Low cost missions to explore the diversity of near Earth objects

We propose a series of low-cost flyby missions to perform a reconnaissance of near-Earth cometary nuclei and asteroids. The primary scientific goal is to study the physical and chemical diversity in these objects. The mission concept is based on the Pegasus launch vehicle. Mission costs, inclusive of launch, development, mission operations, and analysis are expected to be near $50 M per mission. Launch opportunities occur in all years. The benefits of this reconnaissance to society are stressed

Cassini UVIS Observations of the Io Plasma Torus. III. Observations of Temporal and Azimuthal Variability

In this third paper in a series presenting observations by the Cassini Ultraviolet Imaging Spectrometer (UVIS) of the Io plasma torus, we show remarkable, though subtle, spatio-temporal variations in torus properties. The Io torus is found to exhibit significant, near-sinusoidal variations in ion composition as a function of azimuthal position. The azimuthal variation in composition is such that the mixing ratio of S II is strongly correlated with the mixing ratio of S III and the equatorial electron density and strongly anti-correlated with the mixing ratios of both S IV and O II and the equatorial electron temperature. Surprisingly, the azimuthal variation in ion composition is observed to have a period of 10.07 hours--1.5% longer than the System III rotation period of Jupiter, yet 1.3% shorter than the System IV period defined by Brown (1995). Although the amplitude of the azimuthal variation of S III and O II remained in the range of 2-5%, the amplitude of the S II and S IV compositional variation ranged between 5-25% during the UVIS observations. Furthermore, the amplitude of the azimuthal variations of S II and S IV appears to be modulated by its location in System III longitude, such that when the region of maximum S II mixing ratio (minimum S IV mixing ratio) is aligned with a System III longitude of ~200 +/- 15 degrees, the amplitude is a factor of ~4 greater than when the variation is anti-aligned. This behavior can explain numerous, often apparently contradictory, observations of variations in the properties of the Io plasma torus with the System III and System IV coordinate systems.Comment: 35 pages including 12 figures and 2 table

Plasma Sail Concept Fundamentals

The mini-magnetospheric plasma propulsion (M2P2) device, originally proposed by Winglee et al., predicts that a 15-km standoff distance (or 20-km cross-sectional dimension) of the magnetic bubble will provide for sufficient momentum transfer from the solar wind to accelerate a spacecraft to unprecedented speeds of 50 C80 km/s after an acceleration period of 3 mo. Such velocities will enable travel out of the solar system in period of 7 yr almost an order of magnitude improvement over present chemical-based propulsion systems. However, for the parameters of the simulation of Winglee et al., a fluid model for the interaction of M2P2 with the solar wind is not valid. It is assumed in the magnetohydrodynamic (MHD) fluid model, normally applied to planetary magnetospheres, that the characteristic scale size is much greater than the Larmor radius and ion skin depth of the solar wind. In the case of M2P2, the size of the magnetic bubble is actually less than or comparable to the scale of these characteristic parameters. Therefore, a kinetic approach, which addresses the small-scale physical mechanisms, must be used. A two-component approach to determining a preliminary estimate of the momentum transfer to the plasma sail has been adopted. The first component is a self-consistent MHD simulation of the small-scale expansion phase of the magnetic bubble. The fluid treatment is valid to roughly 5 km from the source and the steady-state MHD solution at the 5 km boundary was then used as initial conditions for the hybrid simulation. The hybrid simulations showed that the forces delivered to the innermost regions of the plasma sail are considerably ( 10 times) smaller than the MHD counterpart, are dominated by the magnetic field pressure gradient, and are directed primarily in the transverse direction

Magnetic Flux Circulation in the Rotationally Driven Giant Magnetospheres

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\u27s 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

Current-voltage relation of a centrifugally confined plasma

Observations of Jupiter's auroral regions indicate that electrons are accelerated into Jupiter's atmosphere creating emissions. The acceleration of the electrons intimate that parallel electric fields and field-aligned currents develop along the flux tubes which connect the equatorial plane to the areas with auroral emission. The relationship between the development of parallel electric fields and the parallel currents is often assumed to be the same as that on Earth. However, the relationship is significantly different at Jupiter due to a lack of plasma at high latitudes as large centrifugal forces caused by Jupiter's fast rotation period (about 9.8 h) constrain the magnetospheric plasma to the equatorial plane. We use a 1-D spatial, 2-D velocity space Vlasov code which has been modified to include centrifugal forces to examine the current-voltage relationship that exists at Jupiter. In particular, we investigate this relationship at a distance of 5.9 Jovian radii, the orbital radius of Io, which is coupled with the auroral spot and Io wake auroral emissions

