An MHD model of Ganymede's mini‐magnetosphere suggests that the heliosphere forms in a sub‐Alfvénic flow

Directional fluxes of energetic neutral atoms (ENAs) measured by the Interstellar Boundary Explorer spacecraft reveal a Ribbon of strong emission whose source lies beyond the termination shock. Intense emissions of ENAs (energies 0.1 to 6 keV) appear along an extended arc significantly displaced from the nose of the heliosphere, the point on the boundary defined by the Sun's motion relative to the local interstellar medium (LISM). The locus of the Ribbon differs from expectations based on early models of the interaction of the solar wind with the LISM, assumed to flow at a super‐Alfvénic speed. Here we argue that the distribution of the ENA source can be understood if the flow is sub‐Alfvénic. We use a magnetohydrodynamic model of the mini‐magnetosphere of Ganymede, embedded in the sub‐Alfvénic flow of Jupiter's magnetospheric plasma, to establish where heated ions are distributed on the magnetopause. If the flow of the LISM is sub‐Alfvénic, reconnection would occur along an arc centered away from the nose for an appropriately chosen field orientation. Charge exchange with ions heated by reconnection would produce an ENA source distributed in a manner close to that observed. Heating of ions by reconnection can account also for the way ENA images vary with energy. Sub‐Alfvénic flow implies not only that reconnection on the heliopause can be centered well away from the nose, but also that no bow shock forms upstream of the heliopause. It also seems probable that the configuration of the heliosphere differs from the bullet shape frequently illustrated. Key Points A model for accounting for the ENAs ribbon observed by IBEX LISM flow is sub‐Alfvenic and its B field direction is inferred from the ENAs Magnetic reconnection is a likely generation mechanism for the ENAs ribbonPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/102223/1/jgra50653.pd

Driving Saturn's magnetospheric periodicities from the upper atmosphere/ionosphere: Magnetotail response to dual sources

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95157/1/jgra22221.pd

Dawn‐dusk asymmetries in rotating magnetospheres: Lessons from modeling Saturn

Spacecraft measurements reveal perplexing dawn‐dusk asymmetries of field and plasma properties in the magnetospheres of Saturn and Jupiter. Here we describe a previously unrecognized source of dawn‐dusk asymmetry in a rapidly rotating magnetosphere. We analyze two magnetohydrodynamic simulations, focusing on how flows along and across the field vary with local time in Saturn’s dayside magnetosphere. As plasma rotates from dawn to noon on a dipolarizing flux tube, it flows away from the equator along the flux tube at roughly half of the sound speed (Cs), the maximum speed at which a bulk plasma can flow along a flux tube into a lower pressure region. As plasma rotates from noon to dusk on a stretching flux tube, the field‐aligned component of its centripetal acceleration decreases and it flows back toward the equator at speeds typically smaller than 12Cs. Correspondingly, the plasma sheet remains far thicker and the field less stretched in the afternoon than in the morning. Different radial force balance in the morning and afternoon sectors produce asymmetry in the plasma sheet thickness and a net dusk‐to‐dawn flow inside of L = 15 or equivalently, a large‐scale electric field (E) oriented from postnoon to premidnight, as reported from observations. Morning‐afternoon asymmetry analogous to that found at Saturn has been observed at Jupiter, and a noon‐midnight component of E cannot be ruled out.Key PointsDescribes a previously unrecognized source of dawn‐dusk asymmetry in rapidly rotating magnetospheresAsymmetry arises from the different rates of plasma expansion and contraction along flux tubes prenoon and postnoonThe mechanism explains Saturn’s noon‐midnight E field and may produce analogous effects at JupiterPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135239/1/jgra52440.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/135239/2/jgra52440_am.pd

Control of periodic variations in Saturn's magnetosphere by compressional waves

Many of the periodic variations observed in Saturn's magnetosphere can be linked directly to the presence of a rotating pattern of field‐aligned currents that link the northern and southern ionospheres with each other and with the magnetosphere. Such a current system is incorporated in a magnetohydrodynamic simulation that has previously been shown to reproduce many of the observed periodic properties of the system. Here the simulation is used to investigate a range of phenomena that can be attributed to the effects of compressional waves launched from the rotating current sources. The compressional waves are found to drive the flapping of the plasma sheet and the expansion and contraction of the magnetopause in each rotation period. Because the compressional perturbations weaken as they rotate from morning to evening around the dayside of the magnetosphere, the boundary develops a strong morning‐evening asymmetry. A fit to the shape is provided that may be useful in further investigation of magnetopause properties, but there is already evidence of the proposed asymmetry in the observations of Clarke et al. (2010a). Key Points Investigate effect of compressional waves launched from rotating current sources A range of observed periodic phenomena is controlled by compressional waves Our model predicts dawn‐dusk asymmetry of the magnetopause boundaryPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109570/1/jgra51327.pd

