P. A. Delamere

Cassini UVIS observations of the Io plasma torus: IV. Modeling temporal and azimuthal variability

In this fourth paper in a series, we present a model of the remarkable temporal and azimuthal variability of the Io plasma torus observed during the Cassini encounter with Jupiter. Over a period of three months, the Cassini Ultraviolet Imaging Spectrograph (UVIS) observed a dramatic variation in the average torus composition. Superimposed on this long-term variation, is a 10.07-h periodicity caused by an azimuthal variation in plasma composition subcorotating relative to System III longitude. Quite surprisingly, the amplitude of the azimuthal variation appears to be modulated at the beat frequency between the System III period and the observed 10.07-h period. Previously, we have successfully modeled the months-long compositional change by supposing a factor of three increase in the amount of material supplied to Ios extended neutral clouds. Here, we extend our torus chemistry model to include an azimuthal dimension. We postulate the existence of two azimuthal variations in the number of superthermal electrons in the torus: a primary variation that subcorotates with a period of 10.07 h and a secondary variation that remains fixed in System III longitude. Using these two hot electron variations, our model can reproduce the observed temporal and azimuthal variations observed by Cassini UVIS.

The Solar Wind Around Pluto (SWAP) Instrument Aboard New Horizons

Abstract The Solar Wind Around Pluto (SWAP) instrument on New Horizons will measure the interaction between the solar wind and ions created by atmospheric loss from Pluto. These measurements provide a characterization of the total loss rate and allow us to examine the complex plasma interactions at Pluto for the first time. Constrained to fit within minimal resources, SWAP is optimized to make plasma-ion measurements at all rotation angles as the New Horizons spacecraft scans to image Pluto and Charon during the flyby. To meet these unique requirements, we combined a cylindrically symmetric retarding potential analyzer with small deflectors, a top-hat analyzer, and a redundant/coincidence detection scheme. This configuration allows for highly sensitive measurements and a controllable energy passband at all scan angles of the spacecraft.

Cassini UVIS observations of the Io plasma torus III. Observations of temporal and azimuthal variability

Abstract 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 h—1.5% longer than the System III rotation period of Jupiter, yet 1.3% shorter than the System IV period defined by [Brown, M.E., 1995. J. Geophys. Res. 100, 21683–21696]. 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 and 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 ° , 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.

Pluto's interaction with the solar wind

This study provides the first observations of Plutogenic ions and their unique interaction with the solar wind. We find ~20% solar wind slowing that maps to a point only ~4.5 RP upstream of Pluto and a bow shock most likely produced by comet-like mass loading. The Pluto obstacle is a region of dense heavy ions bounded by a “Plutopause” where the solar wind is largely excluded and which extends back >100 RP into a heavy ion tail. The upstream standoff distance is at only ~2.5 RP. The heavy ion tail contains considerable structure, may still be partially threaded by the interplanetary magnetic field (IMF), and is surrounded by a light ion sheath. The heavy ions (presumably CH4+) have average speed, density, and temperature of ~90 km s−1, ~0.009 cm−3, and ~7 × 105 K, with significant variability, slightly increasing speed/temperature with distance, and are N-S asymmetric. Density and temperature are roughly anticorrelated yielding a pressure ~2 × 10−2 pPa, roughly in balance with the interstellar pickup ions at ~33 AU. We set an upper bound of <30 nT surface field at Pluto and argue that the obstacle is largely produced by atmospheric thermal pressure like Venus and Mars; we also show that the loss rate down the tail (~5 × 1023 s−1) is only ~1% of the expected total CH4 loss rate from Pluto. Finally, we observe a burst of heavy ions upstream from the bow shock as they are becoming picked up and tentatively identify an IMF outward sector at the time of the NH flyby.

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 Saturns 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.

