scholarly journals A model of solar flares

1968 ◽  
Vol 35 ◽  
pp. 471-479 ◽  
Author(s):  
P. A. Sturrock
Keyword(s):  
Magnetic Field ◽  
Solar Flares ◽  
Transverse Dimension ◽  
Growth Time ◽  
Closed Field ◽  
Field Region ◽  
Tearing Mode ◽  
Magnetic Region ◽  
Field Lines ◽  
Photospheric Motion

A model of solar flares is proposed in which the preflare state comprises a bipolar magnetic-field structure associated with a bipolar photospheric magnetic region. At low heights, the magnetic-field lines are closed but, at sufficiently great heights, the lines are drawn out into an open structure comprising a bipolar flux tube containing a ‘neutral sheet’ or ‘sheet pinch’. Such a sheet pinch is probably related to a coronal streamer. The energy stored in the closed-field region is derived from photospheric motion whereas energy stored in the open-field region is derived from the non-thermal energy flux which heats the corona and drives the solar wind.The flare itself is identified with reconnection of magnetic field by the tearing-mode resistive instability. If the thickness of the sheet pinch is determined by resistive diffusion and a growth time of the bipolar region of order 1 day, the transverse dimension will be about 104 cm. The rise time of the tearing-mode instability is then a few seconds, compatible with the characteristic time of Type-III radio bursts. One can understand that the time-scale of the reconnection process is of order 102–103 sec if reconnection proceeds by the Petscheck mechanism, with the modification that resistive diffusion is replaced by the more rapid Bohm diffusion.The evolution of a flare, according to this model, appears to fit a number of the observational characteristics of flares.

scholarly journals Instabilities of Magnetic Fields in Stars

1974 ◽  
Vol 59 ◽  
pp. 177-177
Author(s):  
R. J. Tayler
Keyword(s):  
Magnetic Field ◽  
Gravitational Potential ◽  
Rotation Period ◽  
Region Of Interest ◽  
Alfven Wave ◽  
Closed Field ◽  
Toroidal Plasma ◽  
Wide Range ◽  
Field Lines ◽  
Rotating Star

It has been shown (Markey and Tayler, 1973; Tayler, 1973; Wright, 1973) that a wide range of simple magnetic field configurations in stars are unstable. Although the ultimate effect of the instabilities is unclear, it seems likely that they would lead to enhanced destruction of magnetic flux, so that magnetic field decay would be much more rapid than previously supposed. Instability is almost certain in a non-rotating star containing either a purely toroidal field or a purely poloidal field, which has closed field lines inside the star. In both cases the instability resembles the well known instabilities of cylindrical and toroidal current channels, modified by the constraint that motion must be almost entirely along surfaces of constant gravitational potential.If both toroidal and poloidal fields are present, the problem is more complicated. In a toroidal plasma with a helical field, the worst instabilities are also helical but it is impossible for a helical disturbance to be parallel to a surface of constant gravitational potential everywhere. As a result, the admixture of toroidal and poloidal fields has a stabilizing influence, but it is not at present clear whether the majority of such configurations are completely stable.The effect of rotation has not yet been studied but it will certainly be important if the rotation period is less than the time taken for an Alfvén wave to cross the region of interest. This is true in most stars unless the internal magnetic field is very much stronger than any observed field.

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.

Examining the optical intensity and magnetic field expansion factor in the open magnetic field regions associated with coronal holes

2019 ◽  
Vol 15 (S354) ◽  
pp. 228-231
Author(s):  
Chia-Hsien Lin ◽  
Guan-Han Huang ◽  
Lou-Chuang Lee
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Coronal Holes ◽  
Closed Field ◽  
Logarithmic Scale ◽  
Expansion Factor ◽  
Astrophysical Journal ◽  
Field Lines ◽  
The Relationship ◽  
Open Magnetic Field

AbstractCoronal holes can be identified as the darkest regions in EUV or soft X-ray images with predominantly unipolar magnetic fields (LIRs) or as the regions with open magnetic fields (OMF). Our study reveals that only 12% of OMF regions are coincident with LIRs. The aim of this study is to investigate the conditions that affect the EUV intensity of OMF regions. Our results indicate that the EUV intensity and the magnetic field expansion factor of the OMF regions are weakly positively correlated when plotted in logarithmic scale, and that the bright OMF regions are likely to locate inside or next to the regions with closed field lines. We empirically determined a linear relationship between the expansion factor and the EUV intensity. The relationship is demonstrated to improve the consistency from 12% to 23%. The results have been published in Astrophysical Journal (Huang et al. 2019).

