1. Evolution of Flare Ribbons and Energy Release
Ayumi Asai1,
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
NOAA 9415
1. INTRODUCTION
We estimated the released magnetic energy via reconnection in the corona by using photospheric and chromospheric features. We observed an X2.3 flare which occurred in the active region NOAA9415 on 2001 April 10, in Ha with the Sartorius Telescope (Fig. 1) at Kwasan Observatory. Comparing Ha images with hard X-ray (HXR) images obtained with Yohkoh/HXT, we found the difference of spatial distribution between Ha kernels and HXR sources (Fig. 2): Only two sources accompanied with Ha kernels are seen in HXR.
To explain the difference of appearance, we estimated the energy release rates at each Ha kernels by using photospheric magnetic field strength and separation speed of flare ribbons there.
Fig.1 (Left) Ha full disk image obtained with Flare Monitoring Telescope at Hida Observatory. (Right) Sartorius Telescope at Kwasan Observatory. 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:
B : magnetic field strength
vi : inflow velocity
A : area of reconnection region
3. SEPARATION SPEED OF FLARE RIBBONS
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.
We examined the relation of separation speed of the flare ribbons with energy release rate. Fig. 4 shows the separation of the flare ribbons. The separation speeds are determined at the position of the ribbon-front.
Neutral line
Fig.3 Cartoon of magnetic reconnection.
2. MAGNETIC FIELD STRENGTH
We measured magnetic field strength 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. But this is not large enough to explain the spatial distribution of radiation sources.
Table1.
magnetic field strength
at each source (G)
Time Slice
HXR sources appear when separation speed of flare ribbons slow down. This seems to be inversely dependence of separation speed on the energy release rate. However, it is also known that separation of flare ribbon is decelerated at strong magnetic field regions. It is needed to examine both the dependencies of separation speed and magnetic field strength, simultaneously.
Distance from NL
E1 : 300
E2 : 1350
E3 : 550
E4 : 500
W1 : 300
W2 : 1200
W3 : 500
W4 : 450
05:10 ×
HXR source
Then we examine the possibility that vi has some dependence on B, as some reconnection models suggest.
05:30
vi ∝ B0.5 ⇒ dE/dt ∝ B2.5 : Sweet-Parker type reconnection
vi ∝ B ⇒ dE/dt ∝ B3 : Petschek type reconnection
time
Energy release rates at the HXR sources is times larger than those at the other Ha kernels.
It is larger than the dynamic range of HXT.
Fig.4 Separation of flare ribbons (Ha time slice image) and HXR sources.
Magnetic Field Strength
4. RECONNECTION RATE AND POYNTING FLUX
distance from
neutral line
Reconnection rate (v×B) and Poynting flux (v×B2) are suitable to estimate energy release rate by using photospheric magnetic field and separation of Ha flare ribbons .
Separating Velocity
Those evolutions are well fitted with HXR light curves (Fig. 5).
The estimated reconnection rate and Poynting flux are large enough locally at the HXR sources, and can explain the difference of spatial distributions between HXR and Ha images.
Reconnection rate
Bcoronavi = Bphotospherevfoot
(Conservation of magnetic flux)
Microwave
Poynting Flux
Bcorona2vi ∝ Bphotosphere2vfoot
Reconnection
Rate v×B
(Bcorona∝Bphotosphere is assumed)
HXR
Fig.5 Comparison of reconnection rate and Poynting flux with nonthermal light curves (microwave and HXR).
Poynting Flux
v×B2
We made extensive use of Yohkoh Data Center, and SOHO MDI Data Service.