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Published byAriel Dalton Modified over 9 years ago
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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
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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
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Striations at Earth Menietti et al., JGR, 2000
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The Study Jupiter data from 100-day period during Cassini flyby, distance <1000 R J Saturn data since approach, distance <100 R S 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)
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Striations at Jupiter
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By the Numbers: 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
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Striations at Saturn
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By the Numbers: Saturn 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
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Striation Duration Menietti et al., JGR, 2000
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Negative Drift Rates Menietti et al., JGR, 2000
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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 R E (Mutel et al., 2006) Jupiter = 3000 km/s @ 5.9 ± 0.3 R J (Io) Saturn = 2000 km/s @ 2.2 ± 0.2 R S (Encel. torus) An Upward-Moving, Constant-Velocity Source
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Mutel et al., JGR, 2006
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Why 2.2 R S ? The region around 2.2 R S, 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 R S [Farrell et al., 2008]
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Positive-Drifting Striations
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Summary and Comparison EarthJupiterSaturn Observation Rate 11.5%9.1%10.8% Frequency Range (kHz) 40 – 21525 – 7530 – 80 Avg. Duration (s) 0.812.66.8 Avg. Frequency Drift (kHz) -3.5-14.6-9.9 Avg. Drift Rate (kHz/s) -4.4-1.2-1.6 Avg. Source Speed (km/s) 21330002000 Estimated Source Location 1.5 R E 5.9 ± 0.3 R J 2.2 ± 0.2 R S
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