Yuhong Fan - Academia.edu (original) (raw)

Papers by Yuhong Fan

For our understanding of stellar magnetic activity across the Hertzsprung Russel diagram it is es... more For our understanding of stellar magnetic activity across the Hertzsprung Russel diagram it is essential to study how dynamo generated magnetic field in stellar convection zones is transported toward the stellar surface, reorganized in the near surface layers to form star spots, and expands into the stellar coronae. For a detailed study of the process of flux emergence, the sun plays a central role, since only here the relevant processes can be studied in sufficient resolution and also detailed subsurface information can be derived through means of local helioseismology. Major advancements in this field in the near future are possible due to a combination of high resolution multiwavelength observations, numerical simulations, and advancement in local helioseismic techniques.

... Matthias Rempel1, Yuhong Fan1, Aaron Birch2, Douglas Braun2, Mark Cheung3, William Abbett4 ..... more ... Matthias Rempel1, Yuhong Fan1, Aaron Birch2, Douglas Braun2, Mark Cheung3, William Abbett4 ... The first global models of the flux emergence process in the convection zone were based on the thin flux tube approximation (Choudhuri & Gilman, 1987; Fan et al., 1993, 1994 ...

... zone because of the suba~1iabatic temperature gradient in the overshoot layer. The final magn... more ... zone because of the suba~1iabatic temperature gradient in the overshoot layer. The final magnetic field strength distribution in the leading and the following legs of one of the emerging loop is shown in Fig. 1(b) as a function of the radial distance to the center of the sun. ...

The buoyant rise of a magnetic ux loop originating from

Lecture Notes in Physics

The generation and evolution of astrophysical magnetic fields occurs largely through the action o... more The generation and evolution of astrophysical magnetic fields occurs largely through the action of turbulence. In many situations, the magnetic field is strong enough to influence many important properties of turbulence itself. Numerical simulation of magnetized turbulence is especially challenging in the astrophysical regime because of the high magnetic Reynolds numbers involved, but some aspects of this difficulty can be avoided in weakly ionized systems.

The anelastic approximation results from a scaled variable expansion The anelastic approximation ... more The anelastic approximation results from a scaled variable expansion The anelastic approximation results from a scaled variable expansion of the 3D compressible MHD equations about a of the 3D compressible MHD equations about a nearly nearly adiabatically adiabatically stratified plane-parallel reference atmosphere (which we take to be a stratified plane-parallel reference atmosphere (which we take to be a polytrope). The time-derivative term in the continuity equation is of polytrope). The time-derivative term in the continuity equation is of sufficiently high order in the expansion that it can be neglected, sufficiently high order in the expansion that it can be neglected, thereby filtering fast-moving acoustic waves from the calculation. thereby filtering fast-moving acoustic waves from the calculation. This approximation is well-suited for simulations of sub-sonic This approximation is well-suited for simulations of sub-sonic processes in the high-beta plasma deep in the solar convection zone. processes in the high-beta plasma deep in the solar convection zone. The computational savings inherent in this formulation allows for a The computational savings inherent in this formulation allows for a large exploration of parameter space. large exploration of parameter space.

Public reportingburden for this collection of information is estimated to average 1 hour per resp... more Public reportingburden for this collection of information is estimated to average 1 hour per response, including the time for reviewing inst e data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other a this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0 1 4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing My valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

Proceedings of the International Astronomical Union

Over the past decade, ``thin flux tube'' models have proven successful in explaining ma... more 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. Unfortunately, recent, more sophisticated two-dimensional 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 amount 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 Omega-loop depends strongly on the three-dimensional geometry of the tube --- the greater the apex curvature, the lesser the degree of fragmentation for a fixed amount of initial twist. We also find that the Coriolis force plays a dynamically important role in the evolution and emergence of magnetic flux. We are able to infer general observational characteristics of the emerging flux, and compare our theoretical data with recent observations.

For our understanding of stellar magnetic activity across the Hertzsprung Russel diagram it is es... more For our understanding of stellar magnetic activity across the Hertzsprung Russel diagram it is essential to study how dynamo generated magnetic field in stellar convection zones is transported toward the stellar surface, reorganized in the near surface layers to form star spots, and expands into the stellar coronae. For a detailed study of the process of flux emergence, the sun plays a central role, since only here the relevant processes can be studied in sufficient resolution and also detailed subsurface information can be derived through means of local helioseismology. Major advancements in this field in the near future are possible due to a combination of high resolution multiwavelength observations, numerical simulations, and advancement in local helioseismic techniques.

... Matthias Rempel1, Yuhong Fan1, Aaron Birch2, Douglas Braun2, Mark Cheung3, William Abbett4 ..... more ... Matthias Rempel1, Yuhong Fan1, Aaron Birch2, Douglas Braun2, Mark Cheung3, William Abbett4 ... The first global models of the flux emergence process in the convection zone were based on the thin flux tube approximation (Choudhuri & Gilman, 1987; Fan et al., 1993, 1994 ...

... zone because of the suba~1iabatic temperature gradient in the overshoot layer. The final magn... more ... zone because of the suba~1iabatic temperature gradient in the overshoot layer. The final magnetic field strength distribution in the leading and the following legs of one of the emerging loop is shown in Fig. 1(b) as a function of the radial distance to the center of the sun. ...

The buoyant rise of a magnetic ux loop originating from

Lecture Notes in Physics

The generation and evolution of astrophysical magnetic fields occurs largely through the action o... more The generation and evolution of astrophysical magnetic fields occurs largely through the action of turbulence. In many situations, the magnetic field is strong enough to influence many important properties of turbulence itself. Numerical simulation of magnetized turbulence is especially challenging in the astrophysical regime because of the high magnetic Reynolds numbers involved, but some aspects of this difficulty can be avoided in weakly ionized systems.

The anelastic approximation results from a scaled variable expansion The anelastic approximation ... more The anelastic approximation results from a scaled variable expansion The anelastic approximation results from a scaled variable expansion of the 3D compressible MHD equations about a of the 3D compressible MHD equations about a nearly nearly adiabatically adiabatically stratified plane-parallel reference atmosphere (which we take to be a stratified plane-parallel reference atmosphere (which we take to be a polytrope). The time-derivative term in the continuity equation is of polytrope). The time-derivative term in the continuity equation is of sufficiently high order in the expansion that it can be neglected, sufficiently high order in the expansion that it can be neglected, thereby filtering fast-moving acoustic waves from the calculation. thereby filtering fast-moving acoustic waves from the calculation. This approximation is well-suited for simulations of sub-sonic This approximation is well-suited for simulations of sub-sonic processes in the high-beta plasma deep in the solar convection zone. processes in the high-beta plasma deep in the solar convection zone. The computational savings inherent in this formulation allows for a The computational savings inherent in this formulation allows for a large exploration of parameter space. large exploration of parameter space.

Public reportingburden for this collection of information is estimated to average 1 hour per resp... more Public reportingburden for this collection of information is estimated to average 1 hour per response, including the time for reviewing inst e data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other a this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0 1 4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing My valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

Proceedings of the International Astronomical Union

Over the past decade, ``thin flux tube'' models have proven successful in explaining ma... more 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. Unfortunately, recent, more sophisticated two-dimensional 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 amount 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 Omega-loop depends strongly on the three-dimensional geometry of the tube --- the greater the apex curvature, the lesser the degree of fragmentation for a fixed amount of initial twist. We also find that the Coriolis force plays a dynamically important role in the evolution and emergence of magnetic flux. We are able to infer general observational characteristics of the emerging flux, and compare our theoretical data with recent observations.