scholarly journals An assessment of the role of the centrifugal acceleration mechanism in high altitude polar cap oxygen ion outflow

2008 ◽  
Vol 26 (1) ◽  
pp. 145-157 ◽  
Author(s):  
H. Nilsson ◽  
M. Waara ◽  
O. Marghitu ◽  
M. Yamauchi ◽  
R. Lundin ◽  
...  
Keyword(s):  
Magnetic Field ◽  
High Altitude ◽  
Centrifugal Acceleration ◽  
Polar Cap ◽  
Acceleration Mechanism ◽  
Field Gradients ◽  
Magnetic Field Gradients ◽  
Magnetic Field Model ◽  
The Magnetic Field

Abstract. The role of the centrifugal acceleration mechanism for ion outflow at high altitude above the polar cap has been investigated. Magnetometer data from the four Cluster spacecraft has been used to obtain an estimate of magnetic field gradients. This is combined with ion moment data of the convection drift and the field-aligned particle velocity. Thus all spatial terms in the expression for the centrifugal acceleration are directly obtained from observations. The temporal variation of the unit vector of the magnetic field is estimated by predicting consecutive measurement-points through the use of observed estimates of the magnetic field gradients, and subtracting this from the consecutively observed value. The calculation has been performed for observations of outflowing O+ beams in January to May for the years 2001–2003, and covers an altitude range of about 5 to 12 RE. The accumulated centrifugal acceleration during each orbit is compared with the observed parallel velocities to get an estimate of the relative role of the centrifugal acceleration. Finally the observed spatial terms (parallel and perpendicular) of the centrifugal acceleration are compared with the results obtained when the magnetic field data was taken from the Tsyganenko T89 model instead. It is found that the centrifugal acceleration mechanism is significant, and may explain a large fraction of the parallel velocities observed at high altitude above the polar cap. The magnetic field model results underestimate the centrifugal acceleration at the highest altitudes investigated and show some systematic differences as compared to the observations in the lower altitude ranges investigated. Our results indicate that for altitudes corresponding to magnetic field values of more than 50 nT a test particle model with a steady state magnetic field model, a realistic convection model and an initial velocity of about 20 k m s−1 at 5 RE should be able to reproduce the main part of our observational results.

scholarly journals Cluster survey of the high-altitude cusp properties: a three-year statistical study

2004 ◽  
Vol 22 (8) ◽  
pp. 3009-3019 ◽  
Author(s):  
B. Lavraud ◽  
A. Fedorov ◽  
E. Budnik ◽  
A. Grigoriev ◽  
P. J. Cargill ◽  
...  
Keyword(s):  
Magnetic Field ◽  
High Altitude ◽  
External Boundary ◽  
Plasma Parameters ◽  
Pressure Distributions ◽  
Magnetic Field Model ◽  
Statistical Sense ◽  
Statistical Survey ◽  
Low Magnetic Field ◽  
The Magnetic Field

Abstract. The global characteristics of the high-altitude cusp and its surrounding regions are investigated using a three-year statistical survey based on data obtained by the Cluster spacecraft. The analysis involves an elaborate orbit-sampling methodology that uses a model field and takes into account the actual solar wind conditions and level of geomagnetic activity. The spatial distribution of the magnetic field and various plasma parameters in the vicinity of the low magnetic field exterior cusp are determined and it is found that: 1) The magnetic field distribution shows the presence of an intermediate region between the magnetosheath and the magnetosphere: the exterior cusp, 2) This region is characterized by the presence of dense plasma of magnetosheath origin; a comparison with the Tsyganenko (1996) magnetic field model shows that it is diamagnetic in nature, 3) The spatial distributions show that three distinct boundaries with the lobes, the dayside plasma sheet and the magnetosheath surround the exterior cusp, 4) The external boundary with the magnetosheath has a sharp bulk velocity gradient, as well as a density decrease and temperature increase as one goes from the magnetosheath to the exterior cusp, 5) While the two inner boundaries form a funnel, the external boundary shows no clear indentation, 6) The plasma and magnetic pressure distributions suggest that the exterior cusp is in equilibrium with its surroundings in a statistical sense, and 7) A preliminary analysis of the bulk flow distributions suggests that the exterior cusp is stagnant under northward IMF conditions but convective under southward IMF conditions.

