Suzaku observations of charge exchange emission from solar system objects

Recent results of charge exchange emission from solar system objects observed with the Japanese Suzaku satellite are reviewed. Suzaku is of great importance to investigate diffuse X‐ray emission like the charge exchange from planetary exospheres and comets. The Suzaku studies of Earth's exosphere, Martian exosphere, Jupiter's aurorae, and comets are overviewed (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ionization chemistry in H<Subscript>2</Subscript>O-dominated atmospheres of icy moons

The processes of the formation and dynamics of tenuous gaseous envelopes of icy moons in giant-planet systems are considered. Tenuous exospheres with relatively dense surface layers are likely to form around more massive icy satellites, such as, for example, the Galilean satellites Europa and Ganymede in the Jovian system. Escaping exospheres are formed in the case of low-mass icy moons, as happens for the icy satellite Enceladus in the Saturnian system. The main parent component of such gaseous envelopes is water vapor, which enters into the atmosphere as a result of thermal degassing processes, nonthermal radiolysis, and other active processes and phenomena on the icy surface of a satellite. A numerical kinetic model has been developed to study on a molecular level the processes of the formation, chemical evolution, and dynamics of tenuous gaseous envelopes dominated mainly by H₂O. The ionization processes in such tenuous gaseous envelopes are caused by solar ultraviolet (UV) radiation and solar-wind and/or magnetospheric plasma. The primary processes when ultraviolet solar photons and plasma electrons affect the tenuous gas of the H₂O-dominated atmosphere are responsible for the chemical diversity of the gaseous envelopes of icy moons. Ionization chemistry, including ion-molecular reactions, dissociative recombination of molecular ions, and the reactions of the charge exchange with magnetospheric ions, is important for the formation of chemical diversity in gaseous envelopes of icy satellites. The model considered in the study was used to numerically simulate the formation and development of chemical diversity in the tenuous gaseous envelope of Enceladus. The numerical results were compared to the direct Cassini measurements during its close flyby near Enceladus.     

Accumulation processes in the primitive solar nebula

Particle accumulation processes are discussed for a variety of physical environments, ranging from the collapse phase of an interstellar cloud to the different parts of the models of the primitive solar nebula constructed by Cameron and Pine. Because of turbulence in the collapsing interstellar gas, it is concluded that interstellar grains accumulate into bodies with radii of a few tens of centimeters before the outer parts of the solar nebula are formed. These bodies can descend quite rapidly through the gas toward midplane of the nebula, and accumulation to planetary size can occur in a few thousand years. Substantial modifications of these processes take place in the outer convection zone of the solar nebula, but again it is concluded that bodies in that zone can grow to planetary size in a few thousand years.From the discussion of the interstellar collapse phase it is concluded that the angular momentum of the primitive solar nebula was predominantly of random turbulent origin, and that it is plausible that the primitive solar nebula should have possessed satellite nebulae in highly elliptical orbits. It is proposed that the comets were formed in these satellite nebulae.A number of other detailed conclusions are drawn from the analysis. It is shown to be plausible that an iron-rich planet should be formed in the inner part of the outer nebular convection zone. Discussions are given of the processes of planetary gas accretion, the formation of satellites, the T Tauri solar wind, and the dissipation of excess condensed material after the nebular gases have been removed by the T Tauri solar wind. It is shown that the present radial distances of the planets (but not Bode's Law) should be predicted reasonably well by a solar nebula model intermediate between the uniform and linear cases of Cameron and Pine.     

Stochastic Models of Hot Planetary and Satellite Coronas

The hot planetary and satellite coronas are populated by the suprathermal particles produced in the transition region between the collision-dominated and free-molecule atmospheric layers under the external effects of electromagnetic and corpuscular solar radiation and magnetospheric plasma. We construct a numerical stochastic model to investigate both the local formation and kinetics of suprathermal particles and their transport to exospheric heights from underlying atmospheric layers. In contrast to other commonly used approaches, the suggested numerical model is suitable for studying the flows of atmospheric gas weakly and strongly perturbed by suprathermal particles, i.e., for studying the formation of hot planetary and satellite coronas proper. Highly efficient Monte-Carlo algorithms with weighted particles underlie the numerical implementation of the model. This numerical model is used to investigate the following: (i) the hot oxygen corona of Europa, a Jovian satellite, which is an example of a highly nonequilibrium near-surface atmosphere; and (ii) the nonthermal losses of nitrogen from Titan, a Saturnian satellite, when suprathermal atoms and molecules of nitrogen are only a small admixture to the surrounding thermal molecular nitrogen—the main atmospheric component of Titan.     

