Satellite and Ground Observations of Chorus Emissions

1. Satellite and GroundObservations of Chorus Emissions Prepared by Naoshin Haque Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network

2. 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

3. Listening to Chorus

4. Resonance Condition • Chorus waves play role in both acceleration and precipitation of relativistic electrons through resonant scattering • Resonance condition:

5. Satellite Observations of Chorus • 59 orbits determined by Le Docq et. al [1998] containing chorus: • Upper band: 13 cases, 1,765 wave normals • Lower band: 15 cases, 993 wave normals

6. Upper Band Chorus Cases

7. 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

8. Upper Band Chorus Cases

9. Lower Band Chorus Cases 2/8/1996 near 2.8 MLT from 10:15 to 10:41 UT

10. Lower Band Chorus Cases

11. Ground Observations of Chorus Golkowski and Inan, 2008

12. 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

13. 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

14. Singular Ionospheric Exit Points Golkowski and Inan, 2008

15. Multiple Ionospheric Exit Points Golkowski and Inan, 2008

16. 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

17. 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]

18. 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.