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 Flux Tubes and Dynamos

1993 ◽  
Vol 157 ◽  
pp. 27-39
Author(s):  
M. Schüssler
Keyword(s):  
Field Strength ◽  
Magnetic Flux ◽  
Convection Zone ◽  
Source Region ◽  
Mean Field ◽  
Active Regions ◽  
Dynamo Theory ◽  
Flux Tubes ◽  
Magnetic Structures ◽  
Magnetic Flux Tubes

The structure of solar surface magnetic fields, the way they erupt from the the convection zone below, and processes like flux expulsion and fragmentation instabilities support the view that magnetic flux in a stellar convection zone is in an intermittent, fragmented state which can be described as an ensemble of magnetic flux tubes. Depending on size and field strength, the dynamics of magnetic flux tubes can strongly differ from the behavior of a passive, diffuse field which is often assumed in conventional mean-field dynamo theory. Observed properties of active regions like emergence in low latitudes, Hale's polarity rules, tilt angles, and the process of sunspot formation from smaller fragments, together with theoretical considerations of the dynamics of buoyant flux tubes indicate that the magnetic structures which erupt in an emerging active region are not passive to convection and originate in a source region (presumably an overshoot layer below the convection zone proper) with a field strength of at least 105 G, far beyond the equipartition field strength with respect to convective flows. We discuss the consequences of such a situation for dynamo theory of the solar cycle and consider the possibility of dynamo models on the basis of flux tubes. A simple, illustrative example of a flux tube dynamo is presented.

scholarly journals The Emergence of Magnetic Flux in Active Regions

2001 ◽  
Vol 203 ◽  
pp. 225-228
Author(s):  
W. P. Abbett ◽  
G. H. Fisher ◽  
Y. Fan
Keyword(s):  
Magnetic Flux ◽  
Flux Tube ◽  
Active Regions ◽  
Large Degree ◽  
Solar Interior ◽  
Magnetic Flux Tube ◽  
Initial Field ◽  
Flux Tubes ◽  
Mhd Simulations ◽  
Magnetic Flux Tubes

Over the past decade, “thin flux tube” models have proven successful in explaining many properties of active regions in terms of magnetic flux tube dynamics in the solar interior. On the other hand, recent 2-D MHD simulations of the emergence of magnetic flux have shown that many of the assumptions adopted in the thin flux tube approximation are invalid. For example, unless the flux tubes exhibit a large degree of initial field line twist — and observations of emerging active regions suggest they do not — they will fragment (break apart) before they are able to emerge through the surface. We attempt to resolve this paradox using a number of 3-D MHD simulations (in the anelastic approximation) that describe the rise and fragmentation of twisted magnetic flux tubes. We find that the degree of fragmentation of an evolving Ω-loop depends strongly on the 3-D geometry of the loop, and that the Coriolis force plays a dynamically important role in the evolution and emergence of magnetic flux.

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 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 Rise of Kink-Unstable Magnetic Flux Tubes in the Solar Convection Zone

1998 ◽  
Vol 505 (1) ◽  
pp. L59-L63 ◽  
Author(s):  
Y. Fan ◽  
E. G. Zweibel ◽  
M. G. Linton ◽  
G. H. Fisher
Keyword(s):  
Magnetic Flux ◽  
Convection Zone ◽  
Solar Convection Zone ◽  
Flux Tubes ◽  
Solar Convection ◽  
Magnetic Flux Tubes

scholarly journals Heating of Stellar Chromospheres and Transition Regions

2004 ◽  
Vol 219 ◽  
pp. 437-448
Author(s):  
Zdzislaw E. Musielak
Keyword(s):  
Magnetic Fields ◽  
Magnetic Flux ◽  
Acoustic Waves ◽  
Theoretical Models ◽  
Flux Tubes ◽  
Stellar Convection ◽  
Wave Heating ◽  
Active Stars ◽  
Magnetic Flux Tubes ◽  
Magnetic Waves

To explain the heating of stellar chromospheres and transition regions, two classes of heating mechanisms have been considered: dissipation of acoustic and magnetic waves generated in stellar convection zones; and dissipation of currents generated by photospheric motions of surface magnetic fields. The focus of this paper is on the wave heating mechanisms and on recent results which demonstrate that theoretical models of stellar chromospheres based on the wave heating can explain the “basal flux” and the observed Ca II emission in most stars but cannot account for the observed Mg II emission in active stars. The obtained results clearly show that the base of stellar chromospheres is heated by acoustic waves, the heating of the middle and upper chromospheric layers is dominated by magnetic waves associated with magnetic flux tubes, and that other non-wave heating mechanisms are required to explain the structure of the highest layers of stellar chromospheres and transition regions.

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