Cusp Type Configurations and Particle Energization

1. 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

2. The Geomagnetic Cusps Chapman and Ferraro, ~1930

3. Magnetospheric boundary map into and through the cusps

4. 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

5. Polar Magnetic Field and Particle Observations Cluster/RAPID Protons D’cavity D’cavity M’sheath M’sheath Nykyri et al. Fritz and Chen, 2001 • Are Particles (both magnetosheath and ionospheric sources) locally accelerated? • What is the acceleration mechanism? • Turbulence, Betatron, Fermi, Potential, Other?

6. 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?

7. Region 1 2 3 4 Cusp Closed Field Lines Sharp Transitions between Regions with and without CEP’s Courtesy of K.-H. Trattner

8. Dramatic Change in Cusp Energetic Ions Region 2 1,3

9. By Bz Bx CEP CEP Region 1 2 3 4

10. Reconnection with Southward IMF

11. Region 1 IMF X-LINE

12. Region 1 Q-|| Bow Shock Path of the Sub-solar IMF across the MP and after it was draped across the MP

13. Region 2 Q-|| Bow Shock Region 2 has no magnetic connection to the quasi-parallel bow shock!

14. Can Cusp Turbulence Provide the Particle Acceleration Mechanism?

15. Energy injection Energy dissipation or another source at f_IC? Energy cascade? 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 1) Wave activity/turbulence in the high-altitude cusp: Contribution of low frequency (MHD regime) waves for ion acceleration? Is there a turbulent energy cascade in the cusp operating perpendicularly to |B|?

16. 1) Wave activity in the high-altitude cusp: Poynting flux (S|| (W/m2 Hz)1/2) 10 0.5 f [Hz] 0 1 -0.5 3:00 2:50 Time (UT) Example of Poynting flux calculations in the high-altitude cusp on 9th of March 2001 (from Sundkvist et al. Annales Geophysicae 2005) • Poynting flux changes direction at proton cyclotron frequency (~1.6 Hz) -> waves 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?

17. 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 z x y 3D Simulation: Magnetsheath field strongly northward with small positive By

18. Diamagnetic Cavities • 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 dawn dusk dusk dawn

19. y x B Plasma b: Illustration of geometry and potential: x

20. Test Particle Dynamics -Model Typical particle properties:

21. 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

22. Particle Dynamics: Total/average energy eV eV E total perpend. E║ contrib. of energetic part. parall time eV E┴

23. Pitch angle Particle Dynamics: Evolution inside of simulation domain 90 y z x x

24. Cluster/Rapid: Energetic electron pitchangle distribution Diamagnetic cavity (SC1)

25. Polar Observations: Pitchangle distribution Chen and Fritz, 2004

26. Polar Observations: Particle Fluxes Fritz and Chen, 2003

27. Simulation: Particle Fluxes Protons He++ 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

28. Particle Dynamics: Acceleration 30keV

29. Particle Dynamics: Example 1

30. Particle motion in diamagnetic cavity Plasma b isosurfaces y x

31. Particle motion in Cusp ‘potential’ ‘Potential’ isosurfaces y x

32. Adiabatic vs nonadiabatic particles: Dynamics m = 16mp m = mp/100 40 keV 40 keV

33. Adiabatic vs nonadiabatic particles: Evolution m = mp/100 z y x x y z m = 16mp x x

34. c c c Protons Electrons Adiabatic vs nonadiabatic particles: Cluster/RAPID

35. Gunnar’s Challenge configuration. Magnetic field normalized to 2 Gauss

36. Early evolution, time = 30 tA: • Magnetic field, Velocity, Current density at z = 0 • η = 0.001

37. After neutral point formation, time = 130 tA: • Magnetic field, Velocity, Current density at z = 0 • η = 0.001

38. Pitch Angle Distribution and Evolution of Average Particle Energy: T = 130s T = 110s T = 60s total perpend energetic parallel

39. Particle Locations (10 snapshots) T = 110s T = 130s y y x x Energetic Particles highly localized after neutral point formation! z x

40. Particle Flux: Power law with slope -1.5 to -2 eV

41. Particle orbits – Energy and Location

42. Elctric Field:

43. Energy and Magnetic Moment:

44. 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