Post on 25-Mar-2020
High-speed fluid dynamicsin magnetic reconnection
in a low-beta plasma
Seiji ZENITANINational Astronomical Observatory of Japan
ST06-08-26-A010
• Magnetic reconnection expels a fast outflow jetat the upstream Alfvén speed
• How does the jet interact with an external medium?
• Key parameter
cA =B�4��
1��
�cA
cs
�2�
�V
cs
�2�M2
Motivation
outflow jet
? ?
� � pgas
pmag=
8�p
B2
Typical Mach number
Typical velocity
Sound speed
Branches of fluid dynamics
• Compressible fluid dynamics
• High-speed fluid dynamics• Adiabatic effects
• Shocks
• Shock-shock interaction
Mach 1887tenki.jp 2015
Conventionalfluid dynamics
Shock waves
• Incompressible fluids
Subsonic SupersonicTransonic
1
MHD simulation
+B0
-B0
SymmetryReconnection point
Density
Plasma sheet
β=0.2
Outflow velocity (Vx)
X
Z
z
x
1 23
45 7
6
8 910
Various shocks!
• Extensive analysison shock conditions (Minimum variance analysis; MVA-B)
Petschekslow shocks
Plasmoid (magnetic island)
vx
Normal shock: Analogy to airfoil
(b) t = 250
z
x
Transonic(0.8cs <V)
Subsonic(V << cs)
Supersonic(cs < V)
β=1.0
β=0.2
plasmoid
vx
• MHD slow shock
• Recovery/recompression shock
See also https://www.youtube.com/watch?v=8OIqfCTAZQo
vjet ~ cA
Shock diamond(b) t = 250
z
x Plasmoid frameSimulation frame
Plasma sheet
Plasma sheetOutflow jet
vx
vx
Plasmoid
• (a) Over-expanded flow
Supersonic nozzle problem
• (b) Under-expanded flow
pjet < pambient
pjet > pambient
cs < Vjet
cs < Vjet
Plasmoid frame Plasma sheet
Shapiro 1953
Shock Expansion wave
Expansion wave
Shock diamonds in aeronautics
BBC online http://www.bbc.com/future/story/20130701-flying-the-worlds-fastest-plane
Shock diamonds in video game
Microsoft Flight Simulator X https://www.youtube.com/watch?v=S8QGaiE4yWc
Shock diamonds in astrophysics
PKS 0637-752 Godfrey+ 2012 ApJ
2x106 light years
Vz
Hidden shock-diamonds
• Under-expanded shock-diamonds at the shock crossing point
• Triple-point structure is even more complicated (Zenitani 2015 PoP)
• Contact discontinuity
• Slow expansion fan
β=0.1
X
Z
Petschekslow shocks
vx
Normal shock
A digital catalog of the plasmoid structure
1617
F
G
18
1��
�cA
cs
�2�
�V
cs
�2�M2
• In high Mach number, low-β regime, reconnection system is dominated by shocks
• This regime has long been unexplored!
• Simple, Scalable, and Shock-capturing MHD code• TVD Runge=Kutta method
• TVD MUSCL scheme + HLLD solver
• Hyperbolic divergence cleaning
• Parallel-IO
• Search “OpenMHD” without a space
OpenMHD code
http://th.nao.ac.jp/MEMBER/zenitani/openmhd-e.html
Summary• We have investigated the shock structure of
a reconnection-plasmoid system
• In high Mach-number, low-β regime, several new structures are found:
• Recompression shock
• Over-expanded shock diamonds at the front
• Under-expanded shock diamonds inside the jet
• They are outcomes of high-speed (compressible) fluid effects :
• Code publicly available• References:
• Zenitani, Phys. Plasmas 22, 032114 (2015)• Zenitani & Miyoshi, Phys. Plasmas 18, 022105 (2011)
M > 1
Thank you for your attention.Have a safe return trip!
BBC online http://www.bbc.com/future/story/20130701-flying-the-worlds-fastest-plane