Energy Conversion in the Electron Stagnation Region of Magnetopause Reconnection

2020 ◽  
Author(s):  
James Burch ◽  
James Webster ◽  
Kristina Pritchard ◽  
Kevin Genestreti ◽  
Michael Hesse ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Energy Conversion ◽  
Closed Field ◽  
Stagnation Region ◽  
Strong Electron ◽  
The Earth ◽  
Background Magnetic Field ◽  
Out Of Plane ◽  
Uniform Background ◽  
Field Lines

<p>For reconnection at the Earth’s day side, which is asymmetric, the main energy conversion occurs on closed field lines in the electron stagnation region. Energy conversion, as measured by <strong>J</strong>⦁<strong>E</strong>, occurs where out-of-plane electric field components are embedded within larger regions of out-of-plane current, which is carried by strong electron flows in the M direction of the LMN coordinate system. Bracketing these energy conversion sites are electron jet reversals (along L and -L) and converging  electron flows (along N and -N). These electron flows are like those that surround reconnection X lines, however, in these cases they occur completely within closed field lines. The question then is what, if anything, this energy conversion has to do with local reconnection of magnetic field lines. This paper reports on a study of two events observed by MMS on December 29, 2016 and April 15, 2018. The electron inflows have velocities between 0.05 V<sub>eA</sub> and 0.1 V<sub>eA</sub>, (V<sub>eA</sub> = electron Alfvén speed), which are consistent with predicted reconnection rates. Laboratory measurements and 3D simulation results offer some clues about how reconnecting current sheets can evolve in a uniform background magnetic field.</p>

scholarly journals On large plasmoid formation in a global magnetohydrodynamic simulation

2011 ◽  
Vol 29 (1) ◽  
pp. 167-179 ◽  
Author(s):  
I. Honkonen ◽  
M. Palmroth ◽  
T. I. Pulkkinen ◽  
P. Janhunen ◽  
A. Aikio
Keyword(s):  
Magnetic Field ◽  
Field Line ◽  
Operational Definition ◽  
Closed Field ◽  
Ae Index ◽  
Angular Change ◽  
Line Region ◽  
Field Lines ◽  
Fully Connected ◽  
The Imf

Abstract. We investigate plasmoid formation in the magnetotail using the global magnetohydrodynamic (MHD) simulation GUMICS-4. Here a plasmoid implies a major reconfiguration of the magnetotail where a part of the tail plasma sheet is ejected downstream, in contrast to small Earthward-propagating plasmoids. We define a plasmoid based solely on the structure of the closed (connected to the Earth at both ends) magnetic field line region. In this definition a plasmoid is partly separated from the ordinary closed field line region by lobe field lines or interplanetary field lines resulting from lobe reconnection. We simulate an event that occurred on 18 February 2004 during which four intensifications of the auroral electroject (AE) index occurred in 8 h. Plasmoids form in the simulation for two of the four AE intensifications. Each plasmoid forms as a result of two consecutive large and fast rotations of the interplanetary magnetic field (IMF). In both cases the IMF rotates 180 degrees at 10 degrees per minute, first from southward to northward and some 15 min later from northward to southward. The other two AE intencifications however are not associated with a plasmoid formation. A plasmoid does not form if either the IMF rotation speed or the angular change of the rotation are small. We also present an operational definition for these fully connected plasmoids that enables their automatic detection in simulations. Finally, we show mappings of the plasmoid footpoints in the ionosphere, where they perturb the polar cap boundary in both hemispheres.

scholarly journals Stable Knots and Links in Electromagnetic Fields

2021 ◽  
Author(s):  
Benjamin Bode
Keyword(s):  
Magnetic Field ◽  
Electromagnetic Fields ◽  
Quantum Fluids ◽  
Closed Field ◽  
Special Role ◽  
Elastic Filaments ◽  
Legendrian Links ◽  
Field Lines ◽  
Standard Contact ◽  
The Magnetic Field

