1. H α and hard X-ray observations of solar white-light flares M. D. Ding Department of Astronomy, Nanjing University

2. A brief review WLFs originate from deeper layers than ordinary flares WLFs originate from deeper layers than ordinary flares Many examples support a relationship between WL emission and energetic electrons Many examples support a relationship between WL emission and energetic electrons Heating in the lower layers in WLFs remains unsettled Heating in the lower layers in WLFs remains unsettled

3. Previous radiation and heating mechanisms Hydrogen recombinations (free-bound transition) in chromosphere and negative hydrogen emission in photoshphere Hydrogen recombinations (free-bound transition) in chromosphere and negative hydrogen emission in photoshphere Heating mechanisms: Heating mechanisms: electron beam (Aboudarham & Henoux 1986) electron beam (Aboudarham & Henoux 1986) proton beam (Machado et al. 1978) proton beam (Machado et al. 1978) soft-X-ray irradiation (Henoux & Nakagawa 1977) soft-X-ray irradiation (Henoux & Nakagawa 1977) EUV irradiation (Poland et al. 1988) EUV irradiation (Poland et al. 1988) Alfvén wave dissipation (Emslie & Sturrock 1982) Alfvén wave dissipation (Emslie & Sturrock 1982) Backwarming (Machado et al. 1989) Backwarming (Machado et al. 1989)

4. Electron beam ＋ Backwarming （ Ding et al. 2003)

6. Example I The X10 WLF of 2003 October 29 (S15 W02) by BBSO at 1.56 μm (Xu et al. 2004) The X10 WLF of 2003 October 29 (S15 W02) by BBSO at 1.56 μm (Xu et al. 2004) Blue: RHESSI HXR 50-100 keV Red: NIR continuum Blue: RHESSI HXR 50-100 keV Red: NIR continuum

7. Example II The X5.3 WLF of 2001 August 25 (S17 E34) observed by TRACE (Metcalf et al. 2003) The X5.3 WLF of 2001 August 25 (S17 E34) observed by TRACE (Metcalf et al. 2003) Gray images: TRACE WL White contours: YOHKOH HXR at 33-53 keV Gray images: TRACE WL White contours: YOHKOH HXR at 33-53 keV

8. There are, however, examples showing the WL and HXR kernels are not cospatial The X2.2 WLF of 1991 December 3 (N17 E72) observed by YOHKOH (Sylwester & Sylwester 2000) The X2.2 WLF of 1991 December 3 (N17 E72) observed by YOHKOH (Sylwester & Sylwester 2000) Gray images: WL Gray images: WL Solid contours: HXR 33-53 keV Solid contours: HXR 33-53 keV

9. Multi-wavelength observations of white-light flare The M2.6/2B WLF of 2002 September 29 (N12 E21) The M2.6/2B WLF of 2002 September 29 (N12 E21) The M2.1/1B WLF of 2002 September 30 (N13 E10) The M2.1/1B WLF of 2002 September 30 (N13 E10) Simultaneously observed by the imaging spectrograph of the Solar Tower of Nanjing University and by RHESSI Simultaneously observed by the imaging spectrograph of the Solar Tower of Nanjing University and by RHESSI

10. The 2002 September 29 WLF Black contours: 15-50 keV HXR (06:36:00 — 06:36:30 UT) Black contours: 15-50 keV HXR (06:36:00 — 06:36:30 UT) White contours: WL emission (06:36:16 UT) White contours: WL emission (06:36:16 UT) White-dashed line: MDI magnetic neutral line White-dashed line: MDI magnetic neutral line

11. General features The RHESSI images resolve two conjugate footpoints (A and B) The RHESSI images resolve two conjugate footpoints (A and B) The continuum emission and the HXR emission are basically cospatial The continuum emission and the HXR emission are basically cospatial The magnitude of continuum emission is not simply related to the electron beam flux The magnitude of continuum emission is not simply related to the electron beam flux

12. RHESSI HXR images showing that FP A is harder than B RHESSI HXR images showing that FP A is harder than B

13. Power-law fitting of the HXR spectra for kernels A and B Power-law fitting of the HXR spectra for kernels A and B

14. Comparison of Kernels A and B Kernel A Kernel B Energy flux of electron beam 1 ×10 10 ergs cm -2 s -1 0.8 ×10 10 ergs cm -2 s -1 Continuum enhancement ~4%~8% H α line profile Strong, no central reversal Relatively weak, centrally reversed

15. Continuum Contrast

16. FP A: higher coronal pressure FP A: higher coronal pressure FP B: lower coronal pressure FP B: lower coronal pressure

17. The 2002 September 30 WLF Black contours: 12-25 keV HXR Black contours: 12-25 keV HXR White contours: WL emission White contours: WL emission White-dashed line: MDI magnetic neutral line White-dashed line: MDI magnetic neutral line

19. Motion of the flare footpoint in the continuum White-light & 12-25 keV HXR

20. Conclusions The WLF is basically consistent with the electron beam heating model The WLF is basically consistent with the electron beam heating model First detection of a WL footpoint motion First detection of a WL footpoint motion Heating in the lower atmosphere responsible for the WL emission is achieved through backwarming Heating in the lower atmosphere responsible for the WL emission is achieved through backwarming The energy flux of the electron beam derived from the HXR spectra can explain the WL enhancement The energy flux of the electron beam derived from the HXR spectra can explain the WL enhancement The coronal pressure is crutial to the response of the flare footpoints subject to electron beam heating The coronal pressure is crutial to the response of the flare footpoints subject to electron beam heating