1. Radiation Belt Relativistic Electron Loss Processes
John Sample
UC Berkeley-Space Sciences Lab
Spring AGU 2006
With Thanks to Robyn Millan

2. The Static Radiation Belts
Even the quiet, ‘static’ radiation belts are constrained by losses.
Slot formation by Wave-Particle scattering of electrons into the atmosphere
outer boundary (inside the last closed field line) determined by:
~current sheet scattering
~magnetopause encounters
Hopefully Joe’s talk takes care of much of this sort of thing. This is ~joking, there are no static radiation belts in my mind.

3. The Highly Variable Radiation Belts
Reeves et al. ‘03
Fluxes can change by orders of magnitude on short timescales, especially around magnetic storms
However, the wide variability in radiation belt response to storm conditions (or lack thereof) demonstrates a competition between acceleration and loss processes

4. Tools of the Trade The end of the field line Mid field line
LEO satellites in high inclination orbits
Bremsstrahlung observing Balloons
Sounding Rockets
Mid field line
GEO Satellites
Other Equatorial Orbits
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
mention the band because I don’t talk about them anywhere else, but point out that paul will be talking about them.
Days, Dec. 1999
Frequently, we need multi-point
observations, and must rely on
models to connect the dots

5. A Zoo of Loss Processes
At least three broad categories of flux decrease
pitch angle scattering-atmospheric precipitation
magnetopause encounters
adiabatic decreases
Classification Schemes
Local Time (e.g. Duskside dropouts, Dawnside microbursts)
Energy Dependence (keV microbursts vs. MeV microbursts)
Time profile (microbursts vs. bands, Duskside REP)
Storm-time Dependence (Main phase vs. Recovery
Even observation method (large historical record waiting to be mined)
Different Physics (e.g. Current sheet scattering vs. gyroresonance, when does QL theory break down)
Still to Determine the t, L,MLT, E, and activity dependent relative importance of each
Should be included in any comprehensive model
Losses can constrain acceleration rates required to give measured fluxes
this slide is text heavy, just walk through it. Ask question, are dawnside microbursts the same as those near midnight?

6. The Adiabatic Response
(e.g. Dessler + Karplus ’61
McIlwain ‘66)
Kim and Chan ‘97 looked at fully adiabatic response of equatorially mirroring electrons for Nov. 93 storm
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
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
outward motion of electrons is why we think MP encounters might be more impt.

7. 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
Size of Loss cone is a
function of longitude
set up difference between BLC and DLC alpha-L is defined by sin^2(alpha)=Beq/B100 want a slide about Selesnick paper to make sense later
Equatorial pitch angles
smaller than this
will strike the
atmosphere
Degrees East
after Selesnick 03

8. Available Waves Large number of wave modes available for gyroresonance
Meredith et al.
‘04
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
Summers et al. ‘98
Hiss is found throughout the plasmasphere, not pictured on summers plot. Chorus is found in the low density region outside the plasmapause excited by substorm injected keV electrons between .1 and .8 fce, EMIC temperature ansiotropy in substorm injected protons in high density duskside plasmapause, but are also found at GEO. not pictured in spectrogram

9. Plasmaspheric Hiss Hiss magnetic field amplitude:
(Meredith et al., 2004)
Electron minimum resonant energy
~1 MeV only inside L~3
say where Eres comes from, point out Hiss is slot causer

10. Microburst Precipitation
X-ray
Microbursts
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)

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

12. 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
always hard to match up orbits, coincidence vs. causality

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

14. EMIC Resonance Condition
Meredith et al. 03
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
mostly L-mode, mostly below He gyro-f, but at low heavy ion concentrations can occur below H gyro.
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

15. Combination of waves can be important
Albert 2003 showed lifetimes with hiss alone ~7x longer than with hiss and EMIC
EMIC still needs high ratio of fpe to fce in order to resonate with ~MeV electrons
L=4, 100pT Hiss, 1pT EMIC
(weighted by drift time in relevant region)

16. Energy Selective Precipitation on the 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.
we do see hard, dusk side

17. Energy Selective Precipitation on the 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.
NOON
MIDNIGHT

18. 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

19. >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.

20. Looking for a mechanism
Green et al. ’04 Identified 52 similar events, performed a superposed epoch analysis using GEO,HEO,LEO data
events first observed at dusk, similar LT progression as in Onsager
converting to PSD eliminates adiabatic effects
accounts for local time progression of events
however fluxes remain low well after field returns to normal so real loss occurred
Magnetopause encounters?
losses went in to L ~4,
no similar dropout in proton flux
Precipitation?

21. Looking for a mechanism
Green et al. ’04 Identified 52 similar events, performed a superposed epoch analysis using GEO,HEO,LEO data
Adiabatic? No
Magnetopause encounter? Unlikely
Precipitation?
Do see an increase of Bounce Loss around time of event onset, however data coverage is sparse
Millan et al. ’06
One of the events identified by Green et al. study occurred during the MAXIS balloon campaign of January 2000.
MAXIS observed an extended, intense X-ray burst with sufficient precipitation to account for the flux drop at GEO

22. Quantification of Loss Rates
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.

23. Summary
Wide variety of loss processes act on relativistic electrons in the radiation belts
Each process (subprocess!) has been shown to be dominant in some cases
To actually specifically identify any given event with a particular process usually requires joining multiple satellite data sets with appropriately fit models
It is possible to quantify a loss rate without understanding the details of the loss process
Much work to be done