Chromospheric Magnetic Reconnection from an Observer

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Chromospheric Magnetic Reconnection from an
Observers Point of View

• Jongchul Chae
• Seoul National University, Korea

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What is Chromospheric Reconnection ?

• Magnetic reconnection occurring in the
  chromosphere and photosphere, not in the corona
• Lower Alitudes 0 to 104 km
• Lower temperatures a few 103 K to a few 105 K
• Higher densities
• Small-scale ? low altitude
• Diversities in flow speed, density and
  temperature ? strong stratification
• Driven reconnection
• Flux emergence
• Supergranular flow

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Observational Signatures

• Canceling Magnetic Features
• Jet-like Features seen in Ha and UV/FUV/EUV
• UV/EUV jets, UV explosive events
• H alpha jets/ surges/ H alpha upflow events
• Chromospheric Brightenings
• Ellerman bombs
• Other brightenings in UV/EUV/ H alpha

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Canceling Magnetic Features
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Canceling Magnetic Feature
From Chae, Moon, Park 2003, JKAS 36, S13
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CMF as a CMR event
Corona
UPFLOW
Chromosphere
DOWNFLOW
Photosphere
Interior
FLUX CANCELLATION
CONVERGING MOTION
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Does the flux submerge in CMFs?

• YES! The ASP observations produced the evidence
  for it.

From Chae, Moon, Pevtsov 2004, ApJL, 602, L65
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Observables of CMF

• Rate of Magnetic Flux Loss
• Half length of interface between two poles
• Specific flux loss rate
• Converging speed of each pole toward PIL

From Chae, Moon, Park 2003, JKAS 36, S13
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Summary of CMF Observations
Chae et al. 2002 Case A Chae et al. 2002 Case B Chae et al. 2003 Chae et al. 1998
Flux loss rate Mx/h 3.4 x 1018 2.5 x 1018 1.8x1018 2x1017
Contact length Mm 7.8 3.3 2.5 3
Specific flux loss rate G cm/s 1.2 x 106 1.1x106 2.0x106 2x105
Converging speed km/s 0.27 0.35 0.22
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Chromospheric Jets in Active Regions

• EUV Jets

From Chae, J. 2003, ApJ 584, 1084
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Ha Jets in the same active region
Chae, J. et al. 2000, Solar Physics 195, 333
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EUV/Ha Jets in another AR
From Chae et al. 1999 ApJ 513, L75
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Jet-like features in the quiet Sun

• Ha upflow events

From Chae et al. 1998, ApJ, 504, L123
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Jet-like in the quiet Sun

• UV explosive events

From Chae et al. 1998, ApJ, 479, L109
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Summary of Jet Observations

• Jet-like features occur in strong association
  with canceling magnetic features.
• There is a good correlation between speed and
  temperature in jet-like features.
• Jet-like features with different temperatures
  often occur together at the same place.

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Theoretical Considerations of Chromospheric
Reconnection

• Adiabatic Current Sheet of Sweet-Parker type
• Insights on Chromospheric Reconnection from
  Observations of Jet-like Features
• Insights on Chromospheric Reconnection from
  Observations of Canceling Magnetic Features

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Adiabatic Current Sheet Model of Sweet-Parker
type
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Current Sheet Model of CMR
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Steady-state Current Sheet Model

• Sweet-Parker Model
• Incompressible flow
• Litvinenko (1999)
• Compressible, isothermal flow
• Chae et al. (2003)
• Adiabatic flow
• A generalized approach

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Steady-state Equations

• Induction equation
• Mass conservation
• Momentum conservation
• Adiabatic energy equation

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Solutions

• Basic assumptions
• Density compression factor
• Outflow speed
• Temperature Excess -outflow speed relation

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Insights on Chromospheric Reconnection from
Observations of Jet-like Features
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Are observed jet-like features chromospheric
reconnection jets?

• Temperature excess-outflow speed relation
• Ha jets
• UV Explosive events in the quiet Sun
• EUV Jets

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Are observed jet-like features chromospheric
reconnection jets?

• Yes, very likely as seen from the
  temperature-speed relation.
• Hotter jets are better explained with a smaller
  value of g ( closer to isothermal process )
• g 4/3 fairly well explains the observed
  temperature-speed relations in jet-like features.
  

