scholarly journals Overshoot dependence on the cross-shock potential

2020 ◽  
Vol 38 (1) ◽  
pp. 17-26 ◽  
Author(s):  
Michael Gedalin ◽  
Xiaoyan Zhou ◽  
Christopher T. Russell ◽  
Vassilis Angelopoulos
Keyword(s):  
Magnetic Field ◽  
Critical Reflection ◽  
Pressure Ratio ◽  
Magnetic Pressure ◽  
Distribution Center ◽  
Ion Temperature ◽  
Magnetic Field Increase ◽  
The Cross ◽  
Shock Profile ◽  
The Magnetic Field

Abstract. Coherent downstream oscillations of the magnetic field in shocks are produced due to the coherent ion gyration and quasiperiodic variations of the ion pressure. The amplitude and the positions of the pressure maxima and minima depend on the cross-shock potential and upstream ion temperature. Two critical cross-shock potentials are defined: the critical gyration potential (CGP), which separates the cases of increase or decrease in the component of the velocity of the distribution center along the shock normal, and the critical reflection potential (CRP), above which ion reflection becomes significant. In a weak, very low upstream kinetic-to-magnetic pressure ratio, β, the shocks' CRP exceeds the CGP. For potentials below the CGP, the first downstream maximum of the magnetic field is shifted farther downstream and is larger than the second maximum. For higher potentials, the first maximum occurs just behind the ramp and is lower than the second maximum. With the increase in the upstream temperature, the CGP exceeds the CRP. For potentials below the CRP, the effects of ion reflection are negligible and the shock profile is similar to that of very low-β shocks. If the potential exceeds the CRP, ion reflection is significant, the magnetic field increase toward the overshoot becomes steeper, and the largest peak occurs at the downstream edge of the ramp.

scholarly journals Overshoot dependence on the cross-shock potential

2019 ◽  
Author(s):  
Michael Gedalin ◽  
Xiaoyan Zhou ◽  
Christopher T. Russell ◽  
Vassilis Angelopoulos
Keyword(s):  
Magnetic Field ◽  
Critical Reflection ◽  
Distribution Center ◽  
Normal Velocity ◽  
Ion Temperature ◽  
Field Increase ◽  
Magnetic Field Increase ◽  
The Cross ◽  
Shock Profile ◽  
The Magnetic Field

Abstract. Coherent downstream oscillations of the magnetic field in shocks are produced due to the coherent ion gyration and quasi-periodic variations of the ion pressure. The amplitude and the positions of the pressure maxima and minima depend on the cross-shock potential and upstream ion temperature. Two critical potentials are defined: the critical gyration potential (CGP) which separates the cases of increase or decrease of the normal velocity of the distribution center, and the critical reflection potential (CRP) above which ion reflection becomes significant. In weak very low β shocks CRP exceeds CGP. For potentials below CGP the first downstream maximum of the magnetic field is shifted farther downstream and is larger than the second one. For higher potentials the first maximum occurs just behind the ramp and is lower than the second one. With the increase of the upstream temperature CGP exceeds the CRP. For potentials below CRP the effects of ion reflection are negligible and the shock profile is similar to that of very low β shocks. If the potential exceeds CRP ion reflection is significant, the magnetic field increase toward the overshoot becomes steeper, and the largest peak occurs at the downstream edge of the ramp.

scholarly journals The effects of magnetic field strength on the properties of wind generated from hot accretion flow

2018 ◽  
Vol 615 ◽  
pp. A35 ◽  
Author(s):  
De-Fu Bu ◽  
Amin Mosallanezhad
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Pressure Gradient ◽  
Magnetic Field Strength ◽  
Magnetic Pressure ◽  
Gas Pressure ◽  
Accretion Flow ◽  
Accretion Flows ◽  
Black Hole Accretion ◽  
The Magnetic Field

