scholarly journals The increase in the curvature radius of geomagnetic field lines preceding a classical dipolarization

2020 ◽  
Vol 38 (2) ◽  
pp. 467-479
Author(s):  
Osuke Saka
Keyword(s):  
Field Line ◽  
Electric Fields ◽  
Flux Tube ◽  
Equatorial Plane ◽  
Current System ◽  
Meridian Plane ◽  
Curvature Radius ◽  
Equilibrium Geometry ◽  
Magnetoacoustic Wave ◽  
Field Lines

Abstract. Based on assumptions that substorm field line dipolarization at geosynchronous altitudes is associated with the arrival of high-velocity magnetotail flow bursts referred to as bursty bulk flows, the following sequence of field line dipolarization is proposed: (1) slow magnetoacoustic wave excited through ballooning instability by enhanced inflows in pre-onset intervals towards the equatorial plane; (2) in the equatorial plane, slow magnetoacoustic wave stretching of the flux tube in dawn–dusk directions resulting in spreading plasmas in dawn–dusk directions and reduction in the radial pressure gradient in the flux tube. As a consequence of these processes, the flux tube assumes a new equilibrium geometry in which the curvature radius of new field lines increased in the meridian plane, suggesting an onset of field line dipolarization. The dipolarization processes associated with changing the curvature radius preceded classical dipolarization caused by a reduction of cross-tail currents and pileup of the magnetic fields. Increasing the curvature radius induced a convection surge in the equatorial plane as well as inductive westward electric fields of the order of millivolts per meter (mV m−1). Electric fields transmitted to the ionosphere produce an electromotive force in the E layer for generating a field-aligned current system of Bostrom type. This is also equivalent to the creation of an incomplete Cowling channel in the ionospheric E layer by the convection surge.

scholarly journals The increase of curvature radius of geomagnetic field lines preceding a classical dipolarization

2019 ◽  
Author(s):  
Osuke Saka
Keyword(s):  
Field Line ◽  
Electric Fields ◽  
Flux Tube ◽  
Slow Wave ◽  
Equatorial Plane ◽  
Curvature Radius ◽  
Flux Tubes ◽  
Ballooning Instability ◽  
Field Lines ◽  
Bursty Bulk Flows

Abstract. Downstream observations at geosynchronous altitudes of field line dipolarization exhibit fundamental component of substorms associated with high velocity magnetotail flow bursts referred to as Bursty Bulk Flows. In growth phase of substorms, we found that the magnetosphere at geosynchronous orbit are in unstable conditions for Ballooning instability due to the appreciable tailward stretching of the flux tubes, and for slow magnetoacoustic wave due to the continuing field-aligned inflows of plasma sheet plasmas towards the equatorial plane. We propose following scenario of field line dipolarization in downstream locations; (1) The slow wave was excited through Ballooning instability by the arrival of Dipolarization Front at the leading edge of Bursty Bulk Flows. (2) In the equatorial plane, slow wave stretched the flux tube in dawn-dusk directions, which resulted in the spreading plasmas in dawn-dusk directions and reducing the radial pressure gradient in the flux tube. (3) As a result, the flux tube becomes a new equilibrium geometry in which curvature radius of new field lines increased in meridian plane, suggesting an onset of field line dipolarization. (4) Increasing curvature radius induced inductive electric fields of the order of few mV/m pointing westward in the equatorial plane, as well as radial electric fields associated with stretching flux tubes in dawn-dusk directions. Westward electric fields transmitted to the ionosphere produce a dynamic ionosphere where the E layer contains both dynamo (E · J  0) processes in it for generating field-aligned current system of Bostrom type. The dipolarization processes associated with changing the curvature radius occurred in the transitional intervals lasting for about 10 minutes preceding classical dipolarization composed of reduction of cross-tail currents and pileup of the magnetic fields transported from the tail.

scholarly journals Differential rotation in neutron stars with open and closed magnetic topologies

2020 ◽  
Vol 494 (3) ◽  
pp. 3095-3109
Author(s):  
F Anzuini ◽  
A Melatos
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Open Field ◽  
Flux Tube ◽  
Differential Rotation ◽  
Time Scale ◽  
Outer Sphere ◽  
Closed Field ◽  
Magnetic Tension ◽  
Field Lines

