crossing Time ◽  
Self Consistent ◽  
Normal Polarity ◽  
The Stability ◽  
The Magnetic Field ◽  
Flux Conservation

AbstractWe model vertical prominence dynamics, describing the evolution of the magnetic field in a self-consistent way. Since the photosphere imposes a boundary condition on the field (flux conservation), the Alfvén crossing time τ0/2 between prominence and photosphere has to be taken into account. Using an electrodynamical description of the prominence we are able to compare two basic prominence models: Normal Polarity (NP) and Inverse Polarity (IP).The results indicate that for IP prominences, the stability properties are sensitive to ωτ0 (ω: oscillation frequency of prominence). For ωτ0 ≳ 1 instability results. Forced oscillations of five minutes are efficiently excited in IP prominences that meet certain criteria only. NP prominences on the other hand, are insensitive to the Alfvén crossing time. Forced oscillations of five minutes are difficult to excite in NP prominences.

Paleomagnetism of the Maringouin and Shepody formations, New Brunswick: a Namurian magnetic stratigraphy

1990 ◽  
Vol 27 (6) ◽  
pp. 803-810 ◽  
Author(s):  
V. J. DiVenere ◽  
N. D. Opdyke
Keyword(s):  
Magnetic Field ◽  
High Temperature ◽  
New Brunswick ◽  
Late Paleozoic ◽  
Stratigraphic Sequence ◽  
The North ◽  
Magnetic Stratigraphy ◽  
Normal Polarity ◽  
Reversed Polarity ◽  
The Magnetic Field

Two hundred thirty-five oriented cores were taken in a stratigraphic sequence of 575 m of red sediments from the Namurian Maringouin and Shepody formations. These samples were taken from the south-dipping limb of the Maringouin Anticline on the Maringouin Peninsula in eastern New Brunswick. An additional 59 samples of the Maringouin and Shepody formations were taken from the north-dipping limb for separate fold tests. Progressive thermal demagnetization to temperatures as high as 700 °C permitted the resolution of an intermediate-temperature, syn-folding B component and a high-temperature, pre-folding C component. The C-component mean for the Maringouin Formation yields a north paleopole at 32.4°N, 121.3°E. The Shepody Formation mean yields a paleopole at 35.7°N, 124.3°E. The circles of confidence (α95) for the Maringouin and Shepody formations are 3.8 °and 4.6°, respectively. Plotting sample polarity versus stratigraphie position yields the first coherent magnetic stratigraphy from the late Paleozoic of North America, with five discrete reversed-polarity and five discrete normal-polarity zones. The magnetic field in Namurian times had approximately 50% normal and 50% reversed polarity.

Paleomagnetic constraints on the tectonic history of the Foreland Belt, southern Canadian Cordillera: preliminary results

1997 ◽  
Vol 34 (3) ◽  
pp. 260-270 ◽  
Author(s):  
R. J. Enkin ◽  
P. M. Wheadon ◽  
J. Baker ◽  
K. G. Osadetz
Keyword(s):  
Canadian Cordillera ◽  
Thrust Sheet ◽  
Western Front ◽  
Tectonic History ◽  
Sequence Of Events ◽  
Folded Structure ◽  
Low Latitudes ◽  
Magnetic Period ◽  
Normal Polarity ◽  
History Of

