Type of Project: Numerical modeling and data analysis

Skills/Interest Required: Interest in use of advanced computer modeling tools in the analysis of space-based and ground-based observations. No advanced programming skills required.

Mentor: Dr. Yingna Su and Dr. Adriaan van Ballegooijen

Email: ynsu_at_head.cfa.harvard.edu

Background:

Solar flares, filament eruptions, and coronal mass ejections (CMEs) are spectacular solar eruptions and the primary drivers of "space weather" at the Earth. In particular, fast CMEs generally cause strong disturbances to the Earths environment. It is well accepted that these phenomena are different manifestations of a single physical process thought to be powered by the release of magnetic free energy stored in the corona prior to the activity. Fast CMEs are often associated with large solar flares, which are produced by magneto-hydrodynamic (MHD) instabilities in active regions on the Sun. These instabilities are due to the presence of strong, highly sheared/twisted magnetic fields. Understanding the nature of these instabilities is very important for predicting the "space weather".

Project:

For this project the student will begin by becoming familiar with both ground- and space-based observations of solar eruptions, as well as the numerical modeling technique, i.e., Coronal Modeling System (CMS) developed by van Ballegooijen (2004). In the past, we have made magnetic field models for smaller and simple flares (GOES B- and C- class). For the current project, we want to study a large flare that produces white-light and even gamma-ray emissions. The best candidate event should occur near disk center and have good observational coverage, e.g., Hinode, SDO, RHESSI, and STEREO etc. A possible candidate is the X2.1 flare occurred on Sep 6, 2011. A movie of this event observed by SDO/AIA can be found here. Two magnetic field models of the flaring active region will be created: (1) best-fit nonlinear force-free field model (NLFFF) of the region prior to the eruption; (2) marginally unstable state (MUS) of the magnetic fields present at the onset of the event. The boundary conditions of the magnetic fields are provided by the magnetograms taken by SOLIS and/or SDO/HMI. The best-fit NLFFF model will be constrained by the corona loops observed by Hinode/XRT and SDO/AIA. The comparison of these two models may shed light on the trigger of the event. The MUS model will be compared with the locations of flare footpoints and ribbons at the event onset, and we will attempt to correlate these features (for example, white light and hard X-ray footpoints) with certain field lines in the model. We will try to measure the magnetic field strength near the reconnection region. We can also compare the flare ribbons and footpoints observed at different layers of atmosphere with the Quasi-Separatrix-Layers calculated from the model.

Image:

Figure 1: Comparison of an unstable model with the flare observations at 2010 April 8. SDO/AIA image at 94 Angstrom taken at 02:39 UT is shown in (a)--(b). The red and green contours refer to the positive and negative magnetic fields observed by SDO/HMI at 02:00 UT. The black dashed lines represent the polarity inversion line. The color lines in (b) are the field lines from an unstable model. (c) Vertical slice of electric currents of the unstable, and the field lines are the same as (b). The white circles refer to the location where the field lines are crossing the vertical plane. (d)--(e) AIA image at 193 Angstrom taken at 03:23 UT. (f) The same current plot as (c). The field lines in (e) and (f) are also from the unstable model. The field of view of images in (d) and (e) (0.25 solar radii) is about three times as that in (a) and (b) (0.08 solar radii). This figure is from Su et al. (2011).

References:

Bobra, M. G., van Ballegooijen, A. A., & DeLuca, E. E. 2008, "Modeling Nonpotential Magnetic Fields in Solar Active Regions", ApJ, 672, 1209

Savcheva, A., & van Ballegooijen, A. A. 2009,"Nonlinear Force-free Modeling of a Long-lasting Coronal Sigmoid", ApJ, 703, 1766

Su, Y. N. 2007, "Magnetic Shear in Two-ribbon Solar Flares", PhD thesis

Su, Y. N., van Ballegooijen, A. A., Lites, B. W., Deluca, E. E., Golub, L., Grigis, P. C., Huang, G., & Ji, H. 2009, "Observations and Nonlinear Force-Free Field Modeling of Active Region 10953", ApJ, 691, 105

Su, Y. N., van Ballegooijen, A. A., Schmieder, B., Berlicki, A., Guo, Y., Golub, L., & Huang, G. L. 2009, "Flare Energy Build-up in a Decaying Active Region Near a Coronal Hole", ApJ, 704, 341

Su, Y. N., Surges, V., van Ballegooijen, A. A., Deluca, E. E., & Golub, L. 2011, " Observations and Magnetic Field Modeling the Flare/CME Event on 2010 April 8", ApJ, 734, 53

van Ballegooijen, A. A. 2004, "Observations and Modeling of a Filament on the Sun", ApJ, 612, 519