Understanding solar flares from optical observations Heinzel, P Understanding solar flares from optical observations Heinzel, P. 2003, Adv. Space Res. 32, 2393 S. Kamio Solar seminar 2004.07.05

Brief summary This is a review on the behavior of chromosphere during the impulsive phase of flares, mainly from theoretical aspect. Fast fluctuation of H-alpha and HXR Radiative-Hydrodynamical models of flares Line asymmetries and flare dynamics

H-alpha H-alpha emission in the flare is the responses to particle beams. Kasparova and Heinzel (2002) Line profile depends on the height of maximum energy deposition core wing

Fast fluctuations H-alpha fluctuation is correlated with HXR Dennis et al.(1987) Kundu et al.(1989) Rolli et al.(1998) Asai et al.(2002) Correlation of subsecond fluctuations has not yet been established.

Modeling Non-thermal emission can be modeled with statistical-equilibrium, neglecting dynamics. Radiation Collision Non-thermal collisional rate Heinzel(1991) Electron density variation do not follow temperature Significant response in H-alpha H-alpha intensity drop at pulse onset (sub-second)

RHD simulation Fisher et al.(1985) Chromospheric evaporation depends on energy flux Heating ～ radiation Gentle upflow Heating >> radiation Explosive evaporation Up and down flows Velocity reverse above flare transition region height

up Return current down Electron beams penetrate into high resistivity plasma current heating energy deposit in higher atmosphere Karlicky and Henoux (1992) Strong heating in upper transition region But the flow is highly transient

Red asymmetry Emission enhancement of the red wing in the H-alpha line (Ichimoto and Kurokawa,1984) Caused by downward moving chromospheric condensation (Canfield and Gayley, 1987) Corona Explosive evaporation Chromosphere

Blue asymmetry? Less frequently observed (Heinzel, 1994) Blue asymmetry is seen at onset of a flare and disappears within a few minutes. Found in centrally reversed line profile (Svestka, 1976)

Interpretation Emission in the red wing is absorbed by downward moving plasma. (Heinzel, 1994) Down flow in upper chromosphere can produce a blue asymmetry. (Ding and Fang, 1997) Blue asymmetry is seen in a special condition λ λ

Required optical data Ideally… Sub-second temporal variation of spectral line profiles in the whole 2D field of view. Currently available Spectrograph with fast CCD (1D) Multi spectral line data Wavelength scanning of narrow band filter Also of note Collaboration with RHESSI Impact polarization

Conclusion Various attempts have been made to derive the physical condition of the flare atmosphere from optical line observation. Detailed comparison of time-dependent RHD simulations with high-resolution spectral observation in optical, IR, UV, EUV, and HXR is needed to understand the flare evolution.

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