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Drift Resonant Interactions of Radiation Belt Electrons with ULF waves .

Explore the dynamics and simulations of radiation belts through drift-resonant electron interactions with ULF waves. Analyze electric and magnetic fields, particle motion equations, wave power mapping, and wave field structures.

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Drift Resonant Interactions of Radiation Belt Electrons with ULF waves .

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  1. Drift Resonant Interactions of Radiation Belt Electrons with ULF waves. L. G. Ozeke, I. R. Mann, A. Degeling, V. Amalraj, and I. J. Rae University of Alberta REPW, August 6th – 10th 2007

  2. Radiation Belt Dynamics Simulations Contents • Analytic model of the electric and magnetic fields of a high-m guided poloidal FLR. • 3D adiabatic equations of motion for a charged particle in a dipole field plus the ULF magnetic and electric field perturbations. • Pitch-angle dependence on the drift-resonant radial transport and energisation of ~MeV electrons in the outer radiation belt. ULF Wave Power Map • Development of a ULF wave power map as a function of solar wind from the CARISMA magnetometer network.

  3. Guided Poloidal Wave Equation in a Dipole Field hfEfis the wave electric field, Ef of the guided poloidal wave multiplied by a dipole scale factor, hf W, is an eigenvalue which gives the frequency, w0 of the wave (real only for infinite ΣP). z and y indicates the position along the field line, z=cosqand y=z+z3 The field-aligned density profile, r, varies along the field line as, Polar coordinates

  4. Guided Poloidal Wave Equation Solution If the density varies with z as then So that Here X is a constant, C(L) is a gaussian function with a 180º phase change across the L-shell of the resonant field line. Since the wave is Alfvenic we assume the compressional magnetic field, is zero b||=0, so that The b┴ components are obtained from

  5. Along the field line Wave E-field Structure Across L-shell Ej En • FLR at L=4 with peak amp of 3mV/m. • Period of wave 100 seconds. • Azimuthal wavelength m=15.

  6. Wave b-field Structure Along the field line At the ionosphere b amp nT bn b amp nT bj • Phase of b changes by 180º ~ at the equatorial plane. • H and D components of b may have ~ the same amp on the ground. • Analytic solution

  7. Equations of Motion Taken from, T. G. Northrop, The Adiabatic Motion of Charged Particles

  8. Wave Acceleration of Electrons by Drift Resonance Eastward drifting electrons may be energised by fundamental guided poloidal mode waves via this drift resonance mechanism. Here the electrons azimuthally drift around the Earth at the same phase speed as the wave. m is the waves azimuthal wave number w is the angular frequency of the wave

  9. αeq =18ºαeq = 45º αeq =90º Trapping width for low, medium and high equatorial pitch-angles, αeq (T~2500 sec) L-shell Energy, MeV wave phase mf-wt wave phase mf-wt wave phase mf-wt

  10. Low pitch-angle electrons on trapped orbits, (T~2500 sec). High pitch-angle electrons on open orbits. Electrons are transported inward onto the same L-shell with the same energy. If the wave amplitude decays away the electrons will remain on the same L and with the same energy Pitch-Angle Dependence of Electron Transport for a fixed energy. Initial Conditions W=0.95MeV, L=3.7 20º L-shell 27º 15º 10º 54º wave phase mf-wt

  11. Enhanced Transport of low equatorial pitch-angle electrons may help produce these observed flat-top and butterfly pitch-angle distributions. Pitch-Angle Distributions CRRES(taken from Horne et. al., JGR, 2003) Flat-top distribution Butterfly distribution L=4 L=6

  12. M=7.5MeV/μT, freq=3mHz Degeling, et. al., JGR, 2007 Drift-resonant transport with a decaying wave amplitude producing a peak in phase space density (90º pitch-angles)

  13. Summary of Resonant Transport • Trapping width ~independent of electrons pitch-angle. • Resonant energy is dependent on the equatorial pitch-angle of the electron. • The lower the electrons equatorial pitch-angle the higher the resonant energy. • Possible for low equatorial pitch-angle electrons to be transported inward (and outward) much further than higher pitch-angle electrons. • This may produce energy dependent butterfly pitch-angle distributions (or erode them) as the wave amplitude decays with time.

  14. Future Work • Develop bounce averaged equations of motion from these analytic fields. • Run simulations with distributions of electrons, look at the evolution of PSD.

