Magnetic Reconnection in Solar Activity

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Magnetic Reconnection in Solar Activity
Spiro K. Antiochos, Naval Research
Laboratory

• Corona exhibits both impulsive and quasi-steady
  activity
• All driven by magnetic field

• SOHO EIT Fe XII 195 A, T 1.5MK
• Cadence 12 m
• Resolution 1,500 km
• Wavelet enhancement
• CME/prominence ejection/flare coronal loop
  heating, solar wind
• Reconnection is key process
• Expect great progress with Hinode/STEREO

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The Hinode Mission

• Mission concept High spatial and temporal
  resolution from white light to X-ray
• Launch vehicle ISAS MV
• Orbit Polar, sun synchronous
• Mission lifetime 3 years

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The Solar-B Optical Telescope and Focal Plane
Package

• First space based observations of the Suns
  vector magnetic fields
• Reveals magnetic field photospheric roots and 3-D
  linkage to the corona
• Provides complete time coverage at high
  resolution

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The X-ray Telescope

• Unprecedented combination of spatial resolution,
  field of view, and image cadence.
• Broadest temperature coverage of any coronal
  imager to date.
• Extremely large dynamic range to detect entire
  corona, from coronal holes to X-flares.

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The Extreme-Ultraviolet Imaging Spectrometer (EIS)

• First EUV solar spectrometer capable of obtaining
  high spectral resolution data with both high
  spatial ( 730 km/pixel), and temporal
  resolution (seconds).

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Instrument Features

• Large Effective Area in two EUV bands 170-210 Å
  and 250-290 Å
• - Multi-layer Mirror (15 cm dia ) and
  Grating both with optimized Mo/Si Coatings
• - High QE CCD Two 2048 x 1024 back
  illuminated CCDs
• Spatial resolution 1 arcsec pixels/2 arcsec
  solar resolution
• Line spectroscopy with 25 km/s pixel sampling
• Field of View
• Raster 6 arcmin8.5 arcmin
• FOV centre moveable E W by 800 arcsec
• Wide temperature coverage log T 4.7, 5.4, 6.0
  - 7.3 K
• Simultaneous observation of up to 25 lines

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The STEREO Mission
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STEREO Instruments

• Remote Sensing
• Sun Earth Connection Coronal and Heliospheric
  Investigation (SECCHI) PI Russell Howard, Naval
  Research Laboratory
• STEREO/WAVES (SWAVES) PIJean Louis H. Bougeret,
  Centre National de la Recherche Scientifique,
  Observatory of Paris
• In Situ
• In situ Measurements of Particles and CME
  Transients (IMPACT) PI Janet G. Luhmann,
  University of California, Berkeley
• PLAsma and SupraThermal Ion and Composition
  (PLASTIC) PI Antoinette Galvin, University of
  New Hampshire

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Mission Status

• Hinode launched Sept. 22, 2006
• All instruments operating well and returning data
• EIS spectrometer working nominally
• STREREO launched Oct. 25, 2006
• A spacecraft in final orbit, B has final moon
  encounter on Jan 21
• All instruments operating nominally

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Coronal Reconnection

• Physical properties of Suns Corona
• High Lundquist number 1010
• T 106 K, n 109 cm-3 , B 102 G, L 1010
  cm gt?c
• Low plasma beta 10-2
• Open system
• Line-tying at high-beta photosphere
• Multi-polar magnetic source surface
• Turbulent flows 1 km/s, quasi-static
  driver
• Standard scenario for solar activity
• Photosphere pumps free energy into coronal B
• Current sheets eventually form
• Fast reconnection transfers energy to plasma
• How do current sheets form?
• What is the dissipation mechanism(s)?
• Where does energy go heating, mass motions,
  particles, ?

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How do Current Sheets Form?

