Role of trapped electrons in pic-code simulations of reconnection

J Egedal, MIT, PSFC, Cambridge, MA

Thanks to W Daughton & J F Drake

Fluid descriptionNavier Stoke’s equations for the electron fluid

)()( eeeeee tnmne

vvvpj B vE

Generalized Ohms law:)( eii vvj, vv ne

)()(12 tne

mne

ee

j jvpBjj B vE

|||| jvbEb 2

1nempne

e

b

Pic-code simulation (by Prof. W Daughton)

Why is J|| limited to this small region?

|||| jvbEb 2

1nempne

e

Works, but no equation for j||!

Trapped electron in simulation by Prof. W Daughton

M. Øieroset et al. Nature 412, (2001)

M. Øieroset et al. PRL 89, (2002)

Wind spacecraft observations in distant magnetotail, 60RE

• Measurements within the ion diffusion region reveal: Strong anisotropy in fe.

• Applying f(x0,v0) = f(|v1|) to an X-line geometry consistent with the Wind measurements

• A potential, needed for trapping at low energies

Wind data Thermal electrons trapped

~ -300V~ -700V~ -1000VTheory Wind

J. Egedal et al., (2005) Phys. Rev. Lett. 94, 025006

+ ++ ++

Numerical simulation (by Prof. J. Drake)

Ey

x/di

z/di

0 260

9

0

0.8

-0.8

Jy

x/di

z/di

0 260

9

0

en0vth,e xz (Exz= -xz)

x/di

z/di

0 260

9 1.5 (=6Te)

-1.0

Mass ratio: 225, Te=Ti=0.25, Bg=0.5B0

Why is Jy limited to this small region?

Numerical simulation (by Prof. J. Drake)

|| (E||= -||)

x/di

z/di

0 260

9 1.5 (=6Te)

0

Ey

x/di

z/di

0 260

9

0

0.8

-0.8

Jy

x/di

z/di

0 260

9

0

en0vth,e

x/di

z/di

0 260

9 1.5 (=6Te)

-1.0

xz (Exz= -xz)

Mass ratio: 225, Te=Ti=0.25, Bg=0.5B0

|| (E||= -||)

x/di

z/di

0 26

0

91.5 (=6Te)

0

Escape Energy = 6 Te

x/di

z/di

0 26

0

9

0

1.5

Trajectories of thermal electrons

|| (E||= -||)

Representative electron orbits

J|| is carried by shifting electron orbits.

The shift is controlled by the ions,required for quasi neutrality.