Evidence for chromospheric Evidence for chromospheric heating in the late phase of heating in the late phase of

solar flaressolar flaresDavid Alexander

Lockheed Martin Solar and Astrophysics Lab.

Collaborators: Anja Czaykowska MPI für extraterrestrische PhysikBart De Pontieu LMSAL

Summary of Presentation

• Chromospheric evaporation revisited

• Coronal Diagnostic Spectrometer

• Summary of flare

• Implications for chromospheric heating

• Conclusions

• Results of data analysis

from Cargill & Priest (1983)

• Non-thermalNon-thermal : energy deposition of energetic particles accelerated in flare

Brown (1973) ; Hirayama (1974) ; Nagai & Emslie (1984) ; Fisher, Canfield & McClymont (1985) ; Mariska, Emslie & Li (1989)

• ThermalThermal : energy is transported to chromosphere via thermalconduction fronts of related shocks

Brown (1974) ; Hirayama (1974) ;Antiochos & Sturrock (1978) ; Forbes, Malherbe & Priest (1989) ;Yokoyama & Shibata (1997)

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)- : = 4 ; Ec = 20 keV

Mariska et al. (1989): F ~ (E/Ec)- : = 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

MDI BBSO H

EIT FeXII CDS OV

CDS FeXVI CDS FeXIX

QUICK LOOK AT THE FLAREQUICK LOOK AT THE FLARE

1.5 MK 0.25 MK

2.0 MK 8.0 MK

Sunspot + plage expanding ribbons

CDS DOPPLERGRAMSCDS DOPPLERGRAMS

• distinctive pattern of redshifts and blueshifts• blueshifts confined to leading edges of arcade• redshifts predominate towards neutral line

Interesting differences near sunspot

Vel

ocit

y p

rofi

les

Vel

ocit

y p

rofi

les

• 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/sFeXVI: v ~ 10-20 km/sFeXIX: v ~ 30 km/s

Location of upflow regionsLocation of upflow regions

•Upflows at leadingedge of H ribbon

•Ridge of upflowingplasma moves withH ribbon

•Upflow regionsbecome downflowregions as ribbonsmove outwards

Mach 2 Jet

H loops

UV loops

Current sheet

Termination shockConduction front

Evaporativeupflows Condensation

downflow

Adapted from Forbes and Acton, 1996

Continued heating in late gradual phaseContinued heating in late gradual phase

The CDS observationsprovide direct evidencefor the presence ofcontinuing energisationpresumably due toongoing reconnection

non-thermalnon-thermaloror

thermal?thermal?

Hard X-ray ObservationsHard 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/ssummed over all detectors.Thus, a background subtracted signal strength of 26 cts/s will produce a 2 detection in the integrated HXT L channel.

Hard X-ray production from a Hard X-ray production from a non-thermal electron beamnon-thermal electron beam

Assume that chromosphere acts like a thick-target to a beam of

electrons with energy distribution: F=AE-

1____

22 21

)2/1,2(

4)(

BZ

CR

SAI BH

0

2

1

1, dpdspGIp

p

Convolve photon spectrum with HXT response function toget 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)

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)- : = 4 ; Ec = 20 keV

Mariska et al. (1989): F ~ (E/Ec)- : = 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

Simulated HXR emissionSimulated HXR emissionSingle footpoint

Ec = 20 keV ; N(E<Ec)=E-220 footpoints

Ec = 20 keV ; N(E<Ec)=E-2H

XT

L f

lux

(c

ts/s

)

3 4 5 6 7 83 4 5 6 7 8

Spectral Index

f = 1010

f = 1092 detection

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

Chromospheric Heating: conduction fronts (I)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 104 km):

||pp

5/20

nvcm

Tκw

T = 10 MK, n 2x1010 cm-3, v|| 50km/s,

cp = 2.07x108 cm2s-2K-1 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

Chromospheric Heating: conduction fronts (II)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

Chromospheric Heating: conduction fronts (II)Chromospheric Heating: conduction fronts (II)

w = 9 x 104 km

Thus, our observations suggest that conduction front heating of thechromosphere 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.

• 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)

ConclusionsConclusions

Outstanding questionsOutstanding questions

Late phase particle populationHESSIradio

E < 15 keV << 8protons?

Relative strength of thermal/non-thermal heating with time

HESSICDS