P1253296517rGfAR

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Reconnection process in Sun and Heliosphere
A.C. Das Physical Research Laboratory Ahmedabad
380 009
IHY school for Asia Pacific Region, Kodaikanal,
Dec.10-22, 2007
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• Heliosphere Magnetosphere of our sun
• Interaction of Solar wind and the interstellar
  medium
• Heliopause Balance between interstellar medium
  and solar wind pressure
• Termination shock Solar wind becomes subsonic
  at this point

• Interplanetary medium moving in opposite
  direction becomes subsonic as it collides with
  heliopause Bow shock
• Solar wind, solar flares and coronal mass
  ejection sends materials and fields into the
  heliosphere
• Heliospheric current sheet, ripple in the
  heliosphere

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Solar Flares Coronal Mass Ejections And Closed
Magnetic loop structure in helispheric current
sheet
Generated by a powerful plasma process
Reconnection of magnetic field lines
Giovaneli importance of neutral point in Solar
Flares Dungey Developed a radically different
model in physics of magnetosphre Following his
concept, we will describe the process of
reconnection
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Essential to introduce some basic understanding
of plasma flow and magnetic field structure.
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In absence of plasma valocity, Ohms law
where the magnetic field is secondary and can be
calculated from Amperes law c Curl B 4
For MHD, velocity v and the magnetic field B are
primary and J and E can be calculated from
.
Now for large , can be neglected and
electric field is then driven by the velocity and
magnetic field. Ratio of the 2nd term to the
first term on the right hand side of (5) defines
the magnetic Reynold number given by
is extremely large (1010) in solar
atmosphere
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• Basic Reconnection Process
• Depends on
• Topology of the magnetic field
• Motion of plasma near the neutral point

Magnetic field lines are anti-parrellel One
neutral point, with limiting field lines
Separatrix. Two are going in and two are coming
out Plasma behaviour in absence of pressure E1/C
(VxB) 0 Field lines moving from both sides They
remain field lines. Electric field. Current
enhances, But no reconnection.
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Reconnection-consequence of the break-down of
frozen-in-field approximation. May be caused
because of high current density Finite
Resistivity A different scenario A pair of
inflowing field lines become limiting field lines
and then immediately after that they form
outflowing field lines. Permits Limiting field
lines to cut at neutral point and then reconnect
to form a different set of field lines. Possible
because of violation of frozen in
approximation. This is reconnection in pictorial
form
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Diffusion, and Reconnection In thin region
diffusion is substantial Magnetic Induction
becomes.
In one dimension

where Bx is the magnetic field along
x-direction and z is the vertical direction as
shown in Figure 1.2.
Solution
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Current along y direction Magnetic field lines
are in opposite direction around z0 Magnetic
flux from above as well as from below get dumped
at the separatrix feeding the current. Field
gradient decreases, diffusion slows down process
becomes unproductive. Need to introduce u from
both sides. Can maintain large current Not
physical, unless there is an outflow Finally
reconnection takes place with an outflow Similar
to the picture presented earlier by Dungey.
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• Important Reconnection Models (Steady State)
  MHD Theory
• Sweet-Parker Model
• Magnetic field are anti-parrellel
• Plasma is incompressible
• Plasma flow from both sides with u, current sheet
  length l and width d.
• Conservation of Mass
• ulvd.. (10)
• (consequence of . v 0
• Momentum balance External magnetized
• Internal field free

P is the pressure on the central plane where the
magnetic field is almost zero. Po-pressure
outside, where the magnetic field is B.
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Petschek Model
SP model large l No large rate of Reconnection
because u(d/l) va
Petschek pointed out In MHD flow in the outer
region, Possible that two standing MHD wave
front can be maintained fronts are
shocks Diffusion region can be matched to a
region of standing waves.
a the half angle of the exit flow or the angle
of slow shock such that it remains stationary in
the flow
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In Petschek Model,
u, B are uniform And Electric field also is
uniform Therefore
As a-increases, u has to increase and then B
decreases in the diffusion region and becomes
less than
This is achieved by rotating the magnetic field
vector towards the normal.
Vasyliunas obtained upper limit
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Spontaneous reconnection or Patchy Reconnection
Tearing mode instability
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Growth rate can be estimated shown below
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We have seen that the growth rate depends on the
width of the current sheet and conductivity. Norm
al component of the magnetic field. Bn Electron
Tearing mode disappears. However, ion-tearing
mode can be present. But has limitation on
magnetic field range. External Source LH
turbulence Enhance the growth rate.
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Observational Evidence of the magnetic
Reconnection in solar flares
Top left side Right bottom Cusped shaped loop
structure, Hard X-ray telescope in
Yohkoh Helmet streamer etc. Hard X-ray loop top
above Plasmoid ejection soft X-ray bright loop
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Schematic view of Impulsive flare. Region of
acceleration of particles.
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• Loop-top Hard X-ray source above
• Soft X-ray bright loop
• During SXR loop
• Discovery of loop top HXR
• Made it possible
• Unifying two classes of flare LDE and impulsive
  flare
• Unifying model

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Numerical Simulation of reconnection between
emerging flux and coronal field
Formation of magnetic island that are ejected out
of the current sheet. Localized resistivity seems
to be essential . Tearing mode instability.
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More realistic simulations
Both temperature and density evolution leading to
reconnection and island information. Again
localized resistivity appears to be very
important for fast reconnection.
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Problem of Scale Matching

• Two important aspects-unanswered.
• Local Enhancement of Magnetic Diffusion a
  conjecture
• Enormous gap of scale sizes macro and micro
  features.

• Scale-size of
• the anomalous resistivity
• d ri 10 m

d Thickness of the current sheet
ri Ion Larmor radius

• Scale size of a flare 104 km !

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Interesting turbulent structure in outflow
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MHD-simulation of turbulent reconnection Structur
es of different scales and intensities are seen.
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Solar Maximum Mission (SMM)

• Observation of evidence of Reconnection of
  previously open magnetic structure.
• Appearing as pinching of helmet streamers
  followed by release and acceleration of a large u
  or v-shapped structure.
• Observed sequence of events consistent with
  reconnection across the heliospheric current
  sheet between previously open field lines and
  creation of detached magnetic structure.
• Coronal disconnection events would return
  previously open flux to sun as closed field
  arches.
• Internal magnetic reconnection can also take
  place within the flux rope. As the flux rope
  field lines are sheared, oppositely directed
  field lines are generated which press together
  and reconnect.

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Satellite observation of the Heliospheric
current sheet shows

• Internal structure of the sector boundaries is
  very complex with many directional
  discontinuities in mag field.
• Implies heliospheric current sheet is not a
  single surface constantly changing layer with a
  varying number of current sheets.
• Studied magnetic reconnection caused by resistive
  tearing mode instabilities, multiple current
  sheets 2D MHD simulation.
• Results Complex unsteady reconnection
• NL limits, formation of islands or plasmoids.
• Suggest Occurrence of multi-direction
  discontinuities in the heliosphere.
• May be associated with the magnetic islands and
  plasmoids caused by Reconnection.

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Summary
Magnetic Reconnection is the underlying driver of
giant explosive releases of magnetic energy in
the Suns atmosphere that are observed as solar
flare or CMEs. Many compelling observational
evidences for reconnection which support
reconnection model of solar flares are
presented. Numerical simulation suggests that
the localized resistivity is necessary for
magnetic reconnection. There is still an
enormous gap between the microscale of anomalous
resistivity and the size of solar flares. MHD
turbulence model of reconnection shows
interesting features in various cases and may
play an interesting role in solving the
scale-matching problem.