A fundamental question for the atmospheric evolution of Venus is how much water‐related material escapes from Venus to space. In this study, we calculate the nonthermal escape of H+ and O+ ions ...through the Venusian magnetotail and its dependence on the solar cycle. We separate 8 years of data obtained from the ion mass analyzer on Venus Express into solar minimum and maximum. The average escape of H+ decreased from 7.6 · 1024 (solar minimum) to 2.1 · 1024 s−1 (solar maximum), while a smaller decrease was found for O+: 2.9 · 1024 to 2.0 · 1024 s−1. As a result, the H+/O+ flux ratio decreases from 2.6 to 1.1. This implies that the escape of hydrogen and oxygen could have been below the stoichiometric ratio of water for Venus in its early history under the more active Sun.
Plain Language Summary
An open issue for the atmospheric evolution of Venus is how the presumably large water content was lost. In this study, we use 8 years of data collected by the ion mass analyzer instrument onboard Venus Express. We investigate the escape of hydrogen H+ and oxygen O+ ions, the components of water, from the Venusian atmosphere to space. If water is escaping from Venus entirely as ions, the hydrogen‐to‐oxygen ratio should be close to 2. We find that the ratio of H+/O+ ion escape is 2.6 for the solar minimum period, while the escape is close to 1‐to‐1 for solar maximum, when the Sun is more active. Thus, Venus is currently on average losing water as ions from its atmosphere. However, in the early history of the solar system, when the Sun was in an even more active state, the escape ratio was probably below the water ratio. This implies that the oxygen did not enrich the atmosphere and surface, and instead hydrogen remained in the Venusian system over its history.
Key Points
The escape rate of O+ is almost stable over the solar cycle, while H+ escape rate decreases by a factor of 3.6 from solar minimum to maximum
The H+/O+ escape rate ratio decreases from 2.6 at solar minimum to 1.1 at solar maximum
The historic escape rate has presumably been below the stoichiometric ratio of water
Abstract
The interaction between the solar wind and Venus creates an induced magnetosphere. The regions of the induced magnetosphere are separated by plasma boundaries, where their shapes and sizes ...are influenced by variations in the surrounding environment. Investigations of the boundaries and their variability can help us understand the solar wind’s effect on Venus and unmagnetized planets in general. In this study, the bow shock and ion composition boundary locations are investigated using, for the first time, the full data set of plasma and magnetic field measurements by Venus Express taken during 2006–2014. The locations of the boundaries are examined with respect to upstream conditions and the solar cycle. We find, in agreement with previous studies using Pioneer Venus Orbiter measurements, that the bow shock location is mostly sensitive to the solar cycle and the dynamic pressure and that it exhibits asymmetries in the terminator plane, depending on the direction of the interplanetary magnetic field. The asymmetries are mainly attributed to the asymmetry in the pickup ion distribution and thus mass loading in the magnetosheath. The ion composition boundary on the dayside is found to decrease in altitude with increasing dynamic pressure during solar maximum (2006–2011), but shows no clear trends during solar minimum (2011–2014) conditions.
Several concepts for heliospheric missions operating at heliocentric distances far beyond Earth orbit are currently investigated by the scientific community. The mission concept of the Interstellar ...Probe, e.g., aims at reaching a distance of 1000 au away from the Sun within this century. This would allow the coming generation to obtain a global view of our heliosphere from an outside vantage point by measuring the energetic neutral atoms (ENAs) originating from the various plasma regions. It would also allow for direct sampling of the unperturbed interstellar medium, as well as for many observation opportunities beyond heliospheric science, such as visits to Kuiper Belt objects, a comprehensive view on the interplanetary dust populations, and infrared astronomy free from the foreground emission of the zodiacal cloud. In this study, we present a simple empirical model of ENAs from the heliosphere and derive basic requirements for ENA instrumentation on board a spacecraft at great heliocentric distances. We consider the full energy range of heliospheric ENAs from 10 eV to 100 keV because each part of the energy spectrum has its own merits for heliospheric science. To cover the full ENA energy range, two or three different ENA instruments are needed. Thanks to parallax observations, some insights about the nature of the IBEX ribbon and the dimensions of the heliosphere can already be gained by ENA imaging from a few au heliocentric distance. To directly reveal the global shape of the heliosphere, measurements from outside the heliosphere are, of course, the best option.
Strong depletions of energetic protons (115–244 keV) were observed during Galileo flyby E26 of Europa. We simulate the flux of energetic protons using a Monte Carlo particle backtracing code and show ...that energetic proton depletions during E26 are reproduced by taking into account the perturbations of the electromagnetic fields calculated by magnetohydrodynamic (MHD) simulations and charge exchange with a global atmosphere and plume. A depletion feature occurring shortly after closest approach is driven by plume associated charge exchange, or a combination with plume associated field perturbations. We therefore conclude, with a new method and independent data set, that Galileo could have encountered a plume during E26.
