Solar Atmosphere as a Laboratory for Magnetic Reconnection

Shinsuke Imada

(ISAS/JAXA)

Magnetic Reconnection

Current sheet thickness

Inflow velocity

Outflow velocity

Turbulence/wave

Ion-Electron Heating

Non-thermal acceleration

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.

Slow shock

Plasmas in universe

Comparative studies important

collisionless

weak collision

collisional

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

Ambipolar/Hall = ωci/νin ωci : Ion-cyclotron freq ∝B

νin : Ion-neutral collision freq ∝ n

Chromosphere: ambipolar > Hall, resistivity

by K.A.P. Singh

Assumption: typical magnetic field profile of a vertical flux tube

]4)(

[ Jcc

BBJ

en

BJBV

t

B

nnien

πηρν

−××

+×

−××∇=∂∂

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.

x

By

In the presence of B=0 pointOhmic only

Ambipolar diffusion does not work where B=0

Current is concentrated near B=0 => thinning of current sheet

€

∂∂x

By2 ∂

∂xBy = 0

€

By ∝ x1/ 3

J =∂By

∂x∝ x−2 / 3

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.

x

By

In the presence of B=0 pointAmbipolar only

Ambipolar diffusion does not work where B=0

Current is concentrated near B=0 => thinning of current sheet

€

∂∂x

By2 ∂

∂xBy = 0

€

By ∝ x1/ 3

J =∂By

∂x∝ x−2 / 3

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 diffusionFormation of islands => bursty reconnection

Isobe et al, in preparation

Thus very bursty/sporadic magnetic reconnection can be taken place in chromosphere.

Solar FlareLDE event Impulsive event

Tsuneta et al., 1996 Tsuneta et al.,

1997Let’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

Yohkoh+Hinode We understand very well

We should observe this region!

with spectroscopy

Tsuneta et al., 1996

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 2009Recently 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

α collisional and dielectronic recombination S collisional ionization

FeXIV FeXV FeXVI

ionization recombination

FeXVII FeXVIIIFe13+ Fe14+ Fe15+ Fe16+ Fe17+

These process linearly depend on density

10^9/cc1MKShock angle 85degree

Example of ionization calculation

41MK1200km/s outflowWithout thermal conduction

T=1MK at t<0

T=41MK t>0

Example of ionization calculationin Steady Reconnection model

Example of ionization calculationin case of N~10^8/cc

Emission measure

Black:Jet NEQ、 Red: Jet EQ、 Blue:Post flare loop EQ

Post Flare loop

20 x 1 Mm

10^11/cc15MKIonization 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になるのも電離平衡のタイムスケールとほぼ同じ

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？

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