Synthetic Analysis of a Two-ribbon Microflare

1. Synthetic Analysis of a Two-ribbon Microflare C. Fang, R. L. Jiang, P. F. Chen, Y. H. Tang Department of Astronomy, Nanjing University, Nanjing, China(fangc@nju.edu.cn) Abstract High-resolution Stokes spectral data of Hα, CaII 8542 Å, and FeI 6302.5 Å lines for a two-ribbon microflare (TRMF) were simultaneously observed by THEMIS on 2002 September 5th. We obtained the intensity, velocity and longitudinal magnetic field maps. The hard X-ray emission observed by RHESSI provides evidence of non-thermal particle acceleration in the TRMF. Using the Hα and CaII 8542 Å line profiles and the non-LTE calculation, we obtain the semi-empirical atmospheric models for two brightest kernels of the MTRF. Our result indicates that the maximum temperature enhancement in the chromosphere is about 2500 K, and the particle bombardment is negligible in the heating of low atmosphere. Our numerical simulation shows that the heating is probably caused by magnetic reconnection in the chromosphere. The kinetic and radiative energy of the TRMF are also estimated. Fig. 2 Temperature distribution (left) of the semi-empirical model for the TRMF (dotted lines) at one brightest kernel, comparing with that of our simulation (solid line), and the quiet Sun model VALC (dashed line). The computed (solid lines) and observed (dashed lines) line profiles are also shown (right). With gravity, ionization and radiation considered, two-dimensional time-dependent compressible resistive MHD equations are solved by numerical simulation with a CIP-MOCCT code. We used a modified empirical formula to calculate the radiative loss. The initial magnetic configuration is a force-free field ( a current sheet ) surrounded by a potential field. Line-tying condition was applied to the bottom boundary, and symmetry condition to the left-hand side. The right-hand and the top sides were treated as open boundaries. Data Analysis and Results Using the Hαline profiles and CaII 8542 Å Stokes-V profiles of the TRMF on 2002 5th September, we have obtained the line-of-sight velocity field VII and the longitudinal magnetic field BII respectively. Our analyses indicate that the TRMF is located beside a magnetic neutral line and is accompanied by mass motion (Xia et al. 2007). Using RHESSI data we obtained the HXR image and the power index of the TRMF. Figure 1 shows the results. It indicates that the HXR emission is closely related to the flare, and the energy flux of the electron beam is about 1010 ergs/s/cm2 above 10 keV with a power index of 9.1. Fig. 3 Evolution of magnetic reconnection with the X-point at 800 km and B = 100 G. Colors stand for the temperature, solid lines for magnetic field and arrows for the velocity. The top-left panel shows the initial condition, and the others for the result at different times. The resulted temperature enhancement of our simulation is given in Figure 2. It can be seen that the temperature enhancement of the semi-empirical model of the TRMF can be qualitatively produced by magnetic reconnection in the chromosphere. Fig. 1Hard X-ray andSoft X-ray emission of the TRMF (left), overlap of HXR image on the BII map (middle), power law fit of the HXR emission of the TRMF (right). Conclusions Using non-LTE theory and the observed Hαand CaII 8542 Å line profiles, we computed semi-empirical atmospheric models of the TRMF.Figure 2 gives the model and the comparison between the computed and observed line profiles. Our result indicates that there is a temperature bump in the chromosphere. The maximum temperature enhancement is about 2500 K. Using the semi-empirical models and measured velocities, we computed the radiative and kinetic energies of the TRMF, which is about 3.5 1029 ergs in total. The TRMF is located beside the longitudinal magnetic polarity inversion line and is accompanied by mass motion. The analysis of RHESSI data indicates that energy flux of the electron beam is about 1010 ergs/s/cm2 above 10 keV with a power index of 9.1. The thermal semi-empirical atmospheric model indicates that the maximum temperature enhancement is about 2500 K in the chromosphere. Our simulation implies that the TRMF can be produced by magnetic reconnection in the solar chromosphere.