Space Physics Seminar

The Heliospheric Current Sheet (HCS) separating magnetic fields from the Sun’s north and south magnetic poles is surrounded by the Heliospheric Plasma Sheet (HPS). The current sheet is usually identified as an abrupt reversal in field polarity but also by a reversal in flow direction of the electron heat flux. The plasma sheet is identified as a decrease in field strength (B) and a simultaneous increase in plasma density (n) or by an increase in plasma beta or entropy.

A fundamental aspect of HCS- HPS structure is their thicknesses and how they vary with radial distance. Ulysses observations provided an opportunity to determine these thicknesses at 3 and 5 AU during solar minimum and to then compare them with the thicknesses at 1 AU using ACE data. Surprisingly, HCS thickness is found to decrease with distance whereas HPS thickness increases. These results will be presented along with a discussion of the HCS-HPS identifications and their relation to heat flux.

The Ulysses data near 5 AU also provided an opportunity to investigate the dynamics of the HPS, i.e., the presence or absence of waves or turbulence. Observations of the HPS near 5 AU are advantageous because the spacecraft spends longer intervals inside the thicker plasma sheet. Simultaneous magnetic field and plasma data were analyzed, the resolution of the plasma analyzer limiting the periods under investigation to > 10 minutes. The diagnosis involved distinguishing between various possibilities including non-propagating structures such as mirror modes. Comparison of changes in magnetic field and solar wind velocity components show that the dominant mode is not Alfvenic but involves the anti-correlations between B and n. Therefore, a subsequent analysis was based on relations between magnetic pressure, thermal or kinetic pressure and total pressure that distinguish between the various possibilities. In the examples analyzed, the fluctuations are dominated by the slow magnetosonic mode, an interesting result since slow mode waves are not commonly identified in the solar wind.