Electromagnetic instabilities in non-uniform anisotropic plasmas

1973 ◽  
Vol 10 (2) ◽  
pp. 249-263 ◽  
Author(s):  
B. Butt ◽  
G. S. Lakhina
Keyword(s):  
Magnetic Field ◽  
External Magnetic Field ◽  
Electromagnetic Waves ◽  
Temperature Gradients ◽  
Anisotropic Plasma ◽  
Field Gradients ◽  
Cyclotron Instability ◽  
Magnetic Field Gradients ◽  
Resonant Electron ◽  
The Magnetic Field

Electromagnetic waves propagating perpendicular to an external magnetic field in a non-uniform anisotropic plasma can become unstable due to the excitation of either resonant ion instability or resonant electron instability. The former instability can exist in the absence of both the temperture anisotropy and the temperature gradients, whereas for the excitation of resonant electron instability the presence of at least one of them is necessary. An off-resonance drift cyclotron instability can also get excited if the temperature gradients are much stronger than the magnetic field gradients.

Magnetostrictive gradient in Tb0.27Dy0.73Fe1.95 induced by high magnetic field gradient applied during solidification

2016 ◽  
Vol 09 (01) ◽  
pp. 1650003 ◽  
Author(s):  
Pengfei Gao ◽  
Tie Liu ◽  
Meng Dong ◽  
Yi Yuan ◽  
Kai Wang ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Field Gradient ◽  
High Magnetic Field ◽  
Applied Magnetic Field ◽  
Magnetic Field Gradient ◽  
Gradient Distribution ◽  
Field Gradients ◽  
Magnetic Field Gradients ◽  
The Magnetic Field

We investigated how high magnetic field gradients affected the magnetostrictive performance of Tb[Formula: see text]Dy[Formula: see text]Fe[Formula: see text] during solidification. At high applied magnetic field gradients, the magnetostriction exhibited a gradient distribution throughout the alloy. Increasing the magnetic field gradient also increased the magnetostriction gradient. We attributed the graded magnetostrictive performance to the gradient distribution of (Tb, Dy)Fe2 phase in the alloy and its orientation.

An Adaptive Magnetic Field Source for Magnetic Drug Fixation

2013 ◽  
Vol 1 (4) ◽  
pp. 20-33
Author(s):  
Alexandru Mihail Morega ◽  
Cristina Savastru ◽  
Mihaela Morega
Keyword(s):  
Magnetic Field ◽  
Numerical Models ◽  
Magnetic Drug Targeting ◽  
Spiral Coil ◽  
Field Source ◽  
Tissue Heating ◽  
Field Gradients ◽  
Magnetic Field Gradients ◽  
Transfer Problems ◽  
The Magnetic Field

Magnetic drug targeting (MDT) therapy is usually controlled through the magnetic field produced by a permanent magnet; the solution proposed and assessed here considers a planar spiral coil (PSC) or a system of such coils, as an equally effective magnetic field source. The PSC may be designed to provide proper configurations of the magnetic field gradients, required for the generation of high magnetic body forces and to limit, in the same time, unwanted side effects affecting adjacent tissue (heating, excitable tissue stimulation). Simplified numerical models (2D projections) and more realistic structures (3D representations) are shown and analyzed in the paper; the electromagnetic and heat transfer problems are solved for different powering schemes applied to the coils.

scholarly journals Spectropolarimetric analysis of an active region filament.

2019 ◽  
Vol 625 ◽  
pp. A129 ◽  
Author(s):  
C. J. Díaz Baso ◽  
M. J. Martínez González ◽  
A. Asensio Ramos
Keyword(s):  
Magnetic Field ◽  
Active Region ◽  
Circular Polarization ◽  
Thick Filament ◽  
Model Atmosphere ◽  
Single Component ◽  
Component Model ◽  
Field Gradients ◽  
Magnetic Field Gradients ◽  
The Magnetic Field

Aims. Our aim is to demonstrate the limitations of using a single-component model to study the magnetic field of an active region filament. To do this, we analyzed the polarimetric signals of the He I 10830 Å multiplet, which were acquired with the infrared spectrograph GRIS of the GREGOR telescope (Tenerife, Spain). Methods. After a first analysis of the general properties of the filament using HAZEL under the assumption of a single-component model atmosphere, in this second part we focus our attention on the observed Stokes profiles and the signatures that cannot be explained with this model. Results. We have found an optically thick filament whose blue and red components have the same sign in the linear polarization as an indication of radiative transfer effects. Moreover, the circular polarization signals inside the filament show strong magnetic field gradients. We also show that even a filament with such high absorption still shows signatures of the circular polarization that is generated by the magnetic field below the filament. The reason is that the absorption of the spectral line decays very quickly toward the wings, just where the circular polarization has a larger amplitude. In order to separate the two contributions, we explore the possibility of a two-component model, but the inference becomes impossible to overcome because very many solutions are compatible with the observations.

