HAO 2012 Profiles In Science: Dr. Yuhong Fan
Contact:
303-494-1575
yfan@ucar.edu
Dr. Yuhong Fan is a Senior Scientist at the High Altitude Observatory (HAO) at the National Center for Atmospheric Research (NCAR). She received a B.Sc. in Space Physics from Peking University, China, in 1989, and a Ph.D in Astronomy from the Institute for Astronomy at the University of Hawaii in 1993. Dr. Fan did her postdoctoral research at the National Solar Observatory (NSO) in Tucson, and at the Joint Institute of Laboratory Astrophysics (JILA), University of Colorado at Boulder. She joined the scientific staff of HAO/NCAR in 1998. Dr. Yuhong Fan's research interest is solar and astrophysics magneto-hydrodynamics, with focuses on studying the formation of active region magnetic fields in the solar interior and their emergence into the solar atmosphere, and the evolution of the large scale coronal magnetic fields that results in coronal mass ejections. She has also worked on helioseismic investigation of solar subsurface meridional flows and the interaction between solar p-mode waves and sunspots
Professional Website(s): Yuhong Fan
Summary of Achievements
Over the past year, Fan has been working on improving the thermodynamics of the MHD simulations of coronal mass ejections to incorporate non-adiabatic effects due to heating produced by current sheet formation and thermal conduction along the magnetic field lines. Using these simulations Fan has simulated the quasi-static evolution and the onset of eruption of a coronal magnetic flux rope confined by an arcade field. The simulations show that during the quasi-static rise phase of the flux rope, a current sheet develops underlying the flux rope, and reconnections in the current sheet produce a central hot, low-density channel containing reconnected, twisted fields on top of the current sheet (see Figure 1, top left panel). This would explain the observed feature of an elevated central cavity enclosed by a U-shaped dense shell on top of the dense prominence sheet (see Figure 1) often seen in coronal prominence cavities just prior to eruption. The simulations interpret such feature as the result of “tether cutting” reconnections which reduce the anchoring of the flux rope and buildup the twisted flux of the flux rope (Fan 2012, ApJ, 758, 60).
HAO graduate fellow Maria Weber (Physics Department, Colorado State University), working with Yuhong Fan and Mark Miesch, continues to study the dynamic rise of active region flux tubes in the turbulent solar convective envelope (Weber, Fan, and Miesch 2012), using a thin flux tube model in a rotating spherical shell of turbulent convective flows computed separately from a global convection simulation. By carrying out a large number of simulations that span a wide rage of parameter space in initial field strength (15 kG to 100 kG), initial latitude (0 to 40 degrees), and total flux (1022, 1021, and 1020 Mx), they compare the properties of the emerging Ω-shaped flux loops with the observed properties of solar active regions and put constraints on the field strength for the dynamo generated toroidal magnetic fields at the bottom of the solar convection zone. The study suggests that the initial field strength of active region progenitor flux tubes need to be greater than about 40 kG, in order for them to satisfy the Joy's law trend for mean tilt angles as well as the observed amount of scatter of the tilt angles about the mean Joy's law behavior. Weaker magnetic fields tend to produce too large a scatter to be consistent with the observed results. It is also found that the simulated flux tubes tend to emerge at preferred longitudes, regardless of the initial magnetic field strength. These “active longitudes” often span across the equator between latitudes of 15 degrees to −15 degrees, and can persist and drift pro-grade for multiple solar rotation periods. These active longitudes in the simulations are the result of columnar, rotationally aligned giant cells present in the convection simulation at low latitudes near the equator. If giant convection cells exist in the bulk of the solar convection zone, this phenomenon could in part provide an explanation for the observed active longitudes on the Sun.
