"Memory Chips" Discovered In Earth's Plasma Sheet
The highest resolution measurements of the ions in Earth's plasma sheet
obtained to date with any spacecraft are being currently provided by the
Comprehensive Plasma
Instrumentation (CPI) on board the Geotail
spacecraft. These measurements of ions are obtained in thousands of bins
in direction of arrival and particle speed. The initial analysis has
been accomplished when the spacecraft was positioned at about 10
RE (RE, Earth radius) geocentric radial distance in
the plasma sheet of the magnetotail. It has been previously thought that
the numbers of ions in the direction and velocity bins would not vary
greatly from bin to bin, i.e., a more-or-less featureless
population like that of a Maxwellian distribution. Surprisingly, large
variations of ion populations were found from bin to bin. In fact if the
array of bins are arranged in a matrix organized according to the
direction of the ion motion relative to the local magnetic field and the
magnitude of the ion speed, a "memory chip" for the ions became apparent.
If the bins were colored according to the population density within each
bin then the presence of features resembling "eyes" and "ledges" and
"wings", if you will, were seen. These features are the record of the
past acceleration and transport of individual groups of ions. If the
code for the memory chip can be broken then their contents can be used to
provide remote sensing on a global spatial scale for the dynamics of
Earth's magnetotail.
Reference:
- L.A. Frank, W.R. Paterson, K.L. Ackerson, S. Kokubun, and
T. Yamamoto, Plasma velocity distributions in the near-Earth
plasma sheet: A first look with the Geotail spacecraft,
J. Geophys. Res., 101, pp. 10,627-10,637, 1996.
Breaking the Code for the Plasma-Sheet "Memory Chips"
The recent discovery of "memory chips" in the fine-grained distribution
of positive ions with respect to their population groupings as functions
of their motion relative to the local magnetic field and their speeds has
provided motivation for theorists to break the chip code. A first
attempt at breaking the code has been very encouraging. It was already
recognized by the observers that various sections of "memory chips"
recorded the acceleration and transport histories of diverse populations
of ions at a given time and position in the near-Earth plasma sheet. But
where are the ion source regions and what are the transport highways to
the Geotail spacecraft? In order to answer these questions a realistic
global model of Earth's magnetosphere was used to interpret the contents
of a chip by tracking the ions backwards in time. Although this was only
an initial test of breaking the chip code, it was marvelously successful.
Three specific source regions were identified, i.e., the
Earth's ionosphere, a large region along the flanks of the magnetotail
boundary through which solar wind plasmas were flowing, and, thirdly,
solar plasmas entering the "funnel" in the dayside polar magnetosphere,
otherwise known as the polar cusp. The paths of these ions to the
spacecraft were also identified, including those paths that yield strong,
disorganized accelerations in the great current sheet in the mid-plane of
the magnetotail. Thus the existence of the memory chips and the
successful breaking of their code provide an unexpected, welcome advance
in the ability to remotely study the global dynamics of Earth's
magnetotail.
Reference:
- M. Ashour-Abdalla, M. El-Alaoui, V. Peroomian, J. Raeder,
R.J. Walker, R.L. Richard, L.A. Frank, W.R. Paterson, J.M. Bosqued,
R.P. Lepping, and K.W. Ogilvie, Geophys. Res. Lett., (submitted for
publication), 1996.