Plasma dynamics in Saturn's middle-latitude ionosphere and implications for magnetosphere-ionosphere coupling.

A multifluid model is used to investigate how Saturn's magnetosphere affects ionosphere. The model includes a magnetospheric plasma temperature of 2 eV as a boundary condition. The main results are: (1) H + ions are accelerated along magnetic field lines by ambipolar electric fields and centrifugal force, and have an upward velocity of about 10 km/s at 8000 km; (2) the ionospheric plasma temperature is 10,000 K at 5000 km, and is significantly affected by magnetospheric heat flow at high altitudes; (3) modeled electron densities agree with densities from occultation observations if the maximum neutral temperature at a latitude of 54˚ is about 900 K or if electrons are heated near an altitude of 2500 km; (4) electron heating rates from photoelectrons (≈100 K/s) can also give agreement with observed electron densities when the maximum neutral temperature is lower than 700 K (note that Cassini observations give 520 K); and (5) the ion temperature is high at altitudes above 4000 km and is almost the same as the electron temperature. The ionospheric height-integrated Pedersen conductivity, which affects the magnetospheric plasma velocity, varies with local time with values between 0.4 and 10 S. We suggest that the sub-corotating ion velocity in the inner magnetosphere depends on the local time, because the conductivity generated by dust–plasma interactions in the inner magnetosphere is almost comparable to the ionospheric conductivity. This indicates that magnetosphere–ionosphere coupling is highly important in the Saturn system. [ABSTRACT FROM AUTHOR]