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

Striations at Earth

Menietti et al., JGR, 2000

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)

Striations at Jupiter

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

Striations at Saturn

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

StriationDuration

Menietti et al.,JGR, 2000

NegativeDriftRates

Menietti et al.,JGR, 2000

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

An Upward-Moving,Constant-Velocity Source

Mutel et al., JGR, 2006

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]

Positive-Drifting Striations

Positive-Drifting Striations

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