Interaction of magnetic reconnection and Kelvin‐Helmholtz modes for large magnetic shear: 1. Kelvin‐Helmholtz trigger

At the Earths magnetopause, both magnetic reconnection and the Kelvin-Helmholtz (KH) instability can operate simultaneously for southward interplanetary magnetic field conditions. The dynamic evolution of such a system can be expected to depend on the importance of KH wave evolution versus reconnection and therefore on the respective initial perturbations. In this study, a series of local three-dimensional MHD and Hall MHD simulations are carried out to investigate the situation where the Kelvin-Helmholtz instability is initially the primary process. It is demonstrated that magnetic reconnection is driven and strongly modified by nonlinear KH waves. The highest reconnection rate is close to the Petschek rate, but the total open flux is limited by the size of the nonlinear KH wave. Most of the total open magnetic flux has no flux rope structure and originates from reconnection at thin current layers which connect adjacent vortices. In contrast, complex flux ropes generated by patchy reconnection within the KH vortices dominate the vicinity of the equatorial plane; however, the associated open flux with flux ropes is a minor contribution to the total open flux. Although the presence of Hall physics leads to a fast early increase of the reconnection rate, the maximum reconnection rate and the total amount of open magnetic flux at saturation are the same as in the MHD case.

Current-voltage relation of a centrifugally confined plasma

Observations of Jupiters auroral regions indicate that electrons are accelerated into Jupiters 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 Jupiters 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. Copyright 2009 by the American Geophysical Union.

Saturn's neutral torus versus Jupiter's plasma torus

With the recent discovery of an atmospheric plume of H2O it is thought that Enceladus could deliver as much a 300 kg/s of neutral gas to Saturns inner magnetosphere. Io is the source of roughly 1 ton/s of sulfur and oxygen gas at Jupiter. Despite the apparent similarity, the neutral/ion ratio at Saturn is 3 orders of magnitude higher than at Jupiter. We explore the flow of mass and energy at Saturn and Jupiter using a simplified homogeneous physical chemistry model to understand why these two system are so different. Our results suggest that ionization at Saturn is fundamentally limited by the slower corotational flow velocity at Enceladus, resulting in a factor of 4 lower ion pickup temperature. The net result of cooler ions at Enceladus is a cooler thermal electron population (∼2 eV) that is insufficient to generate significant ionization. Copyright 2007 by the American Geophysical Union.

Pluto’s interaction with its space environment: Solar wind, energetic particles, and dust

