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Local Acceleration and Loss of Relativistic Electrons in the Earth’s Outer Radiation Belt. Nigel P. Meredith British Antarctic Survey. GEM Workshop Zermatt Resort, Utah 22 nd – 27 th June, 2008. Outline. Introduction Local Acceleration Mechanisms Chorus Magnetosonic waves

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Local Acceleration and Loss of Relativistic Electrons in the Earth’s Outer Radiation Belt


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    1. Local Acceleration and Loss of Relativistic Electrons in the Earth’s Outer Radiation Belt Nigel P. Meredith British Antarctic Survey GEM Workshop Zermatt Resort, Utah 22nd – 27th June, 2008

    2. Outline • Introduction • Local Acceleration Mechanisms • Chorus • Magnetosonic waves • Local Loss Mechanisms • Plasmaspheric hiss • EMIC waves • Chorus • Conclusions NASA

    3. E = 876 keV 104 103 flux 102 101 100 1 2 3 4 5 6 7 L shell Earth’s Radiation Belts • Energetic electrons (E > 100 keV) in the Earth’s radiation belts are generally confined to two distinct regions. • Inner radiation belt • 1.2 < L < 2 • exhibits long term stability • Outer radiation belt • 3 < L < 7 • highly dynamic NASA Outer belt Inner belt

    4. Variability of Outer Radiation Belt Electrons • Fluxes change dramatically on a variety of different time scales. • Covers a range of over 4 orders of magnitude. Baker et al., AG, 2008

    5. Variability of Outer Radiation Belt Electrons • Most intense fluxes are seen during the declining phase of the solar cycle. Baker et al., AG, 2008

    6. Importance • Enhanced fluxes of relativistic electrons: • damage satellites, e.g., • 1994: Intelsat K, Anik E1, & E2 • 1997: Telstar 401 • 1998: Galaxy IV • risk to humans in space. Baker et al., AG, 2008

    7. Relevance to the Middle Atmosphere • Energetic electrons can penetrate to low altitudes where they drive changes in atmospheric chemistry primarily in the mesosphere. thermosphere mesosphere stratosphere

    8. Relevance to the Middle Atmosphere • Energetic electrons can penetrate to low altitudes where they drive changes in atmospheric chemistry primarily in the mesosphere. thermosphere mesosphere stratosphere

    9. Relevance to the Middle Atmosphere • Energetic electrons can penetrate to low altitudes where they drive changes in atmospheric chemistry primarily in the mesosphere. mesosphere stratosphere

    10. Relevance to the Middle Atmosphere • Energetic electrons can penetrate to low altitudes where they drive changes in atmospheric chemistry primarily in the mesosphere. mesosphere stratosphere

    11. Relevance to the Middle Atmosphere • Energetic electrons can penetrate to low altitudes where they drive changes in atmospheric chemistry primarily in the mesosphere. • Electron precipitation from the Earth’s radiation belts could thus be very important for communicating solar variability to the Earth’s middle atmosphere [e.g., Kozyra et al., AGU, 2006]. mesosphere stratosphere

    12. Challenge L = 7 L = 3 • Characterise, understand, and ultimately predict the variability of outer radiation belt electrons. • Complex problem involving a variety of source, transport and loss processes. L = 7 L = 3 L = 7 L = 3

    13. Solar Drivers • Relativistic electron variability driven by activity on the Sun. • Two types of solar driver:

    14. Solar Drivers • Relativistic electron variability driven by activity on the Sun. • Two types of solar driver: • Episodic storms caused by solar wind disturbances driven by coronal mass ejections. Imagecourtesy of the LASCO team

    15. Solar Drivers • Relativistic electron variability driven by activity on the Sun. • Two types of solar driver: • Episodic storms caused by solar wind disturbances driven by coronal mass ejections. • Recurrent storms caused by high speed solar wind streams from coronal holes. Imagecourtesy of the LASCO team Reeves, GRL, 1998

    16. Relativistic Electron Variability • The fluxes of relativistic electrons (E>1 MeV) in the outer zone are enhanced by a factor of 2 or more in ~50% of all moderate and intense storms. Reeves et al., GRL, 2003

    17. Relativistic Electron Variability • The fluxes of relativistic electrons (E>1 MeV) in the outer zone are enhanced by a factor of 2 or more in ~50% of all moderate and intense storms. • However, in sharp contrast, ~20% of these storms decrease the fluxes by a factor of 2 or more. • This emphasizes the need to study loss processes as well as acceleration processes. Reeves et al., GRL, 2003

