“Plasma-neutral gas simulations

of reconnection events in cometary tails”

C. Konz, G. T. Birk, & H. Lesch2004, A&A, 415, 791-802

DOI: 10.1051/0004-6361:20031695

S. Tanuma

(Kwasan Observatory)

The formation and dynamical evolution of cometary plasma tails and magnetic boundary layers is studied by the first numerical plasma-neutral gas simulations.

Disconnection Event• “Intreplanetary gas. XXVIII. Plas

ma tail disconnection events in comets: Evidence for magnetic field line reconnection at interplanetary sector boundaried?”, Niedner & Brandt 1987, ApJ, 223, 655

• Comet Morehouse 1908c (Yorkes Observatory)

• (Top) 20:57 GMT, 1908 Sep 30

• (Bottom) 19:43 GMT, Oct 1

Disconnection Event (DE)

• “Intreplanetary gas. XXVIII. Plasma tail disconnection events in comets: Evidence for magnetic field line reconnection at interplanetary sector boundaried?”, Niedner & Brandt 1987, ApJ, 223, 655

• 1974 Jan 21

• Comet Kohoutek

• Des were also observed at the comet Halley 1985-86

Reconnection in cometary tail

Dense, cold, neutral and slab-like comet

Hot, magnetized ambient plasma flow

inflow

× Nightsidereconnection

Current sheet

Reconnection in cometary head

Dense, cold, neutral and slab-like comet

Hot, magnetized ambient plasma flow

inflow

×Dayside reconnection

Current sheet

Secure boundary

Similar Simulations• Reconnection triggered by the comet (reconnection arou

nd the comet, magnetotail; Niednet & Brandt 1979; Niednet, Ionson, & Brandt 1981; Niedner 1982; Ogino, Walker, & Ashour-Adballa 1986; Niednet & Brandt 1987; Brandt & Niednet 1987; Niedner & Schwingennschuh 1987; Ogino 1988; Brandt & Snow 2000)

• Reconnection triggered by the high velocity cloud (Konz, Birk, & Lesch 2004; Konz, Bruns, & Birk 2002)

• Reconnection in the earth’s magnetosphere (Ogino’ papers; Birk, Lesch, & Konz 2004)

• Reconnection triggered by the flux tube

Earth

• “Solar wind induced magnetic field around the unmagnetized Earth”, Birk, Lesch, & Konz 2004, A&A, 420, L15 (pdf)

• See also Ogino’s papers

High Velocity Clouds• “Dynamical evolution of t

he high velocity clouds” Konz, Birk, & Lesch 2004, ApSS, 289, 391 (pdf)

• “Dynamical evolution of high velocity clouds in the intergalactic medium”, Konz, Bruns, & Birk 2002, A&A, 391, 713: Strong radio emission around HVC complex C (pdf; fig)

New point of this paper

• Two fluid (ion and plasma)

• Collisional momentum transfer

• (Birk& Otto 1996, J. Comp. Phys., 125, 513)

• Small and many grid

• Harris-like sheet

Basic Equation (1)

Plasma continuum equation

Neutral gas continuum equation

Plasma momentum equation

Neutral gas momentum equation

Normalization by L, Alfven velocity, Alfven time, magnetic pressure

Basic Equation (2)

Plasma pressure equation

Neutral gas pressure equation

Induction equation

Constraint of momentum conservation

Classical model

Recombination/ionization

Typical parameters in solar wind at 1 AU

• L=5x10^10 cm (The extend of ionosphere of comet at 1 AU)

• No=12 cc

• Bo=5x10^-3 G

• Va=3.15x10^8 cm/s

• ta=159 s

• (Typical quantities at 1 AU)

Initial Condition (Run I)

Dense, cold, neutral and slab-like cometRho_no=1.5x10^4(no=12 cc)T_no=1

Hot, magnetized ambient plasma flowRho_min=1To=100

Vy0=-0.15 (=470 km/s); MA=15

B=Bx =Bo =0.01(=50microG)

-60<x<60-250<y<30303x703 grids

R=2

X=0

Y=30

Y=0

Y=-250X=-30 X=30

Anomalous resistivity model

• They always adopt this model. They assume a background resistivity for the first time.

Eta_=10^-5 (>eta_num=10^-6)Eta_2=0.05Jc=0.1

B, v, resistivity

T=2

T=300

T=900 T=2431

T=2372

T=1786

Anomalous resistivity sets in

Petschek reconnection starts at t=1000

Nightside reconnection

core

Jz

|B|

Results of Run I• Disconnection event (DE): Solar wind magnetic barre

r; Dayside diffusion comet’s ionosphere night side reconnection

• This process is quasi-syclic.• Since we did not include mass-loading of the solar wind

and outgasing from the comet's surface, no plasma density enhancement can be seen in the ejected tail.

• However, increased Ohmic dissipation at the pinched region can account for a brightening of the disconnected tail head.

• Including ionization processes and outgasing of neutrals is necessary to end up with a plasmoid like density enhancement of the tail plasma.

Initial Condition (Run II)

Dense, cold, neutral and slab-like cometRho_no=1.5x10^4(no=12 cc)T_no=1

Hot, magnetized ambient plasma flowRho_min=1To=100

Vy0=-0.15 (=470 km/s); MA=15

B=Bx =Bo =0.01(=50microG)

-60<x<60-250<y<30303x703 grids

R=2

X=0

Y=30

Y=0

Y=-250X=-30 X=30

Harris-type current sheet is assumed at y=25 at t=705ta in Run I. The other conditions are same with Run I.

Y=25

Run I

Second boundary

Strong current

Run I

Results of Run II

• The dayside reconnection occurs violently.

• Dayside reconnection: more dynamic, more violent, 2.5 times higher resistivity, higher reconnection rate, shorter time scale

Conclusion

• The formation and dynamical evolution of cometary plasma tails and magnetic boundary layers is studied by the first numerical plasma-neutral gas simulations.

• It is shown that collisionless interaction between the cometary envelope and the solar wind plasma leads to the formation of a magnetic barrier.

• The dynamics of the magnetotail are governed by multiple magnetic reconnection.

• If the comet encounters a heliospheric current sheet, strong disconnection events characterize the cometary plasma tail.

• But even in the case of homogeneous solar wind conditions, partial disruption of the tail is triggered by dayside reconnection.