Context. Observations indicate that wind can be generated in hot accretion flow. Wind generated from weakly magnetized accretion flow has been studied. However, the properties of wind generated from strongly magnetized hot accretion flow have not been studied. Aims. In this paper, we study the properties of wind generated from both weakly and strongly magnetized accretion flow. We focus on how the magnetic field strength affects the wind properties. Methods. We solve steady-state two-dimensional magnetohydrodynamic equations of black hole accretion in the presence of a largescale magnetic field. We assume self-similarity in radial direction. The magnetic field is assumed to be evenly symmetric with the equatorial plane. Results. We find that wind exists in both weakly and strongly magnetized accretion flows. When the magnetic field is weak (magnetic pressure is more than two orders of magnitude smaller than gas pressure), wind is driven by gas pressure gradient and centrifugal forces. When the magnetic field is strong (magnetic pressure is slightly smaller than gas pressure), wind is driven by gas pressure gradient and magnetic pressure gradient forces. The power of wind in the strongly magnetized case is just slightly larger than that in the weakly magnetized case. The power of wind lies in a range PW ~ 10−4–10−3 Ṁinc2, with Ṁin and c being mass inflow rate and speed of light, respectively. The possible role of wind in active galactic nuclei feedback is briefly discussed.

scholarly journals Mathematical Theory of Cylindrical Isothermal Blast Waves in a Magnetic Field

1981 ◽  
Vol 34 (3) ◽  
pp. 279 ◽  
Author(s):  
I Lerche
Keyword(s):  
Magnetic Field ◽  
Supernova Remnants ◽  
Magnetic Pressure ◽  
Blast Waves ◽  
Physical Domain ◽  
Fluid Flow Velocity ◽  
Magnetic Tension ◽  
Self Similar ◽  
The Magnetic Field ◽  
Similar Flows

An investigation is made of the self-similar flow behind a cylindrical blast wave from a line explosion (situated on r = 0, using conventional cylindrical coordinates r, 4>, z) in a medium whose density and magnetic field both vary as r -w ahead of the blast front, with the assumption that the flow is isothermal. The magnetic field can have components in both the azimuthal B(jJ and longitudinal B, directions. It is found that: (i) For B(jJ =f:. 0 =f:. B, a continuous single-valued solution with a velocity field representing outflow of material away from the line of explosion does not exist for OJ OJ > 0 the governing equation possesses a set of movable critical points. In this case it is shown that the fluid flow velocity is bracketed between two curves and that the asymptotes of the velocity curve on the shock are intersected by, or are tangent to, the two curves. Thus a solution always exists in the physical domain r ~ o. The overall conclusion from the investigation is that the behaviour of isothermal blast waves in the presence of an ambient magnetic field differs substantially from the behaviour calculated for no magnetic field. These results have an impact upon previous applications of the theory of self-similar flows to evolving supernova remnants without allowance for the dynamical influence of magnetic pressure and magnetic tension.

scholarly journals Dependence on ion temperature of shallow-angle magnetic presheaths with adiabatic electrons

2019 ◽  
Vol 85 (6) ◽  
Author(s):  
Alessandro Geraldini ◽  
F. I. Parra ◽  
F. Militello
Keyword(s):  
Magnetic Field ◽  
Fluid Model ◽  
Boltzmann Distribution ◽  
Distribution Functions ◽  
Magnetized Plasma ◽  
Electron Mass ◽  
Ion Temperature ◽  
Ion Distribution ◽  
Ionic Charge ◽  
The Magnetic Field

The magnetic presheath is a boundary layer occurring when magnetized plasma is in contact with a wall and the angle $\unicode[STIX]{x1D6FC}$ between the wall and the magnetic field $\boldsymbol{B}$ is oblique. Here, we consider the fusion-relevant case of a shallow-angle, $\unicode[STIX]{x1D6FC}\ll 1$ , electron-repelling sheath, with the electron density given by a Boltzmann distribution, valid for $\unicode[STIX]{x1D6FC}/\sqrt{\unicode[STIX]{x1D70F}+1}\gg \sqrt{m_{\text{e}}/m_{\text{i}}}$ , where $m_{\text{e}}$ is the electron mass, $m_{\text{i}}$ is the ion mass, $\unicode[STIX]{x1D70F}=T_{\text{i}}/ZT_{\text{e}}$ , $T_{\text{e}}$ is the electron temperature, $T_{\text{i}}$ is the ion temperature and $Z$ is the ionic charge state. The thickness of the magnetic presheath is of the order of a few ion sound Larmor radii $\unicode[STIX]{x1D70C}_{\text{s}}=\sqrt{m_{\text{i}}(ZT_{\text{e}}+T_{\text{i}})}/ZeB$ , where e is the proton charge and $B=|\boldsymbol{B}|$ is the magnitude of the magnetic field. We study the dependence on $\unicode[STIX]{x1D70F}$ of the electrostatic potential and ion distribution function in the magnetic presheath by using a set of prescribed ion distribution functions at the magnetic presheath entrance, parameterized by $\unicode[STIX]{x1D70F}$ . The kinetic model is shown to be asymptotically equivalent to Chodura’s fluid model at small ion temperature, $\unicode[STIX]{x1D70F}\ll 1$ , for $|\text{ln}\,\unicode[STIX]{x1D6FC}|>3|\text{ln}\,\unicode[STIX]{x1D70F}|\gg 1$ . In this limit, despite the fact that fluid equations give a reasonable approximation to the potential, ion gyro-orbits acquire a spatial extent that occupies a large portion of the magnetic presheath. At large ion temperature, $\unicode[STIX]{x1D70F}\gg 1$ , relevant because $T_{\text{i}}$ is measured to be a few times larger than $T_{\text{e}}$ near divertor targets of fusion devices, ions reach the Debye sheath entrance (and subsequently the wall) at a shallow angle whose size is given by $\sqrt{\unicode[STIX]{x1D6FC}}$ or $1/\sqrt{\unicode[STIX]{x1D70F}}$ , depending on which is largest.

