The Space Environment of Io and Europa Bagenal, Fran; Dols, Vincent
Journal of geophysical research. Space physics,
20/May , Letnik:
125, Številka:
5
Journal Article
Recenzirano
The Galilean moons play major roles in the giant magnetosphere of Jupiter. At the same time, the magnetospheric particles and fields affect the moons. The impact of magnetospheric ions on the moons' ...atmospheres supplies clouds of escaping neutral atoms that populate a substantial fraction of their orbits. At the same time, ionization of atoms in the neutral cloud is the primary source of magnetospheric plasma. The stability of this feedback loop depends on the plasma/moon‐atmosphere interaction. The purpose of this review is to describe the physical processes that shape the space environment around the two innermost Galilean moons—Io and Europa—and to show their impact from the planet Jupiter out into interplanetary space.
Key Points
Io and Europa substantially impact the Jovian magnetosphere while interactions of plasma with moons affects their surfaces and atmospheres
Ten components are described, including neutrals, thermal plasma, and energetic particles, spanning from Jupiter to interplanetary medium
Current understanding of the systems and processes are reviewed with a summary of outstanding questions
We present the demographics data for the space physics workforce which are compared with other space sciences fields, physics, plus science and engineering in general. We focus on the early stages of ...college, and draw some lessons from looking beyond the US by discussing this in the context of physics degrees awarded in different countries. We review some of the studies from the National Academies, extracting some relevant recommendations. Studies of the science, technology, engineering and mathematics (STEM) workforce, the physical sciences profession, and specifically the space sciences show that the “pinch point” where the demographics narrow down is at the high school to college stages. We considered the actions that could be made nationally by federal agencies, locally by an institution or individually to enhance and diversify the career pathway through the space sciences.
Centrifugal Equator in Jupiter’s Plasma Sheet Phipps, Phillip; Bagenal, Fran
Journal of geophysical research. Space physics,
January 2021, 2021-01-00, Letnik:
126, Številka:
1
Journal Article
Recenzirano
Odprti dostop
In Jupiter’s magnetosphere, the structure of the plasma sheet depends on the magnetic field geometry and the centrifugal forces on the plasma. We present a simple formulation for the centrifugal ...equator, the farthest point along a magnetic flux tube from the planetary spin axis, for Jupiter’s torus to plasma sheet region (5–30 jovian radii). The formulation is based on a dipole magnetic field and azimuthally symmetric current sheet, both tilted by 9.5° toward System III west longitude of 201°. We find a good fit to such a model with a hyperbolic tangent function varying sinusoidally with longitude. The latitudinal angle of the derived centrifugal equator relative to the jovigraphic equator changes from the dipolar value (2/3 of the dipole tilt) around 5 jovian radii to close to the full dipole tilt at 25 jovian radii.
Key Points
Between 5 and 30 jovian radii, the equatorial current sheet increasingly changes the magnetic field geometry from a dipole to a disk
The centrifugal equator correspondingly changes from 2/3 to the full magnetic dipole tilt
We derive a simple formulation for the centrifugal equator versus distance and System III longitude
We present simple models of the plasma disks surrounding Jupiter and Saturn based on published measurements of plasma properties. We calculate radial profiles of the distribution of plasma mass, ...pressure, thermal energy density, kinetic energy density, and energy density of the suprathermal ion populations. We estimate the mass outflow rate as well as the net sources and sinks of plasma. We also calculate the total energy budget of the system, estimating the total amount of energy that must be added to the systems at Jupiter and Saturn, though the causal processes are not understood. We find that the more extensive, massive disk of sulfur‐ and oxygen‐dominated plasma requires a total input of 3–16 TW to account for the observed energy density at Jupiter. At Saturn, neutral atoms dominate over the plasma population in the inner magnetosphere, and local source/loss process dominate over radial transport out to 8 RS, but beyond 8–10 RS about 75–630 GW needs to be added to the system to heat the plasma.
Key Points
Substantial mass is added to these giant magnetospheres
The plasma is heated significantly as it moves outward
Jupiter's plasma sheet is much more massive and hotter than Saturn's
Jupiter is a planet of superlatives: the most massive planet in the solar system, rotates the fastest, has the strongest magnetic field, and has the most massive satellite system of any planet. These ...unique properties lead to volcanoes on Io and a population of energetic plasma trapped in the magnetic field that provides a physical link between the satellites, particularly Io, and the planet Jupiter. There are strong differences between the magnetospheres of Earth and Jupiter but there are also underlying basic physical principles that all magnetospheres share in common. This paper provides a rough sketch of the magnetosphere of Jupiter, briefly describes the current understanding and lists outstanding issues. As at Earth, a major issue of the jovian system is how the magnetospheric plasma is coupled to the planet's ionosphere.
Preliminary results from NASA's Juno mission are presented in this special issue of Geophysical Research Letters. The data were gathered by nine scientific instruments as the Juno spacecraft ...approached Jupiter on the dawn flank, was inserted into Jupiter orbit on 4 July 2016, and made the first polar passes close to the planet. The first results hint that Jupiter may not have a distinct core, indicate puzzling deep atmospheric convection, and reveal complex small‐scale structure in the magnetic field and auroral processes that are distinctly different from those at Earth.