  15. Global ULF Wave Power Maps • ULF waves may play a critical role in the energisation of the inner magnetosphere and radiation belts. • Characterise ULF Wave power as a function of measurable parameters in the magnetosphere and solar wind. • Provide the input to radial-diffusion driven radiation belt models. • May provide evidence (or lack thereof) of magnetospheric “magic frequencies”

  16. ULF Wave Radial Diffusive Transport Models “MAGNETIC” “ELECTRIC” Compressional Magnetic Field Power Perpendicular (azimuthal) Electric Field Power Both terms are difficult to prescribe with space-based observational data

  17. CARISMA stations TALO to PINA10 years of data (1994-2003). Open field lines L=11.2 L=4.1

  18. Morning Sector Peak ~4mHz FLR 0600 MLT GILL L=6.2 • Power increases with solar wind speed, all frequencies. • Power law decay with frequency. Power~frequency-L

  19. GILL (L=6.6) 12 MLT 18 MLT 06 MLT 24 MLT Evidence of FLR on the day side and morning sector only.

  20. H-component 1-10 mHz(same scale) LOW SW • Red high power purple low power (log scale) • Integrated power 1 mHz to 10 mHz • Power increases across all L-shell with increasing solar wind. • Clear evidence of FLR, enhanced power between L=5-7 in the noon and morning sectors. HIGH SW

  21. D-component 1-10mHz (same scale) LOW SW • Red high power purple low power (log scale) • Integrated power 1 mHz to 10 mHz • Power increases across all L-shell with increasing solar wind. • No clear enhanced power between L=5-7 in the noon and morning sectors. HIGH SW

  22. H-component D-component • Independently scaled (linear) • H and D clearly have different structures. • H-power domanated by morning sector FLRs. • D-power night time (substorms). • Morning to dayside? LOW SW LOW SW HIGH SW HIGH SW

  23. Mapping Guided Alfvén Waves from the Ground to the Ionosphere Hughes and Southwood, JGR, 1976 showed that the magnetic field amplitude on the ground, bg is related to that at the ionosphere, bi by Dqis the latitudinal wavelength at the ionosphere (~half-width of the FLR). m is the azimuthal wavenumber. h is the height of the ionosphere. Wallis and Budzinski, JGR, 1981 At the ionosphere the electric field, Ei , and magnetic field, bi are related by

  24. The Guided Alfvén Wave Equations in a Dipole Field Guided Poloidal modeEj Guided Toroidal modeEn W, is an eigenvalue which gives the frequency,w0and damping factorg of the wave W=((w0+ig)LRE)2/Aeq2 . hjand hnare dipole scale factors We assume that the electric field varies along the field line in the same way both at the FLR location and away from it.

  25. Fundamental Mode Guided Alfvén Wave Eigenfunctions(solid curves L=4.5, dashed curves L=8.5)

  26. Guided Alfvén Wave Eeq/bi as a Function of L-shell • Away from the plasmapause, Eeq/bi gets smaller with increasing L-shell. • FLR’s with the same magnetic field amplitude in the ionosphere, have electric field amplitudes 10 times greater at L=4.5 than at L=8.5. • Eeq/bi does not depend on the ionospheric Pedersen conductance SP

  27. Summary: Global ULF wave maps • ULF diffusive transport (and coherent transport) theories require equatorial electric field amplitudes. • However, in-situ observations of the waves’ electric field are rare in comparison to ground-based measurements of the magnetic field amplitude. • Here we have characterized the ULF wave power on the ground, showing the influence of FLRs and SW speed on the amount of wave power. • By numerically solving the guided toroidal (and guided poloidal) wave equations we showed how bg on the ground can be used to estimate Eeq. Future Work • Extend the Global ULF power maps below L=4. • Determine an empirical function to describe the ULF power maps in terms of physical quantities, L, MLT, SW.

  28. Summary • Drift resonance theories show that the equatorial electric field amplitude determines the amount of energy an electron can receive from a single ULF wave. • However, in-situ observations of the waves’ electric field are rare in comparison to ground-based measurements of the magnetic field amplitude. • By numerically solving the guided poloidal wave equation we showed how bg on the ground can be converted to Eeq. • FLR’s with the same magnetic field amplitude in the ionosphere can have electric field amplitudes ~10 times greater at L=4.5 than at L=8.5. • Consequently, FLR’s which occur on low L-shells may have a significant effect on the energisation and dynamics of radiation belt electrons.

  29. Conjugate Ground and Satellite FLR Observation T89cModel

  30. Ground-Based Magnetometer Observation Toroidal FLR observed on the ground at GILL, dipole L=6.7. ~75 nT peak amplitude m=4 azimuthal wavenumber Latitudinual half-width, Dq~6o Period=667 sec ~6º

  31. In-situ Satellite Observation • Using our mapping model the FLR observed on the ground with a peak amplitude of 75 nT will result in an equatorial electric field amplitude of Eeq=1.6 mV/m • From the conjugate satellite observation it appears that the peak electric field amplitudeEeq is ~1.5 mV/m. • Consequently, the mapping of the ground-based magnetic field to the equatorial electric field is in excellent agreement with the observation.

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