New Horizons unveils the Pluto system In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanitys first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so far: Moore et al. present the complex surface features and geology of Pluto and its large moon Charon, including evidence of tectonics, glacial flow, and possible cryovolcanoes. Grundy et al. analyzed the colors and chemical compositions of their surfaces, with ices of H2O, CH4, CO, N2, and NH3 and a reddish material which may be tholins. Gladstone et al. investigated the atmosphere of Pluto, which is colder and more compact than expected and hosts numerous extensive layers of haze. Weaver et al. examined the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast-rotating, and have bright surfaces. Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these findings massively increase our understanding of the bodies in the outer solar system. They will underpin the analysis of New Horizons data, which will continue for years to come. Science, this issue pp. 1284, 10.1126/science.aad9189, 10.1126/science.aad8866, 10.1126/science.aae0030, & 10.1126/science.aad9045 Pluto modifies its space environment, interacting with the solar wind plasma and energetic particles. INTRODUCTION The scientific objectives of NASA’s New Horizons mission include quantifying the rate at which atmospheric gases are escaping Pluto and describing its interaction with the surrounding space environment. The two New Horizons instruments that measure charged particles are the Solar Wind Around Pluto (SWAP) instrument and the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument. The Venetia Burney Student Dust Counter (SDC) counts the micrometer-sized dust grains that hit the detectors mounted on the ram direction of the spacecraft. This paper describes preliminary results from these three instruments when New Horizons flew past Pluto in July 2015 at a distance of 32.9 astronomical units (AU) from the Sun. RATIONALE Initial studies of the solar wind interaction with Pluto’s atmosphere suggested that the extent of the interaction depends on whether the atmospheric escape flux is strong (producing a comet-like interaction, where the interaction region is dominated by ion pick-up and is many times larger than the object) or weak (producing a Mars-like interaction dominated by ionospheric currents with limited upstream pick-up and where the scale size is comparable to the object). Before the New Horizons flyby, the estimates of the atmospheric escape rate ranged from as low as 1.5 × 1025 molecules s–1 to as high as 2 × 1028 molecules s–1. Combining these wide-ranging predictions of atmospheric escape rates with Voyager and New Horizons observations of extensive variability of the solar wind at 33 AU produced estimates of the scale of the interaction region that spanned all the way from 7 to 1000 Pluto radii (RP). RESULTS At the time of the flyby, SWAP measured the solar wind conditions near Pluto to be nearly constant and stronger than usual. The abnormally high solar wind density and associated pressures for this distance are likely due to a relatively strong traveling interplanetary shock that passed over the spacecraft 5 days earlier. Heavy ions picked up sunward from Pluto should mass-load and slow the solar wind. However, there is no evidence of such solar wind slowing in the SWAP data taken as near as ~20 RP inbound, which suggests that very few atmospheric molecules are escaping upstream and becoming ionized. The reorientation of the spacecraft to enable imaging of the Pluto system meant that both the SWAP and PEPSSI instruments were turned away from the solar direction, thus complicating our analysis of the particle data. Nevertheless, when the spacecraft was ~10 RP from Pluto, SWAP data indicated that the solar wind had slowed by ~20%. We use these measurements to estimate a distance of ~6 RP for the 20% slowing location directly upstream of Pluto. At this time, PEPSSI detected an enhancement of ions with energies in the kilo–electron volt range. The SDC, which measures grains with radii >1.4 µm, detected one candidate impact in ±5 days around its closest approach, indicating a dust density estimate of n = 1.2 km–3, with a 90% confidence level range of 0.6 < n < 4.6 km–3. CONCLUSION New Horizons’s particle instruments revealed an interaction region confined sunward of Pluto to within ~6 RP. The surprisingly small size is consistent with a reduced atmospheric escape rate of 6 × 1025 CH4 molecules s–1, as well as a particularly high solar wind flux due to a passing compression region. This region is similar in scale to the solar wind interaction with Mars’s escaping atmosphere. Beyond Pluto, the disturbance persists to distances greater than 400 RP downstream. Interaction of the solar wind with Pluto’s extended atmosphere. Protons and electrons streaming from the Sun at ~400 km s–1 are slowed and deflected around Pluto because of a combination of ionization of Pluto’s atmosphere and electrical currents induced in Pluto’s ionosphere. CREDIT: STEVE BARTLETT AND NASA’S SCIENTIFIC VISUALIZATION STUDIO The New Horizons spacecraft carried three instruments that measured the space environment near Pluto as it flew by on 14 July 2015. The Solar Wind Around Pluto (SWAP) instrument revealed an interaction region confined sunward of Pluto to within about 6 Pluto radii. The region’s surprisingly small size is consistent with a reduced atmospheric escape rate, as well as a particularly high solar wind flux. Observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument suggest that ions are accelerated and/or deflected around Pluto. In the wake of the interaction region, PEPSSI observed suprathermal particle fluxes equal to about 1/10 of the flux in the interplanetary medium and increasing with distance downstream. The Venetia Burney Student Dust Counter, which measures grains with radii larger than 1.4 micrometers, detected one candidate impact in ±5 days around New Horizons’ closest approach, indicating an upper limit of <4.6 kilometers–3 for the dust density in the Pluto system.

A three-dimensional hybrid code simulation of the December 1984 solar wind AMPTE release

A three-dimensional hybrid code has been developed to study the interaction between small dense plasma clouds and an ambient plasma. The primary advantage of this code is a seamless interface between kinetic particles (plasma cloud) and an MHD fluid (ambient plasma). This interface provides momentum coupling between two distinct ion populations. As a preliminary test of our code, we have simulated the first 3 minutes of the December 1984 AMPTE artificial comet. The results show good agreement with observations of the magnetic field distribution as well as the lateral motion of the comet head. Examples of other applications for this code include comets, coronal mass ejections, Ios plasma torus, and plasma injection experiments.