Analysis of Cassini plasma and magnetic field measurements from 1-7 AU

Cassini was launched in October 1997. Since then it has performed four planetary swing-bys and collected hundreds of days of data in the solar wind. In this thesis we mainly use data from the Cassini Electron Spectrometer (ELS) to discuss low energy electron distributions observed from 1 to 7 AU, and during the Cassini swing-bys of Earth and Jupiter. A method to estimate the spacecraft potential and calculate electron density and temperature by examining one dimensional cuts through the electron distributions is first introduced. These data are used to introduce many equatorial regions of the Earth's magnetosphere as observed by ELS in August 1999. We discuss ELS measurements of the upstream solar wind, the Earth's bow shock, magnetopause, day and nightside magnetosphere, radiation belts, low latitude boundary layer, and the plasmasheet. It is believed the encounter happened whilst reconnection was occurring at the nose of the magnetosphere, and hence we encountered an eroding magnetosphere with signatures of two substorms during the outbound path. The Cassini crossing of the Earth's bow shock is compared to statistical results gained during ~40 crossings by Cassini of Jupiter's bow shock. We find that observations of the electron temperature jump are consistent with that observed at the Earth, suggesting that the same fundamental processes are involved at both systems. Cassini-ELS observations of solar wind electron temperature are then discussed in the context of previous work. Based on ELS data alone we find that the solar wind electron temperature decreases with increasing heliocentric distance and that this decrease is slightly faster than predicted by Parker/two fluid models, slower than expected for adiabatic expansion and toward the steeper end of the scale based on previous observations

Radiolytic Gas-Driven Cryovolcanism in the Outer Solar System

Water ices in surface crusts of Europa, Enceladus, Saturn's main rings, and Kuiper Belt Objects can become heavily oxidized from radiolytic chemical alteration of near-surface water ice by space environment irradiation. Oxidant accumulations and gas production are manifested in part through observed H2O2 on Europa. tentatively also on Enceladus, and found elsewhere in gaseous or condensed phases at moons and rings of Jupiter and Saturn. On subsequent chemical contact in sub-surface environments with significant concentrations of primordially abundant reductants such as NH3 and CH4, oxidants of radiolytic origin can react exothermically to power gas-driven cryovolcanism. The gas-piston effect enormously amplifies the mass flow output in the case of gas formation at basal thermal margins of incompressible fluid reservoirs. Surface irradiation, H2O2 production, NH3 oxidation, and resultant heat, gas, and gas-driven mass flow rates are computed in the fluid reservoir case for selected bodies. At Enceladus the oxidant power inputs are comparable to limits on nonthermal kinetic power for the south polar plumes. Total heat output and plume gas abundance may be accounted for at Enceladus if plume activity is cyclic in high and low "Old Faithful" phases, so that oxidants can accumulate during low activity phases. Interior upwelling of primordially abundant NH3 and CH4 hydrates is assumed to resupply the reductant fuels. Much lower irradiation fluxes on Kuiper Belt Objects require correspondingly larger times for accumulation of oxidants to produce comparable resurfacing, but brightness and surface composition of some objects suggest that such activity may be ongoing

Interchange Injections at Saturn: Statistical Survey of Energetic H+ Sudden Flux Intensifications

We present a statistical study of interchange injections in Saturn’s inner and middle magnetosphere focusing on the dependence of occurrence rate and properties on radial distance, partial pressure, and local time distribution. Events are evaluated from over the entirety of the Cassini mission’s equatorial orbits between 2005 and 2016. We identified interchange events from CHarge Energy Mass Spectrometer (CHEMS) H+ data using a trained and tested automated algorithm, which has been compared with manual event identification for optimization. We provide estimates of interchange based on intensity, which we use to investigate current inconsistencies in local time occurrence rates. This represents the first automated detection method of interchange, estimation of injection event intensity, and comparison between interchange injection survey results. We find that the peak rates of interchange occur between 7 and 9 Saturn radii and that this range coincides with the most intense events as defined by H+ partial particle pressure. We determine that nightside occurrence dominates as compared to the dayside injection rate, supporting the hypothesis of an inversely dependent instability growth rate on local Pedersen ionospheric conductivity. Additionally, we observe a slight preference for intense events on the dawnside, supporting a triggering mechanism related to large‐scale injections from downtail reconnection. Our observed local time dependence paints a dynamic picture of interchange triggering due to both the large‐scale injection‐driven process and ionospheric conductivity.Plain Language SummaryStudying high‐energy particles around magnetized planets is essential to understanding processes behind mass transport in planetary systems. Saturn’s magnetic environment, or magnetosphere, is sourced from a large amount of low‐energy water particles from Enceladus, a moon of Saturn. Saturn’s magnetosphere also undergoes large rotational forces from Saturn’s short day and massive size. The rotational forces and dense internal mass source drive interchange injections, or the injection of high‐energy particles closer to the planet as low‐energy water particles from the inner magnetosphere are transported outward. There have been many strides toward understanding the occurrence rates of interchange injections, but it is still unknown how interchange events are triggered. We present a computational method to identify and rank interchange injections using high‐energy particle fluxes from the Cassini mission to Saturn. These events have never been identified computationally, and the resulting database is now publically available. We find that the peak rates of interchange occur between 7 and 9 Saturn radii and that this range coincides with the highest intensity events. We also find that interchange occurrence rates peak on the nightside of Saturn. Through this study, we identify the potential mechanisms behind interchange events and advance our understanding of mass transport around planets.Key PointsWe developed a novel classification and identification algorithm for interchange injection based on Cassini CHEMS 3–220 keV H+ energetic ionsRadial occurrence rates and maximum partial H+ pressure in interchange peaked between 7 and 9 Saturn radii for all intensity categoriesOccurrence rates peak on the nightside (1800–0600 LT) as compared to the dayside (0600–1800 LT)Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145315/1/jgra54283.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145315/2/jgra54283_am.pd

