Fundamental Technologies

Voyager LECP Pages

Voyager-Related Dissertations and Theses for which LECP Co-Investigator T. P. Armstrong has been Advisor

Analysis of Voyager Observations of Ion and Electron Phase Space Densities in the Magnetospheres of Jupiter and Saturn, Mark Paonessa, Ph.D. Dissertation, Univ. of Kansas, 1983. Abstract:

Data from the Low Energy Charged Particle experiment aboard the Voyager spacecraft were used to calculate ion and electron phase space densities in the magnetospheres of Jupiter and Saturn. These were calculated at constant first and non-zero second adiabatic invariants using real pitch angle and energy spectrum information. These calculations are used to infer the nature of charged particle sources, losses, and transport in the inner magnetospheric regions. Traditional dipolar as well as current, non-dipolar models were used to represent the planetary magnetic fields of Jupiter and Saturn. At Jupiter the general nature of the transport is consistent with inward diffusion from the outer magnetosphere, combined with losses near the orbit of Io. These apparent losses are not consistent with simple satellite sweeping by Io alone. It has been suggested that another loss mechanism is at work, possibly pitch angle scattering in the strong limit. At Saturn the data are also consistent with a picture of inward diffusion from the magnetospheric boundary, though there is evidence for a source of low energy ions in the inner magnetosphere. The O4 and Z3 magnetic field models proposed for Jupiter and Saturn do not change the nature of these conclusions significantly.

Voyager 2 Encounter with Ganymede's Wake: Hydromagnetic and Electrodynamic Process, Gul Tariq, Ph.D. Dissertation, Univ. of Kansas, 1984. Abstract:

Voyager 2's passage through the corotation wake region of Ganymede  found disturbances in the energetic particle and magnetic field data. To explain the nature of the disturbances, an investigation of the interaction of the Jovian plasma with Ganymede is carried out. A series of computer simulations, supported by appropriate theories, are made. Three different aspects of the interaction are studied: (1) A magnetic field model is proposed to describe Alfvenic disturbances caused by Ganymede. Numerical simulations show that the interaction of ensembles of ions with perturbed fields modulates the energies of the ions. The amount of modulation depends on the Alfven mach number of the ambient plasma, the ion energy and the pitch angle of the ions. (2) The electrodynamic processes associated with the plasma-Ganymede interaction and the plasma expansion into the cavity are simulated using a particle-in-cell method. The distribution of ions, potentials, ion and electron thermal and drift energies in the wake region are obtained. (3) Using linear MHD theory, conditions for excitation and growth of the Kelvin-Helmholtz instability are investigated. Theoretical conditions for the existence of magnetosonic waves and transverse Alfven waves are also examined.

An Analysis of the Performance of the Magnetic Deflection System in the Voyager Low Energy Charged Particle Experiment, Sheela Shodhan, Master's Thesis, Univ. of Kansas, 1988. (The entire thesis is online here.) Abstract:

The Beta and the Gamma detectors employed in the Low Energy Magnetospheric Particle Analyzer (LEMPA) sensor subsystem of the Low Energy Charged Particle Experiment (LECP) onboard Voyagers 1 and 2 primarily measure low energy 15 keV electrons.

This thesis based upon the method of Wu [5] presents the responses of these detectors for different incident electron energies by developing numerical models for the magnetic field and sensor subsystem geometry and by developing numerical procedures to follow the particle trajectories inside the sensor subsystem.

We find that for both the detectors, the geometric factors increase with energy, reach a maximum and then decrease with increasing energy. For the Gamma detector, the maximum is at about E=500 keV while for the Beta detector, the maximum is at about E=160 keV.

The Impact of Trapped Radiation on Natural Satellites Imbedded in Planetary Magnetospheres, E. V. Bell II, Ph.D. Dissertation, Univ. of Kansas, 1989. Abstract:

Multiple spacecraft encounters with the magnetospheric regions surrounding the gaseous giants in our solar system have shown time and spatial variations in the durably trapped particle population and the magnetic field itself.  Many of the proposed mechanisms for these variations involve the interaction of the ambient medium with the natural satellites imbedded in the magnetosphere. Several studies have been made of this interaction, with varying degrees of success, but few have combined all of the complexities which the problem has required. Data taken by Voyager 1 during its flyby of Europa's orbit is used as a case study in the complications of traditional approaches. A means of resolving some of these difficulties is presented. A method for tracing particles from their observation point by the spacecraft is used to explain some of the energy and pitch angle dependencies of the Voyager 1 observations.

Observations by Voyager 1 of the Low Energy Ion Component of Anomalous Cosmic Rays from 83 AU to 91 AU, Chris Mosley, Ph.D. Dissertation, Union Institute and University, 2005. Abstract:

Observations made by Voyager 1 of the low energy ion component of the anomalous cosmic rays from 83 AU (astronomical units) to 91 AU, which relates to the years 2002 and 2003, are studied. During this time period, specifically 2002/200 to 2003/044, Voyager 1 encountered a previously unobserved environment, which may be indicative of the first encounter with the termination shock. This period is therefore of extreme importance, and may have a profound impact on current theory pertaining to the termination shock.  Investigation of this period involved the calculation of a count rate for the main particle species (protons, alpha particles, and oxygen) to study changes in the particle density, and the comparison of the resulting fluxes with those done at other energies to see if there is consistency in the energy spectrum. Also, temporal correlations are done to study the count rate behavior between the first and second halves of 2002 at Voyager 1, and for 2002 at Voyager 2 for the three main particle species. The results of these investigations will be used to infer whether or not the signature characteristics of a termination shock crossing, as described by the MHD equations, were present during the 2002/200 to 2003/044 period.

Further, the kinetic pressure components parallel and perpendicular to the interplanetary magnetic field are calculated for the 2002/200 to 2003/044 period to see if the conditions were sufficient for a firehose or mirror plasma instability to occur. If so, then this could give an alternate explanation of the Voyager 1 observations, as well as the need of a MHD-described termination shock.

Results indicate that the observations by Voyager 1 during this period can be interpreted as showing the characteristics of a MHD-described termination shock. However, during this period conditions were sufficient for a firehose instability to occur as well, allowing a possible alternative explanation.

Therefore, a definitive resolution as to what Voyager 1 observed during 2002/200 to 2003/044, whether a MHD-described termination shock or a plasma instability region of some other phenomenon, is still unknown, and continued analysis of this period is needed to arrive at a more conclusive answer.

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Last modified 1/8/07, Tizby Hunt-Ward