Comparative study of ion cyclotron waves at Mars, Venus and Earth

Ion cyclotron waves are generated in the solar wind when it picks up freshly ionized planetary exospheric ions. These waves grow from the free energy of the highly anisotropic distribution of fresh pickup ions, and are observed in the spacecraft frame with left-handed polarization and a wave frequency near the ion’s gyrofrequency. At Mars and Venus and in the Earth’s polar cusp, the solar wind directly interacts with the planetary exospheres. Ion cyclotron waves with many similar properties are observed in these diverse plasma environments. The ion cyclotron waves at Mars indicate its hydrogen exosphere to be extensive and asymmetric in the direction of the interplanetary electric field. The production of fast neutrals plays an important role in forming an extended exosphere in the shape and size observed. At Venus, the region of exospheric proton cyclotron wave production may be restricted to the magnetosheath. The waves observed in the solar wind at Venus appear to be largely produced by the solar-wind-Venus interaction, with some waves at higher frequencies formed near the Sun and carried outward by the solar wind to Venus. These waves have some similarity to the expected properties of exospherically produced proton pickup waves but are characterized by magnetic connection to the bow shock or by a lack of correlation with local solar wind properties respectively. Any confusion of solar derived waves with exospherically derived ion pickup waves is not an issue at Mars because the solar-produced waves are generally at much higher frequencies than the local pickup waves and the solar waves should be mostly absorbed when convected to Mars distance as the proton cyclotron frequency in the plasma frame approaches the frequency of the solar-produced waves. In the Earth’s polar cusp, the wave properties of ion cyclotron waves are quite variable. Spatial gradients in the magnetic field may cause this variation as the background field changes between the regions in which the fast neutrals are produced and where they are re-ionized and picked up. While these waves were discovered early in the magnetospheric exploration, their generation was not understood until after we had observed similar waves in the exospheres of Mars and Venus.     

Hydrogen ENA-cloud observation and modeling as a tool to study star-exoplanet interaction

During the previous years spacecraft observations of so-called Energetic Neutral Atoms (ENAs) have become an important remote-sensing technique in planetary science for analyzing the solar wind plasma flow around the upper atmospheric environments of Solar System bodies. ENAs are produced whenever solar- or stellar wind protons interact via charge exchange with a neutral particle from a planetary atmosphere so that their signals constrain both, ion distributions and neutral gas densities. The observation of ENAs which have been generated due to charge exchange with stellar wind plasma have been used for the indirect mass loss and stellar wind property estimation of Sun-like stars by observing the interaction regions carved out by the collisions between stellar winds and the interstellar medium. In this work we review ENA-observations and data interpretations at Solar System planets and recent hydrogen-cloud observations in UV Lyman-α absorption around hydrogen-rich extra-solar gas giants. We discuss the production of stellar wind related hydrogen ENA-clouds around close-in exoplanets and show how a detailed analysis of attenuation spectra obtained for transiting hydrogen-rich close-in gas giants can be used for the study of the upper atmosphere structure, the planet’s magnetosphere and to obtain information on stellar wind properties. Finally, we discuss how future hydrogen cloud observations around exoplanets by space observatories like the Russia-led World Space Observatory-UV (WSO-UV) together with ESAs planned PLATO mission can be used for the reconstruction of the solar wind history or the test of magnetosphere evolution hypotheses.     

The Use of Small Telescopes for Spectral Imaging of Low-light-level Extended Atmospheres in the Solar System

A 10-cm aperture telescope equipped with coronagraphic capabilities, using occulting masks of various size and material, has been developed to obtain low-light-level, wide-angle (~7^o FOV), narrow-band filtered images of sodium exospheres at Io, the Moon and Mercury. Here we describe new instrument capabilities and recent findings about the extraordinarily long tails of sodium gas discovered in the lunar and hermean exospheres. Spatial and temporal variability patterns captured in such images can be used to study changes in surface sputtering processes and radiation pressure acceleration effects in the inner solar system.     

Acceleration of energetic particles of the outer regions of planetary magnetospheres: Inferences from laboratory and space experiments

High energy particles, with energies above those attainable by adiabatic or steady-state electric field acceleration, have been observed in and around the outer regions of planetary magnetospheres. Acceleration by large amplitude sporadic cross-tail electric fields over an order of magnitude greater than steady-state convection fields is proposed as a source of these particles. It is suggested that such explosive electric fields will occur intermittently in the vicinity of the tail neutral line in the expansive phases of substorms. We use laboratory Double Inverse Pinch Device (DIPD) and satellite evidence to estimate this electric potential for substorms at Earth; values of 500 kV to 2 MV are calculated, in agreement with particle observations. It is further suggested that these particles, which have been accelerated in the night side magnetosphere, drift to the dayside on closed field lines, and under certain interplanetary conditions can escape to regions upstream of the bow shock.     