ABSTRACT Analytic arguments have been advanced that the degree of differential rotation in a neutron star depends on whether the topology of the internal magnetic field is open or closed. To test this assertion, the ideal-magnetohydrodynamics solver pluto is employed to investigate numerically the flow of an incompressible, viscous fluid threaded by a magnetic field with open and closed topologies in a conducting, differentially rotating, spherical shell. Rigid body corotation with the outer sphere is enforced on the Alfvén time-scale, along magnetic field lines that connect the northern and southern hemispheres of the outer sphere. Along other field lines, however, the behaviour is more complicated. For example, an initial point dipole field evolves to produce an approximately closed equatorial flux tube containing at least one predominantly toroidal and approximately closed field line surrounded by a bundle of predominantly toroidal but open field lines. Inside the equatorial flux tube, the field-line-averaged magnetic tension approaches zero, and the fluid rotates differentially, adjusting its angular velocity on the viscous time-scale to match the boundary conditions on the flux tube’s toroidal surface. Outside the equatorial flux tube, the differential rotation increases, as the magnetic tension averaged along open field lines decreases.

scholarly journals Mapping of steady-state electric fields and convective drifts in geomagnetic fields – Part 2: The IGRF

2016 ◽  
Vol 34 (1) ◽  
pp. 67-73 ◽  
Author(s):  
A. D. M. Walker
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Field Line ◽  
Electric Fields ◽  
Rate Of Change ◽  
International Geomagnetic Reference Field ◽  
First Order ◽  
Faraday’S Law ◽  
Geomagnetic Fields ◽  
Field Lines

Abstract. A method of mapping electric fields along geomagnetic field lines is applied to the IGRF (International Geomagnetic Reference Field) model. The method involves integrating additional sets of first order differential equations simultaneously with those for tracing a magnetic field line. These provide a measure of the rate of change of the separation of two magnetic field lines separated by an infinitesimal amount. From the results of the integration Faraday's law is used to compute the electric field as a function of position along the field line. Examples of computations from a software package developed to implement the method are presented. This is expected to be of use in conjugate studies of magnetospheric phenomena such as SuperDARN (Super Dual Auroral Radar) observations of convection in conjugate hemispheres, or comparison of satellite electric field observations with fields measured in the ionosphere.

A re-examination of the role of magnetic field lines in electromagnetic induction

1988 ◽  
Vol 66 (3) ◽  
pp. 245-248
Author(s):  
D. H. Boteler
Keyword(s):  
Magnetic Field ◽  
Reynolds Number ◽  
Field Line ◽  
Electric Fields ◽  
Electromagnetic Induction ◽  
Magnetic Reynolds Number ◽  
Unified View ◽  
Field Lines ◽  
The Magnetic Field

By adopting a view of magnetic fields, originally proposed by Faraday, in which the magnetic field changes by a movement of field lines, it is shown that a changing magnetic field can be described by the relation [Formula: see text] where v is the velocity of the magnetic field lines. These field-line velocities are shown to be the same as material velocities in conditions of infinite magnetic Reynolds number. The "moving field-line" view provides a phenomenological model of a changing magnetic field that is useful in electromagnetic induction studies. It also allows for a unified view of electromagnetic induction in which all induced electric fields can be explained by the v × B force alone.

scholarly journals Optical signatures of auroral arcs produced by field line resonances: comparison with satellite observations and modeling

2003 ◽  
Vol 21 (4) ◽  
pp. 933-945 ◽  
Author(s):  
J. C. Samson ◽  
R. Rankin ◽  
V. T. Tikhonchuk
Keyword(s):  
Phase Shift ◽  
Field Line ◽  
Electric Fields ◽  
Low Frequency ◽  
Optical Data ◽  
Field Line Resonance ◽  
Field Line Resonances ◽  
Line Resonance ◽  
Field Lines ◽  
Auroral Arcs

