1. Evidence for chromospheric heating in the late phase of solar flares David Alexander Lockheed Martin Solar and Astrophysics Lab. Collaborators: Anja CzaykowskaMPI für extraterrestrische Physik Bart De PontieuLMSAL

2. Summary of Presentation Chromospheric evaporation revisited Coronal Diagnostic Spectrometer Summary of flare Implications for chromospheric heating Conclusions Results of data analysis

3. from Cargill & Priest (1983) Non-thermal Non-thermal : energy deposition of energetic particles accelerated in flare Brown (1973) ; Hirayama (1974) ; Nagai & Emslie (1984) ; Fisher, Canfield & McClymont (1985) ; Mariska, Emslie & Li (1989) Thermal Thermal : energy is transported to chromosphere via thermal conduction fronts of related shocks Brown (1974) ; Hirayama (1974) ; Antiochos & Sturrock (1978) ; Forbes, Malherbe & Priest (1989) ; Yokoyama & Shibata (1997)

4. Fisher et al., 1985a,b made distinction between gentle and explosive evaporation  Gentle evaporation  Velocities < 100 km/s Upflow velocities depend crucially on total flux of electrons. Fisher et al. (1985a,b): F ~ (E/E c ) -  :  = 4 ; E c = 20 keV Mariska et al. (1989): F ~ (E/E c ) -  :  = 6 ; E c = 15 keV f = 10 9 ergs/cm 2 /s  V upflow < 30 km/s f =10 10 ergs/cm 2 /s  V upflow ~ 130 km/s f = total incident electron energy flux f=10 10 ergs/cm 2 /s  V upflow  200 km/s

5. MDI BBSO H  EIT FeXIICDS OV CDS FeXVI CDS FeXIX QUICK LOOK AT THE FLARE 1.5 MK 0.25 MK 2.0 MK8.0 MK Sunspot + plage expanding ribbons

6. CDS DOPPLERGRAMS distinctive pattern of redshifts and blueshifts blueshifts confined to leading edges of arcade redshifts predominate towards neutral line Interesting differences near sunspot

7. Velocity profiles Spatial profiles (a) show transition from blue- to red-shift. Line profiles (b) show broad lines but resolvable shifts Different locations along ribbon show similar behaviour Velocity discrimination OV:  v ~ 5-10 km/s FeXVI:  v ~ 10-20 km/s FeXIX:  v ~ 30 km/s

8. Location of upflow regions Upflows at leading edge of H  ribbon Ridge of upflowing plasma moves with H  ribbon Upflow regions become downflow regions as ribbons move outwards

9. Mach 2 Jet H  loops UV loops Current sheet Termination shock Conduction front Evaporative upflows Condensation downflow Continued heating in late gradual phase The CDS observations provide direct evidence for the presence of continuing energisation presumably due to ongoing reconnection non-thermalorthermal?

10. Hard X-ray Observations The ratio of the counts in the two medium energy bands HXT M2/M1 yields a photon spectral index of  4 during the initial decay phase of the flare. All channels show a count rate below background levels by about 17:00 UT, some 40 minutes prior to the first CDS observations. Yohkoh HXT Background level in HXT L channel is 1.25 cts/s/SC or 80 cts/s summed over all detectors. Thus, a background subtracted signal strength of 26 cts/s will produce a 2  detection in the integrated HXT L channel.

11. Hard X-ray production from a non-thermal electron beam Assume that chromosphere acts like a thick-target to a beam of electrons with energy distribution: F=AE -    Convolve photon spectrum with HXT response function to get count rate in HXT L channel:  (  ) is the transmission efficiency of the HXT filter, G( ,p) is the pulse height distribution of the detector s(  ) is the probability that an incoming photon will escape with an energy . Alexander & Metcalf (1999) Mariska, Emslie & Li (1989)

12. Fisher et al., 1985a,b made distinction between gentle and explosive evaporation  Gentle evaporation  Velocities < 100 km/s Upflow velocities depend crucially on total flux of electrons. Fisher et al. (1985a,b): F ~ (E/E c ) -  :  = 4 ; E c = 20 keV Mariska et al. (1989): F ~ (E/E c ) -  :  = 6 ; E c = 15 keV f = 10 9 ergs/cm 2 /s  V upflow < 30 km/s f =10 10 ergs/cm 2 /s  V upflow ~ 130 km/s f = total incident electron energy flux f=10 10 ergs/cm 2 /s  V upflow  200 km/s Observed upflows  10 9  f  10 10 ergs/cm 2 /s

13. Simulated HXR emission Single footpoint E c = 20 keV ; N(E<E c )=E -2 20 footpoints E c = 20 keV ; N(E<E c )=E -2 HXT L flux (cts/s) 3 4 5 6 7 8 Spectral Index  f = 10 10 f = 10 9 2  detection Expected HXT L channel count rates as a function of spectral index Single footpoint means S=10 17 cm 2  1 CDS pixel Electron fluxes necessary to produce observed upflow velocities would also generate detectable hard X-ray signatures

14. Chromospheric Heating: conduction fronts (I) Forbes & Malherbe (1986) Forbes, Malherbe & Priest (1989) Electrons are heated as they diffuse through the conduction front. Fronts stand in front of slow-mode shocks For efficient heating the thermal thickness of the slow shock must exceed the height of the flare loop (~ 5 x 10 4 km): T = 10 MK, n  2x10 10 cm -3, v ||  50km/s, c p = 2.07x10 8 cm 2 s -2 K -1  w = 9 x 10 4 km

15. Velocities Forbes et al. predict very small evaporative flows: v  5 km/s Recent numerical reconnection model of Yokoyama & Shibata (1997) includes conduction and yields evaporative upflows with speeds ~0.2 - 0.3 x the local sound speed: v  40 km/s Chromospheric Heating: conduction fronts (II) w = 9 x 10 4 km

16. Velocities Forbes et al. predict very small evaporative flows: v  5 km/s Recent numerical reconnection model of Yokoyama & Shibata (1997) includes conduction and yields evaporative upflows with speeds ~0.2 - 0.3 x the local sound speed: v  40 km/s Chromospheric Heating: conduction fronts (II) w = 9 x 10 4 km Thus, our observations suggest that conduction front heating of the chromosphere dominates at this stage of the flare. This agrees well with the conclusions of Falchi, Qiu & Cauzzi (1997) who detected 20-30 km/s downflows at the outer edge of Ha ribbons in the decay phase of an M2.6 flare.

17. Reconnection is an ongoing process throughout the entire duration of a solar flare. The dominant consequences of that reconnection transition smoothly(?) from energetic particle production to shock and conduction front formation. cf. Wülser et al (1994) Conclusions Outstanding questions Late phase particle population HESSI radio E < 15 keV  << 8 protons? Relative strength of thermal/non-thermal heating with time HESSI CDS