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Physical Processes
in
Solar and Stellar Flares
Eric Hilton
General Exam
March 17th, 2008
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Outline
• Overview and Flare Observations
• Physical Processes on the Sun
– Standard Two-ribbon Model
– Magnetic Reconnection
– Particle Acceleration
• Stellar Comparison
• Summary
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The Sun
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Magnetic loops
TRACE image
~ 109 cm
Footprints
Flare basics
• Flares are the sudden release of
energy, leading to increased emission in
most wavelength regimes lasting for
minutes to hours.
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Light curves
Kane et al., 1985
X-rays
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Radio
White light
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Time
Moving Footprints
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Sigmoid model
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Moore et. al, 2001
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Data of Sigmoid
RHESSI data
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attribute
Moore et. al, 2001
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Where does the energy come from?
A typical Solar flare emits about 1032 ergs total.
The typical size is
L ~ 3x109 cm, H ~ 2x109 cm, leading to V ~ 2x1028 cm3
Thermal energy?
In the chromosphere, the column density, col is ~0.01
g/cm2 and T~ 1x104 K.
In the corona, it’s 3x10-6 g/cm2, 3x106 K
Eth ≈ 3 colkTL2/mH ≈ 2x1029 ergs for chromosphere
≈ 2x1028 ergs for corona.
Not cutting it.
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Nuclear power?
The corona doesn’t have the temperature
or density, unless…
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No, magnetic energy
EB = VB2/(2o) so , for B = 300-1000 G, you’re at
1x1032-33 erg.
Now, how is the energy released quickly enough?
t ~ L2 o ~ 5x1011 seconds for diffusion, way too
long
So, do it quickly in a current sheet
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Outline
• Overview and Flare Observations
• Physical Processes on the Sun
– Standard Two-ribbon Model
– Magnetic Reconnection
– Particle Acceleration
• Stellar Comparison
• Summary
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Two-Ribbon Flare Model
“Magnetic event”
(reconnection)
Martins & Kuin, 1990
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Two-Ribbon Flare Model
Flow and impaction
of current sheet
Gyrosynchrotron
radio emission
Brehmsstrahlung
hard X-ray
& optical emission
Martins & Kuin, 1990
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Two-Ribbon Flare Model
Chromospheric
evaporation &
condensation
Gyrosynchrotron
radio emission
Blue-shifted UV
(≈100s km/s)
Red-shifted optical
(≈10s km/s)
Martins & Kuin, 1990
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Two-Ribbon Flare Model
Soft X-ray
Corona become
optically thick
Optical
Martins & Kuin, 1990
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Two-Ribbon Flare Model
Post flare emission
(quiescent)
Gyrosynchrotron
radio emission
Optical
Martins & Kuin, 1990
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This model explains…
•
•
•
•
the relationship to CMEs
the Neupert effect
Sunquakes
Radio observations
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Outline
• Overview and Flare Observations
• Physical Processes on the Sun
– Standard Two-ribbon Model
– Magnetic Reconnection
– Particle Acceleration
• Stellar Comparison
• Summary
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Magnetic Reconnection
B-field lines
• Material flows in
vinflow
• v x B gives current
into the page
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• called a ‘current
sheet’ or ‘neutral
sheet’
•current dissipation
heats the plasma
Sweet-Parker
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(1958,1957)
Magnetic Reconnection
vout
B-field lines
vinflow
Pressure is
higher in the
reconnection
region, so
flows out the
ends
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Sweet-Parker
(1958,1957)
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Petschek mechanism
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Priest & Forbes, 2002
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Reconnection Inflow
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Narukage & Shibata, 2006
Outline
• Overview and Flare Observations
• Physical Processes on the Sun
– Standard Two-ribbon Model
– Magnetic Reconnection
– Particle Acceleration
• Stellar Comparison
• Summary
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Particle acceleration
1. DC from E-fields ~ 103 Vm-1 during
reconnection
2. MHD shocks - accelerate more
particles more slowly - can explain the
main phase
3. Highly turbulent environment may give
rise to stochastic acceleration - ie fastmode Alfven-waves.
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Ion beam
• 2.223 MeV is a neutron capture line - ions collide with
atmosphere, producing fast neutrons.
• These neutrons thermalize for ~100 sec before being
captured by Hydrogen.
• Hydrogen is turned into Deuterium, releasing a -ray
• Time profiles (with 100 sec delay) suggest beams happen
at same time.
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Displaced ion and electron beams
Hurford et al.,2006
2003, Oct 28th
flare
4th with
measured
gamma rays all showing
displacement
between - and
hard X-rays.
This is first to
show both
footprints
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Ion and electron beam displacement
•Possible displacement caused by drift of electrons and
ions with different sign of charge. This effect is 2 orders of
magnitude too small.
• Currently, it’s not known why there is displacement.
