Variability of Titan's induced magnetotail: Cassini magnetometer observations

We study the structure and variability of Titan's magnetotail by analyzing Cassini magnetic field observations from all tail crossings between 2005 and 2013. Titan's magnetotail is strongly affected by fluctuations in the ambient magnetospheric field conditions. Therefore, even Titan flybys with nearly identical trajectories may reveal a completely different location and strength of the perturbations in the moon's tail. Short‐scale variations of the ambient magnetospheric field may cause a “fragmentation” of Titan's magnetic lobes, as also seen in Cassini Plasma Spectrometer ion data. By transforming the magnetic field perturbations detected during all available tail crossings to the Draping Coordinate System, we identified the following general characteristics of Titan's plasma interaction: (1) Perpendicular to the background magnetic field and the corotation direction, Titan's magnetotail is confined to a narrow region with a diameter of about 5 Titan radii. Thus, Titan's tail exhibits a rather “flat” structure reminiscent of a delta wing. (2) The plasma incident upon Titan does not possess a significant velocity component along the Saturn‐Titan line. (3) The nonzero component of the background field along the corotation direction generates an asymmetry of Titan's magnetotail which clearly manifests in Cassini magnetometer data.

Influence of fault connectivity on slip rates in southern California: Potential impact on discrepancies between geodetic derived and geologic slip rates

Along the San Bernardino strand of the San Andreas fault (SAF) and across the eastern California shear zone (ECSZ), geologic slip rates differ from those inverted from geodetic measurements, which may partly be due to inaccurate fault connectivity within geodetic models. We employ three‐dimensional models that are mechanically compatible with long‐term plate motion to simulate both fault slip rates and interseismic surface deformation. We compare results from fault networks that follow mapped geologic traces and resemble those used in block model inversions, which connect the San Jacinto fault to the SAF near Cajon Pass and connect distinct faults within the ECSZ. The connection of the SAF with the San Jacinto fault decreases strike‐slip rates along the SAF by up to 10% and increases strike‐slip rates along the San Jacinto fault by up to 16%; however, slip rate changes are still within the large geologic ranges along the SAF. The insensitivity of interseismic surface velocities near Cajon Pass to fault connection suggests that inverse models may utilize both an incorrect fault geometry and slip rate and still provide an excellent fit to interseismic geodetic data. Similarly, connection of faults within the ECSZ produces 36% greater cumulative strike‐slip rates but less than 17% increase in interseismic velocity. When using overconnected models to invert GPS for slip rates, the reduced off‐fault deformation within the models can lead to overprediction of slip rates. While the nature of fault intersections at depth remains enigmatic, fault geometries should be chosen with caution in crustal deformation models.

Spectral description of migrating bed forms and sediment transport

This paper considers the problem of spatiotemporal bed topography evolution and sediment transport estimation in rivers with migrating bed forms of different types and sizes, in statistical equilibrium conditions. Instead of resorting to bed form classification, we propose to evaluate the evolution of multiscale bed topography as the integral of unit contributions defined through a space‒time Fourier decomposition of bed elevations. Using joint 2‒D spectra in the frequency and wave number domain, a functional relationship between the length scales and the timescales in which migrating bed forms are decomposed is proposed and developed into a dimensionless expression for scale‒dependent convection velocities. This formulation highlights the violation of Taylor's hypothesis for migrating bed forms, confirming statistically that larger bed forms travel slower as compared to smaller bed forms. This phenomenological description leads to a spectral extension of the Simons et al. (1965) formula for sediment transport to incorporate a range of multiscale migrating features. Both the scaling of convection velocities and the spectral estimate of sediment transport rate were validated through extensive bed elevation data from laboratory experiments conducted at the St. Anthony Falls Laboratory, for a range of Froude numbers 0.2< F r <0.5, under varying discharge and bed material composition.

