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Chromospheric Flares
Lyndsay Fletcher
Intro – some chromospheric flare characteristics
Diagnosing flare input from chromospheric signatures?
Flare wave energy transport in the chromosphere?
White Light Flares
TRACE broadband ‘WL’ enhancement is visible in flares from X to Cclass
Well-correlated with HXRs in time and space (cf also K. Watanabe)
WL shows locations where flare is energy deposited in lower
atmosphere.
Orange = 25 - 50keV
Blue = TRACE WL
Appearance of chromospheric flare sources
X3.4
FWHM of G-band source = 500km
Hinode SOT (Isobe et al, Krucker et al)
G-band ribbon width (including ‘diffuse’ emission) ~ 2” = 1500 km
G-band ribbon length = 10”=7500km
Area ~ 1.25 x 1017 cm2
Collisional thick-target electron energy flux above 20 keV = 3 x 1029 erg s-1
i.e. chromospheric flare sources compact and intense
TRACE/RHESSI study
TRACE/RHESSI flare study – inferred beam energies from HXR
collisional thick-target model and fluxes.
Sensitive to assumed low energy cutoff for beam – 2 vals. used
WL area
(TRACE)
P > 20keV
ergs s-1
P > 40keV
P/cm2 (> 20)
electrons s-1 ergs cm-2 s-1
P/cm2 (>40)
ergs cm-2 s-1
date
class
07/26/02
M1.0
4.0  1016
2.5 1027
1.5 1025
6.3 1010
3.7 108
10/04/02
M4.0
1.0  1017
4.3 1028
2.0 1027
4.3 1011
2.0 1010
10/05/02
M1.2
7.0  1016
2.6 1027
9.9 1025
3.7 1010
1.4 109
10/23/03
M2.4
1.0  1017
7.9 1028
1.3 1027
7.9 1011
1.3 1010
07/24/04
C4.8
1.3  1017
1.5 1028
7.81026
1.2 1011
6.0 1019
NB, WL area used is an upper limit, not corrected for TRACE pointspread function ~(3px) 2 = 1016cm2.
Questions:
 how well can we estimate total energy input from broadband
UV/optical or from spectroscopy?
(Fletcher et al. tried an estimate based on Balmer-Paschen model
of Metcalf, found higher energy input rates compared to beam)
 Would like to use RMHD simulations with varying energy
deposition profiles to examine changes in line & continuum.
 How far can existing diagnostics (e.g. Gianna’s Ca II data) be
used to understand flare energy deposition - e.g. temperature,
turbulence as function of depth in chromosphere?
 Is it OK to use a static chromospheric model in a flare?
Modes of energy transport – particle beams and waves
Particles: magnetic energy release
accelerates particles in corona, which
then transport energy to
chromosphere.
e.g. Liu et al 2008, turbulent
acceleration in corona and escaping
particle beams.
Waves: reconnection launches
MHD waves which transport
energy through corona and to
chromosphere
Fletcher & Hudson (2008)
MHD simulations – Joachim Birn
• 3D MHD simulations of reconnection in sheared arcade
• Diffusion region assumed small enough that energy in non-thermal
particles accelerated there is negligible (verified by PIC)
• Poynting flux, enthalpy flux, bulk KE flux tracked.
• majority of incoming PF diverted to up/downgoing PF
Poynting flux
in
out
Sheared low-b arcade,
erupting
Poynting
flux in x
direction
Poynting
flux in z
direction
Question:
What happens to a strong Alfvén pulse in the chromosphere?
(1) Could it damp by ion-neutral coupling
 heating and WL, UV production (Emslie & Sturrock 82)
(2) Could it generate a turbulent cascade in k on timescale τ ≈
(λmax/va)(B/δB).
 stochastic electron acceleration in the chromosphere?
(e.g.Hamilton & Petrosian 1992).
(3) Could magnetic stresses, imparted by a wave pulse, drive
formation of multiple field-aligned chromospheric current sheets –
another mode of acceleration? (Turkmani et al 2005)