HAO 2010 PROFILES IN SCIENCE: Dr. Yuhong Fan

Contact

303-494-1575
yfan@ucar.edu

Dr. Yuhong Fan is a Senior Scientist and Corona & Heliosphere Section Head at the High Altitude Observatory (HAO), 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.

Summary of Achievements:

Over the past year, Yuhong Fan's research has continued to focus on 3D magneto-hydrodynamic modeling of the initiation of coronal mass ejections in the solar corona. Through a sequence of 3D simulations of the evolution of the coronal magnetic field resulting from the emergence of a twisted magnetic flux rope into a pre-existing coronal arcade field, she has studied the physical mechanism and condition for the onset of eruption of a coronal flux rope. She has provided the data from the above MHD simulations of CME initiation to university collaborators Yixuan Li, Ju Jing, and Haimin Wang (all at NJIT) for comparison and interpretation of the Hinode SOT observation of the magnetic field changes associated with an eruptive flare.

Fan has also continued investigation of the dynamic evolution of emerging active region flux tubes in the solar convection zone. As a collaborative effort, Aaron Birch and Doug Braun at CoRA/NWRA and Fan have used the simulation data from Fan (2008, ApJ 676, 680) of the rise of buoyant flux tubes in a rotating model solar convective envelope to deduce possible time-distance helioseismic signatures that may be used to detect rising active region flux tubes in the deep solar interior before they emerge at the surface. Furthermore, new HAO graduate fellow Maria Weber (CSU) has started working with Yuhong Fan and Mark Miesch to carry out simulations of the rise of active region flux tubes in the presence of the giant-cell convection and the associated mean flows in the solar convective envelope.

Publication:

(1) Fan, Y. 2010, "On the eruption of coronal flux ropes", The Astrophysical Journal, 719, 728, doi: 10.1088/0004-637X/719/1/728.
Also see: "Lower Solar Atmosphere."

Abstract:

We present three-dimensional MHD simulations of the evolution of the magnetic field in the corona where the emergence of a twisted magnetic flux tube is driven at the lower boundary into a pre-existing coronal potential arcade field. Through a sequence of simulations in which we vary the amount of twisted flux transported into the corona before the emergence is stopped, we investigate the conditions that lead to a dynamic eruption of the resulting coronal flux rope. It is found that the critical condition for the onset of eruption is for the center of the flux rope to reach a critical height at which the corresponding potential field declines with height at a sufficiently steep rate, consistent with the onset of the torus instability of the flux rope. In some cases, immediately after the emergence is stopped, the coronal flux rope first settles into a quasi-static rise with an underlying sigmoid-shaped current layer developing. Preferential heating of field lines going through this current layer may give rise to the observed quiescent X-ray sigmoid loops before eruption. Reconnections in the current layer during the initial quasi-static stage is found to add detached flux to the coronal flux rope, allowing it to rise quasi-statically to the critical height and dynamic eruption of the flux rope then ensues. By identifying field lines whose tops are in the most intense part of the current layer during the eruption, we deduce the evolution and morphology of the post-flare X-ray loops and the flare ribbons at their footpoints.

Figure: Top rows are snapshots of the 3D coronal magnetic field lines and the associated underlying current sheet (represented by the orange surfaces), showing the quasi-static rise and the onset of eruption of a coronal flux rope when the critical height is reached. Bottom rows show side (left panel) and top (middle panel) views of the post-reconnection flare loops and their foot-points (red dots in the right panel), which correspond to the locations of flare ribbons (or the brightening kernels due to precipitation of high energy electrons coming down along the post-reconnection loops) in the chromosphere. The color image in the bottom right panel shows the normal magnetic field on the surface.

Publication:

(2) Li, Y., Jing, J., Fan, Y., and Wang, H. 2010: "Comparison between Observation and Simulation of Magnetic Field Change Associated with Flares", The Astrophysical Journal, submitted.

Abstract:

It has been a long-standing question in solar physics how magnetic field structure changes with eruptive events (e.g., flares and CMEs). In this Letter we present the eruption-associated changes in the magnetic inclination angle, the transverse component of magnetic field and the Lorentz force. The analysis is based on the observation of the X3.4 flare on 2006 December 13 and in comparison to the numerical simulation of Fan (2010). Both observation and simulation show that (1) the magnetic inclination angle in the decayed peripheral penumbra increases, while that in the central area close to flaring polarity inversion line (PIL) deceases after the flare; (2) the transverse component of magnetic field increases at the lower altitude near flaring PIL after the flare. The result suggests that the field lines at flaring neutral line turn to more horizontal near the surface, in agreement with the prediction of Hudson, Fisher & Welsch (2008).

Figure: The left panels show the mean value of the magnetic field inclination angle in the areas of penumbral decay (top) and enhancement (bottom) as a function of height at two time instances before (blue) and after (red) the flare, for active region NOAA 10930 observed by SOT of Hinode. The height variation was based on an NLFFF extrapolation. In comparison, the right panels show the corresponding magnetic field inclination angles from the simulation at the outer peripheral of the bipolar region (top), which corresponds to the area of penumbral decay, and in the central area over the polarity inversion line (bottom), which corresponds to the area of penumbral enhancement, for two time instances just before (green) and after (orange) the eruption. Both the observation and simulation show that the magnetic field in the peripheral of the active region becomes more vertical, producing penumbral decay seen in the observation, while the transverse component of the magnetic field increases at the lower altitude near the flaring polarity inversion line, resulting in the enhancement of the penumbra there after the flare. These changes are due to the lift off of the pre-existing coronal flux rope, and the subsequent implosion of the magnetic field and the inward collapsing of the post-reconnection loops above the polarity inversion line.

Publication:

(3) Birch, A. C.; Braun, D. C.; Fan, Y. 2010: "An Estimate of the Detectability of Rising Flux Tube", The Astrophysical Journal Letters, 723, L190, doi: 10.1088/2041-8205/723/2/L190.

Abstract:

The physics of the formation of magnetic active regions (ARs) is one of the most important problems in solar physics. One main class of theories suggests that ARs are the result of magnetic flux that rises from the tachocline. Time-distance helioseismology, which is based on measurements of wave propagation, promises to allow the study of the subsurface behavior of this magnetic flux. Here, we use a model for a buoyant magnetic flux concentration together with the ray approximation to show that the dominant effect on the wave propagation is expected to be from the roughly 100 m s^-1 retrograde flow associated with the rising flux. Using a B-spline-based method for carrying out inversions of wave travel times for flows in spherical geometry, we show that at 3 days before emergence the detection of this retrograde flow at a depth of 30 Mm should be possible with a signal-to-noise level of about 8 with a sample of 150 emerging ARs.

Figure: Model flow, example inversion result, and an example averaging kernel at about 3 days before emergence. The left panel shows a slice at latitude 17°8 of the Ø component of the flow from the model. The middle panel shows a slice, at the same latitude, through an example inversion result for a particular choice of the regularization parameter. The right most panel shows a slice through the averaging kernel for the inversion at a depth of 40 Mm and longitude of 55°.