Energetics of Solar Flares: Combined Observations and Implications for Chromospheric Evaporation

1. Co-spatial White Light and Hard X-ray Flare Footpoints seen above the Solar Limb:RHESSI and HMI observations Implications for flare energetics and chromospheric evaporation Säm Krucker Space Sciences Laboratory, UC Berkeley University of Applied Sciences Northwestern Switzerland

2. HXR bremsstrahlung thermal bremsstrahlung T ~ 30 MK • Flare loop T, V, n • Spectrum of acc. electrons non-thermal bremsstrahlung accelerated electrons with typical energies above ~10 keV

3. Flare footpoints in WL and HXR Close connection in space, time, and intensity. 2 arcsec Carrington, R. C., 1859

4. Flare ribbons in WL and HXR Close connection in space, time, and intensity. 25-100 keV SOT G-band 2 arcsec Krucker et al. 2011 Carrington, R. C., 1859

5. Heating of flare ribbons SDO/HMI (6173 A) Significant fraction of flare energy is radiated away in optical range

6. Statistical study HMI/RHESSI: all flares have WL footpoints SDO/AIA 171 A (~1 MK) HMI 6173 A increase (Kuhar et al. 2015, TBS) RHESSI 30 keV flux Flares larger than GOES M5 can be generally detected with HMI, for smaller events non-flare related time variations are hiding flare mission.

7. Good correlation strongly suggests that flare-accelerated electrons are involved in the production of the WL emission E0=low energy cutoff in electron spectrum Assuming: thick target (HXR) black body (WL) 40-100 keV SOT G-band Kyoko Watanabe et al. 2010

8. IRIS continuum observations from flare ribbon (GOES X1) IRIS continuum RHESSI 30-100 keV Heinzel & Kleint 2014 Kleint et al. 2015 (to be submitted)

9. IRIS, HMI, FIRS continuum HMI FIRS IRIS Kleint et al. 2015 (to be submitted)

10. IRIS, HMI, FIRS continuum HMI FIRS IRIS Energy in >27 keV is equal to radiative losses in optical range Kleint et al. 2015 (to be submitted)

11. Next step: compare to modeling First attempt: Thick target model Abbett et al. 2015 (to be submitted)

12. Heating & exponential decay (t ~ 10 s) Penn et al. 2015 (to be submitted)

13. Heating of flare ribbons to ~MK De-saturated AIA 171A (Schwartz et al. 2014) SDO/AIA 171 A (~1 MK) Heated ribbon can evaporate hot plasma into corona to form flare loop Thermal conduction from hot coronal loop can also drive evaporation

14. Heating of flare ribbons to ~MK SDO/AIA 171 A (~1 MK) Hot ribbons, but colder than post flare loops. XRT to constrain higher temperatures?

15. Where do flare accelerated electrons heat the chromosphere? • Thick target beam model gives altitudes of HXR source of ~800-3000 km (see Battaglia et al. 2012): • Density • Energy of electrons • Pitch angle • Ionization level • Field line tilt • Since these parameters are mostly unknown, there is no unique prediction. flare-accelerated electrons HXR source in chromosphere due to bremsstrahlung Range for low density models 800 – 1500 km photosphere

16. Stereoscopic observations • Martinez-Oliveros et al. 2012: • RHESSI/HMI/STEREO • Use STEREO EUV ribbon as proxy • Single event • Absolute source height at • 305+-170 km • 195+-70 km • This is surprisingly low: • Very, very low density • Source within Wilson depression • Not thick target beam model flare-accelerated electrons HXR source in chromosphere due to bremsstrahlung Range for low density models 800 – 1500 km indirectly observed photosphere

17. Stereoscopic observations • Martinez-Oliveros et al. 2012: • RHESSI/HMI/STEREO • Use STEREO EUV ribbon as proxy • Single event • Absolute source height at • 305+-170 km • 195+-70 km • This is surprisingly low: • Very, very low density • Source within Wilson depression • Not thick target beam model flare-accelerated electrons HXR source in chromosphere due to bremsstrahlung 800 – 1500 km photosphere

18. Altitude of WL source? • Optical emission is thought to be thermal emission at low temperatures (~10 000 K) • Heated by electrons: co-spatial source with HXRs • Backwarming would predict a lower altitude. • This talk: look at flares that occur within one degree of limb passage (Krucker et al. 2015). • HMI (617.3 nm): • resolution: 1.1” • placing: <0.1” • RHESSI hard X-rays: • resolution: 2.3” • placing: <0.1” flare-accelerated electrons HXR source in chromosphere due to bremsstrahlung Range for low density models 800 – 1500 km ? ? photosphere

19. Emission from the limb is influenced by the opacity of the atmosphere ~350 km disk radiation cannot escape

20. STEREO is used to get flare location relative to limb Projection effects estimated to be less than 100 km for derived altitudes for selected events.

21. Co-spatial HXR and WL footpoints Image+ Non-thermal above the loop top thermal loop top footpoints Image: HMI with pre-flare image subtracted. Black is enhanced emission. 30-80 keV 617.3 nm

22. Co-spatial HXR and WL footpoints Image+ pre-flare derivative pre-flare pre-flare flare footpoint Altitude above photosphere: WL: 810+-70 km HXR: 722+-122 km Low values for TTBM 30-80 keV 617.3 nm

23. Two similar events Image+ 30-80 keV 617.3 nm 30-80 keV 617.3 nm

24. Synchronous source motion in HXR and WL

25. Time evolution of fluxes and altitudes GOES 617.3 nm 30-80 keV 30-80 keV 617.3 nm Consistent results: co-spatial emission below ~1000 km

26. Implications of co-spatial sources • HXR emission comes from relative cold plasma • HXR producing electrons (>30 keV) do not heat chromosphere to millions of degrees • Energy of >30 keV electrons are lost by radiation in the optical range • >30 keV electrons are not responsible for evaporation! • Heating to MK by low energy electrons at higher altitudes? energy goes into evaporated plasma? energy is lost to WL radiation flux lower energies? >30 keV electron energy observation of footpoints at lower energies (<20 keV) very difficult due to limited dynamic range of RHESSI.

27. Low-energy (thermal) emission from footpoints is lost in limited dynamic range Upper limits of footpoint emission at low-energies ? Spectra of footpoint as inferred from images

28. Low-energy (thermal) emission from footpoints is lost in limited dynamic range Upper limits of footpoint emission at low-energies ? Spectra of footpoint as inferred from images HXR focusing optics can overcome this limitation

29. HINODE XRT and SOT observations • XRT hot filters: • constrain high temperatures in footpoints • Time evolution: conduction vs beam heating • Locate GOES fast time variations • is a 2 second cadence to match GOES feasible, at least for some time intervals during the flare? • SOT • Fast time cadence to observe decay on second time scale • Is ~2 second cadence possible in a single filter?

30. t=0 t=19 s t=3 s t=22 s • HINODE SOT RGB • Small source sizes • Footpoint motion? • fast decay • mismatch between spatial and time resolution • Proposition to occasionally run flare mode with higher time cadence, maybe only one filter. Summing over pixel to save telemetry. t=6 s t=25 s

31. Summary • HXR source altitude is low <1000 km • TTBM works only with very low density models, strongly beamed case • Co-spatial WL and HXR sources: • energy of >30 keV electrons is radiated in optical range • >30 keV electrons are not responsible for evaporation! • Unexpected results with implication on our standard picture flare-accelerated electrons Co-spatial HXR and WL sources <1000 Mm Additional source? photosphere