Magnetic Island Formation and

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American Geophysical Union (AGU) Meeting, San
Francisco, Dec 15-19, 2000
Magnetic Island Formation and Arcade Fine
Structure of the Bastille-Day Flare
Markus J. Aschwanden David
Alexander, Richard Nightingale
Lockheed Martin, ATC, Solar Astrophysics Lab.,
Palo Alto, USA
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ABSTRACT
The (second) Bastille-day flare, a GOES
X5.7-class event that occurred on July 14,
2000, represents a classical case of a
two-ribbon flare. The morphology of such flares
was dubbed from H-alpha observations, showing two
bright ribbons that form on each side of a
pre-existing filament and move away continuously
from the neutral line. An arcade of loops that
bridge the two ribbons becomes visible in soft
X-rays during the impulsive flare phase and in
EUV during the post-flare phase. The generally
accepted scenario for such a flare is the
Kopp-Pneuman model, which entails magnetic
reconnection between vertical anti-parallel
magnetic field lines, occurring in an X-type line
above the neutral line, which raises steadily
upward and leaves an arcade of newly-reconnected
loops beneath the reconnection point. The rise
of the reconnection point translates into an
increasing separation of the arcade footpoints,
which is seen like an advancing front of a
bushfire in EUV observations from TRACE. - In
this study we move a step further in theoretical
modeling of such arcade flares by relating the
observed spatial fine structure of the loop
arcade to temporal fine structure of the magnetic
reconnection process. We consider magnetic island
formation that are produced in unsteady
reconnection modes and have been numerically
simulated in sheared magnetic fields. We relate
the islands to the geometric length scales and
recurrence times of expected magnetic islands to
the geometric sizes and separations of adjacent
arcade loops in order to obtain a diagnostic of
the underlying magnetic reconnection process.
Furthermore, we obtain from the propagation speed
of the magnetic zipper effect along the flare
arcade information on the recurrence rate of
magnetic island formation in unsteady
reconnection modes. The intermittent spatial and
temporal finestructure of unsteady reconnection
modes is also believed to be directly related to
elementary burst structure observed in
concomitant hard X-ray pulses and radio bursts.
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Observations
Theoretical Model and Interpretation
Other Flares
Conclusions
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The second Bastille-day flare, 2000-July-14,
1003 UT, TRACE (UVred, 171blue, 195green)
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TRACE, 171 A, 2000-Jul-14, 1011-1059 UT,
cadence42 s
Highpass-filtered movie
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TRACE, 171 A, 2000-Jul-14, 105932 UT
Highpass-filtered image
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Highpass-filtered image, TRACE, 171 A,
2000-Jul-14, 105932 UT
Number of postflare loop structures N
100 Length of arcade
L 180,000 km Average loop separation
L/N1800 km Minimum loop
separation (3 pixels) ?L1100 km
The separation of arcade loops is observed down
to the instrumental resolution !
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Double-Ribbon Hard X-Ray Emission
Yohkoh SXT A difference image showing (bright)
the extended arcade as seen in soft X-rays. This
is a top-down view, so that the basically
circular loops that form the cylinder look more
or less like straight lines, some tilted
(sheared) relative to others. The dark S-shaped
feature is the pre-flare sigmoid structure that
disappeared as the flare developed.
Yohkoh HXT and SXT overlay The SXT image is
taken on 2000-Jul-14 at 102041 UT The HXT
image is in the high-energy band, 53-93
keV, integrated during 101940-102050 UT. The
HXT shows clearly two ribbons ar the footpoints
of the arcade lined out in soft X-rays. This is
the first detection of hard X-ray double ribbons
(see AGU poster by Masuda). Courtesy of Nariaki
Nitta.
Courtesy of Hugh Hudson, Yohkoh Science Nuggets,
Sept 15, 2000
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Eruptive Flare Model (Moore et al. 2000, ApJ)
- Initial bipoles with sigmoidally sheared and
twisted core fields - accomodates confined as
well as eruptive explosion - Ejective eruption
is unleashed by internal tether-cutting
reconnection - Arcade of postflare loops is
formed after eruption of the filament and
magnetic reconnection underneath
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Shear-driven reconnection (Karpen et al. 1995,
1998)
- 2.5-dimensional numerical simulations of
shear-driven reconnection in the X-point region -
For stronger shear, the initial X-point lengthens
upward into a current sheet which reconnects
gradually and undergoes multiple tearing -
Several magnetic islands develop in sequence,
move towards the ends of the sheet and
disappear through reconnection with overlying and
underlying field.
