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Whistler Waves and Related Magnetospheric Phenomena

Denys Piddyachiy

Space, Telecommunications and Radioscience Laboratory Stanford University, Stanford, CA

13 February 2007 PHYS 312 Basic Plasma Physics

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Outline

What is whistler? Examples Mechanism of whistler waves formation Whistler propagation in magnetosphere:

ducted whistlers magnetospherically reflected whistlers

Interaction of whistler waves with energetic particles from Earth’s radiation belts

References

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Whistler! What is this!?

From Merriam-Websterwhis·tler a : any of various birds;

especially : any of numerous oscine birds (especially genus Pachycephala) found chiefly in Australia and South Pacific islands and having a whistling call

b : a large marmot (Marmota caligata) of northwestern North America having a shrill alarm call

c : a broken-winded horse

d : a very-low-frequency radio signal that is generated by lightning discharge, travels along the earth's magnetic-field lines, and produces a sound resembling a whistle of descending pitch in radio receivers

Example of a whistler recorded at Palmer Station, Antarctica

1~ ~groupT or v f

f

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Just another example of whistlers

f, kHz

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Region of interest (filled with plasma)

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Region of interest (filled with plasma)

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Lightning-generated signal

Very good conductors in ELF/VLF range (300 Hz - 30kHz)

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Whistler wave propagation in the magnetosphere

1~ ~groupT or v f

f

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Multi-hop whistlers (Palmer Station, Antarctica)

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Multi-hop whistlers (Palmer Station, Antarctica)

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First order approximation of whistler wave propagation

2 22 1

( cos )pe

ce

kc

Refractive index or dispersion relation:

Group velocity:

cos2 ce

gpe

v ck

Group delay:

gpath

ds DT

v f

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Whistlers aboard a satellite

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Magnetospherically reflected (MR) whistlers

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Magnetospherically reflected vs. ducted whistlers

Hey, dude, what is going on here? I am a physicist. I want an explanation.

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Theoretical background

Ray tracing:

Refractive index:use B field and electron density models

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Refractive index surfaces

Fig. Refractive index surface indicating the resonance cone angle θres. Expanded section shows the refractive index at an angle θ with associated ray direction.

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Modeling of MR whistlers

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Earth’s radiation belts

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Lightning (whistler) induced electron precipitation

Basic physics of Lightning-induced Electron Precipitation (LEP):

1) Lightning discharges produce intense electromagnetic radiation in very low frequency (VLF) range.

2) The VLF waves enter into the magnetosphere

where they propagate almost along B-field lines to a region near the magnetic equator.

3) In the equatorial region VLF waves can

effectively interact with energetic electrons from the radiation belts which leads to the scattering of electrons, i.e. changing of their pitch angles distribution if this condition is satisfied:

ωCe = ω – k║ v║

4) The change of the pitch angles leads to

lowering of electrons reflection points in their bounce motion between conjugate ionospheres and as a consequence to their precipitation into the atmosphere.

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Lightning-induced precipitation detection aboard low earth orbit satellite DEMETER

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Adiabatic motion of electrons in radiation belts

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Mechanism of electron precipitation by whistler waves

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References

Helliwell, R.A., Whistlers and related ionospheric phenomena, Stanford University Press, Stanford, Calif., 1965.

Kivelson, M.G., and C.T. Russel, Introduction to spacephysics, Cambridge University Press, 1995.

Spasojevic, M., Global Dynamics of the Earth's Plasmasphere, Stanford Univ., PhD thesis, 2003.

Bortnik, J., Precipitation of Radiation Belt Electrons by Lightning-Generated Magnetospherically Reflecting Whistler Waves, Stanford Univ., PhD thesis, 2004.

Burgess, W. C., Lightning-Induced Coupling of the Radiation Belts to Geomagnetically Conjugate Ionospheric Regions, Stanford Univ., PhD thesis, 1993.

Carpenter, D.L., Remote sensing the Earth’s plasmasphere, Rad. Sci. Bull, 2004. Inan, U. S., D. Piddyachiy, W. B. Peter, J. A. Savaud, M. Parrot, DEMETER Satellite

observations of lightning-induced electron precipitation bursts, Geophysical Research Letters (under review), 2007.

PhD theses and other related information could be found at http://www-star.stanford.edu/~vlf/