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Are Ellerman bombs chromospheric reconnection
events?

• Ellerman bombs Brightening in the far wing of H
  alpha line profile ? Heating events in the low
  chromosphere DT2000 K
• They may be reconnection events. If so, we have
  a prediction
• It would be important to measure the flow
  associated with Ellerman bombs.
• Note Shimizu et al. 2005 Extremely red-shifted
  magnetic features as high as 10 km/s

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What determines jet temperatures?

• Temperature excess ? Outflow speed ? Alfven speed
  of inflowing region ? magnetic field strength and
  density ? Atmospheric level
• The temperature and speed of reconnection jets
  strongly depend on the atmospheric level where
  reconnection occurs.
• Higher Atmospheric level ? Lower density ? Higher
  Alfven speed ? Higher outflow speed ? Hotter jets

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What determines the degree of compression?

• Specific heat ratio ? the efficiency of radiative
  cooling
• g 1 restores the Litvinenkos (1999) result.
  This is an unrealistic assumption.
• In general , 1 (isothermal) ltg lt5/3 (adiabatic)
• It is likely that g gets bigger with higher
  levels where the medium is more transparent.
• Plasma beta of inflow
• restores the incompressible flow
  assumption of the original Sweet-Parker model
• In practice, this assumption is hard to achieve
  in solar atmosphere, and hence unrealistic.

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What determines the degree of compression?

• Near the photosphere
• In the upper chromosphere

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Insights on Chromospheric Reconnection from
Observations of Canceling Magnetic Features
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Linking observed parameters of CMF and physical
parameters of CMF
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Do CMFs result from reconnection in the
temperature minimum?

• Sturrock (1999) and Litvinenko (1999) ? YES
• We have to say NOT necessarily.
• The speed of reconnection using classical
  conductivity of the inflow region is too slow to
  explain the observed converging speed in
  canceling magnetic features.
• The resistivity of the current sheet should be
  much bigger than that of inflowing region. ?
  anomalous resistivity
• The molecular resistivity of the inflowing region
  is no longer the most important parameter
  characterizing chromospheric reconnection.

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Observational constraints on q

• Too small values of q yield too low inflow speeds
  and too high outflow speeds
• Too big values of q yield too high inflow speeds
  and too low inflow speeds
• The observed inflow speeds and outflow speeds
  constrains the anomalous resistivity factor q

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A reference model
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Is there any preferred height for chromospheric
magnetic reconnection?

• No!
• Classical resistivity is not the major factor.
• Reconnection may occur at any height.
• It may be the geometry of two interacting flux
  systems that determines the reconnection height.

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What we have learned so far

• Canceling magnetic features, Ha jets, UV
  explosive events, EUV jets are nicely fit into
  the picture of chromospheric reconnection.
• An adiabatic current sheet with anomalous
  resistivity factor of about 50 and specific
  ratio of 4/3 may serve as a reference model for
  chromospheric reconection.
• There may be no preferred height of chromospheric
  reconnection, and it may be the geometry of two
  interacting flux systems that determines the
  reconnection height.

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Observational Challenges of Solar-B

• Fine-scale structure of canceling magnetic
  features ? new and more reliable measurements of
  specific cancellation rates and converging speeds
  (SOT)
• Discovery of reconnection outflows in the low
  chromosphere v10 km/s, DT103 K (SOT)
• Simultaneous observations of canceling magnetic
  features, low chromosphere reconnection flows, H
  alpha flows, UV jets, EUV jets, X-ray jets that
  cover diverse speeds, temperatures and
  atmospheric levels (SOT, EIS, XRT)

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Theoretical Challenges

• Impulsive, recurrent (often bursty) occurrence
  (Chae et al. 1998a, b)
• existence of elementary non-steady reconnection
  events ?
• formation of a number of magnetic islands via
  tearing instability?
• Co-occurrence of hot jets and cool jets (Chae
  etal. 1998b, 1999)
• Two step reconnection (formation of magnetic
  islands in the lower atmosphere followed by its
  destruction in the upper atmosphere, Chae 1999)?
• Multi-site reconnection of many thin shredded
  flux sheets at different atmospheric heights (in
  a stratified medium)?