| Yuhong Fan |
| High Altitude Observatory |
| National Center for Atmospheric Research |
| 3450 Mitchell Lane |
| Boulder, CO 80301 |
| phone: 303-497-1575 |
| fax: 303-497-1589 |
| yfan@ucar.edu |
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(September, 2004, the Great Wall) |
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I am a scientist at the High Altitude Observatory (HAO), National Center for Atmospheric Research (NCAR), Boulder, Colorado. I received a B.Sc. in Space Physics from Peking University, China, in 1989, and a Ph.D in Astronomy from the Institute for Astonomy at the University of Hawaii in 1993. I did my postdoctoral research at the National Solar Observatory in Tucson, and at the Joint Institute of Laboratory Astrophysics, University of Corlorado at Boulder. I joined the scientific staff of HAO/NCAR in 1998. Here is a copy of my CV and publication list.
Research interests : solar and astrophysical magnetohydrodynamics; dynamics of rising magnetic flux tubes in the solar interior; physics of solar active region formation; interaction of solar p-modes with active region magnetic flux tubes; helioseismology;
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If the magnetic field seen in sunspots and active regions on the solar surface originates from a strong toroidal magnetic field generated by the dynamo mechanism at the base of the solar convection zone, then the question of how magnetic flux is transported through the convection zone and emerge into the solar atmosphere must be addressed. It is generally thought that magnetic flux rises buoyantly through the convection zone in the form of discrete flux tubes, and that the tubes must maintain reasonable cohesion in order that the emerging flux be organized as active regions, which display a well-defined order as described by Hale's polarity rule and Joy's law of active region tilts. In the past few years, my research has focused on the MHD physics of buoyant magnetic flux tubes and their transport from the bottom of the convection zone to the photosphere to form the observed sunspots and active regions. In collaboration with Ellen Zweibel (JILA/Univ. of colorado), George Fisher and Bill Abbett (SSL/UC Berkeley), Mark Linton (NRL), and Steve Lantz (CTC/Cornell Univ.), I have been working on MHD numerical simulations of the subsurface evolution of rising magnetic flux tubes, and the emergence of magnetic flux tubes through the photosphere. In addition, I have also worked with Doug Braun (CoRA/NWRA) on helioseismic investiation of the large-scale meridional circulation in the solar convectiv enevelope. The following is a summary of recent reserach results: |
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Fan (2001, ApJ, 546, 509) studied the magnetic buoyancy instability (or the Parker instability) of a neutrally buoyant layer of horizontal, unidirectional magnetic field in hydrostatic equilibrium at the base of the solar convection zone. It is found that the non-linear growth of the undulatory Parker instability lead to the formation of buoyant arching flux tubes which rise cohesively through the distance of about 1 density scale height included in the simulation domain. The figure and movie 1 above show the evolution of the magnetic layer perturbed with an unstable undulatory mode. Movies 2 and 3 show (from two different perspective views) a simulation where the magnetic layer is perturbed with a localized velocity field. |
 movie1
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 movie2
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Fan, Abbett, and Fisher (2003, ApJ 582, 1206) have carried out 3D numerical simulations of the dynamic evolution of uniformly buoyant, twisted horizontal magnetic flux tubes in a 3D stratified convective velocity field. Our calculations are relevant to understanding how stratified convection in the deep solar convection zone may affect the rise and the structure of buoyant flux tubes that are responsible for the emergence of solar active regions. It is found that in order for the magnetic buoyancy force of the tube to dominate the hydrodynamic force due to the convective downflows, the field strength B of the flux tube needs to be significantly higher than the field strength Beq in equipartition with the kinetic energy density of the strong downdrafts. For tubes of equipartition field strength, the evolution and the morphology of the tube is largely controlled by the local conditions of the convective flow. Sections of the tube in the paths of strong downdrafts are pinned down to the bottom despite their buoyancy, while the rise speed of sections within upflow regions is significantly boosted. Omega-shaped emerging tubes can form between downdrafts. On the other hand, tubes with 10 times the equipartition field strength are found to rise unimpeded by the downdrafts, and are not significantly distorted by the 3D convective flow. |
 convective velocity field
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 tube with B=Beq
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 tube with B=10Beq
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 animation
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To investigate the origin of the so-called delta-configuration sunspots which are a class of active regions that do not obey the Hale polarity rule and are shown to be flare productive, Fan et al. (1998, ApJ, 505, L59; 1999, ApJ, 521, 460) have performed 3D MHD simulations of the rise of highly twisted, kink-unstable magnetic flux tubes through an adiabatically-stratified model solar convection zone (see the example in the above figure). It is found that the rising flux tube evolves into a highly kinked shape with a large change (> 90 degree) of tube orientation at the apex. The forth panel shows the magnetic field in a horizontal cross-section near the apex of the kinked tube. It shows a compact bipolar structure with inverted polarity order and sheared horizontal field along the neutral line. These properties are similar to those found in delta-spots.
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 animation(1.7Mb)
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To piece together a complete picture of solar active region formation, one has to address the challenging problem of how magnetic flux dynamically emerges from the convectively unstable, pressure dominated plasma of the interior into the stably stratified, tenuous solar atmosphere and corona. The magnetic buoyancy instability is a mechanism through which magnetic flux reaching the photosphere can expand dynamically into the solar atmosphere. Fan (2001, ApJ, 554, L111) carried out 3D MHD simulations of the emergence of a twisted flux tube from the top layer of the solar convection zone into the atmosphere and the corona (see the above figure).It is found that the simulation reproduces several major observed features of a newly developing active region (NOAA AR 5617) described by Strous et al. (1996, A&A, 306, 947), including the orientation of the arch-filament system, the distribution of the vertical magnetic field on the photosphere, the locations of sunspot formation, the downflows along the emerged loops, and the organized shear flow pattern in the photospheric horizontal velocity field.