Radiation Belt Variability John Sample Space Sciences Laboratory

1. Radiation Belt Variability • John Sample • Space Sciences Laboratory • University of California, Berkeley

2. Outline: • Introduction to the Radiation Belts • Structure • Variability • Invariants • Importance of magnetic field models • Acceleration Processes • ULF • Local Wave Heating • Loss Processes • Magnetopause • Precipitation • What will CINEMA see? • Low Energy cutoff of microbursts • Time dispersion • Structure within a burst

3. The earth’s magnetospherearth’s Magnetosphere

4. Plasmasphere • Corotation and crosstail potentials combine to form a region of trapped cold plasma • Ionospheric in origin • T~1eV, n~10^3-10^5 • Boundary shows a strong dependence on activity

6. The Radiation Belts

7. coordinate system

8. Gyration-Bounce-Drift • associated with each type of motion is an adiabatic invariant which together can characterize phase space density. • PSD cannot be directly measured at the satellite Courtesy J. Bortnik ~3kHz ~1 s ~12 min. For a 1 MeV electron at L=4 with eq=45deg.

9. Magnetic storms • Dst index is a measure of the ring current • Commencement and growth: usually triggered by extended Bz-South, CME shock arrival, or high speed stream arrival • minutes-hours • Main phase: Rapid decrease of Dst as ring current increases dramatically • ~few hours • Recovery: gradual decay of the ring current • ~tens of hours to weeks • Ions lost to charge exchange, magnetopause and precipitation

10. Variability in response to Storms • Multiple orders of magnitude variation in days • A delicate balance of acceleration and loss

11. Electron flux pre-storm vs. post storm ~275 storms over an entire solar cycle Reeves et al. 2002

12. Acceleration • two categories of acceleration: internal and external • internal relies on local nonlinear wave particle interactions that violate either the first or second adiabatic invariant: Usually VLF waves • External relies on a source population of 50-100keV electrons in the near tail which are brought into inner magnetosphere by radial diffusion. Usual scenario is ULF waves violate the third invariant while preserving the first two

13. ULF vs. VLF • Tail supplies 100 keV electrons • Electrons produce chorus and keV microbursts on the dawnside • Chorus has cyclotron resonance with electrons = net energy increase • high latitude interaction with chorus gives MeV microbursts • gives rise to a local peak in PSD • High Vsw and magnetic activity produce ULF waves and frequent substorms • Substorms create a seed population and feed ULF wave power • Electrons can be in drift resonance with mHz waves • Third adiabatic invariant is violated, electrons brought to lower L=higher B conserving • must move to higher energy • needs positive PSD gradient, but generally flattens PSD

14. Looking at the trapped flux and trying to separate Acceleration and Loss for a particular value of 1st and 2nd invariants • Phase Space Density is a difficult measurement to make and people have drawn contradictory conclusions about the existence of peaks, the sign of gradients etc. • very much dependant on magnetic field model used • Losses can make external acceleration appear as internal acceleration, • losses also change pitch angle distributions which affects shape of calculated PSD • three published methods • GEO-GEO (local gradient) • GEO-GPS • Polar type orbit L

15. Acceleration by ULF driven Radial Diffusion • With Drift resonance • Particles can move in L and therefore Energy

17. Local Acceleration • Gyroresonant interaction • Usually thought of as diffusive dealt with Quasi-Linear Diffusion • But! Large amplitude waves have now been observed and these require a non-linear analysis

18. Dual Acceleration? • Storms with both ULF and VLF wave power seem to be most effective at acceleration • ULF and VLF power are both stronger and at lower L during storm recovery • VLF influence may be greater at lower L, ULF at higher L • influence of VLF is complicated because it is also a significant loss mechanism

19. Looking at Losses • The end of the field line • LEO satellites in high inclination orbits • Bremsstrahlung observing Balloons • Sounding Rockets • Mid field line • GEO Satellites • Other Equatorial Orbits (CRRES) • Generally looking at Changes in flux: convolves acceleration, loss, and adiabatic changes • Loss cone usually too narrow to resolve • Theory and Modeling • With good models of field and distributions we can connect disparate observations, follow evolution of PSD, and make predictions based on input conditions • still require validation and refinement Blake et al Onsager et al Days, Dec. 1999 Frequently, we need multi-point observations, and must rely on models to connect the dots

20. The Adiabatic Response (e.g. Dessler + Karplus ’61 McIlwain ‘66) • Not Real Loss • changes must occur slower than the drift frequency • Ring current reduces B in inner magnetosphere • electrons move outward to conserve third adiabatic invariant • conservation of the first adiabatic inv. reduces perp. momentum Kim and Chan ’07 for one storm Used symmetric and asymmetric field models Flux decrease occurs because of energy dependence and spatial gradient Found that some real loss was required, suggested magnetopause encounters

21. Real relativistic electron Loss • Loss to the magnetopause • Precipitation or Atmospheric Loss • occurs when a bouncing particle encounters the Earth’s atmosphere at ~70-100km altitude • Two categories identified as important: both thought to be the result of wave-particle scattering • MeV - Microbursts • Dusk-side Relativistic Electron Precipitation

22. Loss out the Magnetopause • Adiabatic drift outward • When Dst drops, particles move outward to conserve their 3rd invariant • Magnetopause compression • Sharp increases in solar wind dynamic pressure • Li et al ‘97 used to explain losses into L=4.6 during November ‘93 storm • Usually thought to be most important at higher L but… • Outward radial diffusion towards a time dependent boundary has recently been found to be capable of rapidly propagating flux drops from MP encounters inward • In some cases magnetopause losses should be recognizable in proton fluxes at similar energies • Very difficult to measure directly

23. Atmospheric loss: the loss cone • The equatorial loss cone is only ~3 deg. wide at GEO (~0.1% of ‘isotropic’ flux) so it is very difficult to separate out electrons that will be lost from stably trapped flux.

