S. Tanuma (Kwasan Observatory)

1. “Plasma-neutral gas simulations of reconnection events in cometary tails”C. Konz, G. T. Birk, & H. Lesch2004, A&A, 415, 791-802DOI: 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.

2. Disconnection Event • “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 • Comet Morehouse 1908c (Yorkes Observatory) • (Top) 20:57 GMT, 1908 Sep 30 • (Bottom) 19:43 GMT, Oct 1

3. 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

4. Reconnection in cometary tail inflow Hot, magnetized ambient plasma flow Dense, cold, neutral and slab-like comet Nightside reconnection × Current sheet

5. Reconnection in cometary head inflow Hot, magnetized ambient plasma flow Dayside reconnection × Current sheet Secure boundary Dense, cold, neutral and slab-like comet

6. Similar Simulations • Reconnection triggered by the comet (reconnection around 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

7. Earth • “Solar wind induced magnetic field around the unmagnetized Earth”, Birk, Lesch, & Konz 2004, A&A, 420, L15 (pdf) • See also Ogino’s papers

8. High Velocity Clouds • “Dynamical evolution of the 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)

9. 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

10. 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

11. Basic Equation (2) Plasma pressure equation Recombination/ ionization Neutral gas pressure equation Induction equation Constraint of momentum conservation Classical model

12. 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)

13. Initial Condition (Run I) Vy0=-0.15 (=470 km/s); MA=15 Hot, magnetized ambient plasma flow Rho_min=1 To=100 Y=30 Y=0 R=2 B=Bx =Bo =0.01 (=50microG) Dense, cold, neutral and slab-like comet Rho_no=1.5x10^4 (no=12 cc) T_no=1 -60<x<60 -250<y<30 303x703 grids Y=-250 X=-30 X=0 X=30

14. 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=0.05 Jc=0.1

15. T=2 B, v, resistivity T=1786 core T=300 T=2372 Anomalous resistivity sets in T=900 T=2431 Nightside reconnection Petschek reconnection starts at t=1000

16. Jz

17. |B|

19. Results of Run I • Disconnection event (DE): Solar wind magnetic barrer; 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.

20. Initial Condition (Run II) Harris-type current sheet is assumed at y=25 at t=705ta in Run I. The other conditions are same with Run I. Vy0=-0.15 (=470 km/s); MA=15 Hot, magnetized ambient plasma flow Rho_min=1 To=100 Y=30 Y=25 Y=0 R=2 Dense, cold, neutral and slab-like comet Rho_no=1.5x10^4 (no=12 cc) T_no=1 B=Bx =Bo =0.01 (=50microG) -60<x<60 -250<y<30 303x703 grids Y=-250 X=-30 X=0 X=30

21. Strong current Second boundary Run I

22. Run I

23. 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

24. 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.