Transcript particle_acceleration

Cusp Type Configurations and Particle Energization
A. Otto and E. Adamson
Motivation: ‘Diamagnetic’ regions at geomagnetic cusps often
show presence of accelerated particle populations (CEP’s).
Particle acceleration in solar magnetic fields.
Can cusp geometry/physics explain a local acceleration
mechanism?
Basic properties.
Acceleration mechanism.
Mass dependence/adiabaticity.
Aspect of magnetic nullpoint physics?
Method: Testparticle model in MHD electric + magnetic fields
Student support: Jason McDonald, George Walker, Univ. Alaska.
Collaborations: Katariina Nykyri, Embry-Riddle, Florida
Poster: Jingnan Guo
The Geomagnetic Cusps
Chapman and Ferraro, ~1930
Magnetospheric boundary map into and through the cusps
Expected Cusp Processes
Vsh
Bsh
Vsh
• Local magnetic shear varies 360 degrees:
• Antiparallel magnetic field => magnetic reconnection
• Parallel and antiparallel field => Kelvin Helmholtz
• Fast (superfast) flow past an obstacle => Turbulence
• Low magnetic field strength
Polar Magnetic Field and Particle Observations
Cluster/RAPID Protons
D’cavity
D’cavity
M’sheath
Fritz and Chen, 2001
M’sheath
Nykyri et al.
• Are Particles (both magnetosheath and ionospheric sources) locally accelerated?
• What is the acceleration mechanism?
Turbulence, Betatron, Fermi, Potential, Other?
Claims:
• Particles (ions) are locally accelerated.
• Accelerated particles provide a source for the ring current
Issues:
Are Particles (both magnetosheath and ionospheric sources) locally
accelerated?
What is the acceleration mechanism?
Turbulence, Betatron, Fermi, Potential, Other?
Region
1
2
3
4
Cusp
Closed Field Lines
Sharp Transitions between Regions with and without CEP’s
Courtesy of K.-H. Trattner
Dramatic Change in Cusp Energetic Ions
Region
2
1,3
By
Bz
Bx
CEP
Region 1
2
CEP
3
4
Reconnection with Southward IMF
Region 1
IMF
X-LINE
Region 1
Q-|| Bow Shock
Path of the Sub-solar IMF across the MP and
after it was draped across the MP
Region 2
Q-|| Bow Shock
Region 2 has no magnetic connection to the quasi-parallel bow shock!
Can Cusp Turbulence Provide the Particle Acceleration Mechanism?
1) Wave activity/turbulence in the high-altitude
cusp:
Energy injection
Energy
cascade?
Energy dissipation or another source at
f_IC?
Origin of MHD-range
fluctuations can be solved
with 3-D cusp model by
A. Otto and E. Adamson
Model spectra
Magnetic field spectra at high-altitude cusp on
17.3.2001 (Cluster trajectory perpendicular to ambient
magnetic field). Nykyri et al. Annales Geophysicae
2006a
Contribution of low frequency (MHD regime) waves for ion acceleration?
Is there a turbulent energy cascade in the cusp operating perpendicularly
1) Wave activity in the high-altitude cusp:
Example of Poynting flux calculations in the high-altitude cusp on 9th of Marc
(from Sundkvist et al. Annales Geophysicae 2005)
Poynting flux (S|| (W/m2 Hz)1/2)
f [Hz]
10
0.5
0
1
-0.5
3:0
Poynting flux changes direction at proton cyclotron0frequency (~1.6 Hz) -> waves
2:50
●
Time (UT)
generated near local proton gyro-frequency in extended region along the flux tube
Ion power flux is 40 times larger than the wave Poynting flux, and waves occur during
enhanced ion power flux -> injected protons are a free energy source for the waves
●
Net Poynting flux of the waves at the vicinity of local proton gyro-frequency is small in
comparison to low frequency waves -> not significant energy transport . But wave-wave
and wave-particle interactions may be important for acceleration and re-distribution of
energy. But by how much?
●
MHD + Test Particle Simulations
Diamagnetic region
• Length scales: L0 = 1 RE
• Density: n0 = 2 cm-3
• Magnetic field: B0 = 40 nT
• Velocity: V0 = 600 km/s
• Time: t0 = 10 s
• Plasma beta: 4
• Mach number: Ms = 0.25, 0.5, 1.0
3D Simulation: Magnetsheath field strongly
northward with small positive By
z
y
x
Diamagnetic Cavities
dusk
•
•
•
dawn
Regions of strongly depressed
magnetic field
Scale: 3 to 4 R_E parallel to
boundary; 1 to 2 RE perpendicular to
boundary
Enhanced pressure and density
dusk
dawn
Plasma b:
Illustration of geometry
and potential:
y
x
B
x
Test Particle Dynamics -Model
dri
 vi
dt
dv i qi
 E  v i  B ri ,t
dt mi
Typical particle properties:
Particle Dynamics:
Initial conditions
- Shell distributions in velocity (e=500eV)
- Random distribution in space
- Color codes max energy (see next slide)
- Number of particles: here 20000
y
z
x
x
Particle Dynamics:
Total/average energy
eV
E
eV
total
perpend.
E
║
contrib. of energetic part.
parall
time
E
eV
┴
Pitch angle
Particle Dynamics:
Evolution inside of simulation domain
90
y
z
x
x
Cluster/Rapid: Energetic electron pitchangle distribution
Diamagnetic cavity (SC1)
Polar Observations: Pitchangle distribution
Chen and Fritz, 2004
Polar Observations: Particle Fluxes
Fritz and Chen, 2003
Simulation: Particle Fluxes
He++
Protons
flux
Energy
Triangles – flux constructed from test particles in the simulation domain (left) and
particles leaving the domain (right)
Dashed – flux corresponding to a Maxwellian with the initial thermal energy
Solid – flux corresponding to a Maxwellian with the maximum thermal energy
30keV
Particle Dynamics:
Acceleration
Particle Dynamics:
Example 1
Particle motion in diamagnetic cavity
Plasma b isosurfaces
y
x
Particle motion in Cusp ‘potential’
‘Potential’ isosurfaces
y
x
Adiabatic vs nonadiabatic particles: Dynamics
m = mp/100
40 keV
m = 16mp
40 keV
Adiabatic vs nonadiabatic particles: Evolution
m = mp/100
z
y
x
x
y
z
m = 16mp
x
x
Protons
Electrons
Adiabatic vs
nonadiabatic
particles:
Cluster/RAPID
c
c
c
Gunnar’s Challenge configuration.
Magnetic field normalized to 2 Gauss
Early evolution, time = 30 tA:

