Solar particles and the dayside limit of closed field lines

Abstract A class of stability criteria is derived for MHD equilibria with closed field lines. The destabilizing perturbations have finite gradients along the field and are localized around a field line, the localiza-tion being stronger on the pressure surface than in the radial direction. By contrast, in sheared configurations the localization is comparable in both directions. The derived stability criteria are less stringent than those obtained for MHD equilibria with shear for similarly localized perturbations in the limit of low shear. These results, obtained from the energy principle, are a particular case of those obtained by solving the linearized resistive MHD equations with an appropriate ansatz and subsequently taking the limit of vanishing shear and resistivity.

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Instabilities of Magnetic Fields in Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900237467 ◽

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.

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Study of the thermal plasma on closed field lines outside the plasmasphere

Planetary and Space Science ◽

10.1016/0032-0633(70)90206-0 ◽

1970 ◽

Vol 18 (8) ◽

pp. 1123-1135 ◽

Cited By ~ 19

Author(s):

H.G. Mayr ◽

J.M. Grebowsky ◽

H.A. Taylor

Keyword(s):

Thermal Plasma ◽

Closed Field ◽

Field Lines

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Dynamical critical scaling of electric field fluctuations in the greater cusp and magnetotail implied by HF radar observations of F-region Doppler velocity

Annales Geophysicae ◽

10.5194/angeo-24-689-2006 ◽

2006 ◽

Vol 24 (2) ◽

pp. 689-705 ◽

Cited By ~ 17

Author(s):

M. L. Parkinson

Keyword(s):

Solar Wind ◽

Open Field ◽

Electric Fields ◽

Spectral Width ◽

Hf Radar ◽

Doppler Velocity ◽

Closed Field ◽

Scaling Exponent ◽

Central Plasma ◽

Field Lines

Abstract. Akasofu's solar wind ε parameter describes the coupling of solar wind energy to the magnetosphere and ionosphere. Analysis of fluctuations in ε using model independent scaling techniques including the peaks of probability density functions (PDFs) and generalised structure function (GSF) analysis show the fluctuations were self-affine (mono-fractal, single exponent scaling) over 9 octaves of time scale from ~46 s to ~9.1 h. However, the peak scaling exponent α0 was a function of the fluctuation bin size, so caution is required when comparing the exponents for different data sets sampled in different ways. The same generic scaling techniques revealed the organisation and functional form of concurrent fluctuations in azimuthal magnetospheric electric fields implied by SuperDARN HF radar measurements of line-of-sight Doppler velocity, vLOS, made in the high-latitude austral ionosphere. The PDFs of vLOS fluctuation were calculated for time scales between 1 min and 256 min, and were sorted into noon sector results obtained with the Halley radar, and midnight sector results obtained with the TIGER radar. The PDFs were further sorted according to the orientation of the interplanetary magnetic field, as well as ionospheric regions of high and low Doppler spectral width. High spectral widths tend to occur at higher latitude, mostly on open field lines but also on closed field lines just equatorward of the open-closed boundary, whereas low spectral widths are concentrated on closed field lines deeper inside the magnetosphere. The vLOS fluctuations were most self-affine (i.e. like the solar wind ε parameter) on the high spectral width field lines in the noon sector ionosphere (i.e. the greater cusp), but suggested multi-fractal behaviour on closed field lines in the midnight sector (i.e. the central plasma sheet). Long tails in the PDFs imply that "microbursts" in ionospheric convection occur far more frequently, especially on open field lines, than can be captured using the effective Nyquist frequency and volume resolution of SuperDARN radars.

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Cluster observations of ELF/VLF signals generated by modulated heating of the lower ionosphere with the HAARP HF transmitter

Annales Geophysicae ◽

10.5194/angeo-22-2643-2004 ◽

2004 ◽

Vol 22 (7) ◽

pp. 2643-2653 ◽

Cited By ~ 14

Author(s):

M. Platino ◽

U. S. Inan ◽

T. F. Bell ◽

J. Pickett ◽

E. J. Kennedy ◽

...

Keyword(s):

High Frequency ◽

Lower Ionosphere ◽

Spectral Signature ◽

Closed Field ◽

Ionospheric Heating ◽

Higher Power ◽

Power Operation ◽

Field Lines ◽

Vlf Waves

Abstract. It is now well known that amplitude modulated HF transmissions into the ionosphere can be used to generate ELF/VLF signals using the so-called "electrojet antenna". Although most observations of the generated ELF/VLF signals have been made on the ground, several low and high-altitude satellite observations have also been reported (James et al., 1990). One of the important unknowns in the physics of ELF/VLF wave generation by ionospheric heating is the volume of the magnetosphere illuminated by the ELF/VLF waves. In an attempt to investigate this question further, ground-satellite conjunction experiments have recently been conducted using the four Cluster satellites and the HF heater of the High-Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska. Being located on largely closed field lines at L≈4.9, HAARP is currently also being used for ground-to-ground type of ELF/VLF wave-injection experiments, and will be increasingly used for this purpose as it is now being upgraded for higher power operation. In this paper, we describe the HAARP installation and present recent results of the HAARP-Cluster experiments. We give an overview of the detected ELF/VLF signals at Cluster, and a possible explanation of the spectral signature detected, as well as the determination of the location of the point of injection of the HAARP ELF/VLF signals into the magnetosphere using ray tracing.

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Differential rotation in neutron stars with open and closed magnetic topologies

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa915 ◽

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.

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

Proceedings of the International Astronomical Union ◽

10.1017/s1743921320000393 ◽

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).

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Transport of energetic solar particles on closed magnetospheric field lines

Space Science Reviews ◽

10.1007/bf00718836 ◽

1975 ◽

Vol 17 (1) ◽

pp. 3-44 ◽

Cited By ~ 16

Author(s):

Manfred Scholer

Keyword(s):

Solar Particles ◽

Field Lines ◽

Magnetospheric Field

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Energy Conversion in the Electron Stagnation Region of Magnetopause Reconnection

10.5194/egusphere-egu2020-3757 ◽

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

For reconnection at the Earth&#8217;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 J&#10625;E, 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 &#160;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 VeA and 0.1 VeA, (VeA = electron Alfv&#233;n 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.

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