AGU Journal highlights -- Jan. 10, 2008

1. Pulses in Saturn's rings

In 2005, the Cassini spacecraft began a series of experiments to profile ring structure and measure the size distribution of particles in Saturn's rings. During these experiments, Cassini flew behind the ring plane and transmitted radio waves through ring particles to Earth. Scientists on Earth analyzed the signals' diffraction patterns to help determine properties of the rings. Thomson et al. study these data and find that in limited regions of rings A and B (A and B lie close to the outside of Saturn's ring system), the diffraction pattern reveals the presence of fine-scale structures that are characterized by periodic radial variation in optical depth. They define specific periods of variation in optical depth for distinct regions of rings A and B. Past research suggests that the dynamic interplay of gravitational and collisional forces leads to the formation of viscous oscillations and gravity wakes in Saturn's rings. The authors speculate that the number density of ring particles contracts and relaxes to form periodic structures that affect the radio signals seen on Earth.

Title: Periodic microstructure in Saturn's rings A and B

Authors:

Fraser S. Thomson and G. Leonard Tyler: Center for Radar Astronomy, Stanford University, Stanford, California, U.S.A.;
Essam A. Marouf: Department of Electrical Engineering, San Jose State University, San Jose, California, U.S.A.;
Richard G. French: Department of Astronomy, Wellesley College, Wellesley, Massachusetts, U.S.A.;
Nicole J. Rappoport: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.

Source:

Geophysical Research Letters (GRL) paper 10.1029/2007GL032526, 2007; http://dx.doi.org/10.1029/2007GL032526

2. Flow velocity controls timing and magnitude of daily fluctuations in streamflow

Studies of streamflow generation and routing typically focus on monitoring single events such as rainfall impulses to characterize whole-watershed response. Wondzell et al. instead analyze daily fluctuations in summertime streamflow from a small watershed in the Cascade Mountains of Oregon. They show that seasonal changes in daily fluctuations in flow volume at the watershed's mouth can be explained by transport of evaporation and transpiration (ET) generated signals down the stream network. When streamflow velocity is high, ET-generated signals transported down the stream network tend to reach the stream gauge in phase, which reinforces the signals and produces strong daily fluctuations. As flow velocity slows over the summer, ET-generated signals become increasingly out of phase, which masks discharge fluctuations. The authors conclude that the pattern of naturally produced fluctuations in discharge could be used to analyze the ecology and hydrology of whole watersheds.

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