Figure 1 caption: Top row panels show the result from a MHD simulation of a coronal flux rope confined by an arcade field prior to its eruption. From left to right, the panels show the 3D coronal magnetic field lines and the associated current sheet (orange iso-surfaces of current density) as viewed above the limb, and the resulting modeled SDO/AIA intensity images in 171, 193, and 212 Angstrom channels. The modeled AIA images show a central cavity enclosed by a U shaped dense shell produced by the upward extension of the dense current sheet. The movie link shows the modeled AIA intensity evolution produced by the MHD model through both the stable and the erupting phases. The lower panels show reversed AIA images in the 171, 193, and 212 Angstrom wavelength channels from the observation of a coronal cavity just prior to its eruption on June 13, 2010 studied by Regnier et al. (2011, A&A, 533, L1), which show a central elevated cavity enclosed in a sharp U shaped dense shell on top of the high density prominence sheet. The features in the modeled AIA images are similar to the observed coronal cavity morphology.
Publications
(1) Fan, Y. 2012: Thermal Signatures of Tether-cutting Reconnections in Pre-eruption Coronal Flux Ropes: Hot Central Voids in Coronal Cavities, ApJ, 758, 60, doi:10.1088/0004-637X/758/1/60.
Abstract: Using a three-dimensional MHD simulation, we model the quasi-static evolution and the onset of eruption of a coronal flux rope. The simulation begins with a twisted flux rope emerging at the lower boundary and pushing into a pre-existing coronal potential arcade field. At a chosen time the emergence is stopped with the lower boundary taken to be rigid. Then the coronal flux rope settles into a quasi-static rise phase during which an underlying, central sigmoid-shaped current layer forms along the so-called hyperbolic flux tube (HFT), a generalization of the X-line configuration. Reconnections in the dissipating current layer effectively add twisted flux to the flux rope and thus allow it to rise quasi-statically, even though the magnetic energy is decreasing as the system relaxes. We examine the thermal features produced by the current layer formation and the associated "tether-cutting" reconnections as a result of heating and field aligned thermal conduction. It is found that a central hot, low-density channel containing reconnected, twisted flux threading under the flux rope axis forms on top of the central current layer. When viewed in the line of sight roughly aligned with the hot channel (which is roughly along the neutral line), the central current layer appears as a high-density vertical column with upward extensions as a "U"-shaped dense shell enclosing a central hot, low-density void. Such thermal features have been observed within coronal prominence cavities. Our MHD simulation suggests that they are the signatures of the development of the HFT topology and the associated tether-cutting reconnections, and that the central void grows and rises with the reconnections, until the flux rope reaches the critical height for the onset of the torus instability and dynamic eruption ensues.
(2) Weber, M.A., Fan, Y., and Miesch, M.S. 2012: Comparing Simulations of Rising Flux Tubes Through the Solar Convection Zone with Observations of Solar Active Regions: Constraining the Dynamo Field Strength, Solar Physics, Online First, DOI: 10.1007/s11207-012-0093-7.
Abstract: We study how active-region-scale flux tubes rise buoyantly from the base of the convection zone to near the solar surface by embedding a thin flux tube model in a rotating spherical shell of solar-like turbulent convection. These toroidal flux tubes that we simulate range in magnetic field strength from 15 kG to 100 kG at initial latitudes of 1 to 40 in both hemispheres. This article expands upon Weber, Fan, and Miesch (Astrophys. J. 741, 11, 2011) (Article 1) with the inclusion of tubes with magnetic flux of 1020 Mx and 1021 Mx, and more simulations of the previously investigated case of 1022 Mx, sampling more convective flows than the previous article, greatly improving statistics. Observed properties of active regions are compared to properties of the simulated emerging flux tubes, including: the tilt of active regions in accordance with Joy's Law as in Article 1, and in addition the scatter of tilt angles about the Joy's Law trend, the most commonly occurring tilt angle, the rotation rate of the emerging loops with respect to the surrounding plasma, and the nature of the magnetic field at the flux tube apex. We discuss how these diagnostic properties constrain the initial field strength of the active-region flux tubes at the bottom of the solar convection zone, and suggest that flux tubes of initial magnetic field strengths of ≤40 kG are good candidates for the progenitors of large (1021 Mx to 1022 Mx) solar active regions, which agrees with the results from Article 1 for flux tubes of 1022 Mx. With the addition of more magnetic flux values and more simulations, we find that for all magnetic field strengths, the emerging tubes show a positive Joy's Law trend, and that this trend does not show a statistically significant dependence on the magnetic flux.