Satellite and GroundObservations of Chorus Emissions

Prepared by Naoshin HaqueStanford University, Stanford, CA

IHY Workshop on Advancing VLF through the Global AWESOME

Network

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Characteristics of Chorus

Whistler-mode chorus most common and most intense emissions in outer magnetosphere

Discrete emissions usually containing rising and falling tones

Often observed in distinct bands: Upper band chorus: f ≥ 0.5fce-eq

Lower band chorus: 0.1fce-eq ≤ f < 0.5fce-eq

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Listening to Chorus

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Resonance Condition

Chorus waves play role in both acceleration and precipitation of relativistic electrons through resonant scattering

Resonance condition:

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Satellite Observations of Chorus

59 orbits determined by Le Docq et. al [1998] containing chorus:

Upper band: 13 cases, 1,765 wave normalsLower band: 15 cases, 993 wave normals

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Upper Band Chorus Cases

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Upper Band Chorus Cases

12/14/1996 near 7.5 MLT from 19:06 to 20:24 UT

7/31/1997 near 4.6 MLT from 16:01 to 16:21 UT

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Upper Band Chorus Cases

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Lower Band Chorus Cases

2/8/1996 near 2.8 MLT from 10:15 to 10:41 UT

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Lower Band Chorus Cases

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Ground Observations of Chorus

Golkowski and Inan, 2008

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Approach

Identify and isolate individual chorus elements at multiple stations

If distance along ground from receiver to directly below exit point less than ionospheric height (~85 km), then single ray is dominant

For distances >85 km but <1000 km from exit point, rays received will include direct ray and rays that have undergone multiple reflections in waveguide

Time of arrival differences between stations only meaningful if individual rays can be identified and number of reflections can be determined

Golkowski and Inan, 2008

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Approach

Identify direct ray at each station For each 2-station pair with

identified chorus elements, time lag accepted as time of arrival difference for direct rays only if cross-correlation coefficient >0.5 and time lag less than direct ray upper bound

Measurements of 2 orthogonal components of magnetic field of wave propagating in Earth-ionosphere waveguide allows for estimate of arrival azimuth by determination of general polarization ellipse

Golkowski and Inan, 2008

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Singular Ionospheric Exit Points

Golkowski and Inan, 2008

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Multiple Ionospheric Exit Points

Golkowski and Inan, 2008

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Multiple Ionospheric Exit Points

Multiple exit point observations presented are unlikely to be ducted chorus waves since this would require concentration of ducts much greater than previously estimated (Carpenter and Sulic, [1988])

Chum and Santolik [2005] show nonducted propagation is possible if equatorial source wave normal angle close to the Gendrin angle. This can yield ray trajectories that reach the topside ionosphere with θ~0°

Golkowski and Inan, 2008

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Future Work

Use IHY Network of AWESOME receivers to determine singular and multiple ionospheric exit points using chorus emissions from multiple receivers

Determine chorus propagation characteristics in magnetosphere

Compare results with those of Golkowski and Inan [2008]

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References

Burton, R. K. and R. E. Holzer (1974), The origin and propagation of chorus in the outer magnetosphere, J. Geophys. Res., 79, 1014–1023.

Gołkowski, M., and U. S. Inan (2008), Multistation observations of ELF/VLF whistler mode chorus, J. Geophys. Res., 113, A08210, doi:10.1029/2007JA012977.

Haque, N., M. Spasojevic, O. Santolik, and U. S. Inan (2010), Wave normal angles of magnetospheric chorus emissions observed on the Polar spacecraft, J. Geophys. Res., in press.

Sazhin, S. S. and M. Hayakawa (1992), Magnetospheric chorus emissions: A review, Planet. Space Sci., 49, 681-697.

Tsurutani, B. E. and E. J. Smith (1974), Postmidnight chorus: A substorm phenomenon, J. Geophys. Res., 79, 118–127.