Bulk properties of plasma ions in the deep Jovian magnetotail
The New Horizons spacecraft flew by Jupiter for a gravity assist in 2007. After the closest approach it followed an essentially tailward trajectory and observed the deep Jovian magnetotail (>200 Rj) for the very first time. The Solar Wind Around Pluto (SWAP) instrument is an electrostatic analyzer onboard New Horizons, observing the plasma ions in the energy-per-charge range from ~21 eV to ~7.8 keV. Using the SWAP data-set, previous studies have identified three distinct regions of the Jovian magnetotail:1) the inside magnetosphere 2) the magnetosheath, and 3) the adjacent boundary layer. We have developed several techniques to analyze the observations and derive the plasma bulk properties (density, temperature and velocity) in those three regions. The derived plasma parameters support that the plasma in the magnetosheath is shocked solar wind re-accelerating as it expands down the distant tail. The derived plasma flow implies a movement and/or compression of the magnetotail that eventually results to the observed magnetopause crossings. In addition, we show that the plasma thermal pressure is essentially constant. In order to analyze the boundary layer ions, we introduce some modifications to the previous model in order to account for uncertainties arising from the plasma flow fluctuation. We used the derived parameters to examine the pressure balance condition within the magnetosheath and the boundary layer in order to estimate the magnetic field in that region. The thermal pressure in the boundary layer is essentially constant as it is in the magnetosheath. We also examine several scenarios regarding the structure and movement of the distant magnetotail, comparing them with the observations. Finally, we analyze the data obtained deep inside magnetotail. The methodology we follow for this analysis is rather complicated since the ion flux in this region is very low and extremely variable. The ion bulk properties in this region are extremely diverse and we occasionally observe sharp discontinuities within plasma regimes. We discuss several cases where two ion populations are observed in the energy range of the instrument and one case where the plasma flow vector rotates ~30°. These new observations reveal that the Jovian magnetotail is probably structured very differently than any Earth-like magnetosphere.