Waves/Turbulence in the Heliospheric Plasma Sheet: Ulysses Observations

The Heliospheric Current Sheet (HCS) and Heliospheric Plasma Sheet (HPS) are the largest structures in the heliosphere and carry important information of fast and slow solar wind interaction and magnetic field variations. The HPS is distinguished by relatively high Beta, slow speed plasma that surrounding the HCS. Here, we report studies of waves/turbulence in the HPS. The data used in the investigation are current sheet and plasma sheet crossings in Ulysses data. The advantage of using these data is that the HPS is thicker at large radial distances and the spacecraft spends longer intervals inside the plasma sheet. We find that, if Alfvén waves are present in the HPS, they are weak and are dominated by variations in the field magnitude, |B|, and solar wind density, NP, that are anti-correlated. To distinguish between slow mode waves, Pressure Balance Structures (PBS) and Mirror Modes, correlations between magnetic, kinetic and total static pressures (pB, pK, and pT) are studied. The slopes of the pB - pK, and pK - pT regression lines are qualitatively consistent with slow mode waves and rule out the other possibilities. In principle, the slopes are measures of the wave speed relative to the Alfvén and sound speeds and imply the direction of propagation with respect to the ambient field.