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In steady state, magnetic flux conservation must be maintained in Saturn’s magnetosphere. The Enceladus plumes add mass to magnetic flux tubes in the inner magnetosphere, and centrifugal force pulls the mass-loaded flux tubes outward. Those flux tubes are carried outward to the magnetotail where they deposit their mass and return to the mass loading region. It may take days for the magnetic flux to be carried outward to the tail, but the return of the nearly empty flux tubes can last only several hours, with speeds of inward motion around 200 km/s. Using time sequences of Cassini particle count rate, the difference in curvature drift and gradient drift is accounted for to determine the return speed, age, and starting dipole L-shell of return flux tubes. Determination of this flux-return process improves our understanding of the magnetic flux circulation at Saturn and provides insight into how other giant planets remove the mass added by their moons.

At the Earth’s magnetopause, flux tubes observed by the Magnetospheric Multiscale (MMS) spacecraft in “entangled” pairs have been interpreted as a precursory stage to the formation of a new pair of...

Flux transfer events (FTEs), as flux ropes (FRs), are considered key agents for solar wind energy to enter the terrestrial magnetosphere. Recent observations identify entangled flux tubes that coll...

Magnetic reconnection is a mechanism that allows rapid and explosive energy transfer from the magnetic field to the plasma. The magnetopause is the interface between the shocked solar wind plasma and Earth’s magnetosphere. Reconnection enables the transport of momentum from the solar wind into Earth’s magnetosphere. Because of its importance in this regard, magnetic reconnection has been extensively studied in the past and is the primary goal of the ongoing Magnetospheric Multiscale (MMS) mission. During magnetic reconnection, the originally anti-parallel fields annihilate and reconnect in a thinned current sheet. In the vicinity of a reconnection site, a prominently increased curvature of the magnetic field (and smaller radius of curvature) marks the region where the particles start to deviate from their regular gyro-motion and become available for energy conversion. Before MMS, there were no closely separated multi-spacecraft missions capable of resolving these micro-scale curvature features, nor examining particle dynamics with sufficiently fast cadence.In this study, we use measurements from the four MMS spacecraft to determine the curvature of the field lines and the plasma properties near the reconnection site. We use this method to study FTEs (flux ropes) on the magnetopause, and the interaction between co-existing FTEs. Our study not only improves our understanding of magnetic reconnection, but also resolves the relationship between FTEs and structures on the magnetopause.

The hot electron beams that Dawn detected at Ceres can be explained by fast-Fermi acceleration at a temporary bow shock. A shock forms when the solar wind encounters a temporary atmosphere, similar to a cometary coma. We use a magnetohydrodynamic model to quantitatively reproduce the 3-D shock surface at Ceres, and deduce the atmosphere characteristics that are required to create such a shock. Our most simple model requires about 1.8 kg/s, or 6×1025/s water vapor production rate to form such a shock. Such an estimate relies on characteristics of the solar wind-Ceres interaction. We present several case studies to show how these conditions affect our estimate. In addition, we contrast these cases with the smaller and narrower shock caused by a subsurface induction. Our multi-fluid model reveals the asymmetry introduced by the large gyroradius of the heavy pickup ions, and further constrains the IMF direction during the events.

Magnetic reconnection occurs in current sheets in various plasma environments, and the reconnection process is controlled by the current sheet characteristics. Most theoretical and simulation studies of magnetic reconnection are based on the neutral current sheet model in which the current is primarily carried by ions. Current sheets in natural plasmas (e.g., in Earth's magnetotail), however, are usually charged with nonzero electric field, and usually the current therein is primarily carried by electrons. Here using particle-in-cell simulations, we study magnetic reconnection in a charged, electron-dominant current sheet and show that reconnection in this current sheet is weaker, has a larger diffusion region, and occurs more easily than reconnection in the neutral, ion-dominant current sheet. Two other current sheet characteristics, the background density and background temperature, also affect the reconnection process significantly.

The nucleus of comet C/2013 A1 (Siding Spring) passed within 141,000 km of Mars on 19 October 2014. Thus, the cometary coma and the plasma it produces washed over Mars for several hours producing significant effects in the Martian magnetosphere and upper atmosphere. We present observations from Mars Atmosphere and Volatile EvolutioN's (MAVEN's) particles and field's instruments that show the Martian magnetosphere was severely distorted during the comet's passage. We note four specific major effects: (1) a variable induced magnetospheric boundary, (2) a strong rotation of the magnetic field as the comet approached, (3) severely distorted and disordered ionospheric magnetic fields during the comet's closest approach, and (4) unusually strong magnetosheath turbulence lasting hours after the comet left. We argue that the comet produced effects comparable to that of a large solar storm (in terms of incident energy) and that our results are therefore important for future studies of atmospheric escape, MAVEN's primary science objective.

