Saturn's Diffuse Core from Ring Seismology

2021 ◽  
Author(s):  
Christopher Mankovich ◽  
Jim Fuller
Keyword(s):  
Magnetic Field ◽  
Gravity Field ◽  
Field Measurements ◽  
Heavy Elements ◽  
Buoyancy Frequency ◽  
Magnetic Field Generation ◽  
Mixing Processes ◽  
Composition Gradients ◽  
Gravity Mode ◽  
Seismic Measurements

<p>Gravity field measurements only weakly constrain the deep interiors of Jupiter and Saturn, stymieing efforts to measure the mass and compactness of these planets' cores, crucial properties for understanding their formation pathways and evolution. However, studies of Saturn's rings by Cassini have revealed waves driven by pulsation modes within Saturn, offering independent seismic probes of Saturn's interior. The observations reveal gravity mode (g mode) pulsations that indicate that a part of Saturn's interior is stably stratified by composition gradients, and the g mode frequencies directly probe the buoyancy frequency within the planet.</p><p>We compare structure models with gravity and new seismic measurements from Cassini to show that the data can only be explained by a diffuse, stably stratified core-envelope transition region in Saturn extending to approximately 60% of the planet's radius. This predominantly stable interior imposes significant constraints on Saturn's intrinsic magnetic field generation. The gradual distribution of heavy elements required by the seismology constrains mixing processes at work in Saturn, and it may reflect the planet's primordial structure and accretion history.</p>

scholarly journals Magnetic field generation from composition gradients in inertial confinement fusion fuel

2020 ◽  
Vol 378 (2184) ◽  
pp. 20200045 ◽  
Author(s):  
James D. Sadler ◽  
Hui Li ◽  
Kirk A. Flippo
Keyword(s):  
Magnetic Field ◽  
Inertial Confinement Fusion ◽  
Source Term ◽  
Hot Spot ◽  
Fusion Energy ◽  
Coulomb Collision ◽  
Magnetic Field Generation ◽  
Field Source ◽  
Field Generation ◽  
Composition Gradients

Experimental asymmetries in fusion implosions can lead to magnetic field generation in the hot plasma core. For typical parameters, previous studies found that the magnetization Hall parameter, given by the product of the electron gyro-frequency and Coulomb collision time, can exceed one. This will affect the hydrodynamics through inhibition and deflection of the electron heat flux. The magnetic field source is the collisionless Biermann term, which arises from the Debye shielding potential in electron pressure gradients. We show that there is an additional source term due to the Z dependence of the Coulomb collision operator. If there are ion composition gradients, such as jets of carbon ablator mix entering the hot-spot, this source term can rapidly exceed the Biermann fields. In addition, the Biermann fields are enhanced due to the increased temperature gradients from carbon radiative cooling. With even stronger self-generated fields, heat loss to the carbon regions will be reduced, potentially reducing the negative effect of carbon mix. This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 1)’.

scholarly journals Magnetospheric accretion & outflows in stars & brown dwarfs: theories and observational constraints

2009 ◽  
Vol 5 (H15) ◽  
pp. 753-753
Author(s):  
S. Mohanty
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Hot Spot ◽  
Brown Dwarfs ◽  
Field Measurements ◽  
Spot Size ◽  
Stellar Rotation ◽  
Magnetic Field Generation ◽  
T Tauri ◽  
Field Geometry

The manner in which young classical T Tauri stars (cTTs) and brown dwarfs accrete gas from their surrounding disks and simultaneously drive jets and outflows is central to star and planet formation and angular momentum evolution, but remains an ill-understood and hotly debated subject. One of the central concerns is the stellar field geometry: while analytic theories assume an idealized stellar dipole, T Tauri fields are observed to be complex multipolar beasts. I present an analytic generalization of the X-wind theory to include such fields. Independent of the precise field geometry, the generalized model makes a unique prediction about the relationship between various cTTs observables. I show that this prediction is supported by observations of accretion rate, hot spot size, stellar rotation and field strength from stellar to brown dwarf masses, including recent detailed spectropolarimetric measurements. I also discuss the unique insights offered by recent magnetic field measurements on accreting brown dwarfs: while they agree with the accretion theory above, they also pose a puzzle for magnetic field generation theory. Resolving this conundrum promises to illuminate our general picture of accretion and angular momentum transport in fully convective objects.