Alternating North‐South Brightness Ratio of Ganymede's Auroral Ovals: Hubble Space Telescope Observations Around the Juno PJ34 Flyby

peer reviewedWe report results of Hubble Space Telescope observations from Ganymede's orbitally trailing side which were taken around the flyby of the Juno spacecraft on 7 June 2021. We find that Ganymede's northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter's magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede's aurora is the momentum of the Jovian plasma sheet north and south of Ganymede's magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and electromagnetic energy fluxes ultimately powering the auroral acceleration process. No clear statistically significant timevariability of the auroral emission on short time scales of 100s could be resolved. We show that electron energy fluxes of several tens of mW m−2 are required for its OI 1,356 Å emission making Ganymede a very poor auroral emitter

New Frontiers-class Uranus Orbiter: Exploring the feasibility of achieving multidisciplinary science with a mid-scale mission

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Neptune Odyssey: A Flagship Concept for the Exploration of the Neptune–Triton System

The Neptune Odyssey mission concept is a Flagship-class orbiter and atmospheric probe to the Neptune-Triton system. This bold mission of exploration would orbit an ice-giant planet to study the planet, its rings, small satellites, space environment, and the planet-sized moon Triton. Triton is a captured dwarf planet from the Kuiper Belt, twin of Pluto, and likely ocean world. Odyssey addresses Neptune system-level science, with equal priorities placed on Neptune, its rings, moons, space environment, and Triton. Between Uranus and Neptune, the latter is unique in providing simultaneous access to both an ice giant and a Kuiper Belt dwarf planet. The spacecraft - in a class equivalent to the NASA/ESA/ASI Cassini spacecraft - would launch by 2031 on a Space Launch System or equivalent launch vehicle and utilize a Jupiter gravity assist for a 12 yr cruise to Neptune and a 4 yr prime orbital mission; alternatively a launch after 2031 would have a 16 yr direct-to-Neptune cruise phase. Our solution provides annual launch opportunities and allows for an easy upgrade to the shorter (12 yr) cruise. Odyssey would orbit Neptune retrograde (prograde with respect to Triton), using the moon's gravity to shape the orbital tour and allow coverage of Triton, Neptune, and the space environment. The atmospheric entry probe would descend in ~37 minutes to the 10 bar pressure level in Neptune's atmosphere just before Odyssey's orbit-insertion engine burn. Odyssey's mission would end by conducting a Cassini-like "Grand Finale,"passing inside the rings and ultimately taking a final great plunge into Neptune's atmosphere

The Auroral Footprint Of Enceladus On Saturn

Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus could lead to electrodynamic coupling between Enceladus and Saturn like that which links Jupiter with Io, Europa and Ganymede. Powerful field-aligned electron beams associated with the Io-Jupiter coupling, for example, create an auroral footprint in Jupiter\u27s ionosphere. Auroral ultraviolet emission associated with Enceladus-Saturn coupling is anticipated to be just a few tenths of a kilorayleigh (ref. 12), about an order of magnitude dimmer than Io\u27s footprint and below the observable threshold, consistent with its non-detection. Here we report the detection of magnetic-field-aligned ion and electron beams (offset several moon radii downstream from Enceladus) with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in a few per cent of the scans of the moon\u27s footprint. The footprint varies in emission magnitude more than can plausibly be explained by changes in magnetospheric parameters-and as such is probably indicative of variable plume activity. © 2011 Macmillan Publishers Limited. All rights reserved