In the southern Canadian Cordillera, the paleomagnetic memory of Paleozoic carbonate strata in the Front Ranges and Inner Foothills of the Foreland thrust and fold belt retains no record of their known deposition at low latitudes. Instead, each folded structure exhibits a similar, but asynchronous, sequence of events including an eastwardly progressing, predeformational chemical remagnetization during the Cordilleran orogeny. The remagnetization of a "western Front Ranges" structure occurs during a period of normal polarity before 130 Ma. The paleomagnetic pole requires that the subsequent deformation of the western Front Ranges is Jurassic or younger. The remagnetization of a Front Ranges structure in the Lewis thrust sheet occurs during a period of normal polarity after 130 Ma but before deformation which, from other evidence, occurred around 75 Ma. The predeformational remagnetization of an "Inner Foothills" structure occurs during a reversed magnetic period that we interpret to be after 75 Ma. An Early Cretaceous sill in the Lewis thrust sheet was remagnetized during a reverse-polarity chron prior to the end of Lewis thrust deformation, when about 70% of the present dip of the sill was acquired. Remagnetization consistently predates deformation, whereas it occurs later at more easterly localities. There are also similarities in character and style of the remagnetizations among localities. When coupled with the eastward progression of the deformation, our observations suggest that an important and pervasive, but hitherto unrecognized and unappreciated, orogenic chemical process affected Paleozoic carbonate strata in the van of the deforming Cordilleran tectonic wedge.

Magnetic field within magnetosheath jets during northward and southward interplanetary magnetic field conditions

2020 ◽  
Author(s):  
Laura Vuorinen ◽  
Heli Hietala ◽  
Ferdinand Plaschke
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Interplanetary Magnetic Field ◽  
Dynamic Pressure ◽  
Time History ◽  
Magnetic Shear ◽  
History Of ◽  
High Dynamic ◽  
The Magnetic Field ◽  
Magnetopause Reconnection

<p>Downstream of the Earth's quasi-parallel shock, transients with higher earthward velocities than the surrounding magnetosheath plasma are often observed. These transients have been named magnetosheath jets. Due to their high dynamic pressure, jets can cause multiple types of effects when colliding into the magnetopause. Recently, jets have been linked to triggering magnetopause reconnection in case studies by Hietala et al. (2018) and Nykyri et al. (2019). Jets have been proposed to affect magnetopause reconnection in multiple ways. Jets can compress the magnetopause and make it thin enough for reconnection to occur. Jets could also affect the magnetic shear either by indenting the magnetopause or via the magnetic field of the jets themselves. Here we want to study whether the magnetic field of jets can statistically affect magnetopause reconnection. In particular, we are interested in whether jets could enhance reconnection during more quiet northward IMF conditions.</p><p>We statistically study the magnetic field within jets in the subsolar magnetosheath using measurements from the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and OMNI solar wind data from 2008–2011. We investigate jets next to the magnetopause and find that the magnetic field within jets is statistically different compared to the non-jet magnetosheath. Our results suggest that during southward IMF, the non-jet magnetosheath magnetic field itself has more variation than the jets. This suggests that jets should have no statistical, neither enhancing nor suppressing, effect on reconnection during southward IMF. However, during northward IMF, the magnetic field within jets is statistically favorable for enhancing magnetic reconnection at the subsolar magnetopause as around 70 % of these jets exhibit southward fields close to the magnetopause.</p>

scholarly journals The Evolution of the Magnetic Fields of Neutron Stars

1992 ◽  
Vol 128 ◽  
pp. 26-34
Author(s):  
Dipankar Bhattacharya
Keyword(s):  
Magnetic Field ◽  
Neutron Star ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Theoretical Models ◽  
Radio Pulsars ◽  
Secular Evolution ◽  
History Of ◽  
Field Decay ◽  
The Magnetic Field

AbstractThe evolution of the magnetic field strength plays a major role in the life history of a neutron star. In this article the observational evidence of field evolution, in particular that of field decay and magnetic alignment, are critically examined. It is concluded that the observed decay of the spindown torque on radio pulsars cannot be caused by a secular evolution of the “obliqueness” of the neutron star, as suggested by some authors. Recent observations provide a strong indication that the decay of the magnetic field strength of a neutron star may be closely related to its evolution in a binary system. Theoretical models for such an evolution are discussed.

scholarly journals Revisiting the magnetization of comet 67P/Churyumov-Gerasimenko