Nuclear magnetic resonance microscopy

1989 ◽  
Vol 47 ◽  
pp. 828-829
Author(s):  
Paul C. Lauterbur
Keyword(s):  
Magnetic Field ◽  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Magnetic Fields ◽  
Signal To Noise ◽  
Field Gradients ◽  
Useful Signal ◽  
Magnetic Field Gradients ◽  
Observation Times ◽  
Nuclear Magnetic

Nuclear magnetic resonance imaging can reach microscopic resolution, as was noted many years ago, but the first serious attempt to explore the limits of the possibilities was made by Hedges. Resolution is ultimately limited under most circumstances by the signal-to-noise ratio, which is greater for small radio receiver coils, high magnetic fields and long observation times. The strongest signals in biological applications are obtained from water protons; for the usual magnetic fields used in NMR experiments (2-14 tesla), receiver coils of one to several millimeters in diameter, and observation times of a number of minutes, the volume resolution will be limited to a few hundred or thousand cubic micrometers. The proportions of voxels may be freely chosen within wide limits by varying the details of the imaging procedure. For isotropic resolution, therefore, objects of the order of (10μm) may be distinguished.Because the spatial coordinates are encoded by magnetic field gradients, the NMR resonance frequency differences, which determine the potential spatial resolution, may be made very large. As noted above, however, the corresponding volumes may become too small to give useful signal-to-noise ratios. In the presence of magnetic field gradients there will also be a loss of signal strength and resolution because molecular diffusion causes the coherence of the NMR signal to decay more rapidly than it otherwise would. This phenomenon is especially important in microscopic imaging.

scholarly journals What Density of Magnetosheath Sodium Ions Can Provide the Observed Decrease in the Magnetic Field of the “Double Magnetopause” during the First MESSENGER Flyby?

Symmetry ◽  
2021 ◽  
Vol 13 (7) ◽  
pp. 1168
Author(s):  
Elena Belenkaya ◽  
Ivan Pensionerov
Keyword(s):  
Magnetic Field ◽  
Analytical Approach ◽  
Field Structure ◽  
Sodium Ions ◽  
Plasma Parameters ◽  
Calculation Results ◽  
Magnetizing Current ◽  
The Magnetic Field ◽  
Density Excess

On 14 January 2008, the MESSENGER spacecraft, during its first flyby around Mercury, recorded the magnetic field structure, which was later called the “double magnetopause”. The role of sodium ions penetrating into the Hermean magnetosphere from the magnetosheath in generation of this structure has been discussed since then. The violation of the symmetry of the plasma parameters at the magnetopause is the cause of the magnetizing current generation. Here, we consider whether the change in the density of sodium ions on both sides of the Hermean magnetopause could be the cause of a wide diamagnetic current in the magnetosphere at its dawn-side boundary observed during the first MESSENGER flyby. In the present paper, we propose an analytical approach that made it possible to determine the magnetosheath Na+ density excess providing the best agreement between the calculation results and the observed magnetic field in the double magnetopause.

Connecting a Star’s Convection Zone with its Corona

1995 ◽  
Vol 12 (2) ◽  
pp. 180-185 ◽  
Author(s):  
D. J. Galloway ◽  
C. A. Jones
Keyword(s):  
Magnetic Field ◽  
Convection Zone ◽  
Flux Rope ◽  
Field System ◽  
Stellar Convection ◽  
Coronal Activity ◽  
Flux Concentration ◽  
Global Understanding ◽  
The Magnetic Field

AbstractThis paper discusses problems which have as their uniting theme the need to understand the coupling between a stellar convection zone and a magnetically dominated corona above it. Interest is concentrated on how the convection drives the atmosphere above, loading it with the currents that give rise to flares and other forms of coronal activity. The role of boundary conditions appears to be crucial, suggesting that a global understanding of the magnetic field system is necessary to explain what is observed in the corona. Calculations are presented which suggest that currents flowing up a flux rope return not in the immediate vicinity of the rope but rather in an alternative flux concentration located some distance away.

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