Plasma Sheet Thickness at Jupiter From Galileo Electron Density Measurements

Electron density has been determined throughout much of the Galileo primary mission at Jupiter (December 7, 1995 to November 6, 1997) by observing plasma waves measured by the plasma wave instrument on board the spacecraft. The density data set is used here to identify spacecraft encounters with Jupiter's magnetotail plasma sheet during the primary mission by assuming that electron density is highest at the center of the plasma sheet. As Jupiter rotates, the spacecraft encounters one pair of plasma sheet crossings during each ten-hour rotation period. Electron density is usually seen to increase as Galileo enters the plasma sheet, reach a maximum value near the center of the plasma sheet, and then decrease as the spacecraft exits the plasma sheet. This signature is clearest in the data at radial distances between 20 RJ, and 50 RJ from Jupiter. Plasma sheet thickness is determined by identifying the z-coordinate of the spacecraft as it enters and exits the plasma sheet. This z-coordinate can be measured with respect to a number of different coordinate systems and magnetic field models. Through analysis of plasma sheet thickness in Jupiter's magnetotail, we observe a thicker plasma sheet in the midnight and dusk sectors and a thinner, more distinct sheet in Jupiter's dawn sector. These observations are consistent with magnetic field signatures reported by the Galileo magnetometer team.