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A. J. Kopf 1,2 and D. A. Gurnett 1 1 University of Iowa 2 University of Florida

A. J. Kopf 1,2 and D. A. Gurnett 1 1 University of Iowa 2 University of Florida Special thanks to J. D. Menietti, R. L. Mutel, and W. M. Farrell. A Statistical study of kilometric radiation fine structure striations observed at Jupiter & Saturn. Striations.

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A. J. Kopf 1,2 and D. A. Gurnett 1 1 University of Iowa 2 University of Florida

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  1. A. J. Kopf1,2 and D. A. Gurnett1 1University of Iowa 2University of Florida Special thanks to J. D. Menietti, R. L. Mutel, and W. M. Farrell A Statistical study of kilometric radiationfine structure striations observed at Jupiter & Saturn

  2. Striations • What Are Kilometric Striations? • Fine-structure features found in kHz-band, generally overlapping kilometric radiation • Appear as narrow, discrete stripes on a frequency-time spectrogram, nicknamed “rain” for appearance • With Cassini, can only be resolved in wideband spectrograms due to resolution • Caused by cyclotron maser instability • Seem to be best explained by upward-traveling ion solitary structures (ion holes) • Why Study These Features? • Test the conclusions from Earth • First time these features have been observed elsewhere • Remote study of source region characteristics • Striations visible 10s-100s of radii from planets

  3. Striations at Earth Menietti et al., JGR, 2000

  4. The Study • Jupiter data from 100-day period during Cassini flyby, distance <1000 RJ • Saturn data since approach, distance <100 RS • Follows the model of Menietti et al. (2000) • Focused on key characteristics, including: • Probability of observation (11.5%) • Frequency range of observations (40-215 kHz) • Duration of striations (0.8 s) • Frequency drift (-3.5 kHz) • Drift rate (-4.4 kHz/s)

  5. Striations at Jupiter

  6. By theNumbers:Jupiter • Observation Frequency: 9.1% • Frequency Range: 25 – 75 kHz • Average Duration: 12.6 s • Average Frequency Drift: -14.6 kHz • Average Drift Rate: -1.2 kHz/s

  7. Striations at Saturn

  8. Observation Frequency: 10.8% • Frequency Range: 30 – 80 kHz • Average Duration: 6.8 s • Average Frequency Drift: -9.9 kHz • Average Drift Rate: -1.6 kHz/s By theNumbers:Saturn

  9. StriationDuration Menietti et al., JGR, 2000

  10. NegativeDriftRates Menietti et al., JGR, 2000

  11. An Upward-Moving,Constant-Velocity Source • Assuming and Approximating: • These planets have a dipole magnetic field • The source radiates at the local electron cyclotron frequency • The source moves upward along the field line at the magnetic latitude of the spacecraft • Average Source Speeds and Locations: • Earth = 213 km/s @ 1.5 RE (Mutel et al., 2006) • Jupiter = 3000 km/s @ 5.9 ± 0.3 RJ (Io) • Saturn = 2000 km/s @ 2.2 ± 0.2 RS (Encel. torus)

  12. An Upward-Moving,Constant-Velocity Source Mutel et al., JGR, 2006

  13. Why 2.2 RS? • The region around 2.2 RS, the inner edge of the Enceladus plasma torus, is a very active zone. • Narrowband Z-mode tones detected near the inner edge of the Enceladus torus [Farrell et al., 2005] • Mass unloading directly observed at 2.2 RS [Farrell et al., 2008]

  14. Positive-Drifting Striations

  15. Positive-Drifting Striations

  16. Summary and Comparison

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