scholarly journals Dynamical Effects in Solar Photospheric Flux Tubes

1996 ◽  
Vol 154 ◽  
pp. 155-158
Author(s):  
S.S. Hasan
Keyword(s):  
Flux Tube ◽  
Ambient Medium ◽  
Resonant Interaction ◽  
Scale Height ◽  
Mhd Equations ◽  
Time Dependent ◽  
Radiative Transport ◽  
Local Pressure ◽  
Flux Tubes ◽  
Pressure Scale

AbstractThe interaction of an intense flux tube, extending vertically through the photosphere, with p-modes in the ambient medium is modelled by solving the time dependent MHD equations in the thin flux tube approximation. It is found that a resonant interaction can occur, which leads to the excitation of flux tube oscillations with large amplitudes. The resonance is not as sharp as in the case of an unstratified atmosphere, but is broadened by a factor proportional to H−2, where H is the local pressure scale height. In addition, the inclusion of radiative transport leads to a decrease in the amplitude of the oscillations, but does not qualitatively change the nature of the interaction.

scholarly journals Time-Dependent Convective Collapse of Flux Tubes

1983 ◽  
Vol 102 ◽  
pp. 73-77
Author(s):  
S. Sirajul Hasan
Keyword(s):  
Flux Tube ◽  
Convection Zone ◽  
Temporal Evolution ◽  
Dynamic Equilibrium ◽  
Mhd Equations ◽  
Time Dependent ◽  
Initial State ◽  
Flux Tubes ◽  
Initial Epoch ◽  
Flow Variables

The time-dependent collapse of a slender flux tube extending vertically in the convection zone of the Sun is modelled. Starting from an initial state in which the flux tube is in hydrostatic equilibrium, the non-linear MHD equations are used to examine its temporal evolution. A detailed study of the flow variables and magnetic field within the tube is presented. It is seen that asymptotically in time a unique state of dynamic equilibrium is established, irrespective of the value of βo (the ratio of the thermal to magnetic pressure at the initial epoch).

scholarly journals Helicity and the Călugăreanu invariant

1992 ◽  
Vol 439 (1906) ◽  
pp. 411-429 ◽  
Keyword(s):  
Flux Tube ◽  
Ambient Medium ◽  
Solenoidal Vector ◽  
Direct Derivation ◽  
Flux Tubes ◽  
Field Distortion ◽  
Solenoidal Vector Field ◽  
Field Lines ◽  
Generic Behaviour

The helicity of a localized solenoidal vector field (i.e. the integrated scalar product of the field and its vector potential) is known to be a conserved quantity under ‘frozen field’ distortion of the ambient medium. In this paper we present a number of results concerning the helicity of linked and knotted flux tubes, particularly as regards the topological interpretation of helicity in terms of the Gauss linking number and its limiting form (the Călugăreanu invariant). The helicity of a single knotted flux tube is shown to be intimately related to the Călugăreanu invariant and a new and direct derivation of this topological invariant from the invariance of helicity is given. Helicity is decomposed into writhe and twist contributions, the writhe contribution involving the Gauss integral (for definition, see equation (4.8)), which admits interpretation in terms of the sum of signed crossings of the knot, averaged over all projections. Part of the twist contribution is shown to be associated with the torsion of the knot and part with what may be described as ‘intrinsic twist’ of the field lines in the flux tube around the knot (see equations (5.13) and (5.15)). The generic behaviour associated with the deformation of the knot through a configuration with points of inflexion (points at which the curvature vanishes) is analysed and the role of the twist parameter is discussed. The derivation of the Călugăreanu invariant from first principles of fluid mechanics provides a good demonstration of the relevance of fluid dynamical techniques to topological problems.

scholarly journals Ion velocity distributions within the LLBL and their possible implication to multiple reconnections

2004 ◽  
Vol 22 (1) ◽  
pp. 213-236 ◽  
Author(s):  
O. L. Vaisberg ◽  
L. A. Avanov ◽  
T. E. Moore ◽  
V. N. Smirnov
Keyword(s):  
Magnetic Field ◽  
Magnetic Flux ◽  
Flux Tube ◽  
Magnetic Flux Tube ◽  
Flux Tubes ◽  
Different Types ◽  
Subsolar Point ◽  
Magnetic Flux Tubes ◽  
Ion Velocity ◽  
Field Lines