Plain Language Summary
We investigate why (normally abundant) fast protons were disappearing during Europa flyby E26 by Galileo. We do this by simulating the proton motion. In some cases we detect few protons because Europa is blocking the field of view. What is new here is that part of the decrease can be explained by charge exchange, a process whereby the protons are removed after they lose their electrical charge in Europa's thin atmosphere. Furthermore, we see that there is a special decrease, which can be explained by an erupting plume of water vapor, thereby providing additional evidence for an active plume during Galileo flyby E26.
Key Points
Energetic proton flux depletions during Galileo flyby E26 are driven by inhomogeneous fields, atmospheric charge exchange, and a plume
Plumes can deplete protons through charge exchange and field perturbations
Plumes are a source of energetic neutral atoms
The formation of electric potential over lunar magnetized regions is essential for understanding fundamental lunar science, for understanding the lunar environment, and for planning human exploration ...on the Moon. A large positive electric potential was predicted and detected from single point measurements. Here, we demonstrate a remote imaging technique of electric potential mapping at the lunar surface, making use of a new concept involving hydrogen neutral atoms derived from solar wind. We apply the technique to a lunar magnetized region using an existing dataset of the neutral atom energy spectrometer SARA/CENA on Chandrayaan‐1. Electrostatic potential larger than +135 V inside the Gerasimovic anomaly is confirmed. This structure is found spreading all over the magnetized region. The widely spread electric potential can influence the local plasma and dust environment near the magnetic anomaly.
Key Points
Large electric potential (>135 V) exists inside a lunar magnetic anomaly
The electric potential is spread over the magnetized region (200 km scale)
We introduce a new technique to image electric potential of the lunar surface
We study the interaction between the Moon and the solar wind using a three-dimensional hybrid plasma solver. The proton fluxes and electromagnetical fields are presented for typical solar wind ...conditions with different magnetic field directions. We find two different wake structures for an interplanetary magnetic field that is perpendicular to the solar wind flow, and for one that is parallell to the flow. The wake for intermediate magnetic field directions will be a mix of these two extreme conditions. Several features are consistent with a fluid interaction, e.g., the presence of a rarefaction cone, and an increased magnetic field in the wake. There are however several kinetic features of the interaction. We find kinks in the magnetic field at the wake boundary. There are also density and magnetic field variations in the far wake, maybe from an ion beam instability related to the wake refill. The results are compared to observations by the WIND spacecraft during a wake crossing. The model magnetic field and ion velocities are in agreement with the measurements. The density and the electron temperature in the central wake are not as well captured by the model, probably from the lack of electron physics in the hybrid model.
Jovian magnetospheric plasma irradiates the surface of Ganymede and is postulated to be the primary agent that changes the surface brightness of Ganymede, leading to asymmetries between polar and ...equatorial regions as well as between the trailing and leading hemispheres. As impinging ions sputter surface constituents as neutrals, ion precipitation patterns can be remotely imaged using the Energetic Neutral Atoms (ENA) measurement technique. Here we calculate the expected sputtered ENA flux from the surface of Ganymede to help interpret future observations by ENA instruments, particularly the Jovian Neutrals Analyzer (JNA) onboard the JUpiter ICy moon Explorer (JUICE) spacecraft. We use sputtering models developed based on laboratory experiments to calculate sputtered fluxes of H2O, O2, and H2. The input ion population used in this study is the result of test particle simulations using electric and magnetic fields from a hybrid simulation of Ganymede's environment. This population includes a thermal component (H+ and O+ from 10 eV to 10 keV) and an energetic component (H+, O++, and S+++ from 10 keV to 10 MeV). We find a global ENA sputtering rate from Ganymede of 1.42 × 1027 s−1, with contributions from H2, O2, and H2O of 34%, 17%, and 49% respectively. We also calculate the energy distribution of sputtered Energetic Neutral Atoms (ENAs), give an estimate of a typical JNA count rate at Ganymede, and investigate latitudinal variations of sputtered fluxes along a simulated orbit track of the JUICE spacecraft. Our results demonstrate the capability of the JNA sensor to remotely map ion precipitation at Ganymede.
Plain Language Summary
Particles trapped by Jupiter's magnetic field interact with Jupiter's moons. Ganymede, the largest of those moons, lacks a dense atmosphere to protect its surface from these energetic Jovian particles, but Ganymede's magnetic field is strong enough to influence their trajectory: charged particles are deflected away from equatorial regions to polar regions, resulting in uneven particle precipitation patterns at the surface of Ganymede. When ions hit the surface of Ganymede, they eject particles from the surface, in a process referred to as sputtering. Those particles are mostly neutral and therefore unaffected by Ganymede's magnetic fields, so we can image where ions hit the surface of Ganymede by measuring ejected neutral particles. The Jovian Neutrals Analyzer (JNA) will fly onboard the JUpiter ICy moon Explorer (JUICE) spacecraft and will measure sputtered neutrals in the vicinity of Ganymede. To help interpret the data to be collected by JNA, we used models derived from laboratory experiments to simulate what JNA will observe at Ganymede. Our results show that JNA will be able to show us where ions hit the surface of Ganymede, which is important as uneven ion precipitation is thought to explain why Ganymede's poles are brighter than its equatorial regions.