    18. Periodic Motions • Energetic particles in the Earth’s radiation belts undergo three types of periodic motion: bounce motion drift motion

    19. Periodic Motions • Energetic particles in the Earth’s radiation belts undergo three types of periodic motion: • They gyrate around the magnetic field bounce motion drift motion gyro motion

    20. Periodic Motions • Energetic particles in the Earth’s radiation belts undergo three types of periodic motion: • They gyrate around the magnetic field bounce motion drift motion gyro motion f 10 kHz T 0.1 ms 1 MeV electron ( = 45o) at L = 4.5

    21. Periodic Motions • Energetic particles in the Earth’s radiation belts undergo three types of periodic motion: • They gyrate around the magnetic field • They bounce between the mirror points drift motion gyro motion bounce motion f 10 kHz 3 Hz T 0.1 ms 0.36 s 1 MeV electron ( = 45o) at L = 4.5

    22. Periodic Motions • Energetic particles in the Earth’s radiation belts undergo three types of periodic motion: • They gyrate around the magnetic field • They bounce between the mirror points • They drift around the Earth drift motion gyro motion bounce motion f 10 kHz 3 Hz 1 mHz T 0.1 ms 0.36 s 15 min 1 MeV electron ( = 45o) at L = 4.5

    23. Adiabatic Invariants • Each periodic motion is associated with a conserved quantity called an adiabatic invariant.

    24. Adiabatic Invariants • Each periodic motion is associated with a conserved quantity called an adiabatic invariant. - First adiabatic invariant is associated with the gyromotion:

    25. Adiabatic Invariants • Each periodic motion is associated with a conserved quantity called an adiabatic invariant. - First adiabatic invariant is associated with the gyromotion: - Second adiabatic invariant is associated with the bounce motion:

    26. Adiabatic Invariants • Each periodic motion is associated with a conserved quantity called an adiabatic invariant. - First adiabatic invariant is associated with the gyromotion: - Second adiabatic invariant is associated with the bounce motion: - Third adiabatic invariant is associated with the drift motion:

    27. Adiabatic Invariants • Each periodic motion is associated with a conserved quantity called an adiabatic invariant. - First adiabatic invariant is associated with the gyromotion: - Second adiabatic invariant is associated with the bounce motion: - Third adiabatic invariant is associated with the drift motion: • A given adiabatic invariant is conserved when forces controlling the motion take place on a timescale that is large compared to the associated period of motion.

    28. Radial Diffusion Radial diffusion is an important transport process in the Earth’s radiation belts: • driven by fluctuations in the Earth’s electric and magnetic fields on timescales of the drift period • enhanced by ULF waves [e.g., Hudson et al., 1999; Elkington et al., 1999] • conserves the first two adiabatic invariants BUT breaks the third adiabatic invariant

    29. Energy (keV) Radial Diffusion • Conservation of first invariant implies: p2= 2meB • Inward radial diffusion leads to significant energisation. L shell courtesy of M. Lam

    30. Radial Diffusion • Conservation of first invariant implies: p2= 2meB • Inward radial diffusion leads to significant energisation. • Outward radial diffusion combined with magnetopause losses can be a significant loss process [Shprits et al., JGR, 2006]. Energy (keV) L shell courtesy of M. Lam

    31. Gyroresonant Wave-Particle Interactions • Recent new results show that gyroresonant wave-particle interactions play a key role in the Earth’s radiation belts. • These interactions can occur when the wave frequency, , is Doppler-shifted to a multiple of the relativistic electron gyrofrequency, e.  - k║v║= ne/ • k║ is the wave number parallel to the magnetic field • v║ is the electron velocity parallel to the magnetic field •  is the relativistic factor

    32. Gyroresonant Wave-Particle Interactions • These interactions break the first and second adiabatic invariants. • Such interactions [e.g., Kennel and Petschek, JGR, 1966] lead to: • pitch angle scattering and potential loss to the atmosphere • energy diffusion

    33. Diffusion Theory • Processes associated with the temporal variability of the radiation belts can usually be treated using diffusion theory [e.g., Schulz and Lanzerotti, 1974]. • Timescales associated with acceleration and loss can be determined from the resulting diffusion coefficients.