Kinetic Alfvén solitons in a low-beta plasma

1997 ◽  
Vol 57 (2) ◽  
pp. 235-245 ◽  
Author(s):  
B. C. KALITA ◽  
R. P. BHATTA
Keyword(s):  
Magnetic Field ◽  
Mach Number ◽  
Solitary Waves ◽  
Magnetic Pressure ◽  
Hot Electrons ◽  
Mass Ratio ◽  
Ion Motion ◽  
Electron Inertia ◽  
Theoretical Investigations ◽  
The Magnetic Field

Kinetic Alfvén solitons with hot electrons and finite electron inertia in a low-beta (β=8πn0T/B2G, the ratio of the kinetic to the magnetic pressure) plasma is studied analytically, with the ion motion being considered dominant through the polarization drift. Both compressive and rarefactive kinetic Alfvén solitons are found to exist within a definite range of kz (the direction of propagation of the kinetic Alfvén solitary waves with respect to the direction of the magnetic field) for each pair of assigned values of β and M (Mach number). Unlike in previous theoretical investigations, β appears as an explicit parameter for the kinetic Alfvén solitons in this case. In addition, consideration of the electron pressure gradient is found to suppress the speed of both the Alfvén solitons considerably for A (=2QM2/βk2z, with Q the electron-to-ion mass ratio) less than unity.

scholarly journals Magnetic fields, stellar feedback, and the geometry of H II regions

2008 ◽  
Vol 4 (S259) ◽  
pp. 25-34
Author(s):  
Gary J. Ferland
Keyword(s):  
Magnetic Field ◽  
Magnetic Pressure ◽  
H Ii Regions ◽  
Line Profiles ◽  
Ionized Gas ◽  
Stellar Feedback ◽  
H Ii Region ◽  
Field Lines ◽  
The Magnetic Field ◽  
Simple Picture

AbstractMagnetic pressure has long been known to dominate over gas pressure in atomic and molecular regions of the interstellar medium. Here I review several recent observational studies of the relationships between the H+, H0 and H2 regions in M42 (the Orion complex) and M17. A simple picture results. When stars form they push back surrounding material, mainly through the outward momentum of starlight acting on grains, and field lines are dragged with the gas due to flux freezing. The magnetic field is compressed and the magnetic pressure increases until it is able to resist further expansion and the system comes into approximate magnetostatic equilibrium. Magnetic field lines can be preferentially aligned perpendicular to the long axis of quiescent cloud before stars form. After star formation and pushback occurs ionized gas will be constrained to flow along field lines and escape from the system along directions perpendicular to the long axis. The magnetic field may play other roles in the physics of the H II region and associated PDR. Cosmic rays may be enhanced along with the field and provide additional heating of atomic and molecular material. Wave motions may be associated with the field and contribute a component of turbulence to observed line profiles.

scholarly journals The Effect of a Magnetic Field on Stellar Pulsations as a Singular Perturbation Problem

