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Evidence for chromospheric heating in the late phase of solar flares

Evidence for chromospheric heating in the late phase of solar flares. David Alexander Lockheed Martin Solar and Astrophysics Lab. Collaborators: Anja Czaykowska MPI für extraterrestrische Physik Bart De Pontieu LMSAL. Summary of Presentation. SOHO/CDS.

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Evidence for chromospheric heating in the late phase of solar flares

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

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

  3. Chromospheric Evaporation: models • 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 : 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) from Cargill & Priest (1983)

  4. Chromospheric heating: non-thermal • 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/Ec)-d : d = 4 ; Ec = 20 keV • Mariska et al. (1989): F ~ (E/Ec)-d : d = 6 ; Ec = 15 keV f = 109 ergs/cm2/s  Vupflow < 30 km/s f =1010 ergs/cm2/s  Vupflow ~ 130 km/s f = total incident electron energy flux f=1010 ergs/cm2/s  Vupflow 200 km/s

  5. QUICK LOOK AT THE FLARE BBSO Ha MDI Sunspot + plage expanding ribbons EIT FeXII CDS OV 0.25 MK 1.5 MK CDS FeXVI CDS FeXIX 2.0 MK 8.0 MK

  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: Dv ~ 5-10 km/s FeXVI: Dv ~ 10-20 km/s FeXIX: Dv ~ 30 km/s

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

  9. Current sheet Mach 2 Jet Termination shock Conduction front UV loops non-thermal or thermal? Ha loops 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

  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 • g~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 2s 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-d  Mariska, Emslie & Li (1989) Convolve photon spectrum with HXT response function to get count rate in HXT L channel: Alexander & Metcalf (1999) h(e) is the transmission efficiency of the HXT filter, G(e,p) is the pulse height distribution of the detector s(e) is the probability that an incoming photon will escape with an energy e.

  12. Chromospheric heating: non-thermal • 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/Ec)-d : d = 4 ; Ec = 20 keV • Mariska et al. (1989): F ~ (E/Ec)-d : d = 6 ; Ec = 15 keV f = 109 ergs/cm2/s  Vupflow < 30 km/s f =1010 ergs/cm2/s  Vupflow ~ 130 km/s f = total incident electron energy flux f=1010 ergs/cm2/s  Vupflow 200 km/s Observed upflows  109 f  1010 ergs/cm2/s

  13. Simulated HXR emission Single footpoint Ec = 20 keV ; N(E<Ec)=E-2 20 footpoints Ec = 20 keV ; N(E<Ec)=E-2 HXT L flux (cts/s) f = 1010 f = 109 2s detection 3 4 5 6 7 8 3 4 5 6 7 8 Spectral Index d Expected HXT L channel count rates as a function of spectral index Single footpoint means S=1017 cm2 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) • 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 104 km): T = 10 MK, n  2x1010 cm-3, v|| 50km/s, cp = 2.07x108 cm2s-2K-1  w = 9 x 104 km Forbes & Malherbe (1986) Forbes, Malherbe & Priest (1989)

  15. Chromospheric Heating: conduction fronts (II) w = 9 x 104 km 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

  16. Chromospheric Heating: conduction fronts (II) w = 9 x 104 km 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 Thus, our observations suggest that conduction front heating of the chromosphere dominates at this stage of the flare. This agrees well with the conclusions ofFalchi, 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. Outstanding questions E < 15 keV d << 8 protons? HESSI radio Late phase particle population HESSI CDS Conclusions • Reconnection is an ongoing process throughout the entire • duration of a solar flare. • The dominant consequences of that reconnection • transitionsmoothly(?)from energetic particle production • to shock and conduction front formation.cf. Wülser et al (1994) Relative strength of thermal/non-thermal heating with time

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