Abstract. We analyze two LLBL crossings made by the Interball-Tail satellite under a southward or variable magnetosheath magnetic field: one crossing on the flank of the magnetosphere, and another one closer to the subsolar point. Three different types of ion velocity distributions within the LLBL are observed: (a) D-shaped distributions, (b) ion velocity distributions consisting of two counter-streaming components of magnetosheath-type, and (c) distributions with three components, one of which has nearly zero parallel velocity and two counter-streaming components. Only the (a) type fits to the single magnetic flux tube formed by reconnection between the magnetospheric and magnetosheath magnetic fields. We argue that two counter-streaming magnetosheath-like ion components observed by Interball within the LLBL cannot be explained by the reflection of the ions from the magnetic mirror deeper within the magnetosphere. Types (b) and (c) ion velocity distributions would form within spiral magnetic flux tubes consisting of a mixture of alternating segments originating from the magnetosheath and from magnetospheric plasma. The shapes of ion velocity distributions and their evolution with decreasing number density in the LLBL indicate that a significant part of the LLBL is located on magnetic field lines of long spiral flux tube islands at the magnetopause, as has been proposed and found to occur in magnetopause simulations. We consider these observations as evidence for multiple reconnection Χ-lines between magnetosheath and magnetospheric flux tubes. Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; solar wind-magnetosphere interactions)

scholarly journals Core overshooting under the light of fluid dynamics

2019 ◽  
Vol 82 ◽  
pp. 153-165
Author(s):  
M. Rieutord
Keyword(s):  
Fluid Dynamics ◽  
Viscous Dissipation ◽  
Mixing Layer ◽  
Mechanical Analysis ◽  
Scale Height ◽  
Early Type ◽  
Pressure Scale ◽  
Early Type Stars ◽  
The One

We discuss the possible contraints that are brought about by a fluid mechanical analysis of the overshooting phenomenon at the interface of convective cores and radiative envelopes of early-type stars. We investigate an improvement of Roxburgh’s criterion by taking into account the viscous dissipation but show that this criterion remains not stringent enough to be predictive. We then discuss the thickness of the overshooting layer and show that all estimates, including the one of Zahn (1991), lead to a very thin mixing layer typically less than a percent of the pressure scale height.

scholarly journals Linking Thin Flux Tube MHD Models to Apparent Stellar Surface Activity

2004 ◽  
Vol 219 ◽  
pp. 546-551
Author(s):  
T. Granzer ◽  
K. G. Strassmeier
Keyword(s):  
Magnetic Flux ◽  
Surface Activity ◽  
Flux Tube ◽  
Convection Zone ◽  
Active Regions ◽  
Similar Model ◽  
Flux Tubes ◽  
Stellar Convection ◽  
Magnetic Flux Tubes ◽  
Spot Formation

We model thin magnetic flux tubes as they rise from the bottom of a stellar convection zone to the photosphere. On emergence they form active regions, i.e. star spots. This model was very successfully applied to the solar case, where the simulations where in agreement with the butterfly diagram, Joy's law, and Hale's law. We propose the use of a similar model to describe stellar activity in the more extreme form found on active stars. A comparison between Doppler-images of well-observed pre-MS stars and a theoretically derived probability of star-spot formation as a function of latitude is presented.

scholarly journals Self-Consistent Models for Small Photospheric Flux Tubes

1983 ◽  
Vol 102 ◽  
pp. 67-71
Author(s):  
W. Deinzer ◽  
G. Hensler ◽  
D. Schmitt ◽  
M. Schüssler ◽  
E. Weisshaar
Keyword(s):  
Magnetic Field ◽  
Flux Tube ◽  
Energy Transport ◽  
Numerical Study ◽  
Momentum Equation ◽  
Mhd Equations ◽  
Compressible Medium ◽  
Solar Photosphere ◽  
Slab Geometry ◽  
Element Technique