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Soft X-Rays
Hard X-Rays
Gamma Rays
Gamma-ray movie
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New model for particle
acceleration
Fletcher & Hudson, 2008
(RHESSI Nugget #68,
Feb 4th, 2008)
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Outline
• Overview and Flare Observations
• Physical Processes on the Sun
– Standard Two-ribbon Model
– Magnetic Reconnection
– Particle Acceleration
• Stellar Comparison
• Summary
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Stellar comparison
• When we look at a star, we lose all spatial
resolution, lots of photons, and continuous
monitoring.
• We can’t observe hard X-rays, and only
observe limited soft X-rays
• We gain new regimes of temperature,
magnetic field generation and configuration,
plasma density, etc.
• We can adopt the Solar analogy, but is it
valid? What observations can we make?
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Stellar Flares
Osten et al., 2005
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Big stellar flares
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Hawley & Pettersen, 1991
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Flare - quiet
Data courtesy of Marcel Agüeros
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X-ray/microwave ratio
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Benz & Gudel, 1994
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The Sun is not a Flare Star!
• Although some parts of the analogy
clearly hold, we would not see flares on
the Sun if it were further away.
• Are the flares we see fundamentally
different?
• We are biased to detecting only the
largest flares, so must be cautious
about extrapolating to rates of smaller
flares.
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Solar vs. Stellar
Aschwanden, 2007
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Magnetic loop lengths
L/R
Mullan et al.,2006
V-I
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EUVE
Flare
rates
Audard et al., 2000
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My Thesis
• I will make hundreds of hours of new
observations of M dwarfs to determine
flare rates
• I am creating model galaxy simulations
to predict flare rates on a Galactic scale
that includes spectral type and activity
level. We can ‘observe’ this model to
predict what LSST will see.
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Summary of Solar Flares
• Magnetic loops become entangled by
motions of the footprints, storing magnetic
energy
• This energy is released through rapid
magnetic reconnection that accelerates
particles.
• Flares emit in all wavelength regimes.
• The general theory is well-established, but
the details continue to be very complex.
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The Sun, in closing
“Coronal dynamics remains an active
research area. Details of the eruption
process including how magnetic energy
is stored, how eruptions onset, and how
the stored energy is converted to other
forms are still open questions.”
- Cassak, Mullan, & Shay
published March 3rd, 2008
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Summary of Stellar Flares
• Many aspects of the Solar model seem
to be true on stars as well.
• Observations have revealed
inconsistencies that have not yet been
resolved.
• Flares are the coolest!
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Thanks
• Thanks to my committee, esp. Mihalis
for coming all the way from Ireland on
St. Patrick’s Day.
• Thanks to my fellow grad students for
feedback on my practice talk.
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The End
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Radio Flares
Osten et al.,2005
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X-ray flares
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Su, Gan, & Li, 2006
Stats of RHESSI flares
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Stats con’t
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Su, Gan, & Li, 2006
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Stats con’td
Su, Gan, & Li, 2006
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Stats con’td
Su, Gan, & Li, 2006
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Statistical motivation for
Avalanche
Charbonneau et al., 2001
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Downward motion of centroid
6-12keV *0.5
25-50keV
Sui, Homan, & Dennis, 2004
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Disproving nano-flare heating
Aschwanden, 2008
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Reconnection & X-ray flux
Jing et al., 2005
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Correlations
Jing et al., 2005
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CMEs
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Jing et al., 2005
Neupert Effect
Veronig et al.,
2005
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Gamma-ray spectroscopy
Smith et al., 2003
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Movie-time
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Loop lengths in active stars
Mullan et al.,2006
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Avalanche
Such models, although a priori far
removed from the physics of
magnetic reconnection and
magnetohydrodynamical evolution of
coronal structures, nonetheless
reproduce quite well the observed
statistical distribution of flare
characteristics. - Belanger, Vincent,
& Charbonneau, 2007
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The model
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Model results
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Charbonneau et al., 2001
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Model results
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Charbonneau et al., 2001
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SOC-Cascades of Loops
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Hughes, et al.,2003
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Cassak et. al, 2008
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Peak values during the flares
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Benz & Gudel, 1994
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Sunquakes
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More complicated reconnection
• Petschek is just a special case of
almost-uniform reconnection
• There are also non-uniform models with
separatrix jets.
• In some cases, the sheet tears, and
enters the regime of impulsive bursty
reconnection
• The 3D models are very complicated 3D
MHD.
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More sigmoid
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Moore et. al, 2001
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Types of Magnetic Emission
Flaring: strong, impulsive
emission that decays rapidly
(minutes to hours), both line
and continuum flux may be
detected (L ≈ 10-3 - 102 Lbol)
Liebert et al. (1999)
Quiescent: steady emission
that persists over long periods,
typically line flux only in
optical (L ≈ 10-6 - 10-3 Lbol)
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Twisted field lines
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Priest & Forbes, 2002
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Solar to stellar - scaling laws
Aschwanden, 2007
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Veronig et al., 2005
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