The Discovery of Large Amplitude Parallel Electric Field Wave Packets at the Plasma Sheet Boundary
by Cattell and J. Dombeck (Univ of Minnesota) and M. Temerin and F. S. Mozer (Univ of Ca, Berkeley)
For the first time, large amplitude electric fields parallel to the Earth's magnetic field have been measured at radial distances greater than 6 Re on magnetic field lines linking the plasma sheet boundary to the auroral zone. The electric field structures move at a velocity on the order of 100 km/s and have a net potential drop of ~100 V. This discovery was enabled by the electric field instrument on the Polar spacecraft which is the first to make very high time resolution measurements of the full 3 components of the electric field in this region of the earth's magnetosphere. Large electric fields on time scales varying from milliseconds to tens of seconds are often observed as the satellite crosses the plasma sheet boundary in a region of field-aligned currents. In this region, an ~30 s burst of high time resolution (800 samples/second) data is often obtained, allowing us to discover the unexpected fact that large parallel electric fields occur at these high altitudes (radia! l distances of ~6 Re). These new data have also revealed that the structure of these parallel electric fields is often very different from the solitary waves and double layers seen at lower altitudes in the region of auroral particle acceleration ( see Nugget 1 for examples of low altitude structures). An example of this new type of structure is plotted in Figure 1 which shows 1s of electric field data obtained on May 20, 1996 at a radial distance of 6.2 Re and a magnetic local time of 22 hours. A unipolar wave packet with a frequency of ~70 Hz can be seen in the top panel which plots the component of the electric field parallel to the earth's dipolae magnetic field. The perpendicular component (shown in the second panel) is approximately ten times smaller. The third panel provides an indication of the density of the plasma. It can be seen that the wave packet occurs within a density hole which was probably dug out by the wave itself. The wave also has associated density fluctuations, implying that it is electrostatic, rather than electromagnetic (like radio waves). This packet resulted in a net parallel potential drop which could accelerate electrons downward, contributing to the energization of auroral electrons, and pulling ions out of the ionosphere and into the tail. By comparing properties of the waves (such as velocity, wavelength and frequency) to theoretical predictions, the waves have been identified as a special type of sound wave which can occur only in a plasma. However, the waves have grown to such large amplitude that simple theoretical models do not apply, similar to the situation of shock waves which can also result from this type of sound wave.