AbstractPersistent topological structures in physical systems have become increasingly important over the last years. Electromagnetic fields with knotted field lines play a special role among these, since they can be used to transfer their knottedness to other systems like plasmas and quantum fluids. In null electromagnetic fields the electric and the magnetic field lines evolve like unbreakable elastic filaments in a fluid flow. In particular, their topology is preserved for all time, so that all knotted closed field lines maintain their knot type. We use an approach due to Bateman to prove that for every link L there is such an electromagnetic field that satisfies Maxwell’s equations in free space and that has closed electric and magnetic field lines in the shape of L for all time. The knotted and linked field lines turn out to be projections of real analytic Legendrian links with respect to the standard contact structure on the 3-sphere.

Acoustic End Effects in Magnetohydrodynamic Submerged Vehicular Propulsors

1992 ◽  
Vol 36 (01) ◽  
pp. 69-76
Author(s):  
John S. Walker ◽  
Gita Talmage ◽  
Samuel H. Brown ◽  
Neal A. Sondergaard
Keyword(s):  
Magnetic Field ◽  
Inertial Force ◽  
Zero Field ◽  
Uniform Field ◽  
Field Region ◽  
Characteristic Ratio ◽  
Fringing Field ◽  
Field Lines ◽  
Channel Configuration ◽  
The Magnetic Field

This paper treats the effects near the ends of the channel on the transmission and reflection of periodic acoustic waves generated at some cross section inside a magnetohydrodynamic (MHD) seawater propulsion system. A region of high uniform magnetic field inside the MHD submerged vehicular propulsor is separated from the essentially zero magnetic field outside the channel by a nonuniform, fringing magnetic field at each end of the channel. The channel configuration chosen here is that of a straight, rectangular duct with electrically insulating top and bottom walls perpendicular to the magnetic field and highly conducting sidewalls parallel to the field. In particular, the mathematical analysis focuses on determining the percentage of the incident wave which is reflected by the fringing-field region back into the uniform-field region and the percentage which is transmitted through the fringing-field region into the zero-field region. The key parameter is the acoustic interaction parameter N, which is the characteristic ratio of the electromagnetic body force opposing motions across magnetic field lines to the inertial "force" in the acoustic wave. Solutions are presented for the fundamental, plane acoustic mode for arbitrary values of Ν and for all acoustic modes for Ν < 1. The amplitudes of the reflected and transmitted waves depend on the wave frequency, the length of the fringing-field region, N, and the type of wave mode. The magnetic field introduces a strong anisotropy with strong damping of modes involving transverse motions across magnetic field lines and with weak damping of modes involving transverse motions along field lines. This is the third in a series of articles on MHD marine propulsion from the David Taylor Research Center MHD propulsion program [Brown et al (1990), Tempelmeyer (1990)].

Examination of the EUV Intensity in the Open Magnetic Field Regions Associated with Coronal Holes

2020 ◽  
Author(s):  
Guan-Han Huang ◽  
Chia-Hsien Lin ◽  
Lou Chuang Lee
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Order Approximation ◽  
Coronal Holes ◽  
Closed Field ◽  
Spatial And Temporal Distributions ◽  
Field Lines ◽  
First Order Approximation ◽  
Log Normal ◽  
Open Magnetic Field

<p>Coronal holes can be identified as the regions with magnetic field lines extending far away from the Sun, or the darkest regions in EUV/X-ray images with predominantly unipolar magnetic fields. A comparison between the locations of our determined regions with open magnetic field lines (OMF) and regions with low EUV intensity (LIR) reveals that only 12% of the OMF regions coincide with the LIRs. The aim of this study is to investigate the conditions leading to the different brightnesses of OMF regions, and to provide a means to predict whether an OMF region would be bright or dark. Examining the statistical distribution profiles of the magnetic field expansion factor (f<sub>s</sub>) and Atmospheric Imaging Assembly 193 Å intensity (I<sub>193</sub>) reveals that both profiles are approximately log-normal. The analysis of the spatial and temporal distributions of f<sub>s</sub> and I<sub>193</sub> indicates that the bright OMF regions often are inside or next to regions with closed field lines, including quiet-Sun regions and regions with strong magnetic fields. Examining the relationship between I<sub>193</sub> and f<sub>s</sub> reveals a weak positive correlation between log I<sub>193</sub> and log f<sub>s</sub> , with a correlation coefficient ≈ 0.39. As a first-order approximation, the positive relationship is determined to be log I<sub>193</sub> = 0.62 log f<sub>s</sub> + 1.51 based on the principle of the whitening/dewhitening transformation. This linear relationship is demonstrated to increase the consistency between the OMF regions and LIRs from 12% to 23%.</p>

scholarly journals Magnetospheric magnetic field modelling for the 2011 and 2012 HST Saturn aurora campaigns – implications for auroral source regions

