John F. Cooper

PROTON IRRADIATION OF CENTAUR, KUIPER BELT, AND OORT CLOUD OBJECTS AT PLASMA TO COSMIC RAY ENERGY

Times for accumulation of chemically significant dosages on icy surfaces of Centaur, Kuiper Belt, and Oort Cloud objects from plasma and energetic ions depend on irradiation position within or outside the heliosphere. Principal irradiation components include solar wind plasma ions, pickup ions from solar UV ionization of interstellar neutral gas, energetic ions accelerated by solar and interplanetary shocks, including the putative solar wind termination shock, and galactic cosmic ray ions from the Local Interstellar Medium (LISM). We present model flux spectra derived from spacecraft data and models for eV to GeV protons at 40 AU, a termination shock position at 85 AU, and in the LISM. Times in years to accumulate dosages ∼100 eV per molecule are computed from the spectra as functions of sensible surface depth less than one centimeter at unit density. The collisional resurfacing model of Luu and Jewitt is reconsidered in the context of depth-dependent dosage rates from plasma, suprathermal, and higher energy protons, and global exposure, by micrometeoroid dust grain impacts, of moderately irradiated red material below a thin crust of heavily irradiated neutral material. This material should be more visible on dynamically ‘cold’ objects in the ∼40 AU region.

Comprehensive analysis of electron observations at Saturn: Voyager 1 and 2

We present a comprehensive analysis of Voyager 1 and 2 electron observations within Saturns magnetosphere. This analysis entails the merging of electron observations from the Plasma Science (PLS) experiment, the Low Energy Charged Particle (LECP) experiment and the Cosmic Ray System (CRS) experiment. For each encounter, the three instruments combined allow us to compute the electron energy spectra over a wide range of energies from 10 eV to ∼ 2MeV between the closest approaches and L = 18.5. The instruments use different technologies, different sensitivities, and different fields of view; however, we observe a surprisingly good matching of the data sets on a 15-min timescale. The PLS-LECP-CRS spectra include the low-energy thermal component of the magnetospheric plasma, the keV suprathermal electrons, and the high-energy tail extending into the MeV energy range. From the combined spectra, we compute a comprehensive set of macroscopic parameters (electron density, pressure, beta factor, and electron current at the spacecraft): the analysis reveals a variety of radial gradients for these quantities and the corresponding electron populations. We also compute phase space densities over a wide range in energy and radial distances, analyzing local time symmetries, electron source distributions, and temporal variations of Saturns magnetosphere. The ultimate goal of this study is to provide a comprehensive empirical model of the charged particle population within Saturns magnetosphere. It will be used to support the development of the Cassini mission and to allow detailed planning of the tour design with regard to charged particle science and radiation hazards.

Energetic nitrogen ions within the inner magnetosphere of Saturn

[1] We investigate the importance of nitrogen ions within Saturn’s magnetosphere and their contribution to the energetic charged particle population within Saturn’s inner magnetosphere. This study is based on the Voyager observations of Saturn’s magnetosphere and Cassini observations. The latter have shown that water group ions dominate both the plasma and energetic particle populations but that nitrogen ions over a broad range of energies were observed at � 5% abundance level. In the outer magnetosphere, methane ions were predicted to be an important pickup ion at Titan and were detected at significant levels in the outer magnetosphere and at Titan. O + ions were found to be the dominant heavy ion in the outer magnetosphere, � 60%, with methane ions being � 30% of the heavy ions and N + being a few percent. The two major sources of nitrogen ions within Saturn’s magnetosphere are Titan’s atmosphere and primordial nitrogen trapped in the icy crust of Saturn’s moons and its ring particles deep within the magnetosphere. It is important to understand the source, transport, and sinks of nitrogen in ordertodeterminewhethertheyhaveaprimordialoriginorarefromTitan’satmosphere.The energetic component is important, since it can come from Titan, be implanted into the surfaces of the icy moons, and reappear at plasma energies via sputtering obfuscating the ultimatesource.Aswewillshow,suchimplantationofnitrogenionscanproduceinteresting chemistry within the ice of Saturn’s moons. The emphasis will be on the nitrogen, but the oxygenandotherwatergroupionsarealsoconsidered.Wearguethatneutralcloudsofheavy atoms and molecules within Saturn’s outer magnetosphere may be the dominant source of energetic heavy ions observed within the inner magnetosphere. Pickup heavy ions in the outer magnetosphere have energies � 1–4 keV when born. If they diffuse radially inward, whileconservingthefirstandsecondadiabaticinvariants,theycanhaveenergiesgreaterthan several hundred keVinside of Dione’s L shell. We will show how observations relate to the various sources and acceleration processes such as ionization, collisions, wave-particle interactions, and radial diffusion.

