Post on 06-Jan-2016
description
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?
Marina Battaglia, Solar Cycle 24, Napa, 2008
Corona
Photosphere
• Dense, hot region in the corona → soft X-rays• hard X-rays from accelerated electrons
How was this dense regionformed?
Marina Battaglia, Solar Cycle 24, Napa, 2008
Corona
Photosphere
2. Particles stream down to chromosphere
Assumption: force freetransport
Marina Battaglia, Solar Cycle 24, Napa, 2008
Corona
Photosphere
3. Stopping in the dense chromosphere
• hard X-ray bremsstrahlung→chromospheric footpoints• heating of chromosphere
footpoints
Marina Battaglia, Solar Cycle 24, Napa, 2008
Corona
Photosphere
4: Expansion of hot plasma (chromospheric “evaporation”) → soft X-ray emission
Marina Battaglia, Solar Cycle 24, Napa, 2008
Expected lightcurve in the Expected lightcurve in the common flare scenariocommon flare scenario
RHESSI 6-12 keV (SXR)
RHESSI 25-50 keV (HXR)
Marina Battaglia, Solar Cycle 24, Napa, 2008
Pre-flares and pre-Pre-flares and pre-heating: a different heating: a different
type of flaretype of flare
Marina Battaglia, Solar Cycle 24, Napa, 2008
RHESSI 25-50 keV (HXR)
RHESSI 6-12 keV (SXR)
RHESSI 25-50 keV (HXR)
~ 3 min
Marina Battaglia, Solar Cycle 24, Napa, 2008
RHESSI 25-50 keV (HXR)
• 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
Marina Battaglia, Solar Cycle 24, Napa, 2008
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
Marina Battaglia, Solar Cycle 24, Napa, 2008
Spectral and spatial Spectral and spatial evolution of pre-heatingevolution of pre-heating
RHESSI 6-12 keV (SXR)
RHESSI 25-50 keV (HXR)
Marina Battaglia, Solar Cycle 24, Napa, 2008
ImagesImages
Marina Battaglia, Solar Cycle 24, Napa, 2008
6-12 keV
25-50 keV
12-25 keV
Time0 min
sola
r dis
c
Marina Battaglia, Solar Cycle 24, Napa, 2008
Time
6-12 keV
25-50 keV
12-25 keV
Time0 min 1 min
sola
r dis
c
Marina Battaglia, Solar Cycle 24, Napa, 2008
Time
6-12 keV
25-50 keV
12-25 keV
Time0 min 1 min 3 min
sola
r dis
c
Marina Battaglia, Solar Cycle 24, Napa, 2008
SpectraSpectra
Beginning: purely thermal non-thermal tail appears
fit range fit range
Marina Battaglia, Solar Cycle 24, Napa, 2008
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
Marina Battaglia, Solar Cycle 24, Napa, 2008
• 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
Marina Battaglia, Solar Cycle 24, Napa, 2008
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)
Marina Battaglia, Solar Cycle 24, Napa, 2008
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”
Marina Battaglia, Solar Cycle 24, Napa, 2008
• 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
Marina Battaglia, Solar Cycle 24, Napa, 2008
Temperature and density space of typical flares with limits for classical heat fluxwith GOES (dots) and RHESSI (triangles) measurements
Marina Battaglia, Solar Cycle 24, Napa, 2008
• 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)
Marina Battaglia, Solar Cycle 24, Napa, 2008
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
Marina Battaglia, Solar Cycle 24, Napa, 2008
• 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
Marina Battaglia, Solar Cycle 24, Napa, 2008
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