"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.

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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.

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