• Photospheric Driving
• Convective motions (and line-tying) directly lead
  to small-scale structure
• Coronal heating, loops,
• Topological Discontinuities
• Separatrices, null-points, QSLs,
• Coronal dynamics, CME initiation, solar wind
  acceleration,
• Ideally driven
• Instability/loss of equilibrium
• Flares, coronal heating?, inflows/outflows,

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1. Photospheric Driven Reconnection

• Current sheets form when photospheric motions
  complex
• Exponential growth of magnetic gradients at
  stagnation points of flow
• Do not need small scale photospheric motions
• Current sheets form as a result of wave
  propagation in spatially varying field phase
  mixing
• Current sheets form as a result of coronal
  turbulence
• All work at some level, but which one heats the
  corona?

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Nanoflare Model

• Motions tangle coronal field resulting in small
  scale current sheets must build up stress
  (implies critical shear angle) (Parker)
• Reconnection relaxes sheets releasing heat
• High repetition frequency produces quasi-steady
  state
• Similar picture for wave or turbulent heating

Electric current sheet
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Nanoflare Mechanism
(Dahlburg et al )

• Tearing mode produces twisted flux tubes
• Amount of twist varies inversely with current
  sheet shear
• Large shear leads to kink-like secondary
  instability

Electric current density at two times in
simulation
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Heating Rate vs. Time
Impulsive !
Switches on at Qcrit 50o
(Dahlburg et al 2005)

• 3D quasi-ideal instability
• Burst of energy release

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Photospherically-Driven Reconnection

• Likely explanation for coronal-loop heating
• ISSUES
• What is correct physical model for loop fine
  structure tangling, waves, turbulence, ?
• What is form of dissipation (resistivity)?
• Can reconnection produce a steady state?
• Where has all the helicity gone?
• Is this really reconnection?
• Role of Hinode/STEREO observations

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Role of STEREO Observations

• 3D active region structure and evolution
• SECCHI/EUVI First Light Images Fe IX

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Role of Hinode Observations

• Quantitative measurement of plasma, T, N, and V
• Active region on 12/1/2006
• EIS raster images

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Role of Hinode Observations

• Quantitative measurement of plasma, T, N, and V
• Distinguish between mechanisms
• Active region on 12/1/2006

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2. Topological Discontinuities

• SOHO found that corona perpetually dynamic
• Associated with reconnection in multi-polar
  topologies
• CME initiation, surges spicules, jets, explosive
  events,
• Also proposed for solar wind acceleration
  (Parker, Axford, .
• Hinode XRT observations of polar coronal hole

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Generic Coronal Topology

• Coronal field extrapolated from photospheric
  data
• 2-dipole topology
• Usual 3D null, separatrix structure

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Topologically-Driven Current Sheets

• 3D AMR simulation (Devore et al)
• Current sheets form due to discontinuity in B
  stress
• Reconnection attempts to decrease stress

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Syrovatskii Null-Point Reconnection
Initial Field
Reconnection relaxation
Plasma stressing
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3D Multi-Polar Reconnection
Vy contours

• Jets Alfven speed, similar to well-known 2D
  results
• But localized in 3D
• See bursty reconnection
• Need high resolution to build up free energy

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Topologically-Driven Reconnection

• Proposed mechanism for vast range of coronal
  activity CME initiation breakout model
• Reconnection of MRX
• ISSUES
• What determines onset of reconnection?
• How much free energy can be built up/released?
• What determines burstiness?
• How large are the generated flows?
• How large is the wave flux generated?
• Hinode/STEREO designed to attack these questions

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3. Eruption-Driven Reconnection

• Current sheets form as a result of ideal
  instability/loss of equilibrium CME
• TRACE observations of 04/21/02 flare
• Growing arcade of 20 MK loops

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Eruption-Driven Reconnection

• Current sheet forms in wake of field line opening
• Main energy release process in CME/flare

(MacNeice et al)
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Eruption-Driven Reconnection

• Widely-accepted model for large flares
• ISSUES
• What is dissipation process?
• Where does the energy go?
• Large fraction in nonthermal particles (10 50
  !)
• Approximately half in electrons!
• Do they escape how?
• Major focus for Hinode

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Flare Reconnection

• EIS raster images ranging from 50,000 to 20MK

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Flare Reconnection

• Unique measurements of flare plasma