Characterized by a cusp-shaped enhancement in the magnetic field strength, the magnetic structure in the solar wind, called an interplanetary field enhancement (IFE), has been investigated since its discovery. To understand its three-dimensional magnetic field geometry, we study an IFE detected by five spacecraft simultaneously. Field lines are seen draping around in the upstream region and rotating in the ambient convection electric field direction in the downstream region. Earlier studies suggest that IFEs are created when the solar wind accelerates newly formed dust clouds. Both signatures found in our study support this hypothesis: the field line draping is caused by dust-solar wind momentum exchange, while the field line rotation is a typical signature of dusty plasma pickup. The force that exchanges the momentum is approximately 106 N. This study illustrates the nature of the interaction between two flowing plasmas of very different mass-to-charge ratio.

Strongly conducting or magnetized obstacles in a flowing plasma generate structures called Alfven wings, which mediate momentum transfer between the obstacle and the plasma. Nonconducting obstacles such as airless planetary bodies can generate such structures, which, however, have so far been seen only in sub-Alfvenic regime. A novel statistical analysis of simultaneous measurements made by two ARTEMIS satellites, one in the solar wind upstream of the Moon and one in the downstream wake, and comparison of the data with results of a three-dimensional hybrid model of the interaction reveal that the perturbed plasma downstream of the Moon generates Alfven wings in super-Alfvenic solar wind. In the wake region, magnetic field lines bulge toward the Moon and the plasma flows are significantly perturbed. We use the simulation to show that some of the observed bends of the field result from field-aligned currents. The perturbations in the wake thus arise from a combination of compressional and Alfvenic perturbations. Because of the super-Alfvenic background flow of the solar wind, the two Alfven wings fold back to form a small intersection angle. The currents that form the Alfven wing in the wake are driven by both plasma flow deceleration and a gradient of plasma pressure, positive down the wake from the region just downstream of the Moon. Such Alfven wing structures, caused by pressure gradients in the wake and the resulting plasma slowdown, should exist downstream of any nonconducting body in a super-Alfvenic plasma flow.

It is well accepted that cold plasma sourced by Enceladus is ultimately lost to the solar wind, while the magnetic flux convecting outward with the plasma must return to the inner magnetosphere. However, whether the interchange or reconnection, or a combination of the two processes is the dominant mechanism in returning the magnetic flux is still under debate. Initial Cassini observations have shown that the magnetic flux returns in the form of flux tubes in the inner magnetosphere. Here we investigate those events with 10 year Cassini magnetometer data and confirm that their magnetic signatures are determined by the background plasma environments: inside (outside) the plasma disk, the returning magnetic field is enhanced (depressed) in strength. The distribution, temporal variation, shape, and transportation rate of the flux tubes are also characterized. The flux tubes break into smaller ones as they convect in. The shape of their cross section is closer to circular than fingerlike as produced in the simulations based on the interchange mechanism. In addition, no sudden changes in any flux tube properties can be found at the “boundary” which has been claimed to separate the reconnection and interchange-dominant regions. On the other hand, reasonable cold plasma loss rate and outflow velocity can be obtained if the transport rate of the magnetic flux matches the reconnection rate, which supports reconnection alone as the dominant mechanism in unloading the cold plasma from the inner magnetosphere and returning the magnetic flux from the tail.

Charged dust exists in various regions in the Solar System. How this charged dust interacts with the surrounding plasma is not well understood. In this study we neglect the charging process and treat the charged dust as a fluid interacting with the ambient magnetized plasma fluid. The model reproduces the expected plasma deceleration with both positively charged and negatively charged dust, but a new effect arises. Negatively charged dust causes the magnetic field to bend in the direction of the convection electric field, while positively charged dust causes the opposite magnetic field bending. Consequently, the interaction does not only result in a perpendicular shift in the downstream current system, but also a rotation in these currents. We present quantitative results using the multi-fluid MHD code BATSRUS for both subsonic and supersonic interactions. We find that the same perpendicular bending exists for all counter-streaming interaction problems, independent of the shape of the dust cloud. The new model can be applied to plasma interaction studies including, but not limited to, charged dust particles in the solar wind, cometary plasma, the Enceladus plume, and active plasma releases, such as the Active Magnetospheric Particle Tracer Experiment (AMPTE) mission. The predicted behavior is consistent with observations at Enceladus.

