HAO January 28 2002

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The Energy Release Process in Solar Flares
Constraints from TRACE observations
Lyndsay Fletcher Department of Physics and
Astronomy University of Glasgow
HAO January 28 2002
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Motivation
To use the high resolution imaging data from the
TRACE satellite, combined with information from
other instruments, to investigate some properties
of the solar flare energy release process.
Overview

• Introduction to Flares
• Strengths of TRACE flare data
• UV/EUV flare kernels
• Flare Ribbons
• Combination with Magnetograms

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Solar Flare Basics

• Solar flares - fast and transient releases of
  energy in the solar atmosphere - are
• one manifestation of the solar magnetic cycle

• The flare energy comes from non-potential energy
  stored in the
• pre-flare coronal magnetic field

• The release of this energy is thought to be
  facilitated by magnetic
• reconnection taking place in the corona.

• The general pattern is
• Rapid release phase, energy in the form of
  fast ions and electrons
• followed by
• Gradual equilibriation phase, possibly further
  slow energy input

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Standard Model
The CSHKP standard model invokes a loop or arcade
of loops Coronal reconnection leads to
material ejection and particle acceleration First
developed when only H-alpha and white light
imaging was available Many further incarnations
include other wavelength observations
Sturrock 1966
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Hard X-ray contours on soft X-ray image (Yohkoh
HXT/SXT)
Ha two ribbon flare (BBSO)
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Characteristic Signatures of a Flare
Impulsive Hard X-rays (33-53keV)
Gradual soft X-rays (1keV)
typical time profile from Yohkoh HXT and SXT
Long duration event with evidence for extended
particle acceleration.
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Energetic particles in Solar Flares
Observed flares span a large range in energies,
from microflares (1024 ergs) to great flares
(1032 ergs)
A large fraction, if not the majority, of energy
released appears in accelerated particles (both
electrons and ions), possibly also in small
events
(Aschwanden et al. 2000, Lin et al., 2001)
(Crosby et al., 1993)
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Energetic particles in Solar Flares
Accelerated electrons (gt 8keV) are present even
in very small GOES C flares
The energy in accelerated ions is comparable to
that in accelerated electrons
Ramaty et al. 1995
Lin et al., 2001
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The Energy Release Process

• Energy for the flare comes from the coronal
  magnetic field
• Are there particular field configurations which
  are more prone to flaring?
• What is the instability - MHD or kinetic -
  leading to flaring?
• Acceleration of particle to relativistic
  energies is intrinsic to the flaring process
• What is the mechanism of particle acceleration?
• How is it linked to other parts of the flare -
  e.g. filament motions ?

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TRACE flare observations I
The TRACE satellite, launched in April 1998, is a
white light and UV/EUV solar imager. It has
observed tens of major X flares, and of the order
of 250 M flares (as well as numerous C flares)

• High spatial resolution
• 0.5 pixels ? 725km resolution

• Dynamic range 1.5 orders of magnitude,
• increased during flares by diffraction
  pattern
• ? extra 2-4 orders dynamic range,

• Good time cadence
• as low as 1s in UV channels (1500/50, 1600
  Å)
• 10s in EUV channels(171, 195 Å)

171 A (Fe IX/X, T 1MK)
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TRACE flare observations II
During Flares, 1600 Å channel dominated by
(pressure sensitive) C IV emission (Brekke et
al. 1995)
1600 Å channel shows kernels, loops and
filaments
TRACE 1600Å 15-Mar-00
TRACE 1600Å 16-Mar-00
HOT (Fe XXIV 15MK) line in the 195 Å passband
(sometimes e--e- bremsstrahlung in 171 Å
passband)
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Impulsive Phase UV/EUV Flare Kernels (I)

• During the impulsive phase, very intense
• UV/EUV emission sources are frequently
• seen, low in the atmosphere.
• Kernel size can be instrumental resolution.
• Variability seconds

