Peter A. Delamere

The NASA New Horizons spacecraft flies past Pluto on July 14, 2015, carrying two instruments that detect charged particles. Pluto has a tenuous, extended atmosphere that is escaping the weak gravity of the planet. The interaction of the solar wind with the escaping atmosphere of Pluto depends on solar wind conditions as well as the vertical structure of the atmosphere. We have analyzed Voyager 2 particles and fields measurements between 25 and 39 AU and present their statistical variations. We have adjusted these predictions to allow for the declining activity of the Sun and solar wind output. We summarize the range of SW conditions that can be expected at 33 AU and survey the range of scales of interaction that New Horizons might experience. Model estimates for the solar wind stand-off distance vary from approximately 7 to 1000 RP with our best estimate being around 40 RP (where we take the radius of Pluto to be RP=1184 km).

NASA's New Horizons spacecraft flies past Pluto on 14 July 2015, carrying two instruments that detect charged particles. Pluto has a tenuous, extended atmosphere that is escaping the planet's weak gravity. The interaction of the solar wind with Pluto's escaping atmosphere depends on solar wind conditions as well as the vertical structure of Pluto's atmosphere. We have analyzed Voyager 2 particles and fields measurements between 25 and 39 AU and present their statistical variations. We have adjusted these predictions to allow for the Sun's declining activity and solar wind output. We summarize the range of SW conditions that can be expected at 33 AU and survey the range of scales of interaction that New Horizons might experience. Model estimates for the solar wind standoff distance vary from ~7 to ~1000 R_P with our best estimate being around 40 R_P (where we take Pluto's radius to be R_P = 1184 km).

The giant‐planet magnetodiscs are shaped by the radial transport of plasma originating in the inner magnetosphere. Magnetic flux transport is a key aspect of the stretched magnetic field configuration of the magnetodisc. While net mass transport is outward (ultimately lost to the solar wind), magnetic flux conservation requires a balanced two‐way transport process. Magnetic reconnection is a critical aspect of the balanced flux transport. We present a comprehensive analysis of current sheet crossings in Saturn's magnetosphere using Cassini magnetometer data from 2004 to 2012 in an attempt to quantify the circulation of magnetic flux, emphasizing local time dependence. A key property of flux transport is the azimuthal bend forward or bend back of the magnetic field. The bend back configuration is an expected property of the magnetodisc with net mass outflow, but the bend forward configuration can be achieved with the rapid inward motion of mostly empty flux tubes following reconnection. We find a strong local time dependence for the bend forward cases, localized mostly in the postnoon sector, indicating that much of the flux‐conserving reconnection occurs in the subsolar and dusk sector. We suggest that the reconnection occur in a complex and patchy network of reconnection sites, supporting the idea that plasma can be lost on small scales through a “drizzle”‐like process. Auroral implications for the observed flux circulation will also be presented.

The internal sources of plasma in the giant magnetospheres of Jupiter and Saturn affect magnetospheric dynamics in terms of magnetosphere‐ionosphere coupling and auroral current systems, as well as the interaction with the solar wind. The radial transport of plasma at Jupiter is well constrained to vary between 300 kg/s and 1200 kg/s over timescales of 20–60 days. Saturn's neutral‐dominated inner magnetosphere has presented a challenge for determining the radial mass transport rates with values ranging between 20 kg/s and 280 kg/s over timescales of 20–80 days. We present an estimate of the radial mass transport rates associated with magnetospheric plasma production for Jupiter and Saturn, assuming that magnetospheric plasma loss (equal to the plasma source) can be treated as mass pickup by the solar wind. That is, the magnetospheric plasma is assumed to be at rest with respect to the solar wind, analogous to cometary mass loading of the solar wind flow. This Alfvénic coupling between the solar wind and the magnetosphere suggests mass loading rates of 500–1500 kg/s for Jupiter and 20–50 kg/s for Saturn, consistent with observations and physical chemistry models. We discuss implications for magnetotail structure by considering the contribution of the viscous interaction of the solar wind with the giant magnetospheres. We suggest that a significant portion of the magnetotail structure can be attributed to this viscous interaction.

Saturn's rapid rotation combined with relatively weak magnetic fields in the outer magnetosphere and sheath lead to conditions that are favorable for the Kelvin‐Helmholtz (KH) instability. A Kelvin‐Helmholtz unstable magnetopause boundary has important consequences for Saturn's interaction with the solar wind due to mass, momentum, and energy transport that can occur at the magnetopause boundary. Previous attempts to identify vortices have been hampered by limited plasma data to unambiguously reveal vortical flow. The magnetic field data, on the other hand, may be able to identify the KH instability due to intense magnetic fluctuations that are associated with KH vortices. We have conducted two‐dimensional hybrid code simulations of Saturn's magnetopause boundary to illustrate the expected magnetic field signatures of KH. Specifically, our simulations show strong field‐aligned current sheet filaments or strong bipolar fluctuations of the in‐plane magnetic field components, bounding the KH vortices. A global search for these characteristic magnetic field signatures near the magnetopause boundary was made of the Cassini mission data from 2004 to 2009. We find that most of the potential KH activity is found on the dusk flank, contrary to expectations. We suggest that KH growth is supported in the prenoon and subsolar regions and that these vortices are transported through coupling to the rotating planet, past noon and tailward on the dusk flank. In addition, we find many instances in the subsolar magnetosphere of possible plasmoid formation (B_z northward) in conjunction with these intense magnetic field fluctuations.