Abstract. We show two examples from the CANOPUS array of the optical signatures of auroral arcs produced by field line resonances on the night of 31 January 1997. The first example occurs during local evening at about 18:00 MLT (Magnetic Local Time), where CANOPUS meridian scanning photometer data show all the classic features of field line resonances. There are two, near-monochromatic resonances (at approximately 2.0 and 2.5 mHz) and both show latitudinal peaks in amplitude with an approximately 180 degree latitudinal phase shift across the maximum. The second field line resonance event occurs closer to local midnight, between approximately 22:00 and 22:40 MLT. Magnetometer and optical data show that the field line resonance has a very low frequency, near 1.3 mHz. All-sky imager data from CANOPUS show that in this event the field line resonances produce auroral arcs with westward propagation, with arc widths of about 10 km. Electron energies are on the order of 1 keV. This event was also seen in data from the FAST satellite (Lotko et al., 1998), and we compare our observations with those of Lotko et al. (1998). A remarkable feature of this field line resonance is that the latitudinal phase shift was substantially greater than 180 degrees. In our discussion, we present a model of field line resonances which accounts for the dominant physical effects and which is in good agreement with the observations. We emphasize three points. First, the low frequency of the field line resonance in the second event is likely due to the stretched topology of the magnetotail field lines, with the field line resonance on field lines threading the earthward edge of the plasma sheet. Second, the latitudinal phase structure may indicate dispersive effects due to electron trapping or finite ion gyroradius. Third, we show that a nonlocal conductivity model can easily explain the parallel electric fields and the precipitating electron energies seen in the field line resonance.Key words. Magnetospheric physics (electric fields; energetic particles precipitating; current systems)

On the formation of a plasma pressure anisotropy in the dayside magnetosheath

1995 ◽  
Vol 13 (3) ◽  
pp. 237-241
Author(s):  
B. V. Rezhenov ◽  
V. V. Safargaleev ◽  
W. B. Lyatsky
Keyword(s):  
Magnetic Field ◽  
Equatorial Plane ◽  
Momentum Equation ◽  
Plasma Pressure ◽  
Meridian Plane ◽  
Small Pitch ◽  
Pressure Anisotropy ◽  
Subsolar Point ◽  
Field Lines ◽  
The Magnetic Field

Abstract. We present a numerical solution for the momentum equation of the magnetosheath particles that describes the distribution of the pressure anisotropy of the magnetosheath plasma in the midday meridian plane. The pressure anisotropy is a maximum near the magnetopause subsolar point (p⊥/p\\Vert ≌ 10). The pressure anisotropy is caused by two factors: particles with small pitch angles (V\\Vert>V⊥) which travel along the magnetic field lines away from the equatorial plane of the magnetosheath; and particles, after crossing the bowshock, which reach the bulk velocity component directed along the magnetic field lines again, away from the magnetosheath equatorial plane. This velocity increases with increasing distance from the subsolar point of the bowshock, and does not permit particles with large pitch angles (V⊥>V\\Vert) to move toward the equatorial plane.

scholarly journals Signatures of the midnight open-closed magnetic field line boundary during balanced dayside and nightside reconnection

2002 ◽  
Vol 20 (10) ◽  
pp. 1617-1630 ◽  
Author(s):  
M. L. Parkinson ◽  
P. L. Dyson ◽  
M. Pinnock ◽  
J. C. Devlin ◽  
M. R. Hairston ◽  
...  
Keyword(s):  
Field Line ◽  
Electric Fields ◽  
Plasma Sheet ◽  
Spectral Width ◽  
Doppler Velocity ◽  
Electron Content ◽  
Closed Field ◽  
Total Electron ◽  
Central Plasma ◽  
Field Lines

Abstract. The geomagnetic conditions were moderately disturbed (Kp = 2) during magnetic midnight on 10 December 1999, when the Tasman International Geospace Environment Radar (TIGER), a Southern Hemisphere HF SuperDARN radar, observed a persistent, sharp latitudinal decrease (~ 90 km) in spectral width near - 69°L. The line-of-sight Doppler velocity also rapidly declined across this spectral width boundary (SWB). The region poleward of the SWB was characterized by high spectral widths (>200 m/s), and the start of bursty equatorward and eastward flows (>500 m/s), which rapidly expanded equatorward. The relationships between familiar ionospheric and magnetospheric regions were inferred by comparing TIGER data with spectrograms calculated from precipitating particles measured on board the Defence Meteorology Satellite Program (DMSP) F14 satellite. The high spectral width scatter is often observed, and on this evening it was associated with irregularities forming on the open (but soon to be reconnected) field lines threading the polar cap ionosphere to the southern tail lobe. The region equatorward of the SWB was characterized by very low spectral widths (<50 m/s) and generally slower, more zonal flows (<300 m/s). This kind of scatter is more transient, and was associated with irregularities residing on the closed field lines threading the discrete and diffuse auroral oval to the plasma sheet boundary layer (PSBL) and central plasma sheet (CPS). Hence, the SWB was a reasonable proxy for the open-closed field line boundary, and the equatorward limit of the region, with low spectral width, was probably aligned with the poleward wall of the main ionospheric trough. The SWB was observed to contract poleward and expand equatorward on time scales of ~10 min, much as would be expected during balanced dayside and nightside reconnection. Total electron content (TEC) measurements made at Macquarie Island (- 65°L) and Hobart (- 54°L), and the ionograms recorded at the same stations, as well as at Bundoora (- 49°L), also helped to validate the interpretation.Key words. Ionosphere (auroral ionosphere; electric fields and currents; ionosphere-magnetosphere interactions)

