200 research outputs found
Modeling the Enceladus plume--plasma interaction
We investigate the chemical interaction between Saturn's corotating plasma
and Enceladus' volcanic plumes. We evolve plasma as it passes through a
prescribed H2O plume using a physical chemistry model adapted for water-group
reactions. The flow field is assumed to be that of a plasma around an
electrically-conducting obstacle centered on Enceladus and aligned with
Saturn's magnetic field, consistent with Cassini magnetometer data. We explore
the effects on the physical chemistry due to: (1) a small population of hot
electrons; (2) a plasma flow decelerated in response to the pickup of fresh
ions; (3) the source rate of neutral H2O. The model confirms that charge
exchange dominates the local chemistry and that H3O+ dominates the water-group
composition downstream of the Enceladus plumes. We also find that the amount of
fresh pickup ions depends heavily on both the neutral source strength and on
the presence of a persistent population of hot electrons.Comment: 10 pages, 1 table, 2 figure
The roles of charge exchange and dissociation in spreading Saturn's neutral clouds
Neutrals sourced directly from Enceladus's plumes are initially confined to a
dense neutral torus in Enceladus's orbit around Saturn. This neutral torus is
redistributed by charge exchange, impact/photodissociation, and neutral-neutral
collisions to produce Saturn's neutral clouds. Here we consider the former
processes in greater detail than in previous studies. In the case of
dissociation, models have assumed that OH is produced with a single speed of 1
km/s, whereas laboratory measurements suggest a range of speeds between 1 and
1.6 km/s. We show that the high-speed case increases dissociation's range of
influence from 9 to 15 Rs. For charge exchange, we present a new modeling
approach, where the ions are followed within a neutral background, whereas
neutral cloud models are conventionally constructed from the neutrals' point of
view. This approach allows us to comment on the significance of the ions'
gyrophase at the moment charge exchange occurs. Accounting for gyrophase: (1)
has no consequence on the H2O cloud; (2) doubles the local density of OH at the
orbit of Enceladus; and (3) decreases the oxygen densities at Enceladus's orbit
by less than 10%. Finally, we consider velocity-dependent, as well as
species-dependent cross sections and find that the oxygen cloud produced from
charge exchange is spread out more than H2O, whereas the OH cloud is the most
confined.Comment: Accepted to the Journal of Geophysical Research, 49 pages, 10 figure
Characterizing the limitations to the coupling between Saturn's ionosphere and middle magnetosphere
Observations of Saturn's ultraviolet and infrared aurora show structures that, when traced along the planetary magnetic field, map to the inner, middle, and outer magnetosphere. From low to high latitudes the structures seen in the UV are the Enceladus footprint, which maps to an equatorial radius of 4 R S (Saturn radii); a diffuse emission that maps to a broad equatorial region from 4-11 RS on the nightside; and a bright ring of emission that maps to ∼15 RS. With the exception of the Enceladus spot, the magnetospheric drivers for these auroral emissions are not yet fully understood. We apply a 1D spatial, 2D velocity space Vlasov solver to flux tubes mapping from equatorial radii of 4, 6, 9, and 13 RS to Saturn's southern atmosphere. The aim is to globally characterize the field-aligned potential structure and plasma density profiles. The ionospheric properties - the field-aligned current densities at the ionospheric boundary, energy intensity profiles and fluxes of the electrons precipitating into the ionosphere - are also determined. We then couple our results to an ionospheric model to calculate the Pedersen conductances at the foot of the relevant flux tubes. We find that for a zero net potential drop between the ionosphere and magnetosphere, there exists a sharp potential drop at ∼1.5 RS along the magnetic field line as measured from the planetary center. The strength of this potential drop is approximately equal to that of the ambipolar potential resulting from the centrifugal confinement of the heavy, cold magnetospheric ion population. We also find that the ionospheric properties respond to changes in the magnetospheric plasma population, which are reflected in the nature of the precipitating electron population. For the flux tube mapping to 9 RS (-70), the incident electron energy flux into the ionosphere resulting from a magnetospheric plasma population with a small fraction of hot electrons is an order of magnitude less than that inferred from observations, implying that significant high-latitude field-aligned potentials (up to 1.5 keV) may exist in the saturnian magnetosphere. Calculated ionospheric Pedersen conductances range from 3.0-18.9 mho, and are thus not expected to limit the currents flowing between the ionosphere and magnetosphere
Current-voltage relation for the Saturnian system
