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Chromospheric Evaporation. Peter Gallagher University College Dublin Ryan Milligan Queen’s University Belfast. Canonical Flare Model. Step 1: Acceleration. Reconnection produces power-law electron distribution. Step 2: Propagation.
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Chromospheric Evaporation Peter Gallagher University College Dublin Ryan Milligan Queen’s University Belfast Peter Gallagher (UCD)
Canonical Flare Model • Step 1: Acceleration. • Reconnection produces power-law electron distribution. • Step 2: Propagation. • Electrons spiral along magnetic fields from corona to chromosphere. • Step 3: Heating. • Electrons deposit energy in chromosphere via Coulomb collisions. • Step 4: Evaporation. • Dense chromosphere radiates and may expand. Peter Gallagher (UCD)
T1: Nonthermal Electrons T3: VUP T2: Impulsive Heating T3: VDOWN Chromospheric Response • How does the chromosphere respond to nonthermal electrons? • Assume power-law electron spectrum: • f(E) ~ E-electrons cm-2 s-1 Loop leg Density Peter Gallagher (UCD)
Chromospheric Response • Chromospheric response depends on properties of accelerated electrons: • Low-energy cut-off (Ec) • Lower Ec=> more energy => more rapid and pronounced response. • Power-law index () • Harder spectrum => high energy electrons penetrate deeper where chromospere better able to radiate => less rapid and pronounced response. • Total flux • Higher flux => more energy => more rapid and pronounced response. EC f(E) thermal nonthermal E Peter Gallagher (UCD)
Gentle vs Explosive Evaporation Peter Gallagher (UCD)
Gentle vs Explosive Evaporation Peter Gallagher (UCD)
RHESSI Spectral Coverage Peter Gallagher (UCD)
CDS and TRACE: 26 March 2002 Flare • SOHO/CDS • He I (0.03 MK) • O V (0.25 MK) • Mg X (1.1 MK) • Fe XVI (2.5 MK) • Fe XIX (8 MK) • TRACE 17.1 nm • Fe IX/X (1.0 MK) Peter Gallagher (UCD)
RHESSI Integrated Spectrum Peter Gallagher (UCD)
Footpoint Downflows • Loops are not static. • Downflows <50 km s-1, upflows >100 km s-1 • Loops cool via conduction, radiation, and flows. Peter Gallagher (UCD)
M2.2 Flare – CDS/EIT/GOES Peter Gallagher (UCD)
M2.2 Flare – CDS/EIT/GOES Peter Gallagher (UCD)
RHESSI Lightcurve Peter Gallagher (UCD)
RHESSI Spectrum • Thermal: • T ~ 20 MK • EM ~ 1049cm-3 • Nonthermal: • Ec ~ 24 keV • ~ 7.3 • HXR Area <1018cm2 • => Nonthermal Electron Flux >3x1010 ergs cm-2 s-1 Peter Gallagher (UCD)
6 - 12 keV (dashed line) Thermal 25 – 50 keV (solid line) Non-thermal Peter Gallagher (UCD)
Stationary Fe XIX Component Blueshifted Fe XIX Component Evidence for Upflows Doppler shifts measured relative to a stationary component: v/c = (- 0)/ 0 In Fe XIX v = 270 km s-1 Peter Gallagher (UCD)
Flow velocity vs. Temperature Peter Gallagher (UCD)
Future Work • How does the chromospheric response depend on the nonthermal electron properties? • We only have one event! • Nonthermal electrons => F>3x1010 ergs cm-2 s-1 • Response => ~ -30 km s-1 and 270 km s-1 • Is there a threshold for explosive evaporation? • Heating < expansion => 3kT / Q < L/cs • => need large number of CDS/RHESSI flares Peter Gallagher (UCD)