The Deep Space 1 (DS1) spacecraft passed the sunward side of Comet 19P/Borrelly in 2001. Along its relatively north–south orbit, a set of plasma density and velocity measurement revealed a northward shift of the plasma boundaries and the mass loading peak. Both onboard and ground based telescopes found evidence for asymmetric distribution of the dust and neutrals. In this paper, five mass-loading patterns are studied to present the first study of the effect of non-spherical neutral distribution profiles on the solar wind-cometary plasma interaction environment. Using magnetohydrodynamic simulations, it is found that a combination of Gaussian and cosine neutral jet distribution, with cosine being the major part, can fit the DS1 general plasma measurement well, with a total gas production rate of around 5 × 10 28 s −1 . These model-data comparisons indicate that the general plasma distribution around Comet Borrelly can be explained with its aspherical neutral jet distribution. However, such neutral jets by themselves are insufficient to produce the density offset in the central peak. Kinetic effects, such as finite gyroradius may be required to create the offset plasma peak.

On 2007 January 12, comet C/2006 P1 (McNaught) passed its perihelion at 0.17 AU. Abundant remote observations offer plenty of information on the neutral composition and neutral velocities within 1 million kilometers of the comet nucleus. In early February, the Ulysses spacecraft made an in situ measurement of the ion composition, plasma velocity, and magnetic field when passing through the distant ion tail and the ambient solar wind. The measurement by Ulysses was made when the comet was at around 0.8 AU. With the constraints provided by remote and in situ observations, we simulated the plasma environment of Comet C/2006 P1 (McNaught) using a multi-species comet MHD model over a wide range of heliocentric distances from 0.17 to 1.75 AU. The solar wind interaction of the comet at various locations is characterized and typical subsolar standoff distances of the bow shock and contact surface are presented and compared to analytic solutions. We find the variation in the bow shock standoff distances at different heliocentric distances is smaller than the contact surface. In addition, we modified the multi-species model for the case when the comet was at 0.7 AU and achieved comparable water group ion abundances, proton densities, plasma velocities, and plasma temperatures to the Ulysses/SWICS and SWOOPS observations. We discuss the dominating chemical reactions throughout the comet-solar wind interaction region and demonstrate the link between the ion composition near the comet and in the distant tail as measured by Ulysses.

[1] Applying a 2.5-D hybrid simulation, we have studied the particle acceleration caused by magnetic reconnection. The results show that the reconnection process will not only heat all the particles, but also accelerate a few particles to comparatively high velocities (about 2.0vA). Such selective accelerating makes the velocity distribution of all particles changed from the Maxwellian distribution, to a shell or quasi-shell distribution. In addition, this shape varies with the change of particle positions. In order to study the accelerating process occurring near the X-type neutral point, we have observed the time-variation of velocities and positions of selected particles. Among the particles flowed out inside the reconnection region, some are trapped in the magnetic mirror, with a convolution radius that outsteps the width of the reconnection region. These particles can thus construct the lowspeed part of the fluid near the boundary of outflow regions. On the other hand, there are three kinds of drifting trajectory for particles entering the reconnection region beside the X-type neutral point: flee along the field lines, be trapped in the magnetic mirror, and traverse the local field lines. The ratios of particles undergoing the three kinds of trajectory are about 70%, 20%, 10%, respectively.

[1] In 2009 and 2010, the Cassini spacecraft made seven targeted Enceladus flybys along trajectories parallel to the equatorial plane. The magnetometer on board Cassini made a complete set of vector field distribution measurements on these tracks and observed significant perturbations that further constrained the location of the momentum-exchange region. These observations were made not only above and below the moon Enceladus, but also above and below the momentum-loading center. The observed field perturbations are consistent with previous interpretations of the interaction. Southernly-biased Io-type, Alfven-wing signatures are penetrated by Cassini. In addition to the draping due to the slowing down, the magnetic field drapes away from Saturn, indicating that the dust in the plume is predominantly negatively charged. No Saturnward or anti-Saturnward tilt of the plume is identified.