Coupled SKR Emissions in Saturn’s Northern and Southern Ionospheres

Kilometric radiation (SKR) emitted above Saturn’s auroral ionosphere is modulated in intensity at periods close to the planetary rotation period; SKR periods differ slightly for sources in the north and in the south. Although there is good evidence that the signals are generated independently in the two hemispheres, it is also well established that during southern summer power emitted from the northern hemisphere is modulated in intensity not only at the northern period but also at the southern period, an observation that requires an explanation. We examine the idea that the signal in the north at the southern period is a secondary effect of the strong field‐aligned current system centered at 270° that drives the southern signal. Basing our analysis on studies of field‐aligned current systems in the terrestrial and Jovian magnetospheres, we argue that the parallel electric fields that drive electrons into the southern auroral ionosphere and generate SKR are, at least in part, bidirectional and thus capable of accelerating electrons toward the opposite hemisphere where the secondary signal is detected with intensity lower than that of the locally generated signal. This interpretation implies that the atmospheric process that modulates the periodic responses can operate independently in each hemisphere.Plain Language SummaryRadio frequency signals with wavelengths of order 1 km emitted from high latitudes at Saturn vary in intensity at close to the planetary rotation period. Signals emitted from the southern and northern hemispheres are modulated at slightly different periods. It has been shown that these signals are generated in regions above the atmosphere where electrons accelerated to high velocities move toward the planet along the planetary magnetic field, generating intense electric current. Refined analysis has shown that sometimes the emissions are modulated not only at the dominant period for that hemisphere but also at the period of the opposite hemisphere. The mechanism for generating, for example, southern period emissions in the northern hemisphere has not been established. We propose that where electrons are accelerated in the southern hemisphere, they are accelerated both downward and upward along the planetary magnetic field. The upward moving electrons from the south move downward as they approach the northern hemisphere end of the magnetic field line, generating emissions with an intensity modulated at the southern period. This model implies that the peak emission at the southern period should occur at the same time north and south, a feature that has not yet been tested.Key PointsWe describe the generation of coupled north‐south SKR emissions observed at SaturnThe SKR emission in the north at the southern period is interpreted as a secondary effect of the strong field‐aligned current system that drives the southern signalThe signals at the southern period should appear at 270° southern PPO phase in both hemispheresPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143770/1/grl57125.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/143770/2/grl57125_am.pd

Global MHD simulations of the Response of Jupiter’s Magnetosphere and Ionosphere to Changes in the Solar Wind and IMF

We have developed a new global magnetohydrodynamic (MHD) model for Jupiter’s magnetosphere based on the BATSRUS code and an ionospheric electrodynamics solver. Our model includes the Io plasma torus at its appropriate location and couples the global magnetosphere with the planetary ionosphere through field‐aligned currents. Through comparisons with available particle and field observations as well as empirical models, we show that the model captures the overall configuration of the magnetosphere reasonably well. In order to understand how the magnetosphere responds to different solar wind drivers, we have carried out time‐dependent simulations using various kinds of upstream conditions, such as a forward shock and a rotation in the interplanetary magnetic field (IMF). Our model predicts that compression of the magnetosphere by a forward shock of typical strength generally weakens the corotation enforcement currents on the dayside and produces an enhancement on the nightside. However, the global response varies depending on the IMF orientation. A forward shock with a typical Parker‐spiral IMF configuration has a larger impact on the magnetospheric configuration and large‐scale current systems than with a parallel IMF configuration. Plasmoids are found to form in the simulation due to tail reconnection and have complex magnetic topology, as they evolve and propagate down tail. For a fixed mass input rate in the Io plasma torus, the frequency of plasmoid occurrence in our simulation is found to vary depending on the upstream solar wind driving.Key PointsA new global MHD model is introduced for Jupiter’s magnetosphere that self‐consistently includes the Io plasma torus at the right locationTime‐dependent simulations show that the global magnetosphere responds differently to different types of drivers in the solar windPlasmoids form in the tail with occurrence frequency dependent on the external drivingPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151316/1/jgra55090.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151316/2/jgra55090_am.pd