Saturn's magnetosphere is populated by plasma created from neutrals ejected by the moon Enceladus. These neutrals are ionized and picked up by the planetary magnetic field requiring large amounts of angular momentum to be transferred from Saturn's upper atmosphere to the magnetospheric plasma. The resulting upward currents that supply this angular momentum are associated with electrons, which travel toward the planetary atmosphere. At high magnetic latitudes along the flux tube, parallel electric fields may develop to enhance the field-aligned current density flowing between the two regions. We show that, similar to the Jovian system, the current-voltage relation in the Saturnian system must be evaluated at the top of the acceleration region, which occurs at ~1.5 RS along the magnetic field line as measured from the center of the planet. Owing to the large abundance of protons in the Saturnian system, cold electrons carry the majority of the field-aligned current for net potential drops less than 500 V. For the flux tube intersecting the equatorial plane at 4 RS, field-aligned potentials of 50-130 V are consistent with the energy fluxes inferred from the Enceladus emission. In the middle magnetosphere, field-aligned potentials of ∼1.5 kV produce ionospheric electron energy fluxes of 0.3 mW/m2 when hot electrons comprise 0.3% of the magnetospheric electron population. Key Points Current-voltage relation must be evaluated at high magnetic latitudes. Cold electrons contribute strongly to field-aligned current density. Full Knight (1973) current-voltage relation must be applied to Saturnian system
A Sensitivity Study of the Enceladus Torus
We have developed a homogeneous model of physical chemistry to investigate
the neutral-dominated, water-based Enceladus torus. Electrons are treated as
the summation of two isotropic Maxwellian distributionsa thermal component
and a hot component. The effects of electron impact, electron recombination,
charge exchange, and photochemistry are included. The mass source is neutral
HO, and a rigidly-corotating magnetosphere introduces energy via pickup of
freshly-ionized neutrals. A small fraction of energy is also input by Coulomb
collisions with a small population ( 1%) of supra-thermal electrons. Mass
and energy are lost due to radial diffusion, escaping fast neutrals produced by
charge exchange and recombination, and a small amount of radiative cooling. We
explore a constrained parameter space spanned by water source rate, ion radial
diffusion, hot-electron temperature, and hot-electron density. The key findings
are: (1) radial transport must take longer than 12 days; (2) water is input at
a rate of 100--180 kg s; (3) hot electrons have energies between 100 and
250 eV; (4) neutrals dominate ions by a ratio of 40:1 and continue to dominate
even when thermal electrons have temperatures as high as 5 eV; (5)
hot electrons do not exceed 1% of the total electron population within the
torus; (6) if hot electrons alone drive the observed longitudinal variation in
thermal electron density, then they also drive a significant variation in ion
composition.Comment: 9 pages text, 3 tables, 9 figure
Ergocalciferol in New-onset Type 1 diabetes: A Randomized Controlled Trial
Background: The impact of the anti-inflammatory and immunomodulatory actions of Vitamin D on the duration of partial clinical remission (PR) in youth with type 1 diabetes (T1D) is unclear. Objective: To determine the effect of adjunctive ergocalciferol on residual β-cell function (RBCF) and PR in youth with newly-diagnosed T1D who were maintained on a standardized insulin treatment protocol. Hypothesis: Ergocalciferol supplementation increases RBCF and prolongs PR. Methods: A 12-month randomized, double-blind, placebo-controlled trial of 50,000 IU of ergocalciferol per week for 2 months, and then once every 2 weeks for 10 months, versus placebo in 36 subjects of ages 10-21years(y), with T1D ofmonths, and a stimulated C-peptide (SCP) level of ≥0.2nmol/L (≥0.6ng/mL). The ergocalciferol group had 18 randomized subjects (10m/ 8f), mean age 13.3±2.8y; while the control group had 18 subjects (14m/4f), age 14.3±2.9y. Results: The ergocalciferol treatment group had significantly higher serum 25-hydroxyvitamin D at 6 months (p=0.01) and 9 months (p=0.02) than the placebo group. At 12 months, the ergocalciferol group had a significantly lower serum TNF-α concentration (p=0.03). There were no significant differences between the groups at each timepoint from baseline to 12 months for SCP concentration (p=0.08), HbA1c (p=0.09), insulin-dose-adjusted A1c (IDAA1c), or total daily dose of insulin. Temporal trends for rising HbA1c (p=0.044) and IDAA1c (p=0.015) were significantly blunted in the ergocalciferol group. Conclusions: Ergocalciferol significantly reduced serum TNF-α concentration and the rates of increase in both A1c and IDAA1c suggesting a protection of RBCF and PR in youth with newly-diagnosed T1D
Observational signatures of the weak lensing magnification of supernovae
Due to the deflection of light by density fluctuations along the line of
sight, weak lensing is an unavoidable systematic uncertainty in the use of type
Ia supernovae (SNe Ia) as cosmological distance indicators. We derive the
expected weak lensing signatures of SNe Ia by convolving the intrinsic
distribution in SN Ia peak luminosity with magnification distributions of point
sources. We analyze current SN Ia data, and find marginal evidence for weak
lensing effects. The statistics is poor because of the small number of observed
SNe Ia. Future observational data will allow unambiguous detection of the weak
lensing effect of SNe Ia. The observational signatures of weak lensing of SNe
Ia that we have derived provide useful templates with which future data can be
compared.Comment: Including 4 color figures. Expanded and modified version. JCAP
accepte
Characterizing the limitations to the coupling between Saturn's ionosphere and middle magnetosphere
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