Pre-flares observed by RHESSI

Pre-flares observed Pre-flares observed by RHESSIby RHESSI

Marina Battaglia Marina Battaglia ETH ZETH Zürichürich

Lyndsay FletcherLyndsay FletcherGlasgow UniversityGlasgow University

Arnold O. BenzArnold O. BenzETH ZETH Züürichrich

Marina Battaglia, Solar Cycle 24, Napa, 2008

1. Energy release and particle acceleration

Corona

?

?

• Energy release due to reconnection of magnetic field lines

Photosphere

B0

Where is the actual acceleration site?


Corona

Photosphere

• Dense, hot region in the corona → soft X-rays• hard X-rays from accelerated electrons

How was this dense regionformed?


Corona

Photosphere

2. Particles stream down to chromosphere

Assumption: force freetransport


Corona

Photosphere

3. Stopping in the dense chromosphere

• hard X-ray bremsstrahlung→chromospheric footpoints• heating of chromosphere

footpoints


Corona

Photosphere

4: Expansion of hot plasma (chromospheric “evaporation”) → soft X-ray emission


Expected lightcurve in the Expected lightcurve in the common flare scenariocommon flare scenario

RHESSI 6-12 keV (SXR)

RHESSI 25-50 keV (HXR)


Pre-flares and pre-Pre-flares and pre-heating: a different heating: a different

type of flaretype of flare





~ 3 min



• SXR emission starts 3 minutes before SXR emission starts 3 minutes before the HXR emissionthe HXR emission

• There must be hot material already There must be hot material already present present

andand

the amount of hot material seems to the amount of hot material seems to increaseincrease


Observations of pre-heatingObservations of pre-heating

• Analyze 4 events displaying Analyze 4 events displaying pronounced pre-heating, using data pronounced pre-heating, using data from the RHESSI and GOES X-ray from the RHESSI and GOES X-ray satellitessatellites

• Analyze the time evolution of the Analyze the time evolution of the early flare phase in images and early flare phase in images and spectraspectra


Spectral and spatial Spectral and spatial evolution of pre-heatingevolution of pre-heating




ImagesImages


6-12 keV

25-50 keV

12-25 keV

Time0 min

sola

r dis

c


Time

6-12 keV

25-50 keV

12-25 keV

Time0 min 1 min

sola

r dis

c


Time

6-12 keV

25-50 keV

12-25 keV

Time0 min 1 min 3 min

sola

r dis

c


SpectraSpectra

Beginning: purely thermal non-thermal tail appears

fit range fit range


Spectral and spatial Spectral and spatial evolution of pre-heatingevolution of pre-heating

Lightcurves

Emission measure

Temperature

Density

GOES

RHESSI

6-12 keV

25-50 keV

assuming constantvolume


• Emission measure and density increase: Emission measure and density increase:

material is added to the coronal source material is added to the coronal source

region (assuming constant source region (assuming constant source

volume)volume)

• Most likely origin: chromosphereMost likely origin: chromosphere

• No footpoints, no HXR → standard No footpoints, no HXR → standard

electron beam model not applicableelectron beam model not applicable


Energy transport by Energy transport by heat conductionheat conduction

• Coronal temperature ~ 20 MKCoronal temperature ~ 20 MK• Typical chromospheric temperature Typical chromospheric temperature

~ 10000 K~ 10000 K• → → large temperature gradient, drives large temperature gradient, drives

heat fluxheat flux• Classical: Classical:

(Spitzer conductivity)


Conductively driven Conductively driven evaporation vs beam driven evaporation vs beam driven

evaporationevaporationelectron beam

energy deposition in chromosphere

heat flux

heating of chromospheric plasma / HXR emission

density increase in coronal source

energy deposition in chromosphere

heating of chromospheric plasma

“chromospheric evaporation”

density increase in coronal source

“chromospheric evaporation”


• Very large temperature gradients Very large temperature gradients result in heat flux that the plasma is result in heat flux that the plasma is unable to carry → flux saturationunable to carry → flux saturation

• Threshold for saturation is Threshold for saturation is determined by the electron mean determined by the electron mean free path free path (n,T) and the temperature (n,T) and the temperature scale length T/∆T≈ Lscale length T/∆T≈ Llooploop


Temperature and density space of typical flares with limits for classical heat fluxwith GOES (dots) and RHESSI (triangles) measurements


• The heat flux in the presented events The heat flux in the presented events is limitedis limited

• Classical treatment is not applicable Classical treatment is not applicable – contrary to widely made – contrary to widely made assumptionassumption

• Use the limited heat flux FUse the limited heat flux Fsatsat==FFcondcond

where where ((/L/Llooploop) is a correction factor ) is a correction factor derived by Campbell (1984)derived by Campbell (1984)


Conductively driven Conductively driven evaporationevaporation

• Energy is deposited to the Energy is deposited to the chromosphere due to thermal chromosphere due to thermal conductionconduction

• → → heating, evaporation, but no HXR heating, evaporation, but no HXR emissionemission


• From the time evolution of the From the time evolution of the density, the energy flux Fdensity, the energy flux Fevapof the of the evaporated material was computedevaporated material was computed

• FFevapevap < F < Fsatsat → the conductive energy → the conductive energy input causes enough evaporated input causes enough evaporated material to account for the observed material to account for the observed density increasedensity increase


Summary, Summary, Conclusions & open Conclusions & open

questionsquestions• The pre-heating phase in flares The pre-heating phase in flares

observed by RHESSI was studiedobserved by RHESSI was studied• The observed increase in density can The observed increase in density can

be explained by conductively driven be explained by conductively driven chromospheric evaporationchromospheric evaporation

• Contrary to often made assumptions, Contrary to often made assumptions, the conductive heat flux is saturated the conductive heat flux is saturated and a non-classical treatment is and a non-classical treatment is necessarynecessary

Pre-flares observed by RHESSI

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