2014 ◽  
Vol 32 (6) ◽  
pp. 689-704 ◽  
Author(s):  
E. S. Belenkaya ◽  
S. W. H. Cowley ◽  
C. J. Meredith ◽  
J. D. Nichols ◽  
V. V. Kalegaev ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Ring Current ◽  
Hubble Space Telescope ◽  
Current System ◽  
Outer Edge ◽  
Local Times ◽  
Closed Field ◽  
Principal Connection ◽  
Magnetic Field Model ◽  
Field Lines

Abstract. A unique set of images of Saturn's northern polar UV aurora was obtained by the Hubble Space Telescope in 2011 and 2012 at times when the Cassini spacecraft was located in the solar wind just upstream of Saturn's bow shock. This rare situation provides an opportunity to use the Kronian paraboloid magnetic field model to examine source locations of the bright auroral features by mapping them along field lines into the magnetosphere, taking account of the interplanetary magnetic field (IMF) measured near simultaneously by Cassini. It is found that the persistent dawn arc maps to closed field lines in the dawn to noon sector, with an equatorward edge generally located in the inner part of the ring current, typically at ~ 7 Saturn radii (RS) near dawn, and a poleward edge that maps variously between the centre of the ring current and beyond its outer edge at ~ 15 RS, depending on the latitudinal width of the arc. This location, together with a lack of response in properties to the concurrent IMF, suggests a principal connection with ring-current and nightside processes. The higher-latitude patchy auroras observed intermittently near to noon and at later local times extending towards dusk are instead found to straddle the model open–closed field boundary, thus mapping along field lines to the dayside outer magnetosphere and magnetopause. These emissions, which occur preferentially for northward IMF directions, are thus likely associated with reconnection and open-flux production at the magnetopause. One image for southward IMF also exhibits a prominent patch of very high latitude emissions extending poleward of patchy dawn arc emissions in the pre-noon sector. This is found to lie centrally within the region of open model field lines, suggesting an origin in the current system associated with lobe reconnection, similar to that observed in the terrestrial magnetosphere for northward IMF.

scholarly journals Overlapping ion structures in the mid-altitude cusp under northward IMF: signature of dual lobe reconnection?

2012 ◽  
Vol 30 (3) ◽  
pp. 489-501 ◽  
Author(s):  
F. Pitout ◽  
C. P. Escoubet ◽  
M. G. G. T. Taylor ◽  
J. Berchem ◽  
A. P. Walsh
Keyword(s):  
Magnetic Field ◽  
Interplanetary Magnetic Field ◽  
Low Energy ◽  
Closed Field ◽  
Opposite Hemisphere ◽  
Cluster Ion ◽  
Low Energy Tail ◽  
Field Lines ◽  
Lobe Reconnection ◽  
Cooling Model

Abstract. On some rare occasions, data from the Cluster Ion Spectrometer (CIS) in the mid-altitude cusp reveal overlapping ion populations under northward interplanetary magnetic field (IMF). While the poleward part of the cusp exhibits the expected reverse dispersion due to lobe reconnection, its equatorward part shows a second ion population at higher-energy that coexists with the low energy tail of the dispersion. This second population is either dispersionless or slightly dispersed with energies increasing with increasing latitudes, indicative of lobe reconnection as well. Our analysis of a case that occurred 3 September 2002 when the IMF stayed northward for more than two hours suggests that the second population comes from the opposite hemisphere and is very likely on newly-closed field lines. We interpret this overlap of cusp populations as a clear mid-altitude signature of re-closed magnetic field lines by double lobe reconnection (reconnection in both hemispheres) under northward IMF. This interpretation is supported by modelling performed with the Cooling model and an MHD model.

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