Energetic helium isotopes trapped in the magnetosphere

Helium nuclei have been measured within the magnetosphere (L ≤ 6) during the 1990/1991 CRRES mission over an energy range of ∼40–100 MeV nucleonl−1. The Office of Naval Research 604 instrument resolves helium isotopes with a mass resolution of ∼0.1 amu. Each helium isotope can be represented by a power law energy spectrum, and the energy spectrum of the 3He is different from that of 4He; that is, the 3He/4He ratio is energy dependent. In the energy range 51–86 MeV/nucleon the 3He/4He ratio is 7.4 ± 2.6 for L = 1.15–1.3 and 2.2±0.6 for L = 1.8–2.15. All observed helium events at L < 2.65 show pitch angle distributions consistent with equatorially trapped ions, whereas the distributions at larger L values are more uniform, within a factor of 2. For 2.15 < L < 6, as L decreases, the 3He/4He ratio increases and the helium energy spectrum softens. A possible origin of the geomagnetically trapped helium isotopes at L <2.15 is proton interactions in the residual atmosphere. The CRRES satellite was operational during the large geomagnetic storm on March 24, 1991, and we have divided the analysis into prestorm and poststorm time periods. Following the storm, the magnetosphere became more dynamic, and at L ∼ 2.3 an enhanced helium flux was observed. Over the energy range of 51–86 MeV nucleonl−1 both the average 3He and 4He fluxes at L = 2.15–2.65 increased by a factor of 6 after the storm relative to the prestorm period, while at L = 1.8–2.15 the average 3He and 4He fluxes decreased by factors of 3 and 11, respectively, following the storm. The implications of these observed results are discussed.

Local time asymmetry of drift shells for energetic electrons in the middle magnetosphere of Saturn

Abstract Saturn encounter trajectories of Pioneer 11, Voyager 1, and Voyager 2 passed through the noon-dusk quadrant of the middle magnetosphere during approach and through dawn-midnight outbound. Intensities of energetic electrons (≥0.6 MeV) were an order of magnitude higher inbound than outbound at 6–15 R s , even though the same (Pioneer 11) or higher (Voyager 1 and 2) intensities were expected outbound for stably trapped electrons with azimuthally symmetric distributions. Explanations include electron drift shell asymmetry arising from (1) magnetopause compression by the solar wind and/or (2) dusk-to-dawn electric field associated with solar wind driven convection within the magnetosphere. Theoretical and numerical calculations indicate that maximum effects of the electric field are expected at L -dependent drift resonance energies in the range of the maximum observed asymmetries. Maximum intensities would then be expected on the dusk side of the middle magnetosphere where “bannana”-shaped drift shells are expected near the resonance energies. Since some of the asymmetric drift shells would not extend around to the dawn side, minimum intensities would be expected there as observed. Further analysis and modeling will be needed to assess the relative effects of the electric field magnetopause compression, and other possible causes. Full local time coverage of energetic particles by the Cassini Orbiter mission would be invaluable for additional constraints on the magnetospheric electric and magnetic fields.

Jovian plasma torus interaction with Europa. Plasma wake structure and effect of inductive magnetic field: 3D hybrid kinetic simulation

Abstract The hybrid kinetic model supports comprehensive simulation of the interaction between different spatial and energetic elements of the Europa moon–magnetosphere system with respect to a variable upstream magnetic field and flux or density distributions of plasma and energetic ions, electrons, and neutral atoms. This capability is critical for improving the interpretation of the existing Europa flyby measurements from the Galileo Orbiter mission, and for planning flyby and orbital measurements (including the surface and atmospheric compositions) for future missions. The simulations are based on recent models of the atmosphere of Europa ( Cassidy et al., 2007 , Shematovich et al., 2005 ). In contrast to previous approaches with MHD simulations, the hybrid model allows us to fully take into account the finite gyroradius effect and electron pressure, and to correctly estimate the ion velocity distribution and the fluxes along the magnetic field (assuming an initial Maxwellian velocity distribution for upstream background ions). Photoionization, electron-impact ionization, charge exchange and collisions between the ions and neutrals are also included in our model. We consider the models with O ++ and S ++ background plasma, and various betas for background ions and electrons, and pickup electrons. The majority of O2 atmosphere is thermal with an extended non-thermal population ( Cassidy et al., 2007 ). In this paper, we discuss two tasks: (1) the plasma wake structure dependence on the parameters of the upstream plasma and Europas atmosphere (model I, cases (a) and (b) with a homogeneous Jovian magnetosphere field, an inductive magnetic dipole and high oceanic shell conductivity); and (2) estimation of the possible effect of an induced magnetic field arising from oceanic shell conductivity. This effect was estimated based on the difference between the observed and modeled magnetic fields (model II, case (c) with an inhomogeneous Jovian magnetosphere field, an inductive magnetic dipole and low oceanic shell conductivity).