scholarly journals RF-Carpets that Compress Ions to High Density Beams

2015 ◽  
Vol 21 (S4) ◽  
pp. 84-89
Author(s):  
H. Wollnik ◽  
F. Arai ◽  
Y. Ito ◽  
P. Schury ◽  
M. Wada
Keyword(s):  
Phase Space ◽  
Electric Field ◽  
Electric Fields ◽  
Ion Beams ◽  
High Density ◽  
Ion Density ◽  
Field Lines

AbstractIons that are moved by electric fields in gases follow quite exactly the electric field lines since these ions have substantially lost their kinetic energies in collisions with gas atoms or molecules and so carry no momenta. Shaping the electric fields appropriately the phase space such ion beams occupy can be reduced and correspondingly the ion density of beams be increased.

scholarly journals The development of magnetic field line wander by plasma turbulence

2017 ◽  
Vol 83 (4) ◽  
Author(s):  
Gregory G. Howes ◽  
Sofiane Bourouaine
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Magnetic Field Line ◽  
Large Scale ◽  
Magnetic Energy ◽  
Plasma Turbulence ◽  
Alfven Wave ◽  
Astrophysical Plasmas ◽  
Field Lines ◽  
The Magnetic Field

Plasma turbulence occurs ubiquitously in space and astrophysical plasmas, mediating the nonlinear transfer of energy from large-scale electromagnetic fields and plasma flows to small scales at which the energy may be ultimately converted to plasma heat. But plasma turbulence also generically leads to a tangling of the magnetic field that threads through the plasma. The resulting wander of the magnetic field lines may significantly impact a number of important physical processes, including the propagation of cosmic rays and energetic particles, confinement in magnetic fusion devices and the fundamental processes of turbulence, magnetic reconnection and particle acceleration. The various potential impacts of magnetic field line wander are reviewed in detail, and a number of important theoretical considerations are identified that may influence the development and saturation of magnetic field line wander in astrophysical plasma turbulence. The results of nonlinear gyrokinetic simulations of kinetic Alfvén wave turbulence of sub-ion length scales are evaluated to understand the development and saturation of the turbulent magnetic energy spectrum and of the magnetic field line wander. It is found that turbulent space and astrophysical plasmas are generally expected to contain a stochastic magnetic field due to the tangling of the field by strong plasma turbulence. Future work will explore how the saturated magnetic field line wander varies as a function of the amplitude of the plasma turbulence and the ratio of the thermal to magnetic pressure, known as the plasma beta.

scholarly journals Mapping of steady-state electric fields and convective drifts in geomagnetic fields – Part 1: Elementary models

2016 ◽  
Vol 34 (1) ◽  
pp. 55-65 ◽  
Author(s):  
A. D. M. Walker ◽  
G. J. Sofko
Keyword(s):  
Magnetic Field ◽  
Steady State ◽  
Electric Field ◽  
Electric Fields ◽  
Magnetic Field Line ◽  
Geomagnetic Field ◽  
Geomagnetic Field Models ◽  
Spatial Derivatives ◽  
Field Lines ◽  
The Magnetic Field

Abstract. When studying magnetospheric convection, it is often necessary to map the steady-state electric field, measured at some point on a magnetic field line, to a magnetically conjugate point in the other hemisphere, or the equatorial plane, or at the position of a satellite. Such mapping is relatively easy in a dipole field although the appropriate formulae are not easily accessible. They are derived and reviewed here with some examples. It is not possible to derive such formulae in more realistic geomagnetic field models. A new method is described in this paper for accurate mapping of electric fields along field lines, which can be used for any field model in which the magnetic field and its spatial derivatives can be computed. From the spatial derivatives of the magnetic field three first order differential equations are derived for the components of the normalized element of separation of two closely spaced field lines. These can be integrated along with the magnetic field tracing equations and Faraday's law used to obtain the electric field as a function of distance measured along the magnetic field line. The method is tested in a simple model consisting of a dipole field plus a magnetotail model. The method is shown to be accurate, convenient, and suitable for use with more realistic geomagnetic field models.

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