Key Points
Juno's polar, eccentric orbit provides a unique close‐up view of Jupiter
Juno's science payload addresses key issues of the magnetosphere, atmosphere, and deep interior of Jupiter
The first passes are revealing unexpected atmospheric structure and auroral processes
The Io torus produces ultraviolet emissions diagnostic of plasma conditions. We revisit data sets obtained by the Voyager 1, Galileo, and Cassini missions at Jupiter. With the latest version (8.0) of ...the CHIANTI atomic database we analyze UV spectra to determine ion composition. We compare ion composition obtained from observations from these three missions with a theoretical model of the physical chemistry of the torus by Delamere et al. (2005). We find ion abundances from the Voyager data similar to the Cassini epoch, consistent with the dissociation and ionization of SO2, but with a slightly higher average ionization state for sulfur, consistent with the higher electron temperature measured by Voyager. This reanalysis of the Voyager data produces a much lower oxygen:sulfur ratio than earlier analysis by Shemansky (1988), which was also reported by Bagenal (1994). We derive fractional ion compositions in the center of the torus to be S+/Ne ~ 5%, S++/Ne ~ 20%, S+++/Ne ~ 5%, O+/Ne ~ 20%, O++/Ne ~ 3%, and Σ(On+)/Σ(Sn+) ~ 0.8, leaving about 10–15% of the charge as protons. The radial profile of ion composition indicates a slightly higher average ionization state, a modest loss of sulfur relative to oxygen, and Σ(On+)/Σ(Sn+) ~ 1.2 at about 8 RJ, beyond which the composition is basically frozen in. The Galileo observations of UV emissions from the torus suggest that the composition in June 1996 may have comprised a lower abundance of oxygen than usual, consistent with observations made at the same time by the EUVE satellite.
Key Points
Reanalysis of Voyager UVS observations of the Io plasma torus shows composition similar to Cassini UVIS
Analysis of Cassini UVIS spectra with latest CHIANTI 8.0 does not produce significant changes in composition
The torus composition in June 1996 exhibited lower oxygen abundance
Abstract
The NASA New Horizons Venetia Burney Student Dust Counter (SDC) measures dust particle impacts along the spacecraft’s flight path for grains with mass ≥10
−12
g, mapping out their spatial ...density distribution. We present the latest SDC dust density, size distribution, and flux measurements through 55 au and compare them to numerical model predictions. Kuiper Belt objects (KBOs) are thought to be the dominant source of interplanetary dust particles in the outer solar system due to both collisions between KBOs and their continual bombardment by interstellar dust particles. Continued measurements through 55 au show higher than model-predicted dust fluxes as New Horizons approaches the putative outer edge of the Kuiper Belt (KB). We discuss potential explanations for the growing deviation: radiation pressure stretches the dust distribution to further heliocentric distances than its parent body distribution; icy dust grains undergo photosputtering that rapidly increases their response to radiation pressure forces and pushes them further away from the Sun; and the distribution of KBOs may extend much further than existing observations suggest. Ongoing SDC measurements at even larger heliocentric distances will continue to constrain the contributions of dust production in the KB. Continued SDC measurements remain crucial for understanding the Kuiper Belt and the interpretation of dust disks around other stars.
The plasma science (PLS) Instrument on the Galileo spacecraft (orbiting Jupiter from December 1995 to September 2003) measured properties of the ions that were trapped in the magnetic field. The PLS ...data provide a survey of the plasma properties between approx. 5 and 30 Jupiter radii R(sub J) in the equatorial region. We present plasma properties derived via two analysis methods: numerical moments and forward modeling. We find that the density decreases with radial distance by nearly 5 orders of magnitude from approx. 2 to 3000 cm(exp.-3) at 6R(sub j) to approx. 0.05cm(sub -3) at 30 R(sub j). The density profile did not show major changes from orbit to orbit, suggesting that the plasma production and transport remained constant within about a factor of 2. The radial profile of ion temperature increased with distance which implied that contrary to the concept of adiabatic cooling on expansion, the plasma heats up as it expands out from Io's orbit (where TI is approx.60-80 eV) at approx. 6R(sub j) to a few keV at 30R(sub j).There does not seem to be a long-term, systematic variation in ion temperature with either local time or longitude. This latter finding differs from earlier analysis of Galileo PLS data from a selection of orbits. Further examination of all data from all Galileo orbits suggests that System Ill variations are transitory on timescales of weeks, consistent with the modeling of Cassini Ultraviolet Imaging Spectrograph observations. The plasma flow is dominated by azimuthal flow that is between 80% and 100% of corotation out to 25 R(sub j).
Io's atmospheric oxygen atoms are heated by atmospheric sputtering and escape from Io's gravity, forming a neutral oxygen cloud around Io's orbit. This neutral cloud is important as a source of the ...Io oxygen plasma torus. Previous studies derived the distribution and density of the equilibrium neutral oxygen cloud. However, little is known about the evolution of the neutral cloud. In this study, we analyzed Hisaki satellite observations of the spatial distribution of OI 130.4 nm emissions around Io's orbit during transient strong density enhancement in the torus in 2015 (called high density period). Comparing time variations of OI and OII 83.4 nm emissions, we estimated that the lifetimes of O+ in this period were about 21 days in the high density period and 41 days in the normal density period. Hisaki observations are consistent with a decrease in the lifetime of O+ when the density in the torus increases. The radial distribution showed the neutral oxygen cloud spread outward up to 8.6 Jupiter radii during the high density period. We also show that during the high density period, the neutral oxygen number density at Io's orbit (where north‐south thickness is assumed to be 1.2 Jupiter radii) increased to
91−25+29 cm−3, more than three times the value during the normal density period (
27−7+8 cm−3). The azimuthal distribution showed a dense region around Io and a longitudinally uniform, diffuse region distributed along Io's orbit that enlarges during the high density period.
Key Points
The lifetime of O+ was estimated from time variations of O and O+, and it shortened during the density enhancement in 2015
Io's neutral oxygen cloud spread outward from Jupiter during the high density period
The number density of O at Io's orbit during the high density period is three times larger than that during the normal density period
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