Fundamental Technologies

Galileo Spacecraft Pages

Investigation of the Magnetosphere of Ganymede with Galileo's Energetic Particle Detector
Ph.D. dissertation by Shawn M. Stone, University of Kansas, 1999.

Chapter 5. The Simulation: Time Reversed Particle Following

In Chapter 1, rate profiles were displayed showing evidence of the absorption of particles by Ganymede. These absorption, or loss cone, signatures are present in all species and energy channels for the G2 and G7 encounters. As has been discussed, the trajectories of particles are guided by the strength and geometry of the fields around them. The field geometry around Ganymede is such that loss cones are observed whenever the EPD instrument views down the magnetic field line towards Ganymede for both ions and electrons. The electrons also exhibit an absorption feature whenever EPD peers down the field line away from Ganymede [Williams et al.,1997]. To test the range of validity of the magnetic field models described in Chapter 4, particles are traced back in time from the EPD instrument. The trajectories that intersect the surface of Ganymede are removed from the distribution of particles. In this manner, a simulated rate profile can be constructed with absorption features derived from the candidate model field.

A simulation has been written called Particle_Follow that accomplishes this task. It recreates the trajectory of Galileo and look directions of the EPD instrument in GSII coordinates within the region of Ganymede for the G2 and G7 encounters. Particles are traced back in time from the EPD instrument through the candidate field model. Other fields and mechanisms can be added such as parallel electric field or pitch angle scattering. Due to the fact that loss cones depend on the pitch angle, the calculated pitch angle from the model field may affect the shape and depth of the absorption features. To test this, Particle_Follow has a mode that enforces the measured pitch and phase angles to the model field.

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Updated 10/22/02, T. Hunt-Ward