2019 ◽  
Vol 630 ◽  
pp. A46 ◽  
Author(s):  
P. Heinisch ◽  
H.-U. Auster ◽  
I. Richter ◽  
K. H. Glassmeier
Keyword(s):  
Magnetic Field ◽  
Magnetic Measurements ◽  
Field Observations ◽  
Upper Limit ◽  
Background Magnetic Field ◽  
Surface Magnetization ◽  
Cometary Material ◽  
History Of ◽  
Comet 67P ◽  
The Magnetic Field

Context. The landing of the Philae probe as part of the ESA Rosetta mission made it possible to study the magnetization of comet 67P/Churyumov-Gerasimenko (67P) by combining observations from the lander and orbiter. In this work, we revisit the magnetic properties with information gained during the progression of the mission for a comprehensive understanding of the circumstances of Philae’s descent and landing. Aims. The aim is to derive a limit for any possible magnetization of the cometary material on the surface of 67P. To achieve this, the surface contacts of Philae were analyzed. Combined with a more detailed understanding of the background magnetic field, this allows us to interpret the underlying magnetic measurements in detail. Methods. We combined magnetic field observations from the ROMAP magnetometer on board Philae with observations from the RPC-MAG instrument on board the Rosetta orbiter. To facilitate this, a correlation analysis was used to correct phase shifts between the observed signals. Additionally, in-flight calibration of the ROMAP offsets was performed using information about the dynamics of Philae during flight. These corrections made it possible to use the orbiter measurements as reference for the comet-based Philae observations. We assumed a simple dipole model and used the magnetic field observations to derive an upper limit for the magnetization of the cometary material. Results. An upper limit of 0.9 nT for the observed magnetic field on the surface of 67P was derived for any contribution from surface magnetization. For homogeneously magnetized pebbles with a size of typical aggregates in the range of ~5 cm, this translates into an upper limit of ~5 × 10−5 Am2 kg−1 for the specific magnetic moment. Depending on the exact history of formation, this results in an upper limit of 4 μT for the magnitude of the magnetic field in the solar nebula during the formation of comet 67P.

scholarly journals Plasma transport into the duskside magnetopause caused by Kelvin–Helmholtz vortices in response to the northward turning of the interplanetary magnetic field observed by THEMIS

2019 ◽  
Author(s):  
Guang Qing Yan ◽  
George K. Parks ◽  
Chun Lin Cai ◽  
Tao Chen ◽  
James P. McFadden ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Boundary Layer ◽  
Interplanetary Magnetic Field ◽  
Time History ◽  
Plasma Transport ◽  
Low Latitude ◽  
Unique Event ◽  
History Of ◽  
The Magnetic Field ◽  
Low Latitude Boundary Layer

Abstract. A train of Kelvin–Helmholtz (K–H) vortices with plasma transport across the magnetopause has been observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) when the interplanetary magnetic field (IMF) abruptly turns northward. This unique event occurred without pre-existing denser boundary layer to facilitate the instability. Two THEMIS spacecraft, TH-A and TH-E, separated by 3 Re, periodically encountered the duskside magnetopause and the low-latitude boundary layer (LLBL) with a period of 2 minutes and tailward propagation of 194 km/s. There was no high-velocity low-density feature, but the rotations in the bulk velocity observation, distorted magnetopause with plasma parameter fluctuations and the magnetic field line stretching, indicate the formation of rolled-up K–H vortices at the duskside magnetopause. A mixture of magnetosheath ions with magnetospheric ions and enhanced energy flux of hot electrons is identified in the K–H vortices. This mixture region appears more periodic at the upstream spacecraft and more dispersive at the downstream location, indicating a significant transport can occur and evolve during the tailward propagation of the K–H waves. There is still much work to fully understand the Kelvin–Helmholtz mechanism. The observations of direct response to the northward turning of the IMF, the unambiguous plasma transport within the vortices, involving both ion and electron fluxes can provide additional clues to the K–H mechanism.

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