Abstract This study analyzes the emission of energetic neutral atoms (ENAs), generated by charge exchange between energetic protons and Ganymede's atmosphere. We also constrain the observability of such ENAs by an imaging instrument aboard a spacecraft. Our approach employs tracing tools that calculate the trajectories of magnetospheric parent protons near Ganymede. We determine the ENA flux through a hypothetical spherical detector encompassing the moon's atmosphere. We additionally generate synthetic ENA images, as seen by a point‐like detector with a finite field of view. The complexity of Ganymede's electromagnetic environment is successively increased; we consider (i) uniform Jovian fields, (ii) the superposition of the moon's internal dipole with Jupiter's field, and (iii) draped fields from a hybrid model of Ganymede's plasma interaction. Our major results are: (a) In uniform fields, the ENA flux is elevated within a circular band on the detector sphere. Synthetic ENA images record a cluster of high flux near the moon's limb, with the position of this enhancement determined by the viewing geometry. (b) When including Ganymede's internal dipole, the flux through the sphere displays a localized increase above the ramside apex, mainly generated by protons on open field lines at mid‐latitudes. In the synthetic images, the reduced ENA emissions from the closed field line region produce local flux depletions along the equator. (c) Pile‐up of Jupiter's field significantly reduces the ENA flux from Ganymede's ramside atmosphere. (d) At energies above several keV, the emissions from Ganymede's atmosphere clearly exceed the ENA flux released from the moon's surface.
AbstractWe study the generation and evolution of spatially localized dynamic plasma pressure enhancements in the magnetosheath (high‐speed jets) by carrying out three‐dimensional hybrid simulations of the Earth's dayside magnetosphere with a novel, space‐time adaptive code, HYPERS. High‐speed jets are shown to occur downstream of quasi‐parallel bow shocks under southward and northward quasi‐radial interplanetary magnetic field conditions. The physical properties and three‐dimensional morphology of simulation jets are found to be consistent with general statistical knowledge acquired from the satellite observations. We discuss a “magnetokinetic” mechanism for jet origin whereby the compression of solar wind plasma and its penetration into the magnetosheath is tied to the turbulence‐driven magnetic field perturbations. We compare three‐dimensional jets to dynamic pressure structures observed in two‐dimensional hybrid simulations and demonstrate the impact of large jets on the magnetopause and the cusp.
Abstract In the publication Troshichev et al. (2006) ( https://doi.org/10.1029/2005JA011402 ) on the polar cap (PC) indices, PCN and PCS, an error was made by using components of the interplanetary magnetic field (IMF) in their geocentric solar ecliptic (GSE) representation instead of the prescribed geocentric solar magnetospheric (GSM) representation for calculations of index scaling parameters. The mistake has caused a trail of incorrect relations and wrong conclusions extending since 2006 up to now (2020) which should be discontinued, for instance, by issuing a corrigendum note from the authors. The present contribution explains the error and discusses in an extended example its consequences for one of the publications that have referred to the invalid scaling parameter set. Further investigations reported here of the PC index versions recommended by the International Association for Geomagnetism and Aeronomy (IAGA) indicate occurrences of similar problems in the present derivation of index scaling parameters.
AbstractThe goal of our study is to present a systematic modeling framework for the identification of water vapor plumes in plasma and magnetic field data from spacecraft flybys of Europa. In particular, we determine the degree to which different plume configurations can be obscured by the interaction of Jupiter's magnetospheric plasma with Europa's induced dipole field and its global atmosphere. We apply the hybrid model AIKEF (kinetic ions, fluid electrons) to investigate the effect of inhomogeneities in Europa's atmosphere (plumes) on the plasma interaction with the Jovian magnetosphere. To systematically assess the magnitude and structure of the perturbations associated with the plume‐plasma interaction at Europa, we vary the plume location across Europa's surface while considering different symmetric and asymmetric density profiles of the moon's global atmosphere. To isolate the impact of a plume on Europa's magnetospheric environment, we also conduct model runs without any global atmosphere. To quantify the magnetic perturbations caused by plumes, we analyze the field components along hypothetical spacecraft trajectories through each plume. Conclusions of our study are (1) localized regions of stagnant flow are most indicative of the presence of a plume. (2) The visibility of plumes in the magnetic field strongly depends on the density profile (whether it is symmetric or asymmetric) of the global atmosphere. (3) The presence of an induced dipole complicates the identification of magnetic signatures associated with a plume and dominates Europa's magnetic environment in its intermediate vicinity. (4) Complex fine structures are visible in the tail of escaping plume ions.
AbstractIn the absence of a global magnetic field at Venus, its ionosphere is the obstacle to the flow of the solar wind resulting in the formation of a smaller bow shock and foreshock. Spacecraft observations and global hybrid (kinetic ions, fluid electrons) simulations have demonstrated that despite its smaller size, the foreshock at Venus has properties similar to those seen at its much larger terrestrial counterpart. This study employs global hybrid simulations and Venus Express observations to demonstrate the formation of foreshock bubbles (FBs) at Venus and examine their impacts on the Venusian ionosphere. Discovered at Earth, FBs form in response to backstreaming ion beams in the foreshock interacting with solar wind rotational or tangential discontinuities. They exhibit a hot, tenuous, and low magnetic field core bounded by a fast magnetosonic shock wave and the associated plasma sheath. Their sizes scale with the width of the foreshock and they are carried by the solar wind toward the bow shock. The results of the hybrid simulations show that despite the smaller size of the Venusian foreshock, FBs should form at this planet. A preliminary examination of the Venus Express data has shown the presence of a number of FB candidates, one of which is discussed here. Using the results of hybrid simulations we examine the impacts of FBs on the ionosphere and demonstrate their global consequences which exceed those of hot flow anomalies also formed as a result of the interaction between solar wind discontinuities and the Venusian bow shock.
AbstractSpace physics is the study of Earth's home in space. Elements of space physics include how the Sun works from its interior to its atmosphere, the environment between the Sun and planets out to the interstellar medium, and the physics of the magnetic barriers surrounding Earth and other planets. Space physics is highly relevant to society. Space weather, with its goal of predicting how Earth's technological infrastructure responds to activity on the Sun, is an oft‐cited example, but there are many more. Space physics has important impacts in formulating public policy.
AbstractSaturn's magnetospheric periodicities are commonly thought to have a dual nature, one period originating from the southern hemisphere and a slightly different period from the northern. Both periods vary a few percent over time intervals of years and apparently merged a few months after Saturn equinox. These periodicities have not been explained. The dual‐period waveform is generally represented as the superposition of two sinusoids with nearly equal periods. However, dual‐period waves can also result from an amplitude modulation of a carrier periodicity, from frequency modulation of the carrier, from random phase jumps in the carrier, and from small, random changes in the period itself. While such simple phenomena are well known in the radio community, they can serve as possible explanations for how a single planetary period can appear as the dual (or multiple) periods observed in Saturn's magnetosphere. Candidates for modulation and randomization include the solar wind pressure and speed, the orbital periods of moons of Saturn, or even the trajectory of the Cassini orbiter itself.