# Presentation: 270953

## 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

**Summary of Presentation**

** ****Chromospheric evaporation revisited**

** ****Coronal Diagnostic Spectrometer**

** ****Summary of flare **

** ****Implications for chromospheric heating**

** ****Conclusions**

- SOHO/CDS

** ****Results of data analysis**

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

**from Cargill & Priest (1983)**

** ****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)**** **

- Chromospheric Evaporation: models

## 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/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**

## QUICK LOOK AT THE FLARE

**MDI**

**BBSO H******

**EIT FeXII**

**CDS OV**

**CDS FeXVI**

**CDS FeXIX**

**QUICK LOOK AT THE FLARE**

**1.5 MK**

**0.25 MK**

**2.0 MK**

**8.0 MK**

**Sunspot + plage**

**expanding ribbons**

## CDS DOPPLERGRAMS

**CDS DOPPLERGRAMS**

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

**Interesting differences **
**near sunspot**

## Velocity profiles

**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**

## 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**

## Continued heating in late gradual phase

**Continued heating in late gradual phase**

**The CDS observations**
**provide direct evidence**
**for the presence of**
**continuing energisation**
**presumably due to**
**ongoing reconnection**

## 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.**** **

## 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)**

## 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/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**

## 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**

**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**

## 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**

## Chromospheric Heating: conduction fronts (II)

**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**

**w = 9 x 10****4 ****km**

## Chromospheric Heating: conduction fronts (II)

**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**

**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.**

## Conclusions

**  ****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**

**Relative strength of thermal/non-thermal **
**heating with time**