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Dynamic Magnetic Reconnection (Kliem et al. 2000)
- 2-dimensional numerical simulation of
magnetic reconnection in large-scale current
sheet - Reconnection is dominated by repeated
formation and subsequent coalescence of
magnetic islands (secondary tearing or
impulsive bursty regime) - Magnetic island
formation modulates the local electromagnetic
fields, particle acceleration, and the
resulting intermittent radio and hard X-ray
pulses.
(Kliem, Karlicky, Benz 2000, AA 360, 715)
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Bursty Reconnection
- Numeric simulations show quasi-periodic
formation of magnetic islands of tR200 tA
which corresponds to 13 Alfven crossing times
of the current sheet length. - Extrapolation to
solar flare conditions B70 G, n109 cm-3 -gt
tR1-13 s B70 G, n1010 cm-3 -gt tR0.4-4 s -
The predicted time scales agree with
quasi-periods of radio bursts (metric type III
bursts, decimetric pulsations) and hard X-ray
pulses.
Top Electric field at the main X points
(reconnection rate) vs. time. Thick line
Petschek-like reconnection with a central
X-point (t70-200) and forced reconnection by
initial island coalescence at the origin
(t200-300). Medium line width (solid and
dotted) main new X-points. Initial perturbation
is at time 0. Bottom Location of the dominant X
point.
Kliem, Karlicky, Benz 2000, AA 360, 715
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Spatial Mapping of Magnetic Islands to Arcade
Loops
Time
Each arcade loop is interpreted as a magnetic
field line connected with a magnetic
island generated in the (intermittent)bursty
regime of the tearing mode instability.
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Modulation of Hard X-Ray Time Structures
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Spatial Fragmentation of Flare Radio Emission
Decimetric millisecond spikes, 1990-June-24,
082003-082040 UT, ETH Zurich
Decimetric radio pulsations, 1993-Jan-07,
090920-091500 UT, ETH Zurich
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Spatial Witnesses of Fragmented Energy release ?
TRACE, 171 A, 1998-Sept-30, 143005 UT

MOVIE 1998-Sep-30 142124-145907 UT (34
frames)
- Temporal Pattern of Energy release occurs
intermittent, quasi-periodic, fragmented - EUV
images show sequential arcade loops after cooling
of flare loops down to 1 MK - Spatial
discreteness of arcade loops witness either
fragmented energy release or cooling time
differences
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Flare 1998-Aug-24
TRACE 171 A, Postflare image 98-Aug-25
001313 UT Flare start 98-Aug-24 2130
UT GOES class X1
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Flare 2000-Nov-8
TRACE 171 A 2000-Nov-9, 0005 UT Flare start
Nov 8, 2242 UT GOES class M7.4 NOAA AR
9213 Associated with CME
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Conclusions
1) Hard X-ray and radio emission in flares
generally show temporal fine structure with
pulses and quasi-periods of order T0.5-5
sec. 2) Spatio-temporal mapping of
quasi-periodic time structure to
quasi-periodic spatial structures in soft X-ray
or EUV maps has never been demonstrated for
flare events. 3) We conjecture that the temporal
intermitency of magnetic reconnection, i.e.
the magnetic island formation in shear-driven
reconnection processes, maps to the spatial
pattern of arcade loops in two-ribbon flare
events. 4) We propose a flare scenario with a
propagation of intermittent magnetic
reconnection events along the neutral line, below
an erupting filament, where magnetic
islands become subsequently formed by
magnetic tearing. The electromagnetic field of
dissipated magnetic islands produces pulses
of accelerated particles, which cause
chromospheric evaporation into sequential
arcade loops. 5) The use of EUV images as a
diagnostic is complicated by the time delay
of loop filling and cooling (down to EUV
temperatures).