    34. PADIE(Pitch Angle and energy Diffusion of Ions and Electrons) • The PADIE code [Glauert and Horne, JGR, 2005] has been developed to model the effects of electromagnetic waves on charged particles trapped in a magnetic field. • The code calculates the bounce-averaged, relativistic, diffusion coefficients

    35. Important Wave Modes magnetopause plasmaspheric hiss plasmaspause • Plasma waves that can lead to efficient gyroresonant wave particle interactions with relativistic electrons include: • - Chorus waves • - Magnetosonic waves • - Plasmaspheric hiss • - EMIC waves. Sun EMIC waves chorus magnetosonic waves

    36. Whistler Mode Chorus • Whistler mode chorus is an intense electromagnetic emission observed outside of the plasmapause in the frequency range 0.1fce < f < 0.8fce. • The waves are generated by plasma sheet electrons injected during substorms and/or enhanced convection.

    37. plasmapause plasmapause fuhr fce chorus waves flhr Wave Data Meredith et al., JGR, 2004

    38. Gyroresonant Wave Particle Interactions • Enhanced storm-time convection electric fields provide a seed population of outer zone electrons with energies up to a few hundred keV [e.g.,Baker et al., ASR, 1998; Obara et al., EPS, 2000]. • Gyroresonant wave-particle interactions with whistler-mode chorus then provide a mechanism for accelerating these seed electrons to relativistic energies [e.g., Horne and Thorne, GRL, 1998].

    39. October 9th 1990 Storm Recovery phase associated with: Iles et al., JGR, 2006 Meredith et al., JGR, 2002

    40. October 9th 1990 Storm Recovery phase associated with: • enhanced AE activity Meredith et al., JGR, 2002

    41. October 9th 1990 Storm Recovery phase associated with: • enhanced AE activity • enhanced levels of whistler mode chorus Meredith et al., JGR, 2002

    42. October 9th 1990 Storm Recovery phase associated with: • enhanced AE activity • enhanced levels of whistler mode chorus • gradual acceleration of electrons to relativistic energies Meredith et al., JGR, 2002

    43. Phase Space Density Analysis • Important information on the nature of the acceleration process can be found through phase space density analysis.

    44. Phase Space Density Analysis Radial Diffusion • Important information on the nature of the acceleration process can be found through phase space density analysis. • Acceleration by inward radial diffusion driven by positive gradients in the phase space density. phase space density L* NASA

    45. Phase Space Density Analysis Radial Diffusion • Important information on the nature of the acceleration process can be found through phase space density analysis. • Acceleration by inward radial diffusion driven by positive gradients in the phase space density. • Local acceleration produces peaks in phase space density. phase space density L* NASA Local Acceleration phase space density L* NASA

    46. Phase Space Density Analysis 9 10 11 12 13 UT date (September 1990) = 550 MeV/G; K = 0.11 G1/2RE 10-7 10-8 f(,K,L*) (cm MeV/c)-3 10-9 3.5 4.0 4.5 5.0 5.5 6.0 6.5 3.0 L* Iles et al., JGR, 2006

    47. Evidence for a developing peak in the electron phase space density at ~ MeV energies. Local acceleration plays a key role during the recovery phase of this storm. Phase Space Density Analysis 9 10 11 12 13 UT date (September 1990) = 550 MeV/G; K = 0.11 G1/2RE 10-7 10-8 f(,K,L*) (cm MeV/c)-3 10-9 3.5 4.0 4.5 5.0 5.5 6.0 6.5 3.0 L* Iles et al., JGR, 2006

    48. Developing peaks in the electron PSD have also been observed by Polar. Data from multiple satellites show frequent and persistent peaks in the equatorial PSD. Local acceleration is an important mechanism for radiation belt dynamics. Phase Space Density Analysis Green and Kivelson, JGR, 2004 Chen et al., Nature Physics, 2007

    49. Survey of 26 Geomagnetic Storms L = 5 All Substorms Large Substorms Relativistic electron flux change tAE>300 (days) tAE>100 (days) Seed Electrons Chorus Waves Relativistic electron flux change Chorus power (pT2day) Seed flux Meredith et al., JGR, 2003

    50. Survey of 26 Geomagnetic Storms L = 5 All Substorms Large Substorms • Trend for larger relativistic electron flux enhancements to be associated with: • longer durations of prolonged AE activity Relativistic electron flux change tAE>300 (days) tAE>100 (days) Seed Electrons Chorus Waves Relativistic electron flux change Chorus power (pT2day) Seed flux Meredith et al., JGR, 2003