Heliospheric cosmic ray irradiation of Kuiper Belt comets

Abstract Irradiation by galactic cosmic ray particles at energies above 0.1 GeV has been shown to be a significant source of energy for chemical modification of ices in Oort Cloud comets. However, these ions have minimal efficiency for surface modification, since their energy is deposited over depths of many meters, and they must act over the lifetime of the Solar System to produce an appreciable radiation-induced mantle. Recent measurements by the Voyager and Pioneer spacecraft, now moving outward from the Sun beyond 40 – 60 A.U. into the Kuiper belt region, have found a radially increasing intensity of the “anomalous” cosmic ray component, consisting of interstellar hydrogen and heavier ions accelerated at energies up to 10 2 MeV/nucleon at the solar wind termination shock at 65 – 100 A.U. from the Sun. Significant intensities of the anomalous cosmic ray ions are expected throughout the inner Kuiper belt region beyond the termination shock and out to the heliopause. These ions would significantly affect mantle formation on Kuiper Belt comets at column depths less than 10 grams/cm 2 in the outer surface layer of material accessible to direct and remote sensing measurements.

Proton irradiation environment of solar system objects in the heliospheric boundary regions

Two classes of outer solar system objects, Scattered Disk Objects and comets, include many known members that are natural counterparts of the Voyager 1 spacecraft now traversing the heliosheath enroute to the heliopause. Thirty‐two Scattered Disk Objects and about ten times more comets, as cataloged by the Smithsonian Astrophysical Observatory’s Minor Planets Center, have orbits passing partly or wholly through the heliosheath. Objects passing from the supersonic heliosphere, upstream of the solar wind termination shock, through the heliosheath and out into the local interstellar medium may undergo increasing surface irradiation intensities as suggested by Voyager 1 measurements and limits on interstellar proton flux spectra. Correspondingly, volume dosage rates for chemically significant surface irradiation at micron to meter depths could also increase with potential cumulative effects on surface composition, albedo, and atmospheric evolution of these objects.

MeV proton flux predictions near Saturn's D ring

Abstract Radiation belts of MeV protons have been observed just outward of Saturns main rings. During the final stages of the mission, the Cassini spacecraft will pass through the gap between the main rings and the planet. Based on how the known radiation belts of Saturn are formed, it is expected that MeV protons will be present in this gap and also bounce through the tenuous D ring right outside the gap. At least one model has suggested that the intensity of MeV protons near the planet could be much larger than in the known belts. We model this inner radiation belt using a technique developed earlier to understand Saturns known radiation belts. We find that the inner belt is very different from the outer belts in the sense that its intensity is limited by the densities of the D ring and Saturns upper atmosphere, not by radial diffusion and satellite absorption. The atmospheric density is relatively well constrained by EUV occultations. Based on that we predict an intensity in the gap region that is well below that of the known belts. It is more difficult to do the same for the region magnetically connected to the D ring since its density is poorly constrained. We find that the intensity in this region can be comparable to the known belts. Such intensities pose no hazard to the mission since Cassini would only experience these fluxes on timescales of minutes but might affect scientific measurements by decreasing the signal‐to‐contamination ratio of instruments.

Science of opportunity: Heliophysics on the FASTSAT mission and STP-S26

The FASTSAT spacecraft, which was launched on November 19, 2010 on the DoD STP-S26 mission, carries three instruments developed in joint collaboration by NASA GSFC and the US Naval Academy: PISA, TTI, and MINI-ME.1,2 As part of a rapid-development, low-cost instrument design and fabrication program, these instruments were a perfect match for FASTSAT, which was designed and built in less than one year. These instruments, while independently developed, provide a collaborative view of important processes in the upper atmosphere relating to solar and energetic particle input, atmospheric response, and ion outflow. PISA measures in-situ irregularities in electron number density, TTI provides limb measurements of the atomic oxygen temperature profile with altitude, and MINI-ME provides a unique look at ion populations by a remote sensing technique involving neutral atom imaging. Together with other instruments and payloads on STP-S26 such as the NSF RAX mission, FalconSat-5, and NanoSail-D (launched as a tertiary payload from FASTSAT), these instruments provide a valuable “constellation of opportunity” for following the flow of energy and charged and neutral particles through the upper atmosphere. Together, and for a small fraction of the price of a major mission, these spacecraft will measure the energetic electrons impacting the upper atmosphere, the ions leaving it, and the large-scale plasma and neutral response to these energy inputs. The result will be a new model for maximizing scientific return from multiple small, distributed payloads as secondary payloads on a larger launch vehicle.