171 Å
Do they indicate beam precipitation sites?
(e.g. Woodgate et al 83)
So TRACE flare kernel observations might
complement HXR observations but with rather high
spatial precision
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Impulsive Phase UV/EUV Flare Kernels (II)
Example Bastille Day 2000 Flare (Fletcher
Hudson 2002, Masuda et al. 2002)
movie
During the flare impulsive phase, EUV and UV
footpoints are well-correlated in space and time
with HXR emission.
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Energy Input
Assume that energy input in beam goes to heat
plasma and produce 195Å emission thermally
?E nek ?T
From the TRACE counts/pixel/s we have an estimate
of the total emission measurem e, per pixel
e 1044 cm-3 ne2 V ne2 Apixel
l where l is the depth over which the emission
is formed. If we further assume that the bulk
of the energy was in electrons of energy 40 keV
and above (consistent with HXR observations) then
the emission depth is approximately equal to
their stopping depth, lstop.
From beam physics the collisional column depth N
is given by
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Energy Input
From the expression for emission measure we can
rearrange to get an expression for the
chromospheric electron density
Putting in numbers gives ne 1016 cm-3 , l
40km, i.e. a chromospheric source The energy
input rate is then 5 x 1012 ergs cm-2
corresponding to a very large flare. Similar
exercises can of course be done only using hard
X-ray data, but in principle TRACE data can be
used to examine the energy input on a per-pixel
basis.
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Impulsive Phase UV/EUV Flare Kernels (III)
Pixels are 325km x 325 km
In those events where (E)UV/HXR are well
correlated, (E)UV can be used to estimate beam
precipitation areas (like Ha, but better) ?
precipitation areas as small as 1016cm2 ?
small bundles of fieldlines active for 1 min
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Limits on the Flare Beam
Very small scales of beam fragmentation become a
problem for the beam propagation A beam of
electrons, if not charge-neutralised by an
accompanying ion beam, becomes balanced by a
locally-generated return current
(induction equation) nbeam vbeam qe
ncor v r.c. qion 0 If the return current
velocity is too high (higher than the ion sound
speed) ion acoustic turbulence sets in and halts
the beam propagation (Brown Melrose 1977) The
return current velocity is determined by the beam
flux and the local density.
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Limits on the Flare Beam
Assuming a coronal origin for the beam, the small
precipitation areas demand either Coronal
conditions such that T 107K and n 1011
cm-3 or Accompanying charge-neutralising
proton beam, implying mp/me times more energy
in protons than in electrons
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Flare Ribbons (I)
By overlaying TRACE UV images of 2-ribbon flares
on magnetograms, estimates can be made of total
magnetic flux reconnected, and reconnection rates
(Fletcher Hudson 2001, Gäng et al. 2001,
Forbes Lin 2000)
195 Å channel
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Ribbons and Field
As reconnection progresses in the corona, the
footpoints of just reconnected fieldlines are
illuminated (by particles/ heat conduction)
leading to the appearance of EUV ribbons. The
footpoint ribbons sweep across the magnetic field
Predictions Footpoint ribbons move faster
through weak field Equal flux swept out in
unit time on either side of the neutral line
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Flare Ribbons III
Longitudinal fluxes on both sides of the neutral
line are not equal
In some instances, ribbons move more slowly
through low field regions.
Total longitudinal flux swept out by ribbons
(1020 Mx)
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Possible Reasons for Discrepancy in Longitudinal
Fluxes

• Field in Northern ribbon predominantly
  horizontal
• i.e. component measured by MDI is 50 of
  total
• ? inclination 60o to LOS
• Not all footpoints in North illuminated
• particle precipitation/conduction inhibited in
  this direction.
• Not all illuminated pixels in South linked to all
  reconnected fieldlines
• field structured below resolution of TRACE or
  MDI, patchy reconnection
• Northern ribbon dominated by sub-pixel, extremely
  strong flux elements
• linearity of MDI for very high B??

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Flare Ribbons (IV)
Gäng et al (2002) have repeated the exercise
using high resolution magnetogram data and TRACE
1600 A ribbon observations.
In both studies, more flux of one sign than the
other is swept out, by a factor 2. The
mismatch decreases as time goes on (see also
Forbes Harvey) Gäng et al reconnection rate
5x1017 Mx/s in an M1 event Fletcher Hudson
find 3x1018 Mx/s in X5.7 event
Figure from Gäng et al.
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Flare Ribbons (V)
We can also estimate the coronal electric field
(assuming 2D symmetry) Poletto Kopp 1986,
Forbes 1999 Electric field given by the
rate at which closed magnetic flux A increases
E0

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3-D fields
In reality, one must take into account the actual
distribution of magnetic sources in the
photosphere and construct the 3-D field - even
in the apparently axisymmetric case of 2-ribbon
flares
Sweet (1959) demonstrated presence of nulls and
separators in multipolar field
In 3-d this gives separatrix surfaces and their
intersections, separators (figure from Priest
Schrijver 2000)
ITP Workshop Jan-18-2002
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Combining with Magnetogram Data (I)
Many flares are do not have an obvious 2-ribbon
symmetry. The magnetic topology may be
significantly more complicated. To attempt to
understand individual flares, TRACE data can be
combined with magnetogram data to infer magnetic
geometry in the corona (e.g. Aulanier et al,
2000 Fletcher et al 2001)
Aulanier et al 2000 identify a persistent bright
loop-like feature in TRACE 171 Å with a spine
field line in 3-D (using Mees data)
and infer the presence of a null point in the
corona, at which reconnection may take place
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Combining with Magnetogram Data (II)
Fletcher et al. (2001) study an M1.9 flare,
using TRACE, MDI and Yohkoh data. A potential
field extrapolation identifies the likely
location of a coronal null above an isolated
positive polarity
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Combining with Magnetogram Data (III)
A series of extrapolations from a discrete-source
representation of the evolving magnetic sources
strongly suggests that this flare is the result
of 3-D reconnection through a coronal null.
SXT with contours of TRACE 171 Å (black) and HXT
e.g. Jet - slingshot effect of opening
field Bright TRACE/SXT emission - separator where
field closes in
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Conclusions
The signatures of accelerated particles in TRACE
flare data can be used to estimate some basic
properties of the acceleration region - beam
precipitation area - flare energy input rate -
magnetic reconnection rate - electric field in
the reconnection region A first-order
examination of data with this in mind has thrown
up some interesting inconsistencies in the data
with respect to the standard model, but also
revealed possibilities for detailed studies in
the future.
HAO January 28 2002