[1] Between May and October in 2007 and 2008, the five THEMIS spacecraft recorded a total of 3701 instances of bipolar magnetic variations in the magnetopause normal direction associated with enhancements of field magnitude that are interpreted as flux transfer events (FTEs) on the magnetopause and/or associated perturbations in the background magnetosphere and magnetosheath. When spacecraft traversed the FTE structures, the velocity components tangential to the magnetopause were generally antisunward, consistent with the sheath flow direction. On the other hand, when the spacecraft were located within the low-latitude boundary layer (LLBL) in the magnetosphere and remotely sensed the perturbations related to FTEs on the magnetopause, the velocity tangential to the magnetopause was found to be antisunward near the magnetopause but sunward further in from the magnetopause. The normal component variations for both groups had the same bipolar structure with inward flows followed by outward flows. This pattern has the form of a flow vortex just inside the magnetopause associated with an FTE moving in an antisunward direction at or outside of the magnetopause. A 2-dimensional magnetohydrodynamic (MHD) simulation code has been developed to understand the flow perturbations outside an FTE. Our simulation starts from an explicit solution, in which it is assumed that the plasma is inviscid and incompressible and no flow vortex is present. Only when we impose finite viscosity near the FTEs do flow vortices develop. However, the origin of this viscosity remains unknown.

[1] The major momentum‐loading source in Saturn’s magnetosphere, Enceladus, has been studied with seven Cassini flybys between 2005 and 2008. In this paper, we first use parameter tests with our 3‐D magnetohydrodynamic simulation to demonstrate and determine the sensitivity of the interaction to both electron impact rates and charge‐ exchange rates. We also investigate the reasons behind our previous discovery that in the plume, within about two Enceladus radii of the plume’s source, the momentum‐loading rates per unit ion and neutral density are orders of magnitude lower than at greater distances. We find that depletion of hot electrons and variations in charge‐exchange rates are two possible explanations for such a reduction of the momentum‐loading rates. Assisted by the Cassini observations, we use our understanding of the plasma interaction to determine the temporal variation of Enceladus’ neutral plume, which is important in understanding its origin, as well as the geological evolution of this icy moon. We base our study on magnetometer observations during all seven flybys to present the first comparative analysis to all flybys in 2005 and 2008. It is found that the maximum variation in gas production rates is one third the largest rate. The plasma momentum‐ loading rate ranges from 0.8 to 1.9 kg/s, which is consistent with previous studies.

[1] In 2008, the Cassini spacecraft made four close Enceladus flybys along similar trajectories. During these flybys the magnetometer recorded the time-varying magnetic field associated with the plasma interaction with Enceladus and its plume. Close to Enceladus, the Cassini magnetometer observed 4% to 7% enhancement in the magnetic field magnitude, associated with the slowing down of the ambient plasma. Herein we examine these four flybys, estimate the deceleration of the flow, locate the momentum-loading center for each pass, and compare their pass to pass variability. Even though the spacecraft trajectories were similar, two different types of perturbations were observed at distances greater than 5 Enceladus radii downstream, and to the north of the moon. Ion-cyclotron waves were observed during each of the flybys, with pass to pass wave amplitudes varying in a similar manner as the enhancement of the field magnitude.

[1] The interaction of Saturn's inner magnetosphere with its moons ranges from the addition of significant quantities of gas, dust, and plasma, causing significant consequences for the dynamics and energetics of the entire Saturnian magnetosphere, to the simple absorption of plasma and energetic particles by the icy moons with non-electrically conducting interiors. The interaction with these moons is complex with the contribution of many physical processes, depending on the geometry of any plume, the structure of the atmosphere, and its interaction with the surface and interior of the moon, the latter by induced fields. Our ultimate goal is to understand the complexities of this interaction and its temporal variations, especially at Enceladus. In this paper we use magnetohydrodynamics (MHD) code for addressing the flow around obstacles that are simpler than the Enceladus interaction. These simulations both help us understand the interaction with other icy moons and prepare us for the simulation of the flow around Enceladus. The processes involved include ordinary collisions, impact ionization, photoionization, and charge exchange. We examine a series of simple canonical interactions before we later apply our simulation where the multiple processes are occurring simultaneously with asymmetric geometries. We apply our 3-D MHD model to simulate the interaction between the Saturnian corotational plasma flow for the following cases: an absorbing body having an insulating surface; ion pickup via photo and impact ionization from a spherically symmetric neutral cloud; charge exchange with such a neutral cloud; and ion pickup at an insulating, absorbing body with an atmosphere acted upon by the sum of the three ionization processes. In addition to validating the model and obtaining a deeper understanding of the consequences of each interaction, we can immediately make some conclusions about the Enceladus interaction. We find that the magnetometer data are most consistent with the surface of Enceladus being absorbing and insulating, rather than the surface being reflecting and electrically conducting. For the conditions in the corotating flow at Enceladus, the perturbation to the plasma flow produced by photo/impact ionization is an order of magnitude smaller than that produced by charge exchange. Moreover, the perturbation to the magnetic field Bz component by a spherically symmetric mass loading source alone is an order of magnitude smaller than that observed in the neighborhood of the plume. Thus, the perturbation observed in the magnetometer data is primarily due to the mass loading in the plume, which is primarily ion-neutral charge exchange. The geometry and source strength of the plume are investigated in a following paper.

