Flare Energy Build-Up in a Decaying Active Region

Near a Coronal Hole

Yingna SuSmithsonian 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

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 RopeFriction

Van Ballegooijen 2004; Bobra et al. 2008; Su et al. 2009

PF model NLFFF ModelMagneto

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 2008-05-16 11:42 UT 2008-05-17 08:03 UT

model1 model2

Fpol ϕaxi Fpol ϕaxi

1. 11010 7(15)1019 11010 7(15)1019

2. 0 7(15)1019 0 15(20)1019

3. 0 15(20)1019

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 (Fpol) and axial flux (ϕaxi) 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.

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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