Magnetic field, Velocity,
Current density at z = 0

η = 0.001
After neutral point formation, time = 130 tA:

Magnetic field, Velocity,
Current density at z = 0

η = 0.001
Bz
t
   Bz u -
 rj
r
r
Pitch Angle Distribution and Evolution of Average Particle Energy:
T = 110s
T = 60s
total
perpend
energetic
parallel
T = 130s
Particle Locations (10 snapshots)
T = 110s
T = 130s
y
y
x
x
Energetic Particles highly localized after
neutral point formation!
z
x
Particle Flux:
Power law with slope -1.5 to -2
eV
Particle orbits – Energy and Location
Elctric Field:
Energy and Magnetic Moment:
Mechanism and Scaling:
• Highly efficient particle trapping.
• Particle motion combination of gradient curvature and ExB drift.
• Gradient curvature drift has a net motion along the electric field component
• Energy gain scales with electric field E~B2 and length scale L0
• Energization not confined to inertial length scales.
• Primary energization in perpendicular direction.
• Parallel electric field = 0.
• Particle trapping + perpendicular electric field natural for high beta regions
(magnetic neutral points, diamagnetic cavities, ..)
Issues:
• Total energy gain far in access of MHD approximation
• Processes that lead to pitch angle scattering
• Temporal evolution of electric + magnetic fields