Latest Measurements of the Enceladus Plume: MHD model of field and plasma

Cassini has probed the icy Saturn moon, Enceladus with seven flybys during 2005 and 2008. These flybys are primarily north-south flybys that pass the moon with its closest approach both on the upstream and downstream sides. The observations have revealed that the primary mass loading source originates from an extensive and asymmetric water plume centered at the south polar region. Such a persistent gas production results in a plasma torus centered at the Enceladus orbit and constantly altered by the force balance disturbed after the intense pick-up in the Enceladus plume. Past study has found time variations both in the torus plasma density and gas production rate around Enceladus, while some features in the Cassini data are still not well explained. In November 2009 Cassini visits the moon again with two south-polar passes in a different geometry: Cassini flies from the upstream to the upstream side of the plume with relatively constant north-south distances of 1.5 and 7.5 RE from the equatorial plane of Enceladus, respectively. To understand the interaction between Enceladus and the corotational torus plasma, both the Cassini magnetometer and plasma (CAPS) data are investigated with our self-consistent torus-moon interaction model, considering the photoionization, electron impact ionization and charge exchange in our MHD code. By constraints from the observed plasma velocity, temperature, and magnetic field perturbations around Enceladus, we present the size, structure and time variations of the plume.