Thank you, regarding Spike's message

From: Amara Graps (
Date: Wed Mar 28 2001 - 08:43:51 MST

Dear Extropes,

I know that my evil twin, Spike Jones (*), sent a sweet and supportive
message regarding my birthday, so I'll say a public 'Thank You' regarding
his message. Since he mananged to pass through _his_ milestone birthday
with his goofy self apparently intact, I'll trust that I'll pass through
my birthday in a similarly goofy-intact way.

See: my mood these days:

(*) I went through a wormhole on the way out, so my birth was
delayed 5 months

My birthday unfortunately comes at a time when my current funding
and my current Germany visa expires and my thesis is due, making my
life a bit more interesting, but my near-term future in Heidelberg
at MPI-K as a postdoc looks good (as long as I get my dissertation
into the university in the next few weeks).

If you're interested in solar system dust, then you might be
interested in the following (a synopsis).



by Amara Lynn Graps

Dissertation to the Naturwissenschaftlich-Mathematische
Gesamtfakult\"at of Ruprecht-Karls-Universit\"at Heidelberg
Spring 2001

The Jovian dust streams are high-rate (at least 250~\kms) bursts of
submicron-sized particles travelling 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, travelling on its way to Saturn, recorded more
high-rate bursts 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 three spacecraft data show the dust
streams to be at a much higher rate (``dust storms") {\it outside}
of the Jovian magnetosphere, than {\it inside} of the Jovian
magnetosphere, yet the source of the Jovian dust streams is the
same. The source of the Jovian dust streams is Jupiter's moon, Io,
in particular, dust from Io's volcanoes. Charged Io dust, travelling
on trajectories from Io's location, is shown in this work to have
some particular signatures in real space, in frequency space, inside
of Jupiter's magnetosphere, and outside of Jupiter's magnetosphere.

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, 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,
Earth's and Saturn'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 +1V 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, and a marked difference in dynamics with only slightly
changing the particle size, the density, and the secondary electron
emission current parameters.

In order to reach some of the speeds 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: 5-6 nanometers, density: 1-2~gr-cm$^3$,
initial potential: 1-3V, secondary electron emission yield: 3,
dependent on a maximum electron energy 300~eV, and a photoelectron
emission yield: 1.

The Jovian dust stream dynamics in the frequency-transformed Galileo
dust measurements show two different signatures, depending whether
the dust detector is located outside of the Jovian magnetosphere or
inside of the Jovian magnetosphere. Inside of the magnetosphere, the
frequency signature is that of {\it amplitude modulation} with
Jupiter's magnetic field modulating Io's orbital frequency signal.
Outside of Jupiter's magnetosphere, Jupiter's magnetic field
signature has disappeared, with Io's orbital frequency remaining as
the prominent signature. This time-frequency analysis is the first
direct evidence that Io is the source of the Jovian dust streams.


Amara Graps email:
Computational Physics vita: finger
Multiplex Answers URL:
"Whenever I see an adult on a bicycle, I do not despair for the
future of the human race." -- H. G. Wells

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