The promise and limitations of improved-accuracy gravity field measurements for Uranus and Neptune

2021 ◽  
Author(s):  
Naor Movshovitz ◽  
Jonathan Fortney
Keyword(s):  
Gravity Field ◽  
Field Measurements ◽  
Density Profiles ◽  
Refractory Elements ◽  
Wide Range ◽  
Base Functions ◽  
Empirical Density ◽  
Homogeneous Composition ◽  
Composition Gradients ◽  
Improved Accuracy

<p>Uranus and Neptune present unique challenges to planetary modelers. The<br>composition of the so-called ice giants is very uncertain, even more so than the<br>composition of the gas giants. For instance, it is far from clear that either<br>planet's composition is dominated by water. Instead, the composition of Uranus and<br>Neptune likely includes water and other refractory elements in large quantities as<br>well as a substantial H/He envelope. Furthermore, formation models also predict<br>that composition gradients are likely in the interiors of these planets, rather<br>than a neat differentiation into layers of homogeneous composition. (See Helled<br>and Fortney 2020 and references within.)</p><p>A key question that impacts the science case for a potential orbiting mission to<br>Uranus or Neptune is how will more precise measurements of the gravitational field<br>better constrain either planet's interior density profile and composition.<br>Surprisingly, there is yet no published answer to this question.  Here, we present<br>new work that explores this issue, using a Bayesian framework that allows<br>exploration of a wide range of interior density profiles.</p><p>Our approach, which builds off our previous work for Saturn (Movshovitz et al.,<br>2020) and that of others  (e.g. Marley et al., 1995, Helled et al., 2011) takes a<br>relatively unbiased view of the interior structure by employing so-called<br>empirical density profiles. A parameterization is applied to the density profiles<br>directly (via mathematical base functions) instead of to an assumed layered<br>composition (H/He, water, rocks). While some of these empirical density profiles<br>may imply unrealistic compositions, they can also probe solutions that would be<br>missed by the standard layered-composition approach.</p><p>Here we will present models of Uranus and Neptune constructed with this approach,<br>and ask two questions: 1) How large is the space of possible solutions today? 2)<br>How much will it be reduced should a future mission to Uranus and Neptune improve<br>the precision on their gravity field measurements by several orders of magnitude,<br>to the level now available for Jupiter and Saturn?</p>

scholarly journals An 84-μG Magnetic Field in a Galaxy at Z=0.692?

2008 ◽  
Vol 4 (S254) ◽  
pp. 95-96
Author(s):  
Arthur M. Wolfe ◽  
Regina A. Jorgenson ◽  
Timothy Robishaw ◽  
Carl Heiles ◽  
Jason X. Prochaska
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Faraday Rotation ◽  
Large Scale ◽  
Dynamo Model ◽  
Magnetic Field Generation ◽  
Interstellar Gas ◽  
Splitting Technique ◽  
Average Value ◽  
The Magnetic Field

AbstractThe magnetic field pervading our Galaxy is a crucial constituent of the interstellar medium: it mediates the dynamics of interstellar clouds, the energy density of cosmic rays, and the formation of stars (Beck 2005). The field associated with ionized interstellar gas has been determined through observations of pulsars in our Galaxy. Radio-frequency measurements of pulse dispersion and the rotation of the plane of linear polarization, i.e., Faraday rotation, yield an average value B ≈ 3 μG (Han et al. 2006). The possible detection of Faraday rotation of linearly polarized photons emitted by high-redshift quasars (Kronberg et al. 2008) suggests similar magnetic fields are present in foreground galaxies with redshifts z > 1. As Faraday rotation alone, however, determines neither the magnitude nor the redshift of the magnetic field, the strength of galactic magnetic fields at redshifts z > 0 remains uncertain.Here we report a measurement of a magnetic field of B ≈ 84 μG in a galaxy at z =0.692, using the same Zeeman-splitting technique that revealed an average value of B = 6 μG in the neutral interstellar gas of our Galaxy (Heiles et al. 2004). This is unexpected, as the leading theory of magnetic field generation, the mean-field dynamo model, predicts large-scale magnetic fields to be weaker in the past, rather than stronger (Parker 1970).The full text of this paper was published in Nature (Wolfe et al. 2008).

Magnetic field measurements in space

1963 ◽  
Vol 1 (3) ◽  
pp. 399-414 ◽  
Author(s):  
Laurence J. Cahill
Keyword(s):  
Magnetic Field ◽  
Field Measurements ◽  
Magnetic Field Measurements
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