Generation of parallel electric fields in the Jupiter-Io torus wake region

Infrared and ultraviolet images have established that auroral emissions at Jupiter caused by the electromagnetic interaction with Io not only produce a bright spot, but an emission trail that extends in longitude from Io's magnetic footprint. Electron acceleration that produces the bright spot is believed to be dominated by Alfvén waves whereas we argue that the trail or wake aurora results from quasi-static parallel electric fields associated with large-scale, field-aligned currents between the Io torus and Jupiter's ionosphere. These currents ultimately transfer angular momentum from Jupiter to the Io torus. We examine the generation and the impact of the quasi-static parallel electric fields in the Io trail aurora. A critical component to our analysis is a current-voltage relation that accounts for the low-density plasma along the magnetic flux tubes that connect the Io torus and Jupiter. This low-density region, ∼2 Rj from Jupiter's center, can significantly limit the field-aligned current, essentially acting as a "high-latitude current choke." Once parallel electric fields are introduced, the governing equations that couple Jupiter's ionosphere to the Io torus become nonlinear and, while the large-scale behavior is similar to that expected with no parallel electric field, there are substantial deviations on smaller scales. The solutions, bound by properties of the Io torus and Jupiter's ionosphere, indicate that the parallel potentials are on the order of 1 kV when constrained by peak energy fluxes of a few milliwatts per square meter. The parallel potentials that we predict are significantly lower than earlier reports

Local Time Asymmetry of Saturn\u27s Magnetosheath Flows

Using gross averages of the azimuthal component of flow in Saturn\u27s magnetosheath, we find that flows in the prenoon sector reach a maximum value of roughly half that of the postnoon side. Corotational magnetodisc plasma creates a much larger flow shear with solar wind plasma prenoon than postnoon. Maxwell stress tensor analysis shows that momentum can be transferred out of the magnetosphere along tangential field lines if a normal component to the boundary is present, i.e., field lines which pierce the magnetopause. A Kelvin‐Helmholtz unstable flow gives rise to precisely this situation, as intermittent reconnection allows the magnetic field to thread the boundary. We interpret the Kelvin‐Helmholtz instability acting along the magnetopause as a tangetial drag, facilitating two‐way transport of momentum through the boundary. We use reduced magnetosheath flows in the dawn sector as evidence of the importance of this interaction in Saturn\u27s magnetosphere

The Roles of Flux Tube Entropy and Effective Gravity in the Inward Plasma Transport at Saturn

The inward plasma transport at the Saturnian magnetosphere is examined using the flux tube interchange stability formalism developed by Southwood & Kivelson. Seven events are selected. Three cases are considered: (1) the injected flux tube and ambient plasmas are nonisotropic, (2) the injected flux tube and ambient plasmas are isotropic, and (3) the injected flux tube plasma is isotropic, but the ambient plasma is nonisotropic. Case 1 may be relevant for fresh injections, while case 3 may be relevant for old injections. For cases 1 and 2, all but one event have negative stability conditions, suggesting that the flux tube should be moving inward. For case 3, the injections located at L \u3e 11 have negative stability conditions, while four out of five of the injections at L \u3c 9 have positive stability conditions. The positive stability condition for small L suggests that the injection may be near its equilibrium position and possibly oscillating thereabouts-hence the outward transport if the flux tube overshot the equilibrium position. The flux tube entropy plays an important role in braking the plasma inward transport. When the stability condition is positive, it is because the entropy term, which is positive, counters and dominates the effective gravity term, which is negative for all the events. The ambient plasma and drift-out from adjacent injections can affect the stability and the inward motion of the injected flux tube. The results have implications for inward plasma transport in the Jovian magnetosphere, as well as other fast-rotating planetary magnetospheres