Flare Energy Build-Up in a Decaying Active Region Near a Coronal Hole Yingna Su Smithsonian Astrophysical Observatory Collaborators: A. A. van Ballegooijen, B. Schmieder, Berlicki, A., Y. Guo, L. Golub, G. L. Huang XRT Team Meeting Talk, Cambridge, MA, June 24, 2009

Introduction  Solar eruptions free energy sheared/twisted field  Question: When and under which condition a solar eruption occurs?  Evolution of sheared fields prior to the flare  3D pre-flare magnetic configuration  Method: Study an eruptive flare in a small decaying region  Observations: EUV (TRACE, STEREO), X-rays (XRT), H-alpha (BBSO, THEMIS), Magnetogram (THEMIS, MDI)  Modeling: None Linear Force Free Field Modeling (NLFFF)

Observations

Observations of a Flare on May 17, 2008 Two sets of highly sheared loops before the flare The flare (B1.7): near coronal hole, quasi-circular ribbons, coronal dimming, filament eruption, and CME Nearly potential loops after the flare

Following a filament eruption around 20:15 UT on May 16, the two short loop systems corresponding to the southern filament evolved into one long J-shaped loop system. Evolution of Coronal Loops before the Flare

Formation of bright sheared loops in the south-eastern part of the region as observed by STEREO-B. This may be evidence of an extension of the southern filament channel.

Evolution of Photospheric Magnetic Field Several hours before the flare: flux cancellations in boxes 1, 2, and 3; flux emergence in box 2. No significant evolution of the LOS magnetic field is observed closely associated with the B1.7 flare.

Modeling

Flux Rope Insertion Method Insert Flux Rope Friction Van Ballegooijen 2004; Bobra et al. 2008; Su et al PF modelNLFFF Model Magneto

Model Constraints Model Free Parameters: Poloidal and Axial Fluxes Model is constrained by observed highly sheared coronal loops

Comparison of Two NLFFF Models

Flux Rope :42 UT :03 UT model1 model2 F pol ϕ axi F pol ϕ axi 1. 1  (15)   (15)  (15)  (20)  (20)  Table 1. Parameters for three NLFFF models. Models 1 and 2 contain two flux ropes, and model 3 contains three flux ropes. The poloidal flux (F pol ) and axial flux ( ϕ axi ) of the flux rope are in units of Mx/cm and Mx, respectively. The upper limit of the axial flux of the flux ropes is given in brackets. The axial flux of the flux rope in the NLFFF model on May 17 is twice that on May 16, and the model on May 17 is only marginally stable. Model Parameters

Comparison of Horizontal Field PF NLFFF There is a significant difference between the observed (blue) and modeled (black) vectors at the photospheric level. No significant difference between the PF model and the NLFFF model at photosphere. However, the NLFFF is further away from the PF at the chromosphere.

Flare ribbons and Separatrix The outer flare ribbons are associated with the separatrix between open and closed fields.

Discussion

Null-Point and Fan-Separatrix Topology A magnetic null (more like a line of nulls) exists in the corona of the active region prior to the B1.7 flare. This flare may be triggered by reconnection at the null point.

Conclusions  A “large” B1.7 Flare  Build-Up of free energy prior to the flare Observations: formation of southern long J loops and sheared loops in SE Pre-flare NLFFF modeling--increase of axial flux, and an additional flux rope  Flare onset: reconnection at the null point Null point in the corona. Flare ribbon ---Separatrix Surface  Horizontal field in the photosphere: not sensitive to the non-potential field in the corona.

Thank you

Different Flare/CME Models Internal Tether Cutting (2.5D, Su et al. 2006) Internal Tether Cutting (3D, Moore et al. 2001) Break-out Model (Antiochos 1999; MacNeice et al. 2004) Loss-of-equilibrium (Forbes & Priest 1995)

Null-Point and Fan-Separatrix Topology Antiochos 1998; Pariat et al. 2009