Investigation of the Magnetosphere of Ganymede with Galileo's Energetic Particle Detector

Ph.D. dissertation by Shawn M. Stone, University of Kansas, 1999.

Copyright 1999 by Shawn M. Stone.  Used with permission.

Chapter 1. Introduction

The focus of this study is to investigate the newly discovered magnetic field of the Jovian moon Ganymede using particle data from the Energetic Particle Detector (EPD) instrument aboard the Galileo satellite. Present in the energetic particle data are absorption signatures due to impact with Ganymede. Since particles are guided by the magnetic field, these signatures can be used to evaluate any magnetic field model proposed for the moon and its environment. The magnetic field of Ganymede has been modeled by Kivelson et al. [1996,1997,1998] as a simple dipole excluding external field sources (namely a field due to magnetopause and magnetotail currents) superpositioned on the Jovian background magnetic field. This study presents a 3D magnetopause and tail field configuration, along with an internal multipole model for Ganymede, that was derived by refitting the internal and external source model coefficients to the magnetic field data using the methods of Choe and Beard [1973].

A numerical simulation has been written and presented here that follows a distribution of particles backward in time from the EPD instrument in these two field models. Particles that intersect the surface of Ganymede are removed from the distribution. In this manner, a simulated rate profile is constructed and compared to the real EPD particle data at Ganymede. These results show that the magnetopause and tail contributions are important. It appears that the magnetic field lines of the vacuum superposition model, model M1, map to regions on Ganymede’s surface with field strengths that are too high. Also from the simulation, limits on the effects of corotational electric field, parallel electric field, and pitch angle diffusion on the absorption signatures are determined.

The characterization of planetary magnetic fields is an important part of space physics. Magnetic field models play an essential role in the study of charged particle transport, energization, loss, and absorption by rings and satellites within magnetospheres [Acuna et al., 1983]. The first planetary field discovered was, of course, that of the Earth. Observations of the terrestrial magnetic field had been gathered by mariners and explorers for hundreds of years. William Gilbert used this data in 1600 to show that the Earth’s magnetic field is much like that of a spherical lodestone, proceeding from within and uniformly magnetized. Gauss presented the formalism in 1839 that spherical harmonics could be used to represent the surface field of the Earth [Chapman and Bartels, 1940]. With the advent of rocket technology, it was possible to send spacecraft into orbit around Earth equipped with magnetometers to measure the field strength and direction not only in latitude and longitude but also in altitude. MAGSAT, GEOS, and ATS-6 are such satellites that provided field measurements distributed over the globe. There are now 40 years’ of data that detail the temporal and spatial extent of the geomagnetic field. This wealth has given rise to many models, static as well as time dependent [Olson, 1979; Tsygananke, 1995].

In contrast, our knowledge of the magnetic fields of other planets is based on a few spacecraft flybys (summarized in Table 1.1; below--please scroll down). The observations are sparse in both spatial distribution and time due to the fact that the data is taken local to the spacecraft and each spent only a day or so within a given magnetosphere. This makes it difficult to model accurately because, in magnetospheres, there may be current systems contributing to the magnetic field not sampled by the spacecraft. For example, the exclusion of a ring current system leaves it to the internal field model to fit the data, incorrectly estimating its contribution. To help remedy this, charged particle measurements provide information about the large scale geometry of the magnetic field at regions remote from the spacecraft because the particles are constrained by adiabatic invariants that depend on the magnetic field everywhere along the particle trajectory. As the magnetic field guides the trapped particles around the planet they can impact a satellite in a more or less predictable manner. It is then possible to use absorption signatures of trapped radiation by satellites to study and improve magnetic field models, as has been done for Jupiter and Saturn [Chenette et al., 1982; McKibben and Connerney, 1986; Acuna et al., 1988; Carbary et al., 1983].

Table 1.1 Planets discovered to have an intrinsic magnetic field by visiting spacecraft. Shown is the year of the encounter. *The magnetic field of Jupiter was actually discovered by the observation of the decametric radiation emanating from a region near Jupiter [Burke and Franklin, 1955; Berge and Gulkis, 1976].

With the arrival of the Galileo orbiter at Jupiter on December 7, 1995, the prospects of improving our knowledge of the Jovian magnetic field were certain. The nominal two year mission was designed to study Jupiter’s atmosphere, satellites, and surrounding magnetosphere in great detail. The possibility of discovering a Galilean moon with its own magnetic field was also a tantalizing prospect. The trajectories of the Pioneer and Voyager spacecraft did not approach closely enough to measure an unambiguous intrinsic magnetic field for either of these moons. Table 1.2 shows the distance of closest approach for the Voyager 1 and 2 spacecraft. Galileo, on the other hand, was slated to approach very closely to each moon (<1000 km).

Table 1.2 Closest approach of the Voyager spacecraft to the Galilean moons. The distances are given in kilometers and the numbers in parenthesis are the number of radii for the respective moon.

On September 6, 1996, Galileo came within 838 km of Ganymede and measured for the first time the intrinsic magnetic field of a planetary satellite [Kivelson et al., 1996]. The model put forward by Kivelson et al. [1996] is a simple dipole superimposed on the background field of Jupiter (details below). The model does not take into account other possible current sources in the region, namely a magnetopause and tail current. The particle environment around Ganymede is also very interesting. In all ion and electron channels there are absorption signatures whenever Galileo’s Energetic Particle Detector (EPD) peers down the magnetic field line towards Ganymede and anti-loss cones for the electrons whenever peering away [Williams et al., 1997]. Unfortunately, EPD was not functioning properly during this first pass (G1). During the second (G2), third (G7), and fourth (G8) passes, however, both the magnetometer and EPD instrument were functioning properly.

• 1.1 Anatomy of a Magnetosphere
• 1.2 Satellites Embedded Within Magnetospheres
• 1.3 Discovery of Ganymede's Magnetic Field
• 1.4 The Magnetosphere of Ganymede
• 1.5 Charged Particles in the Vicinity of Ganymede
• 1.6 Thesis Overview

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