A. J. Kopf1,2 and D. A. Gurnett1
1University of Iowa2University of Florida
Special thanks to J. D. Menietti, R. L. Mutel, and W. M. Farrell
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 instabilitySeem 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 elsewhereRemote study of source region characteristics
○ Striations visible 10s-100s of radii from planets
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)
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
By theNumbers: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
Assuming and Approximating:These planets have a dipole magnetic fieldThe source radiates at the local electron cyclotron
frequencyThe 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)
An Upward-Moving,Constant-Velocity Source
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]
Summary and Comparison
Earth Jupiter Saturn
Observation Rate 11.5% 9.1% 10.8%
Frequency Range (kHz) 40 – 215 25 – 75 30 – 80
Avg. Duration (s) 0.8 12.6 6.8
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) 213 3000 2000
Estimated Source Location 1.5 RE 5.9 ± 0.3 RJ 2.2 ± 0.2 RS