Measurement of the Reconnection Rate in Solar Flares

1. Measurement of the Reconnection Rate in Solar Flares H. Isobe 2004/12/6 Taiyo-Zasshikai

2. Chromospheric brightenings during solar flares are • Observation of

3. Reconnection rate Magnetic flux reconnected per unit time is given by BVin = E (Electric field) or in dimensionless form, Vinflow/VA The electric field E can be inferred from the motion of flare footpoint emission (flare ribbons) and magnetograms.

4. “Mortion of Flare Footpoint Emission and Inferred Electric Field in Reconnecting Current Sheet” Qui, J., Lee, J., Gary, D. E., Wang, H. (2002, ApJ, 565, 1335) C9.0 flare on 2000 Mar. 16 (NOAA8906) Ha (BBSO) HXR (Yohkoh)

5. Qui et al. 2002 • E=VxB=9000 V/m (maximum) • Temporal correlation with HXR • 2D approximation seems bad.

6. “Study of Ribbon Separation of a Flare Associated with a Quiescent Filament Eruption” Wang, H., Qui, J., Jing, J. & Zhang, H (2003, ApJ, 593, 564) M1 flare 2000 Sep. 12 Quiet region E=100 V/m (10 V/m in decay phase) H alpha image from Kanzelhohe Solar Observatory

7. Wang et al. 2003 Good correlation with ribbon separation and filament/CME acceleration.

8. “Magnetic Reconnection and Mass Acceleration in Flare-Coronal Mass Ejection Event” Qui, Wang, Cheng, & Gary (2004, ApJ, 604, 900) X1.6 flare 2001 Oct. 19 NOAA 9661 E=580 V/m M1.0 flare 2000 Sep. 12 Decaying AR E=50 V/m

9. Qui et al. 2004 Good correlation with filament/CME acceleration and the reconnection rate (E)

10. “Tracking of TRACE Ultraviolet Flare Footpoints”Fletcher, Pollock, & Potts (2004, Sol. Phys., 222, 279) M8.5 flare 2002 Jul. 17 • Focus on the fine structure of the ribbon and motion of each kernel. • Found evidence that UKV kernels move along the boundary of granules.

11. How to calculate the reconnection rate(Isobe et al. 2002, ApJ, 566, 528) • Energy release rate by reconnection is given by the Poynting flux into the reconnection region; Lz Ly H: heating rate (erg/s), Vin: inflow velocity, Bc: magnetic field strength in the corona. Ly, Lz: size of the reconnection region ~ size of the flare arcade • Conservation of the magnetic flux; Bfoot: magnetic field strength at the photosphere Vfoot: separation velocity of the flare ribbons

12. Energy release rate H • Temperature and emission measure (and hence density, with assumption of the line-of-sight depth) calculated from Yohkoh/SXT data using filter ratio method. • To estimate the cooling terms Isobe et al. (2002) used following approximate forms: Radiative cooling Conductive cooling Possible errors: (1) conduction may be overestimated, (2) enthalpy flux from chromospheric evaporation not considered. =>we carried out 1D numerical simulation of a flare loop.

13. Numerical simulation of 1D flare loop • 1D hydrodynamical simulations • Flare energy injected at loop top • Nonlinear (Spitzer) heat conduction, radiative cooling, chromospheric evaporation Pseudo observation of the simulation result 1.From the temperature and density (emission measure) obtained from simulations, soft X-ray emissions detected by the bandpass filters of Yohkoh/SXT are calculated. 2. From the spatially integrated SXT count rates of the simulation result for Be and Thick Al. filters, temperature, emission measure, and hence thermal energy are calculated. 3. The rate of thermal energy increase during the impulsive phase is compared withthe energy injection rate in the simulation.

14. Scaling law of the dEth/dt and energy release rate • A series of simulations with different loop length and flare energy allows us to derive a scaling law that gives the ratio of thermal energy that would beobserved by SXT to the real energy release rate. • The ratio is 0.3〜0.8 depending on loop length and input heat flux. Ratio to input energy fux

15. Events X2.3 class 2000/11/24 M3.7 class 2000/07/14 C8.9 class 2000/11/16 GOES X-ray light curve TRACE 1600 A image

16. Data analyses 1. Vfoot and Bfoot TRACE 1600 A image SOHO/MDI Time slice of the flare ribbons => Vfoot Coalignment with magnetogram => Bfoot

17. Data analyses 2. Energy release rate Thick Al. Be SXT count rates Eth • Energy release rate H is calculated from Eth obtained from SXT data and the scaling law.

18. Results 1. Reconnection Rate

19. Results 2. Electric field • Electric field in the current sheet is larger in C class than in M class. • The length along the flare ribbon is nearly the same in these flares. Therefore the total voltage drop along the current sheet is also smaller in the M class flare. • Acceleration is not by direct electric field?

20. Summary • We propose an indirect method to determine the inflow velocity, coronal magnetic field, and hence reconnection rate from observational data. • Applying the method to three two-ribbon flares, we found that reconnection rate is 0.003-0.08. These values are consistent with previous studies. • The macroscopic electric field in the reconnecting current sheet inferred from separation velocity of the flare ribbons shows no correlation with hard X-ray count rate, though data samples are too few. • It is likely that the macroscopic electric field influences the spectrum of the high energy particles. Statistical study using RHESSI data may answer the question.

23. Asai et al. 2003, 2004 • 1230 Gauss, 63km/s, 7.7 kV/s • GOES X2.3 HXT Hband 80/s/sc (only after peak) • Measure vfBf and vfBf^2 (proxy of Poynting flux in the corona). Their difference along the ribbon is large enough to explain the localization of HXR source.

24. Electric Field vs GOES class SXR flux (GOES) Electric field (V/m) Results from Isobe, Takasaki, Shibata in prep, Qui et al. 2002, 2004, Wang et al. 2003, Asai et al. 2003