Solar Atmosphere as a Laboratory
for Magnetic Reconnection
Shinsuke Imada
(ISAS/JAXA)
Magnetic Reconnection
Inflow velocity
Ion-Electron
Heating
Non-thermal
acceleration
Outflow velocity
Current sheet
thickness
Turbulence/wave
Magnetic field
Density
Original idea is converting magnetic field energy to plasma kinetic energy.
Recently plasma heating, particle acceleration and wave excitation are also
well discussed in the category of magnetic reconnection. These energy
conversion rates are heavily depending on the plasma conditions. In that sense
it is very interesting to discuss magnetic reconnection in various plasma
conditions.
Plasmas in universe
weak collision
collisionless
collisional
Comparative studies important
This figure shows the density and temperature relationship in the Sun, Earth, Lab,
and astro plasmas. The magnetic reconnection is studied in various conditions,
from collisionless to collisional regime.
What plasma parameter controls?
What is the goal of magnetic reconnection study? I already mentioned that the
importance of understanding of energy conversion. This figure shows the illustration of
the energy conversion in two plasma conditions. In my opinion to answer this puzzles
and answer what plasma condition controls the each energy conversion rates.
Dynamic activities seen in the chromosphere
Many magnetic reconnection seem to be
taken place in chromosphere.
One of the most important findings by Hinode!
I want to mention one of the most important findings by Hinode. Hinode observed
very dynamical chromospheric activity which may be associated with magnetic
reconnection. Thus now chromosphere is also a target of magnetic reconnection. Is
magnetic reconnection the same as coronal reconnection?
Relative importance of diffusivities
Assumption: typical magnetic field
profile of a vertical flux tube
Ambipolar/Hall = ωci/νin
ωci : Ion-cyclotron freq ∝B
νin : Ion-neutral collision freq ∝ n
B
J  B ( J  B)  B 4
   [Vn  B 


J]
t
ene
c ni  n
c
by K.A.P. Singh
Chromosphere: ambipolar > Hall,
resistivity
May be not.. Recently Isobe-san studied the magnetic reconnection in chromosphere.
He discusses relative importance of magnetic diffusivity between Hall and ambipolar
diffusivity in chromosphere, and found ambipolar diffusion is important in chromosphere.
In the presence of B=0 point
Ohmic only
By
x
Ambipolar diffusion does not
work where B=0
Current is concentrated near
B=0
=> thinning of current sheet
By  x1/ 3
 2 
By
By  0
By
x x
J
 x 2 / 3
x
This movie shows the current sheet development in case of only ohmic diffusivity.
The upper panel shows the magnetic field variation in space, and the bottom is
current density. In the case of only ohmic diffusivity, the current will be diffusing.
In the presence of B=0 point
Ambipolar only
By
x
Ambipolar diffusion does not
work where B=0
Current is concentrated near
B=0
=> thinning of current sheet
By  x1/ 3
 2 
By
By  0
By
x x
J
 x 2 / 3
x
On the other hand, in the case of ambipolar diffusivity, the magnetic field gradient
go steeper and forms the this current sheet. This is because the ambipolar
diffusion cannot work magnetic field is zero.
2.5D simulation (still preliminary)
Ohmic diffusion only
Ohmic+Ambipolar
Thin current sheet created by ambipolar diffusion
Formation of islands => bursty reconnection
Isobe et al, in preparation
Thus very bursty/sporadic magnetic reconnection can be taken place in chromosphere.
LDE event
Tsuneta et al.,
1996
Solar Flare
Tsuneta et al.,
1997
Impulsive event
Let’s move to coronal magnetic reconnection which we have a large amount of
knowledge. This is the very famous observation of solar flares, one is LDE and
impulsive event. We can see many structure of flare loops, but we cannot see
reconnection region itself in many event.
Standard model for Solar Flare
We should observe this region!
with spectroscopy
Yohkoh+Hinode
We understand
very well
Tsuneta et al.,
What we need?
• Typical dynamical timescale of reconnection
10(Mm)/1000(km/s) ~ 10s
• To observe steady reconnection, we should
take one image within 10 sec
• Another important point is understanding of
Thermal Non-Equilibrium plasma, to
understand rapid and strong plasma heating.
Thermal Non-Equilibrium Plasma
• Non-Gaussian Distribution function
→ Power-law distribution, beam plasma
time scale for equilibrium is very short
(kinetic regime or e-e or p-p collision)
• Different temperature in different species
→ Tp>Te
time scale for equilibrium is relatively long
(e-p collision)
• Ionization non-equilibrium
→ strong heating or flare
time scale for equilibrium is relatively long
Ion Temperature
Imada et al., APJL 2009
Recently Imada et al proposed a method for estimating an ion temperature by using
emission lines from different atomic species. This method do not assume Ionization
equilibrium. This method will apply to solar flare near future.
Ionization Process
Fe13+
FeXIV
Fe14+
FeXV
ionization
α
S
Fe15+
FeXVI
Fe16+
FeXVII
Fe17+
FeXVIII
recombination
collisional and dielectronic recombination
collisional ionization
These process linearly depend on density
Example of ionization calculation
10^9/cc
1MK
Shock angle 85degree
41MK
1200km/s outflow
Without thermal conduction
T=1MK at t<0
T=41MK t>0
Example of ionization calculation
in Steady Reconnection model
Example of ionization calculation
in case of N~10^8/cc
Emission measure
Post Flare loop
20 x 1 Mm
10^11/cc
15MK
Ionization EQ
Black:Jet NEQ、Red: Jet EQ、Blue:Post flare loop EQ
Spatial resolution
The necessary spatial resolution is the separation of each line peaks. ~1arcsec
Necessary for next generation
Solar observation
• To observe the dynamics of magnetic
reconnection, we need high throughput
spectrometer (This is most important!)
• We need several emission lines to diagnose
inside the reconnection region because of
NIEQ (line ratio or filter ratio may not work)
• To diagnose electron temperature, it is
important to observe continuum in X-ray range
(photon counting in X-ray range is useful)
• Spatial resolution needs 0.1~0.5 arcsec
• Dynamical range is also important to remove
the effect of bright post flare loops
(Occulter may be useful)
• If we can change the direction of slit, it is
very useful!
We want to see
this region!
熱伝導有りだと
Ti＜Teになるはず
本当か？？
そもそも本当にSS
は等温衝撃波か？
Ti=Teになるのも電
離平衡のタイムス
ケールとほぼ同じ
もしSSの加熱がイオ
ン電子で異なれば違
いが見られる筈