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September 21, 2005 Peter Gallagher (UCD)
Chromospheric EvaporationChromospheric Evaporation
Peter GallagherUniversity College Dublin
Ryan MilliganQueen’s University Belfast
September 21, 2005 Peter Gallagher (UCD)
Canonical Flare ModelCanonical Flare Model
o Step 1: Acceleration.o Reconnection produces power-law
electron distribution.
o Step 2: Propagation.o Electrons spiral along magnetic fields
from corona to chromosphere.
o Step 3: Heating.o Electrons deposit energy in chromosphere
via Coulomb collisions.
o Step 4: Evaporation.o Dense chromosphere radiates and may
expand.
September 21, 2005 Peter Gallagher (UCD)
Chromospheric ResponseChromospheric Response
o How does the chromosphere respond to nonthermal electrons?
o Assume power-law electron spectrum:
o f(E) ~ E- electrons cm-2 s-1
T1: NonthermalElectrons
T2: Impulsive Heating
T3: VUP
T3: VDOWN
Density
Loop leg
September 21, 2005 Peter Gallagher (UCD)
Chromospheric ResponseChromospheric Response
o Chromospheric response depends on properties of accelerated electrons:
o Low-energy cut-off (Ec)o Lower Ec => more energy => more rapid and
pronounced response.
o Power-law index ()o Harder spectrum => high energy electrons
penetrate deeper where chromospere better able to radiate => less rapid and pronounced response.
o Total fluxo Higher flux => more energy => more rapid and
pronounced response.
EC
E
f(E)
nonthermalthermal
€
P = f (E)dEEc
∞
∫
September 21, 2005 Peter Gallagher (UCD)
Gentle Explosive
Flux (ergs cm-2 s-1) <1010 >3 x 1010
T (K) <106 >107
P (dyn cm-2) x10 x100-1000
Upflows (km s-1) 10’s 100’s
Downflows (km s-1) 0 10’s
Gentle vs Explosive EvaporationGentle vs Explosive Evaporation
September 21, 2005 Peter Gallagher (UCD)
Gentle vs Explosive EvaporationGentle vs Explosive Evaporation
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
September 21, 2005 Peter Gallagher (UCD)
CDS and TRACE: 26 March 2002 FlareCDS and TRACE: 26 March 2002 Flare
o SOHO/CDSo He I (0.03 MK)o O V (0.25 MK)o Mg X (1.1 MK)o Fe XVI (2.5 MK)o Fe XIX (8 MK)
o TRACE 17.1 nmo Fe IX/X (1.0 MK)
September 21, 2005 Peter Gallagher (UCD)
Footpoint DownflowsFootpoint Downflows
o Loops are not static.
o Downflows <50 km s-1, upflows >100 km s-1
o Loops cool via conduction, radiation, and flows.
September 21, 2005 Peter Gallagher (UCD)
RHESSI SpectrumRHESSI Spectrum
• Thermal:
T ~ 20 MK
EM ~ 1049 cm-3
• Nonthermal:
Ec ~ 24 keV
~ 7.3
• HXR Area <1018 cm2
=> Nonthermal Electron Flux >3x1010 ergs cm-2 s-1
September 21, 2005 Peter Gallagher (UCD)
6 - 12 keV (dashed line)
Thermal
25 – 50 keV (solid line)
Non-thermal
September 21, 2005 Peter Gallagher (UCD)
Evidence for UpflowsEvidence for Upflows
Stationary Fe XIX Component
Blueshifted Fe XIX Component
Doppler shifts measured relative to a stationary component:
v/c = (- 0)/ 0
In Fe XIX
v = 270 km s-1
September 21, 2005 Peter Gallagher (UCD)
Future WorkFuture Work
o How does the chromospheric response depend on the nonthermal electron properties?
o We only have one event!o Nonthermal electrons => F>3x1010 ergs cm-2 s-1
o Response => ~ -30 km s-1 and 270 km s-1
o Is there a threshold for explosive evaporation? Heating < expansion => 3kT / Q < L/cs
o => need large number of CDS/RHESSI flares