24. Precipitation into the atmosphere • Equatorial pitch angles smaller than αL will encounter the atmosphere before mirroring • Wave Particle Interactions • Gyroresonance requires • w-k||v|| = ±nW/g • Chorus (~100Hz- few kHz) • EMIC (~Hz, below H+,He+,O+) • Hiss (~100Hz- few kHz) • Current sheet scattering • when electron gyro-radius is comparable to radius of curvature of the field line. • should be most important near midnight • should also affect protons • Bounce Loss Cone vs. Drift Loss Cone Degrees East

25. Drift Loss vs. Bounce Loss Size of Loss cone is a function of longitude (and L) Equatorial pitch angles smaller than this will strike the atmosphere

26. Available Waves • Large number of wave modes available for gyroresonance • Hiss vs. Chorus separation occasionally ambiguous • Waves have local time, activity dependence • many times we do not have wave measurements at the same time as particles Meredith et al. ‘04 Summers et al. ‘98

27. Microburst Precipitation X-ray Microbursts from precipitating electrons Anderson and Milton, 1964 • Early balloon observations of ~100 keV microbursts attributed to scattering by VLF whistler-mode waves (Parks, 1978, Rosenberg et al., 1990) Sampex • Relativistic electron microbursts: first reported by Imhof ‘92, studied extensively with SAMPEX, large geometric factor HILT which measures >1MeV electrons allows for time resolution up to 20ms. ~100s (Lorentzen et al., 2001)

28. Microburst precipitation Occur between L=4-6 0200-1000 MLT Outside plasmapause MLT / L Distribution Microbursts (Courtesy T. P. O’Brien) • Whistler Chorus • Also occurs on dayside • Outside plasmapause Meredith et al. ‘02

29. MeV microbursts and Whistler Chorus • local time circumstantial evidence led Lorentzen ’01 to look for conjunction of POLAR (Wave data) with SAMPEX observations of MeV microbursts. • no 1-1 correspondence was found, but Chorus risers having duration similar to single microbursts were seen at similar times,L-Shells, MLT • Demonstrated that resonance with MeV electrons would have to occur off equatorially or at a higher harmonic

30. Energy Selective Precipitation: Dusk Side • We do see strong scattering losses on the duskside • Thorne and Andreoli ‘80 • 3 events in 14 months of s3-3 data (1% of all precipitation events observed) • Imhof ’86 • 41 events using S-(72,78,81)-1 data. Some very intense. • Filtered on narrow L structures • e-foldings >500keV • Concluded such events were rare, but poor MLT coverage may have missed peak. • Foat ’96, Millan ’02 • Observed Bremsstrahlung X-ray bursts from similarly high energy electron precipitation on the dusk side. MIDNIGHT

31. Distribution of EMIC waves • Peak near dusk meridian ~colocated with bulge in plasmasphere (from Meredith et al., 2003 CRRES) (from Erlandson and Ukhorskiy, 2001 DE1)

32. EMIC Resonance Condition • In order for EMIC waves to be resonant with electrons, the electrons must overtake the wave at high doppler shift • this enforces a minimum • v|| minimum Eres • small pitch angles will resonate at a smaller total E • Resonance condition depends strongly on wave velocity • Dispersion relation requires high density or low magnetic field to resonate with ~1-2MeV electrons • will also resonate with ~few keV protons Meredith et al. 03 Imhof et al ’86 found ~30% of pre-midnight selective high energy events studied using S81 and other satellites were related to or embedded in regions of proton precipitation

33. >2MeV flux dropouts • Sudden drops in high energy flux at GEO identified by Onsager et al. ’02 • Events begin with tailward stretching of nightside magnetopause, an asymmetric process that can occur before Dst drops • Fluxes aren’t equilibrated in local time within one electron drift.

34. Quantification of Loss Rates Every Mechanism is apparently Important • Lorentzen ’01 Demonstrated that microburst loss from a single storm could flush the radiation belts of MeV electrons • O’Brien ’04 Principal losses occur during main phase, losses continue through recovery, but with less cumulative effect. • Millan ’02 Losses from Duskside REP discussed earlier were shown to be capable of emptying outer belt in a few days • Current work by Shprits et al. has shown outward radial diffusion and MP encounters capable of similar high loss rates • Ukorhisky et al. used test particles in TS05 to show strong MP losses during Sep. 7, ’02 storm • Selesnick ’06 By using a model to compare drift loss to trapped population using SAMPEX able to calculate daily energy dependent atmospheric loss rates. • Misses very rapid Bursts

35. What CINEMA will see at high latitude

36. Microburst flux appears in higher energy instrument, but not in lower energy instrument • Only several rocket observations of this phenomena

37. Again, no microbursts in low energy instrument (larger gap in Energy coverage)

38. Microburst Size: From George’s Talk Microburst size as small as ~15 km. Average is ~100 km KHU’s sats will resolve the Size question with much greater accuracy while close together

39. Auroral X-rays from PIXIE • Auroral X-rays will present CINEMA with some ambiguity, because they interact with the instrument in a manner similar to ENAs • But… • With effort and occultation, we can get better spatial resolution than existing measurements

40. Direct Observations of Auroral electrons • CINEMA will see very high fluxes of auroral electrons accelerated above the payload by U-shaped potentials The parallel electric field can accelerate electrons into the STEIN energy range KHU can look for small structures