HAO 2011 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.
Professional Website(s): Yuhong Fan
Summary of Achievements
Over the past year, Yuhong Fan has begun to perform three-dimensional (3D) magneto-hydrodynamic (MHD) simulations to model realistic coronal mass ejection (CME) events using observational inputs for both the pre-existing coronal magnetic field and lower boundary driving conditions. Specifically she has carried out MHD simulations that qualitatively model the coronal magnetic field evolution associated with the eruptive flare that occurred on 2006 December 13 in the emerging δ-sunspot region NOAA 10930 observed by the Hinode satellite (see Figure 1, and Fan 2011, ApJ, 740, 68). The simulations show that the eruption can result from the emergence of an east-west-oriented twisted flux rope whose positive emerging flux "butts against"the southern end of the pre-existing negative sunspot. The simulated coronal magnetic field reproduces several key features of the eruptive flare, including the morphology of the X-ray loops and the evolution of the flare ribbons as observed by Hinode. These simulations represent initial steps towards determining the 3D magnetic field structure of CME ejecta originating from the lower solar corona in realistic CME events.
As part of her Ph.D. thesis research, HAO graduate fellow Maria Weber (Colorado State University) working with Yuhong Fan and Mark Miesch, has completed a large set of thin flux tube simulations that model the dynamic rise of active region scale magnetic flux tubes in a solar-like global convective velocity field computed from the Anelastic Spherical Harmonics (ASH) code. These simulations show that the effects of the convective flow allow flux tubes with weaker field strengths (below 50 KG) at the bottom of the solar convection zone to develop emerging loops with properties that are consistent with the observed properties of solar active regions (see Figure 2, and Weber, Fan and Miesch 2011, ApJ, 741, 11). Such weaker field strengths are more consistent with the values suggested by the recent dynamic dynamo models. Work is continuing with the development of the finite difference spherical anelastic MHD (FSAM) code to model solar-like global convection and the associated mean flows with significantly lower numerical diffusivities.
Figure 1 caption: (a) Modeled coronal magnetic field lines just before the onset of eruption, produced from an MHD simulation where an east–west-oriented magnetic flux rope emerges into a pre-existing coronal field constructed based on the Solar and Heliospheric Observatory/MDI full-disk magnetogram at 20:51:01 UT on 2006 December 12. The simulated pre-eruption coronal magnetic field shows morphology that is in qualitative agreement with that seen in the Hinode soft X-ray observation shown in (b). An inverse S-shaped current sheet forms in the simulated magnetic field, and the field lines (shown as the pink field-lines) going through the current sheet are expected to be preferentially heated, which explains the formation of the inverse-S shaped bright X-ray loops in the X-ray image.
Figure 2 caption: Tilt angle as a function of emergence latitude for modeled rising flux tubes with a range of initial magnetic field strength and for cases with (plus signs) and without (diamond points) the influence of convection. The black (gray) lines are the linear least-squares fit to the results with (without) the presence of convection. The convective flow promotes tilt angles that are consistent with the observed mean tilt of solar active regions because of the mean kinetic helicity in the flow.
Publications
(1) Fan, Y. 2011: A Magnetohydrodynamic Model of the December 13 2006 Eruptive Flare, ApJ, 740, 68, doi:10.1088/0004-637X/740/2/68.
Abstract: We present a three-dimensional magnetohydrodynamic simulation that qualitatively models the coronal magnetic field evolution associated with the eruptive flare that occurred on 2006 December 13 in the emerging δ-sunspot region NOAA 10930 observed by the Hinode satellite. The simulation is set up to drive the emergence of an east-west-oriented magnetic flux rope at the lower boundary into a preexisting coronal field constructed from the Solar and Heliospheric Observatory/Michelson Doppler Imager full-disk magnetogram at 20:51:01 UT on 2006 December 12. The resulting coronal flux rope embedded in the ambient coronal magnetic field first settles into a stage of quasi-static rise and then undergoes a dynamic eruption, with the leading edge of the flux rope cavity accelerating to a steady speed of about 830 km s−1. The pre-eruption coronal magnetic field shows morphology that is in qualitative agreement with that seen in the Hinode soft X-ray observation in both the magnetic connectivity as well as the development of an inverse-S-shaped X-ray sigmoid. We examine the properties of the erupting flux rope and the morphology of the post-reconnection loops, and compare them with the observations.
(2) Weber, M., Fan, Y., and Miesch, M. 2011: The Rise of Active Region Flux Tubes in the Turbulent Solar Convective Envelope, ApJ, 741, 11, doi:10.1088/0004-637X/741/1/11.
Abstract: We use a thin flux tube model in a rotating spherical shell of turbulent convective flows to study how active region scale flux tubes rise buoyantly from the bottom of the convection zone to near the solar surface. We investigate toroidal flux tubes at the base of the convection zone with field strengths ranging from 15 kG to 100 kG at initial latitudes ranging from 1" to 40" with a total flux of 1022 Mx. We find that the dynamic evolution of the flux tube changes from convection dominated to magnetic buoyancy dominated as the initial field strength increases from 15 kG to 100 kG. At 100 kG, the development of Ω-shaped rising loops is mainly controlled by the growth of the magnetic buoyancy instability. However, at low field strengths of 15 kG, the development of rising Ω-shaped loops is largely controlled by convective flows, and properties of the emerging loops are significantly changed compared to previous results in the absence of convection. With convection, rise times are drastically reduced (from years to a few months), loops are able to emerge at low latitudes, and tilt angles of emerging loops are consistent with Joy's law for initial field strengths of 40 kG or greater. We also examine other asymmetries that develop between the leading and following legs of the emerging loops. Taking all the results together, we find that mid-range field strengths of ~40—50 kG produce emerging loops that best match the observed properties of solar active regions.