[1] The Saturnian moons in the inner magnetosphere are immersed in a plasma disk that rotates much faster than the moon's Keplerian speed. The interaction of the rotating plasma with the moons results in a disturbance in the Saturnian magnetospheric plasma that depends on the nature of obstacle that the moon represents. In particular at Enceladus, such perturbations in the magnetic field and flowing plasma enable us to infer the 3-D shape of the Enceladus plume and its outgassing rate. In this paper, we apply our 3-D magnetohydrodynamic model to extensively study the effects of different plume and disk plasma conditions on the interaction. By finding the best agreement with the observations of two diagnostic flybys, one with its point of closest approach on the upstream side and the other on the downstream side, we determine the plume intensity and configuration. We find that mass loading in the plume is less efficient close to the surface of the moon, where the neutral density is the highest. For E2 and E5, the opening angle of the plume is about 20°, and the plume is tilted toward the corotating direction. The upstream density has a significant effect on the mass loading rate, while its effect on the magnitude of the magnetic perturbation is less significant. An upstream velocity component in the Saturn direction helps to explain the observed magnetic perturbation in the By component and signals the need to consider Enceladus's effect on the global plasma circulation in addition to the local effect. Quantitative comparisons of the simulated and observed interaction are provided.

By traversing the plume erupting from high southern latitudes on Saturn's moon Enceladus, Cassini orbiter instruments can directly sample the material therein. Cassini Plasma Spectrometer, CAPS, data show that a major plume component comprises previously-undetected particles of nanometer scales and larger that bridge the mass gap between previously observed gaseous species and solid icy grains. This population is electrically charged both negative and positive, indicating that subsurface triboelectric charging, i.e., contact electrification of condensed plume material may occur through mutual collisions within vents. The electric field of Saturn's magnetosphere controls the jets' morphologies, separating particles according to mass and charge. Fine-scale structuring of these particles' spatial distribution correlates with discrete plume jets' sources, and reveals locations of other possible active regions. The observed plume population likely forms a major component of high velocity nanometer particle streams detected outside Saturn's magnetosphere. Citation: Jones, G. H., et al. (2009), Fine jet structure of electrically charged grains in Enceladus' plume, Geophys. Res. Lett., 36, L16204, doi:10.1029/2009GL038284.

Two Sun-grazing comets were recently imaged in the low solar corona by space telescopes in unprecedented detail, revealing a wide range of new phenomena. This sparked growing interest in the interaction of comets with the coronal plasma and magnetic field and their diagnostic potential as solar probes. However, interpretation of such rich observational data requires profound understanding of relevant physical processes in an unexplored regime. Here advanced numerical modeling can provide critical clues. To this end, we present a prototype, multi-fluid, magnetohydrodynamic model of a steady-state comet in the low solar corona. These simulation results are compared with previously modeled comets in the solar wind environment. By inspecting their projection and column densities, we find a dominance of O6 + ions in the cometary tail, which can explain the observed extreme ultraviolet emission. The tail is found to be comparable to recent EUV images of these comets. In addition, the comet tail appears wider when the observer's line of sight is perpendicular rather than parallel to the local magnetic field. This is opposite to the trend in the interplanetary space permeated in the solar wind, because the ratio between dynamic pressure and magnetic pressure is an order of magnitude smaller than at 1 AU. On the other hand, we find that iron ions in the comet head build up to a density comparable to that of oxygen ions, but are unlikely to form a visible tail because of the shorter mean free paths of the neutrals.