Key Points
A new method for calculating sputtered fluxes at Ganymede is introduced
The energy spectra of sputtered H2O, O2, and H2 Energetic Neutral Atoms (ENAs) are calculated for the first time
The Jovian Neutrals Analyzer on JUpiter ICy moon Explorer (JUICE) can remotely map ion precipitation at Ganymede
Velocity distribution functions (VDFs) are a key to understanding the interplay between particles and waves in a plasma. Any deviation from an isotropic Maxwellian distribution may be unstable and ...result in wave generation. Using data from the ion mass spectrometer IMA (Ion Mass Analyzer) and the magnetometer (MAG) onboard Venus Express, we study proton distributions in the plasma environment of Venus. We focus on the temperature anisotropy, that is, the ratio between the proton temperature perpendicular (T⊥) and parallel (T‖) to the background magnetic field. We calculate average values of T⊥ and T‖ for different spatial areas around Venus. In addition we present spatial maps of the average of the two temperatures and of their average ratio. Our results show that the proton distributions in the solar wind are quite isotropic, while at the bow shock stronger perpendicular than parallel heating makes the downstream VDFs slightly anisotropic (T⊥/T‖ > 1) and possibly unstable to generation of proton cyclotron waves or mirror mode waves. Both wave modes have previously been observed in Venus's magnetosheath. The perpendicular heating is strongest in the near‐subsolar magnetosheath (T⊥/T‖≈3/2), which is also where mirror mode waves are most frequently observed. We believe that the mirror mode waves observed here are indeed generated by the anisotropy. In the magnetotail we observe planetary protons with largely isotropic VDFs, originating from Venus's ionosphere.
Key Points
We present maps of the perpendicular and parallel proton temperatures and their ratio in the plasma environment around Venus
The largest perpendicular temperature anisotropy with a median temperature ratio of about 3/2 is found in the near‐subsolar magnetosheath
The region with the largest observed temperature anisotropy coincides with observations of proton cyclotron and mirror mode waves
With a newly developed technique and magnetic field measurements obtained by the magnetometer on Venus Express, we study the flapping motion of the Venusian magnetotail. We find that the flapping ...motion generally comprises contributions both from a nonpropagating steady flapping and a propagating kink‐like flapping. The flapping motion tilts the current sheet normal significantly in the plane perpendicular to the Venus‐Sun line. The kink‐like flapping waves traveling along solar wind electric field or its antidirection can be found in either magnetotail hemisphere where solar wind electric field pointing toward/away. The traveling behaviors suggest that the locations of the triggers for kink‐like flappings are near the boundaries between magnetotail current sheet and magnetosheath, not near the central region of magnetotail as is for the Earth's magnetotail.
Key Points
Current sheet normals are varied in the YZ plane during flapping period
The flapping motions compromise the nonpropagating and propagating type
Sources for the kink-like flapping probably locate at the magnetotail flanks
Since the Moon is not shielded by a global magnetic field or by an atmosphere, solar wind plasma impinges onto the lunar surface almost unhindered. Until recently, it was assumed that almost all of ...the impinging solar wind ions are absorbed by the surface. However, recent Interstellar Boundary Explorer, Chandrayaan‐1, and Kaguya observations showed that the interaction process between the solar wind ions and the lunar surface is more complex than previously assumed. In contrast to previous assumptions, a large fraction of the impinging solar wind ions is backscattered as energetic neutral atoms. Using the complete Chandrayaan‐1 Energetic Neutral Analyzer data set, we compute a global solar wind reflection ratio of 0.16 ± 0.05 from the lunar surface. Since these backscattered neutral particles are not affected by any electric or magnetic fields, each particle's point of origin on the lunar surface can be determined in a straight‐forward manner allowing us to create energetic neutral atom maps of the lunar surface. The energetic neutral atom measurements recorded by the Chandrayaan‐1 Energetic Neutral Analyzer cover ∼89% of the lunar surface, whereby the lunar farside is almost completely covered. We analyzed all available energetic neutral atom measurements recorded by the Chandrayaan‐1 Energetic Neutral Analyzer to create the first global energetic neutral hydrogen maps of the lunar surface.
Key Points
From the Chandrayaan-1/CENA data we compute a global ENA albedo of 0.16+/-0.05
We present the first global energetic hydrogen map of the lunar surface
The high energy ENA map correlates well with magnetic field surface maps
PDF