We give a short summary of some results of a numerical study of magnetic field concentrations in the solar photosphere and upper convection zone. We have developed a 2D time dependent code for the full MHD equations (momentum equation, equation of continuity, induction equation for infinite conductivity and energy equation) in slab geometry for a compressible medium. A Finite-Element-technique is used. Convective energy transport is described by the mixing-length formalism while the diffusion approximation is employed for radiation. We parametrize the inhibition of convective heat flow by the magnetic field and calculate the material functions (opacity, adiabatic temperature gradient, specific heat) self-consistently. Here we present a nearly static flux tube model with a magnetic flux of ∼ 1018 mx, a depth of 1000 km and a photospheric diameter of ∼ 300 km as the result of a dynamical calculation. The influx of heat within the flux tube at the bottom of the layer is reduced to 0.2 of the normal value. The mass distribution is a linear function of the flux function A: dm(A)/dA = const. Fig. 1 shows the model: Isodensities (a), fieldlines (b), isotherms (c) and lines of constant continuum optical depth (d) are given. The “Wilson depression” (height difference between τ = 1 within and outside the tube) is ∼ 150 km and the maximum horizontal temperature deficit is ∼ 3000 K. Field strengths as function of x for three different depths and as function of depth along the symmetry axis are shown in (e) and (f), respectively. Note the sharp edge of the tube.

scholarly journals Infrared variability due to magnetic pressure-driven jets, dust ejection and quasi-puffed-up inner rims

2020 ◽  
Vol 493 (3) ◽  
pp. 4022-4038 ◽  
Author(s):  
Kurt Liffman ◽  
Geoffrey Bryan ◽  
Mark Hutchison ◽  
Sarah T Maddison
Keyword(s):  
Accretion Rate ◽  
Jet Flow ◽  
Flow Speed ◽  
Magnetic Pressure ◽  
Scale Height ◽  
Time Dependent ◽  
Gas Flows ◽  
Centrifugal Acceleration ◽  
The Face ◽  
Disc Region

ABSTRACT The interaction between a YSO stellar magnetic field and its protostellar disc can result in stellar accretional flows and outflows from the inner disc rim. Gas flows with a velocity component perpendicular to disc mid-plane subject particles to centrifugal acceleration away from the protostar, resulting in particles being catapulted across the face of the disc. The ejected material can produce a ‘dust fan’, which may be dense enough to mimic the appearance of a ‘puffed-up’ inner disc rim. We derive analytical equations for the time-dependent disc toroidal field, the disc magnetic twist, the size of the stable toroidal disc region, the jet speed, and the disc region of maximal jet flow speed. We show how the observed infrared variability of the pre-transition disc system LRLL 31 can be modelled by a dust ejecta fan from the inner-most regions of the disc whose height is partially dependent on the jet flow speed. The greater the jet flow speed, the higher is the potential dust fan scale height. An increase in mass accretion on to the star tends to increase the height and optical depth of the dust ejection fan, increasing the amount of 1–8 µm radiation. The subsequent shadow reduces the amount of light falling on the outer disc and decreases the 8–40 µm radiation. A decrease in the accretion rate reverses this scenario, thereby producing the observed ‘see-saw’ infrared variability.

scholarly journals Flux Tube Shredding and its Infrared Signature

1994 ◽  
Vol 154 ◽  
pp. 459-463
Author(s):  
M. Bünte ◽  
O. Steiner ◽  
S.K. Solanki ◽  
V.J. Pizzo
Keyword(s):  
Magnetic Flux ◽  
Flux Tube ◽  
Flux Tubes ◽  
Infrared Signature ◽  
Magnetic Tension ◽  
Magnetic Flux Tubes ◽  
Interchange Instability

The interchange instability of solar magnetic flux tubes and possible stabilization mechanisms are reviewed. Special attention is paid to the influence of magnetic tension forces and the internal atmosphere, both of which were neglected in earlier studies of this instability. It is found that whirl flows with velocities of only 2.2 km s–1 are strong enough to stabilize the flux tubes. However, their absence or the excitation of other instabilities might lead to a shredding of the tubes. The observability of such a scenario in the infrared is briefly discussed.

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.

Sign in / Sign up

Export Citation Format

Share Document