A mechanistic‐stochastic formulation of bed load particle motions: From individual particle forces to the Fokker‐Planck equation under low transport rates

Bed load transport is a highly complex process. The probability density function (PDF) of particle velocities results from the local particle momentum variability in response to fluid drag and interactions with the bed. Starting from the forces exerted on a single particle under low transport rates (i.e., rolling and sliding regimes), we derive here the nonlinear stochastic Langevin equation (LE) to describe the dynamics of a single particle, accounting for both the deterministic and the stochastic components of such forces. Then, the Fokker‐Planck equation (FPE), which describes the evolution of the PDF of the ensemble particle velocities, is derived from the LE. We show that the theoretical PDFs of both streamwise and cross‐stream velocities obtained by solving the FPE under equilibrium conditions have exponential form (PDFs of both positive and negative velocities decay exponentially), consistent with the experimental data by Roseberry et al. ([Roseberry, J. C., 2012]). Moreover, we theoretically show how the exponential‐like PDF of an ensemble of particle velocities results from the forces exerted on a single particle. We also show that the simulated particle motions using the proposed Langevin model exhibit an emergent nonlinear relationship between hop distances and travel times (power law with exponent 5/3), in agreement with the experimental data, providing a statistical description of the particles' random motion in the context of a stochastic transport process. Finally, our study emphasizes that the motion of individual particles, described by the LE, and the behavior of the ensemble, described by the FPE, are connected within a statistical mechanics framework.

Finite amplitude bars in mixed bedrock‐alluvial channels

We present a nonlinear asymptotic theory of fully developed flow and bed topography in a wide channel of constant curvature to describe finite amplitude perturbations of bottom topography, subject to an inerodible bedrock layer. The flow field is evaluated at the leading order of approximation as a slowly varying sequence of locally uniform flows, slightly perturbed by a weak curvature‐induced secondary flow. Using the constraint of constant fluid discharge and sediment flux, we calculate an analytical solution for the cross‐sectional profile of flow depth and bed topography, and we determine the average slope in the bend necessary to transport the sediment supplied from a straight, alluvial, upstream reach. Both fully alluvial bends and bends with partial bedrock exposure are shown to require a larger average slope than a straight upstream reach; the relative slope increase is much larger for mixed bedrock‐alluvial bends. Curvature and sediment supply are shown to have a strong effect on the characteristics of the point bars in mixed bedrock‐alluvial channels. Higher curvature bends produce bars of larger amplitude and more bedrock exposure through the cross section, and increasing the sediment supply leads to taller and wider point bars. Differences in the relative roughness of sediment and bedrock have a smaller, secondary effect on point bar characteristics. Our analytical approach can potentially be extended to the case of arbitrary, yet slowly varying, curvature, and should ultimately lead to an improved understanding of the formation of meanders in bedrock channels.

Drilling Glacial Deposits in Offshore Polar Regions

High latitudes are of fundamental importance in the Earth's climate system—they house ice sheets that govern global sea level heights, influence how much solar energy is reflected back to space, and create deep and bottom waters that drive the ocean's ability to circulate energy and nutrients across the globe.

Discontinuities in the magnetic field near Enceladus

The plasma interaction of Saturn's icy moon Enceladus generates a hemisphere coupling current system that directly connects the giant planet's northern and southern polar magnetosphere. Based on Cassini magnetometer observations from all 20 targeted Enceladus flybys between 2004 and 2014, we study the magnetic field discontinuities associated with these hemisphere coupling currents. We identify a total number of 11 events during which the magnetic field was discontinuous at the surface of the Enceladus flux tube (defined by the bundle of magnetic field lines tangential to the solid body of the moon). A minimum variance analysis is applied to calculate the surface normals of these discontinuities. In agreement with theoretical expectations, the normals are found to be perpendicular to the surface of the Enceladus flux tube. The variation of the hemisphere coupling currents with Enceladean longitude leaves a clear imprint in the strengths of the observed magnetic field jumps as well.

Appendix B: Basic Tools

Appendix C: Introduction to the Mathematics of Fixed‐Income Pricing

Appendix D: About the Companion Website