Driving Saturn's magnetospheric periodicities from the upper atmosphere/ionosphere

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95345/1/jgra21800.pd

Studying dawn-dusk asymmetries of Mercury's magnetotail using MHD-EPIC simulations

MESSENGER has observed a lot of dawn-dusk asymmetries in Mercury's magnetotail, such as the asymmetries of the cross-tail current sheet thickness and the occurrence of flux ropes, dipolarization events and energetic electron injections. In order to obtain a global pictures of Mercury's magnetotail dynamics and the relationship between these asymmetries, we perform global simulations with the magnetohydrodynamics with embedded particle-in-cell (MHD-EPIC) model, where Mercury's magnetotail region is covered by a PIC code. Our simulations show that the dawnside current sheet is thicker, the plasma density is larger, and the electron pressure is higher than the duskside. Under a strong IMF driver, the simulated reconnection sites prefer the dawnside. We also found the dipolarization events and the planetward electron jets are moving dawnward while they are moving towards the planet, so that almost all dipolarization events and high-speed plasma flows concentrate in the dawn sector. The simulation results are consistent with MESSENGER observations

Embedded Kinetic Simulation of Ganymede’s Magnetosphere: Improvements and Inferences

The largest moon in the solar system, Ganymede, is also the only moon known to possess a strong intrinsic magnetic field and a corresponding magnetosphere. Using the new version of Hall magnetohydrodynamic with embedded particle‐in‐cell model with a self‐consistently coupled resistive body representing the electrical properties of the moon’s interior, improved inner boundary conditions, and the flexibility of coupling different grid geometries, we achieve better match of magnetic field with measurements for all six Galileo flybys. The G2 flyby comparisons of plasma bulk flow velocities with the Galileo Plasma Subsystem data support the oxygen ion assumption inside Ganymede’s magnetosphere. Crescent shape, nongyrotropic, and nonisotropic ion distributions are identified from the coupled model. Furthermore, we have derived the energy fluxes associated with the upstream magnetopause reconnection of ∼10−7W/cm2 based on our model results and found a maximum of 40% contribution to the total peak auroral emissions.Key PointsHall MHD‐EPIC model of Ganymede’s magnetosphere uses realistic inner boundary conditions and energy‐conserving PIC schemeIon‐scale kinetics at upstream magnetopause are fully resolved, shown by the nongyrotropic/anisotropic distributionsElectron precipitation of ∼10−7 W/cm2 shows up to half of the peak emission brightness contributed by upstream reconnectionPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151299/1/jgra55029_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151299/2/jgra55029.pd

Magnetohydrodynamic modelling of star-planet interaction and associated auroral radio emission

We present calculations of auroral radio powers of magnetised hot Jupiters orbiting Sun-like stars, computed using global magnetohydrodynamic (MHD) modelling of the magnetospheric and ionospheric convection arising from the interaction between the magnetosphere and the stellar wind. Exoplanetary auroral radio powers are traditionally estimated using empirical or analytically-derived relations, such as the Radiometric Bode's Law (RBL), which relates radio power to the magnetic or kinetic energy dissipated in the stellar wind-planet interaction. Such methods risk an oversimplification of the magnetospheric electrodynamics giving rise to radio emission. As the next step toward a self-consistent picture, we model the stellar wind-magnetosphere-ionosphere coupling currents using a 3D MHD model. We compute electron-cyclotron maser instability-driven emission from the calculated ionospheric field-aligned current density. We show that the auroral radio power is highly sensitive to interplanetary magnetic field (IMF) strength, and that the emission is saturated for plausible hot Jupiter Pedersen conductances, indicating that radio power may be largely independent of ionospheric conductance. We estimate peak radio powers of $10^{14}$ W from a planet exposed to an IMF strength of $10^3$ nT, implying flux densities at a distance of 15 pc from Earth potentially detectable with current and future radio telescopes. We also find a relation between radio power and planetary orbital distance that is broadly consistent with results from previous analytic models of magnetosphere-ionosphere coupling at hot Jupiters, and indicates that the RBL likely overestimates the radio powers by up to two orders of magnitude in the hot Jupiter regimeComment: 13 pages, 10 figure