The 3D Sun and Inner Heliosphere: The STEREO View

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The 3-D Sun and Inner Heliosphere The STEREO
View   Peter Bochsler Physikalisches
Institut University of Bern Sidlerstrasse
5 CH-3012 Bern/Switzerland  
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Stereoscopic 3-D Reconstruction of Coronal
Structures

• Light
• Straight pathways
• (Almost) no absorption or scattering
• Doppler shift indicates velocity of source

• Particles
• Charged particles follow magnetic field lines
• Time delays depending on particle energy
• Ionization state and nuclear properties provide
  information on origin.

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3-D reconnection in magneto-hydrodynamic
turbulence A case study by H. Politano, A.
Pouquet, and P.L. Sulem Phys. Plasmas 2 (1995)
2931-2939
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Soon after beginning (at t0.4) Symmetric
current sheets connect magnetic neutral points
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t0.8 The strongest current sheets are no
longer related to magnetic neutral points
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t1.0 A strong, new, current has evolved in the
center
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t1.2 More couples of current sheets evolve,
each related to magnetic nulls
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t1.8 Current sheets disrupt into many complex
structures
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t2.8 Complete disrupture of current sheets and
vorticity sheets into small-scale structures
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Which signatures should we expect from particle
observations with STEREO ?
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Particle acceleration at an X-type
reconnection site with a parallel magnetic
field P.K. Browning and G.E. Vekstein Department
of Physics, University of Manchester Institute of
Science and Technology, Manchester, England,
United Kingdom Journal of Geophysical Research
106 (2001) 18677-18692
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From the Introduction the energy spectrum of
these charged particles provides a key diagnostic
of the reconnection process, and we focus on this
aspect of reconnection here
From the Introduction the mass/charge
spectrum of these charged particles provides a
key diagnostic of the reconnection process, and
we focus on this aspect of reconnection
here ???
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Assume a two-dimensional magnetic configuration
with an X-type neutral point and a uniform
magnetic field transverse to the X-point plane
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E Electric field along z direction, driving
reconnection (reconnection occurs when ohmic
resistance is non-negligible) Particles are
magnetized on the global scale, L, if their
larmor radius is much smaller than the length
scale
L
r
Bz, Ez
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E Electric field along z direction, driving
reconnection (reconnection occurs when ohmic
resistance is non-negligible) Particles are
magnetized on the global scale, L, if their
larmor radius is much smaller than the length
scale Around a region of size
particles are susceptible to acceleration by the
driving electric field. Typical drift velocity
E-field acceleration distance r Hence
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If Bz is large, also the escaping particles are
magnetized.

• Motion near X-point (r lt gL)
• Time scale
• Gyrofrequency
• Gyromotion along Bz dominates E/B-drift
  
• if g gt e1/3

r
Bz, Ez
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)
d
l
e
i
f

c
i
r
! simplified example
t
c
e
l
1.0
e

o
n
(
particle bounces

n
o
i
t
c
e
j
n
I
z
0.0
-1.0
-1.0
0.0
1.0
x
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Case of strong magnetic moment Particle bounces
between mirror points and drifts slowly under the
influence of the electric force towards the
X-point. There, it is de-magnetized and moves
quickly into an another quadrant where it begins
to bounce again.
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Final speeds of particles depend crucially on
their approach to the neutral point. Particles,
which come very close will reach the highest
end-velocity before they leave the (x,y)-box
Particles with large final energies leave east or
west
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Career of the most energetic particle The
particle with the largest final energy (among 107
particles) enters near the vertex near x0, hits
the center of the X-type neutral region, and
experiences a strong and almost undisturbed
uplift due to the ambient recon-nection electric
field. It leaves the field through the east exit.
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The fate of the two most energetic 4He (from
107) particles
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Slope -3/4
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Abundance ratios of accelerated particles show
strong directional features.
Relevant for STEREOAre these features
detectable at 1 AU ?
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N W
E S
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particle outflow
particle inflow
particle outflow
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Simplistic explanation Random walk through a
long pipe under the influence of
gravity Experiment Consider particles undergoing
a random walk process in x and y direction while
falling through a long cylindrical pipe. The
final energy of a particle is registered when it
hits the wall.

• Result
• Most particles will hit the wall rather soon,
  their energy remains small because they
  experience a short acceleration time.
• Very few particles manage to stay for a long
  time and, therefore, gain a lot of energy.
• The energy distribution of these particles
  approaches a power law

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Analytic Random Walk Model Number of particles
remaining in the reconnection site can be
estimated from the diffusion equation Originally
No particles contained in a site of size L2. Then
diffusion sets in mean
size of cloud at time t after time t, the
probability of finding a particle still within L2
is Typical energy W(t) of particle after time
t (E electric field) Hence
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Analytic Random Walk Model (improved) .. Typic
al energy W(t) of particle after time t (E
electric field) However, depends also
on energy l Larmor radius, vp approaches
energy in z-direction
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H.S. Ji, M.T. Song, G.L. Huang Exact
Solutions for two dimensional steady state
magnetic reconnection in partially ionized
plasmas Astrophys. J. 548 (2001) 1087-1092
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Ion flow function Magnetic flux
function Neutral flow function increasing
coupling between neutrals and ions gtgtgt
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increased coupling between neutrals and ions gtgtgt
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ion flux field
magnetic flux field
neutral flux field
Degree of ionization strong
medium weak
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Ji, Song and Huang, ApJ. 548 (2001) 1087
Ion flow
Neutral flow
100 ionized 10 ionized
0.1 ionized
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Fractionation at work!

• Reconnection in the photosphere is a process
  which occurs in a weakly ionized medium with
  strong coupling of neutrals and ions.
• When the coupling is not strong (upper
  chromosphere, transition region), ions can slip
  through moving neutrals and bring the magnetic
  field toward the magnetic null point (Þ...FIP
  enrichment)
• In a strong coupling case (photosphere), ions
  bring both, magnetic field and neutrals forward
  (Þ...no FIP enrichment)

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Minor species at work!

• The degree of ionization helps shaping the
  reconnection process and the reconnection
  configuration.
• Strong coupling of neutrals to ions makes fast
  reconnection in dense, partially ionized media
  possible
• Reconnection in the weakly ionized photosphere is
  much less violent than in the corona, owing to
  the much larger load of neutral gas (Ji, Song,
  and Huang, 2001).
• In a partially ionized medium, field cancellation
  may lead to a runaway process by increasing the
  amount of low-FIP ions, thereby enhancing the
  low-FIP content of outflowing species.

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The End
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