Synopsis of Amara Lynn Graps PhD thesis: 
"Io Revealed in the Jovian Dust Streams"


The Jovian dust streams are high-speed (at least 200~\kms) collimated
streams of submicron-sized particles traveling in the same direction
from a source in the Jovian system.  They were discovered in
March~1992 by the cosmic dust detector instrument onboard the Ulysses
spacecraft, when the spacecraft was just past its closest approach to
Jupiter.  Observations of the Jovian dust stream phenomena continued
in the next nine years. A second spacecraft, Galileo, now in orbit
around Jupiter, is equipped with an identical dust detector instrument
to Ulysses' dust instrument.  Before and since the Galileo
spacecraft's arrival in the Jupiter system in December~1995,
investigators recorded more dust stream observations. In July and
August 2000, a third spacecraft with a dust detector (combined with a
chemical analyzer), Cassini, traveling on its way to Saturn, recorded
more high-speed streams of submicron-sized particles from the Jovian
system. The many years-long successful Jovian dust streams
observations reached a pinnacle on December~30,~2000, when both the
Cassini and Galileo dust detectors accomplished a coordinated set of
measurements of the Jovian dust streams inside and outside of
Jupiter's magnetosphere.

The work in this thesis describes an emerging electrodynamical picture
of the Jovian dust streams as they appear inside and outside of the
Jupiter environment. The source of the Jovian dust streams is
Jupiter's moon, Io, in particular, dust from Io's volcanoes. Charged
Io dust, traveling on trajectories from Io's location, is shown in
this work to have some particular signatures in real space and in
frequency space.

The Jovian dust stream dynamics in the frequency-transformed Galileo
dust measurements show different signatures, varying, orbit-to-orbit
during Galileo's last 29 orbits around Jupiter. The varying
frequencies from orbit-to-orbit are dependent on the spacecraft and
dust detector geometry, on Jupiter's magnetosphere/plasma conditions,
but also on Io itself, most likely its volcanoes' activity.  The
presence of Io's orbital rotation frequency demonstrates that Io is a
localized source of charged dust particles because charged dust from
diffuse sources would couple to Jupiter's magnetic field and appear in
frequency space with Jupiter's rotation frequency and its harmonics. A
confirmation of Io's role as a localized charged dust source arises
through the modulation effects.  This time-frequency analysis is the
first direct evidence that Io is the source of the Jovian dust
streams. I provide additional frequency evidence of Io recorded by
Cassini and Galileo during an August~2000 dust streams `storm'.

One key to understanding the Jovian dust stream trajectories in real
space is the dust particle's {\it variable} charging. If the dust
particle is small enough (submicron-sized), then its trajectory is
dominated by Lorentz forces. If the dust particle's charge varies
during its travels, then its Lorentz-force-dominated dynamics vary, as
well.  The dust particle's charge varies via currents generated as the
particle samples the plasma through which it travels.

Results from numerical charging experiments here show that the dust
particle rarely reaches an equilibrium potential as it travels. The
equilibrium charging times for the dust particles in Jupiter's
magnetosphere are on the order of hours to days, therefore the dust
particle accumulates more and more charges, which can dramatically
influence its dynamical behavior. Numerical charging experiments here
also show that the secondary electron emission current, which was
previously thought not to have an effect in the placid calm of the
interplanetary solar wind, contributes at least +1~V potential to the
particle's overall potential in the solar wind. Dynamical simulations
of the dust stream particles using variable charging show several
interesting effects: sensitivity of the particle's dynamics to the
harmonic expansion of the magnetic field model, the velocities of
variable charged particles increase over a much longer distance than
fixed charge particles and the simulations show marked difference in
dynamics with only slightly changing the particle size, the density,
and the secondary electron emission current parameters. One effect
studied is the size of ejected particles with different material
properties. Here, smaller particles are ejected that have lower
secondary electron emission energies/yields, and therefore the
ejection of small sulfur particles are favored over the ejection of
small silicate particles.

In order to meet the time-of-flight that the dust particles achieved
during the December~2000 joint Galileo-Cassini dust stream
measurements, the smallest dust particles could have the following
range of parameters: size: 6~nanometers, density: 1.35--1.75~g/cm^3
initial charge potential: 1--4~V, secondary electron emission yield:
3.0, dependent on a maximum electron energy 300~eV, a photoelectron
emission yield: 0.1--1.0, which produce dust particle speeds:
220\450~\kms\ (Galileo\Cassini) and charge potentials: 5.5\6.3~V
(Galileo\Cassini).