Electron Acceleration in the Van Allen Radiation Belts by Fast Magnetosonic Waves

Richard B. Horne1

R. M. Thorne2, S. A. Glauert1, N. P. Meredith1 D. Poktelov3, and O. Santolik4

1. British Antarctic Survey2. University of California, Los Angeles

3. University of Bath4. Charles University, Prague

R.Horne@bas.ac.uk

REPW, Rarotonga, 7 August, 2007

The Problem[Baker and Kenekal, 2007]

• Solar wind velocity related to electron flux variations inside the Van Allen radiation belts

• Flux variations are due to acceleration, transport and loss inside the magnetosphere

• How do you produce >1 MeV electrons from a source of ~ keV electrons?

Magnetosonic Waves

• Magnetosonic waves propagate across Bo, fcH < f < fLHR

• Intense

• Generated by proton ring distributions [e.g., Boardsen et al. 1992]

Low Frequency Propagation Perpendicular to B

• Fast compressional magnetosonic wave

– B field and plasma compressions

• Bw is along Bo, and Ew is perpendicular to Bo and k

Latitude Distribution of MS Waves from CLUSTER

Nemec et al. PSS [2005]

Ion Ring Distributions

• Ion ring distributions form during magnetic storms

• Energy dependent drift– Slow drift - loss

• Injection into existing population

• Boundary between open and closed drift paths

Fok et al. JGR, [1996]

Weak Storm Event

Resonant Diffusion

• Solve with dispersion relation

– Not field-aligned !

• Cyclotron resonance >3 MeV

– unlikely to contribute

• Landau resonance possible

– Energy diffusion

• Higher energies at larger pitch angles

• For a band of waves with spread of directions

– Landau resonance extended over pitch angles

Fit to CLUSTER Data

• Band of waves – Quasi-linear diffusion approach

• Diffusion coefficients – use PADIE code

– [Glauert and Horne, 2005]

– estimate acceleration and loss timescales

• Least squares fit to CLUSTER data

– Gaussian distribution of power

– Propagation at 89o with angular spread

– Landau and 5 cyclotron resonances

– Bounce average over 3o latitude

Angular Power Spectral Density

• Wave normal angle distribution is confined to large angles to be consistent with propagation within +- 3-5 degrees latitude

Diffusion RatesOutside plasmapause Inside plasmapause

Chorus – MSonic Comparison• Magnetosonic waves, L=4.5

• x0.6 for bounce and drift averageChorus, L=4.5, Bounce and drift averaged

Horne et al., JGR [2005]

Conclusions

• Magnetosonic waves do not cause loss by precipitation, but accelerate electrons inside the magnetic field

• Acceleration possible from ~ 30 keV to a few MeV

• Occurrence rate is ~ 60% between 3.9 and 5 Re [Santolik et al., 2004]

• Assuming present for 60% of the drift orbit– Diffusion rates are comparable to those for whistler mode chorus– Needs a full wave survey!!

• Suggest they contributed to acceleration during 25 Nov 2002

• Since the waves are generated by protons, and acceleration electrons– Energy transfer from ring current to radiation belts

CRRES Initial Survey of MSonic Waves• Outside plasmapause only – Substorm related phenomena

Fine Structure

Frequency Distribution of MS Waves from CLUSTER

Nemec et al. PSS [2005]

Growth of Magnetosonic Waves

Magnetosonic Wave Generation

• L=4.95• Ring peaks at 25 keV and 10 keV• Data from Gloeckler et al. [1985]

• Model, from observed electron distribution

Protons Electrons

Growth Rates

• Growth peaked near harmonics of fcH

• Plasmasheet electrons restricts growth

– Landau damping

Horne et al., JGR [2000]

Propagation and Growth

• Confined to equatorial region by Landau damping

– electron acceleration

• Propagate across plasmapause

• Can also propagate around in MLT

– Guided by plasmapause

Horne et al., JGR [2000]

Ion Diffusion

• Msonic waves also diffuse ions

• Ion heating near 90 degrees

• 10-100 keV

• Tries to remove the ring distribution

• Does this help excite EMIC waves?

Summary

• Magnetosonic waves may be as effective as chorus for electron acceleration

• Propagate at large angles to Bo

– Cannot use Danny’s approximation to calculate diffusion rates

• Appear to be substorm related

– Generated by ion ring distributions

– Substorm ion injection provides the seed population

• Transfer energy from Ring current to radiation belt

– But no simple relation between ring current and radiation belt

• Cause ion diffusion and heating

– Does this help excite EMIC waves? – See Vania

• Need to survey the wave power in MLT to determine effectiveness

The End

Ion Cyclotron Absorption

• Inward radial propagation from L=6.5

• Cyclotron resonant absorption by protons

• Absorption increases with proton temperature

• N=18 resonance shown as an example

• Waves diffuse and heat ions

Dispersion, Multi-ion Plasma