Evolution of Flare Ribbons and Energy Release Ayumi Asai (浅井 歩)1, Satoshi Masuda2, Takaaki Yokoyama3, Masumi Shimojo3, Takako T. Ishii1, Hiroaki Isobe1, Kazunari Shibata1, and Hiroki Kurokawa1 1:Kwasan and Hida Observatories, Kyoto University 2:Solar-Telestorial Environment Laboratory, Nagoya University 3:Nobeyama Radio Observatory, NAOJ 2nd Korea-Japan-China Joint Workshop, Rikubetsu, Japan, 2-4 October 2002 1. INTRODUCTION To solve the questions about particle acceleration, it is important to investigate when and where energy release occurs and the amount of the released magnetic energy during solar flares. We estimated the released energy via magnetic reconnection in the corona by using photospheric and chromospheric features. We observed an X2.3 flare which occurred in active region NOAA9415 on 2001 April 10 (Fig. 1), in Ha with the Sartorius Telescope at Kwasan Observatory. To explain the different appearance, we estimated the energy release rates at each Ha kernels by using photospheric magnetic field strength and separation speed of flare ribbons. W1 HXR sources E1 W2 W3 E2 E3 E4 W4 Fig.2 Ha image and HXR image. Energy release rate (dE/dt) is written as: NOAA 9415 B : magnetic field strength vi : inflow velocity A : area of reconnection region Comparing the Ha images with the hard X-ray (HXR) images obtained with Yohkoh/HXT, we found the difference between the spatial distribution of the Ha kernels and that of the HXR sources (Fig. 2): only two sources accompanied with Ha kernels are seen in HXR. The dynamic range of HXT is low (～10). If energy release rates at the HXR sources are (at least) 10 times larger than those at the other Ha kernels, the difference of appearance can be explained. Fig.1 Ha full disk image obtained with Flare Monitoring Telescope at Hida Observatory. Fig.3 Cartoon of magnetic reconnection. 2. MAGNETIC FIELD STRENGTH 3. SEPARATION SPEED OF FLARE RIBBONS We measured the photospheric magnetic field strengths at each Ha kernel.They are 3 times larger at the HXR sources than the other Ha kernels (Table 1). If vi has no dependence on B, then dE/dt ∝ B2. This shows that dE/dts at the HXR sources are about 9 time larger. This is not large enough. Neutral line We examined the relation between the separation speed of the flare ribbons with the energy release rate. Fig. 4 shows the separation of the flare ribbons. Table1. magnetic field strength at each source (G) Time Slice E1 : 300 E2 : 1350 E3 : 550 E4 : 500 W1 : 300 W2 : 1200 W3 : 500 W4 : 450 HXR sources appear when the separation speed of the flare ribbons reduces. This seems that the energy release rate inversely depend on the separation speed. However, it is also known that the separation of the flare ribbon is decelerated by strong magnetic field. It is needed to examine both the dependencies of separation speed and magnetic field strength, simultaneously. Distance 05:10 × Then we examine the possibility that vi has some dependence on B, as some reconnection models suggest. HXR source vi ∝ B0.5 ⇒ dE/dt ∝ B2.5 : Sweet-Parker type reconnection vi ∝ B ⇒ dE/dt ∝ B3 : Petschek type reconnection 05:30 The estimated energy release rates at the HXR sources is 16 and 27 times larger than those at the other Ha kernels, respectively. It is larger than the dynamic range of HXT. time Fig.4 Separation of the Ha flare ribbons (Ha time slice image). The cross shows an HXR source. Magnetic Field Strength 4. RECONNECTION RATE AND POYNTING FLUX We estimated the temporal evolutions of the reconnection rate and the Poynting flux along a slit put on an HXR source, like sec.3. The temporal evolutions of the reconnection rate and the Poyntin flux are well fitted with the HXR light curve (Fig. 5). Comparing the evolutions at each slit, we found that they are also locally large enough at the HXR sources, and can explain the difference of spatial distributions between HXR and Ha images. distance from neutral line Reconnection rate (v×B) and Poynting flux (v×B2) are suitable to estimate the energy release rate by using the photospheric magnetic field strengths and the separation speed of the Ha flare ribbons . Separating Velocity Microwave Bcoronavi = Bphotospherevfoot Bcorona2vi ∝ Bphotosphere2vfoot Reconnection rate Poynting Flux (Conservation of magnetic flux) (Bcorona∝Bphotosphere is assumed) Reconnection Rate v×B HXR Poynting Flux v×B2 We made extensive use of Yohkoh Data Center, and SOHO MDI Data Service. Fig.5 Comparison of the reconnection rate and the Poynting flux with nonthermal light curves (microwave and HXR).