Exploring the Nature of Magnetic Reconnection in Solar Transients

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Exploring the Nature of Magnetic Reconnection in
Solar Transients
David Alexander Rice University
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Basic Tenet
Hard accelerated
magnetic X-rays particles
reconnection
(e-, p)
Flares require reconnection to be fast
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Observational Evidence for reconnection is
Indirect
Yokoyama et al., 1999
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Reconnection Issues in Solar Transients

• Magnetic Topology
• Temporal and spatial development
• Particle acceleration vs. bulk energisation
• Driving mechanism

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Magnetic Topology
Potential Field (a 0) Metcalf et al., 2003
a 1.75x 10-3 Mm-1 a 3.5x 10-3 Mm-1
Alexander and Metcalf, 2004
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Magnetic Topology
Longcope

• Attempting to relate particle production
  (spectra, flux, location) to
• topology (separatrices/separator/ QSL, e.g.
  Demoulin, Longcope, )
• augmented by time-of-flight analyses (Aschwanden
  et al, 1995)
• total magnetic energy release
• rate of energy release (e.g. Fletcher and
  Hudson, 2001, Qiu et al, 2004).
• derived from flux swept up by flare ribbons

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Temporal and Spatial Development
Gradual phase of flare shows signs of energy
release with no evidence of particle signatures
implying transition from acceleration to heating.
Czaykowska, Alexander and De Pontieu, 2001
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Temporal and Spatial Development
Hard X-ray emission is generally fairly localized
despite the large volume involved in the full
flare.
Energy
Time
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Temporal and Spatial Development
Aschwanden Alexander, 2001
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Particle Source Functions
Isotropic particle source function
Anisotropic particle source function
Asymmetries show a marked energy dependence.
Fokker-Planck Simulations (McClements
Alexander, 2004)
Observations (Alexander Metcalf, 2003)
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Particle Source Functions
Effect of non-uniform field in loop
Accelerated electron source function
Asymmetric Field
anisotropic isotropic
Anisotropic Source Function
E (keV)
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The role of resistivity

• Plane Harris Sheet
• Tearing Unstable
• Fast reconnection (Hall physics)

• Arcade
• Tearing Stable
• Slow reconnection

• Sweet-Parker rate is insufficient in arcades
• Current sheet reconnects at the fast enough rate
  if
• Anomalous Resistivity is present
• Reconnection is driven

Lapenta and Knoll, 2003
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Driven Reconnection KHI
Brackbill Knoll (2001)

• Cross field velocity shear is introduced
• Velocity shear induces the KHI
• The KHI kinks current
• Kinks cause local compression
• Local compression drives reconnection

Scaling Insensitive to resistivity Anomalous
resistivity NOT required
Cf. classical Tearing instability with scaling
Anomalous resistivity required
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Driven Reconnection KHI

• KHI Field lines are compressed and distorted
• At a different degree at different heights
• Reconnection is driven by KHI

Brackbill, Knoll Lapenta (2001)
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Magnetic Stochasticity
Allowing for small level of MHD turbulence in
field leads to a generation of multiple current
sheets which significantly enhances reconnection
rate (decrease in L/d, increase in number of
reconnection regions) without recourse to
anomalous resistivity.
Lx
l??
l?
Local reconnection happens on small scale
l?? rather than Lx, accelerating reconnection.
Lazarian Vishniac (1999)
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Anomalous resistivity and particle source
functions
Ad hoc anomalous resistivity is frequently
invoked to generate fast enough reconnection in a
variety of solar phenomena (flares, jets etc).
e.g. Yokoyama Shibata (200?)
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Anomalous resistivity and particle source
functions
Current-driven ion-acoustic waves (CDIAW) can
drive anomalous resistivity in a manner which
more directly relates observations of the
consequences of reconnection to a physical
source, e.g. Watt et al (2002), Petkaki et al.
(2003).
?pet?380
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Time signatures in particles
Recent work on the role of the electron inertial
term is proving useful in explaining observations
of energy release and particle acceleration on
very short timescales (lt 1s) in solar flares.
McClements et al. (2004) Spectrum of
current-free magnetic X-point determined, taking
into account resistivity electron inertia.
For finite collisionless skin depth, spectrum
has discrete continuous components continuum
modes have intrinsic damping For solar
coronal parameters, this yields energy release
time short enough to account for impulsive phase
of flare
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Future Observations 3D Detectors
Stern et al. (2004)
Ongoing collaboration between LMSAL, Stanford,
NIST and Rice.
Transition Edge Sensor and other
micro-calorimeter technologies are paving the way
for high-sensitivity 3D detectors (temporal,
spatial and spectral) aimed at observing
reconnection at work in solar flares.
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Conclusions

• Magnetic Reconnection has to be at work on the
  Sun, especially in solar flares.
• While observational signatures are indirect, the
  data are good enough to place some constraints on
  how the reconnection proceeds.
• Coupling theoretical developments (e.g. 3D
  reconnection) to observational consequences is
  necessary to make progress in not only
  understanding the energy release in solar flares
  but the reconnection process itself.