1980 ◽  
Vol 58 ◽  
pp. 661-666
Author(s):  
M. Goossens ◽  
D. Biront
Keyword(s):  
Magnetic Field ◽  
Magnetic Pressure ◽  
Matched Asymptotic Expansion ◽  
Perturbation Scheme ◽  
Regular Perturbation ◽  
Perturbation Problem ◽  
Stellar Pulsations ◽  
Thermodynamic Pressure ◽  
The Magnetic Field ◽  
Adiabatic Oscillation

AbstractThe perturbation problem that describes the effect of a weak magnetic field on stellar adiabatic oscillation is considered. This perturbation problem is singular when the magnetic field does not vanish at the stellar surface, and a regular perturbation scheme fails where the magnetic pressure is comparable to the thermodynamic pressure. The application of the Method of Matched Asymptotic Expansion is used to obtain expressions for the eigenfunctions and the eigenfrequencies.

scholarly journals Global properties of magnetotail current sheet flapping: THEMIS perspectives

2009 ◽  
Vol 27 (1) ◽  
pp. 319-328 ◽  
Author(s):  
A. Runov ◽  
V. Angelopoulos ◽  
V. A. Sergeev ◽  
K.-H. Glassmeier ◽  
U. Auster ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Current Sheet ◽  
Growth Phase ◽  
Timing Analysis ◽  
Wave Length ◽  
Global Properties ◽  
The Cross ◽  
Magnetic Field Variations ◽  
The Magnetic Field ◽  
Magnetotail Current Sheet

Abstract. A sequence of magnetic field oscillations with an amplitude of up to 30 nT and a time scale of 30 min was detected by four of the five THEMIS spacecraft in the magnetotail plasma sheet. The probes P1 and P2 were at X=−15.2 and −12.7 RE and P3 and P4 were at X=−7.9 RE. All four probes were at −6.5>Y>−7.5 RE (major conjunction). Multi-point timing analysis of the magnetic field variations shows that fronts of the oscillations propagated flankward (dawnward and Earthward) nearly perpendicular to the direction of the magnetic maximum variation (B1) at velocities of 20–30 km/s. These are typical characteristics of current sheet flapping motion. The observed anti-correlation between ∂B1/∂t and the Z-component of the bulk velocity make it possible to estimate a flapping amplitude of 1 to 3 RE. The cross-tail scale wave-length was found to be about 5 RE. Thus the flapping waves are steep tail-aligned structures with a lengthwise scale of >10 RE. The intermittent plasma motion with the cross-tail velocity component changing its sign, observed during flapping, indicates that the flapping waves were propagating through the ambient plasma. Simultaneous observations of the magnetic field variations by THEMIS ground-based magnetometers show that the flapping oscillations were observed during the growth phase of a substorm.

scholarly journals Magnetic relaxation and the Taylor conjecture

2015 ◽  
Vol 81 (6) ◽  
Author(s):  
H. K. Moffatt
Keyword(s):  
Magnetic Field ◽  
Initial Phase ◽  
Magnetic Relaxation ◽  
Magnetic Energy ◽  
Magnetic Pressure ◽  
Second Phase ◽  
One Dimensional ◽  
Induction Equation ◽  
Two Phases ◽  
The Magnetic Field

A one-dimensional model of magnetic relaxation in a pressureless low-resistivity plasma is considered. The initial two-component magnetic field $\boldsymbol{b}(\boldsymbol{x},t)$ is strongly helical, with non-uniform helicity density. The magnetic pressure gradient drives a velocity field that is dissipated by viscosity. Relaxation occurs in two phases. The first is a rapid initial phase in which the magnetic energy drops sharply and the magnetic pressure becomes approximately uniform; the helicity density is redistributed during this phase but remains non-uniform, and although the total helicity remains relatively constant, a Taylor state is not established. The second phase is one of slow diffusion, in which the velocity is weak, though still driven by persistent weak non-uniformity of magnetic pressure; during this phase, magnetic energy and helicity decay slowly and at constant ratio through the combined effects of pressure equalisation and finite resistivity. The density field, initially uniform, develops rapidly (in association with the magnetic field) during the initial phase, and continues to evolve, developing sharp maxima, throughout the diffusive stage. Finally it is proved that, if the resistivity is zero, the spatial mean $\langle (\boldsymbol{b}\boldsymbol{\cdot }\boldsymbol{{\rm\nabla}}\times \boldsymbol{b})/b^{2}\rangle$ is an invariant of the governing one-dimensional induction equation.

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