ASYMMETRIC THIN CURRENT SHEETS: A 1-D TEST PARTICLE MODEL AND COMPARISON WITH SW DATA
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ASYMMETRIC THIN CURRENT SHEETS: A 1-D TEST
PARTICLE MODEL AND COMPARISON WITH SW DATA
J. Chen1 and R. A. Santoro2
1 Plasma Physics Division, Naval Research Laboratory
2 Lockheed Martin Management and Data System
The Second Workshop on Thin Current Sheets
19—21 April 2004 University of Maryland
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NRL Plasma Physics Division
COLLISIONLESS CURRENT SHEETS
Observations of Collisionless Current Sheets
• Extensively observed in the magnetosphere (also in laboratory)
– Plasma and magnetic field data
– Key new observations: CLUSTER
• Model-data comparisons using magnetospheric data
– But, no in situ data in the corona or astrophysical plasmas
Objective: Construct a quantitative model of asymmetric collisionless
current sheets and test it against solar wind data
– Observational identifiers—magnetic field and plasma particle properties
– Collilsionless but generally asymmetric
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NRL Plasma Physics Division
BASIC STRUCTURE
Current Sheets in Space Plasmas
• Collisionless:
• Magnetotail current sheet observations: e.g., Fairfield [1984], McPherron et al. [1987],
Mitchell et al. [1990], Lui et al. [1992], Sergeev et al. [1993], Asano et al. [2003]
– Thickness: “Thin”
, ( )i i i mfp sysL L
a
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NRL Plasma Physics Division
MODELS AND PARAMETERS
Symmetric Current Sheets
• Analytic Harris model: [Harris, 1962]
– magnetic field with Bn = 0
– Sharply peaked particle density:
– Uniform average velocity in the current sheet
• Important parameters: particle distributions of the asymptotic sources
1. Two basic regimes:
2. The form of f, in particular, the size
of the high-energy tail;
e.g., the distribution
/ 1 . / 1D th D thv v v v v
tanh( / )z 2
0 sech ( / )n n z
Sf
function
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NRL Plasma Physics Division
PREVIOUS WORK (1)
The vD/vth > 1 Regime
• Harris-like models: [e.g., Eastwood 1972, 1974; Francfort and Pellat 1976; Burkhart et al.
1992; Pritchett and Coroniti 1992]
– magnetic field
– Sharply peaked particle density
–
tanh( / )z
/ 0.5D thv v
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NRL Plasma Physics Division
PREVIOUS WORK (2)
The vD/vth << 1 Regime [Holland and Chen 1993; Sitnov et al. 2000]
• Current sheet properties—non-Harris-like
– magnetic field
– Particle density is nearly constant (10—20%)
– Velocity is peaked in the current sheet
– Pressure tensor is nondiagonal and anisotropic
• Observed quiet-time magnetotail current sheet properties
[McComas et al. 1986; Sergeev et al. 1993]
– Magnetic field:
– Particle density is nearly constant
– Velocity is peaked in the current sheet
• New work: Extend Holland and Chen [1993] to asymmetric current sheets
tanh( / )z
tanh( / )z
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NRL Plasma Physics Division
MODEL: 1-D Asymmetric Thin Current Sheets
IONS: Vlasov Equilibrium
• Individual ion trajectories are calculated
– Static magnetic (and electric) field with
– Motion is nonintegrable: transient, stochastic, and integrable orbits
• Ion contributions to J(x3), n(x3), V(x3) are calculated on a grid
• Obtain new B(x3). Iterate until convergence.
ELECTRONS:
• Mass-less fluid equations
– Momentum equation (me = 0)
– Quasi-neutrality
– Polytropic equation of state
0nB
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NRL Plasma Physics Division
MODEL
MODEL SPECIFICATION
• Asymptotic source particle distributions
– : both are -function distributions; n0, U
• Parameters: Bn/Ba and Ti / Te for each asymptotic region
MODEL OUTPUT
• Converged B1(x3), J(x3), n(x3), T(x3), P(x3)
• is satisfied
f f
aB
1=0
c J B P
nB
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NRL Plasma Physics Division
MODEL RESULTS
SS
Demand that the solution satisfy specified n, T, V, and B.
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NRL Plasma Physics Division
FORCE BALANCE: PRESSURE TENSOR
Pressure tensor: nondiagonal and anisotropic inside the current sheet
Anisotropic and nearly diagonal outside
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NRL Plasma Physics Division
SOLAR WIND DATA
Time resolution: 3 sec (diamonds). 0.044 sec (thin line)
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NRL Plasma Physics Division
MODEL—DATA COMPARISON
Model: lines. Data: diamonds
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NRL Plasma Physics Division
MODE—DATA COMPARISON
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NRL Plasma Physics Division
NONLOCAL NATURE
• “Kinetic thinning” [Harold and Chen 1996]
– Source distributions: Increase vD/vth, more field-aligned, increased high-energy
tail in f thinner current sheets
– Bifurcated current sheets
• Increase asymmetry, , of the sources (this work)
thinner current sheets
• Increasing fraction of transient orbits [Chen and Palmadesso 1986]
f f
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NRL Plasma Physics Division
SUMMARY
• Current sheet current:
• For Te ~ Ti in the solar wind, J2e is ~50% of J2i.
• It is possible to match both magnetic field and plasma data with good agreement
– Force balance is satisfied in all three directions
• Current sheets are not Harris-like: density is relatively flat, pressure tensor is
nondiagonal inside the current sheet
• Current sheet structure can be “remotely” determined via source distributions
– A purely kinetic effect
– Associated with increased flows, more field-line aligned distribution
– Formation of bifurcated current sheets
• Implications: anisotropic (ion) tearing mode can be strongly unstable [Chen and
Palmadesso, 1984]
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TJ J
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