X-rays from solar system objects

During the last few years our knowledge about the X-ray emission from bodies within the solar system has significantly improved. Several new solar system objects are now known to shine in X-rays at energies below 2 keV. Apart from the Sun, the known X-ray emitters now include planets (Venus, Earth, Mars, Jupiter, and Saturn), planetary satellites (Moon, Io, Europa, and Ganymede), all active comets, the Io plasma torus (IPT), the rings of Saturn, the coronae (exospheres) of Earth and Mars, and the heliosphere. The advent of higher-resolution X-ray spectroscopy with the Chandra and XMM-Newton X-ray observatories has been of great benefit in advancing the field of planetary X-ray astronomy. Progress in modeling X-ray emission, laboratory studies of X-ray production, and theoretical calculations of cross-sections, have all contributed to our understanding of processes that produce X-rays from the solar system bodies.At Jupiter and Earth, both auroral and non-auroral disk X-ray emissions have been observed. X-rays have been detected from Saturn's disk, but no convincing evidence of an X-ray aurora has been observed. The first soft (0.1–2 keV) X-ray observation of Earth's aurora by Chandra shows that it is highly variable. The non-auroral X-ray emissions from Jupiter, Saturn, and Earth, those from the disk of Mars, Venus, and Moon, and from the rings of Saturn, are mainly produced by scattering of solar X-rays. The spectral characteristics of X-ray emission from comets, the heliosphere, the geocorona, and the Martian halo are quite similar, but they appear to be quite different from those of Jovian auroral X-rays. X-rays from the Galilean satellites and the IPT are mostly driven by impact of Jovian magnetospheric particles.This paper reviews studies of the soft X-ray emission from the solar system bodies, excluding the Sun. Processes of production of solar system X-rays are discussed and an overview is provided of the main source mechanisms of X-ray production at each object. A brief account on recent development in the area of laboratory studies of X-ray production is also provided.     

Stellar occultation measurements of molecular oxygen in the lower thermosphere

Stellar ultraviolet light near 1500 Å is attenuated in the Earth's upper atmosphere due to strong absorption in the Schumann-Runge continuum of molecular oxygen. The intensity of stars in the Schumann-Runge continuum region has been monitored by the University of Wisconsin stellar photometers aboard the OAO-2 satellite during occultation of the star by the Earth's atmosphere. These data have been used to determine the molecular oxygen number density profile at the occultation tangent point. The results of 14 stellar occultations obtained in low and middle latitudes are presented giving the night-time vertical number density profile of molecular oxygen in the 140–200 km region. In general, the measured molecular oxygen number density is about a factor of 2 lower than the number densities predicted by the CIRA 1965 model. Also, the number density at a given height appears to decrease with decreasing solar activity. Measurements taken at low latitudes during the August 1970 geomagnetic storm showed a decrease in the molecular oxygen number density at a given height several days after the peak of the storm followed by a slow recovery to pre-storm densities.     

Neutral composition changes during a period of increasing magnetic activity

Neutral composition data obtained by the gas analyzer aboard the satellite ESRO 4 are investigated for a period of increasing magnetic activity. The prominent feature observed is the development of localized disturbance zones in the high-and mid-latitude regions which show a significant enhancement in argon and nitrogen densities and a simultaneous decrease in helium densities. The behavior of oxygen within these zones is complex, and both increases and decreases are observed. Both the latitudinal extent and the magnitude of the disturbed regions increase with growing magnetic activity. In contrast the low-latitude region exhibits moderate enhancements of all constituents. Using the AE index as an indicator of magnetic activity, we find that at higher latitudes the atmospheric response time is of the order of one orbital revolution or less. Comparisons with other observations and with a theoretical model by Mayr and Volland (1974) show good agreement.     

Self-consistent modelling of Mercury’s exosphere by sputtering, micro-meteorite impact and photon-stimulated desorption

A Monte-Carlo model of exospheres (Wurz and Lammer, 2003) was extended by treating the ion-induced sputtering process, photon-stimulated desorption, and micro-meteorite impact vaporisation quantitatively in a self-consistent way starting with the actual release of particles from the mineral surface of Mercury. Based on available literature data we established a global model for the surface mineralogy of Mercury and from that derived the average elemental composition of the surface. This model serves as a tool to estimate densities of species in the exosphere depending on the release mechanism and the associated physical parameters quantitatively describing the particle release from the surface.Our calculation shows that the total contribution to the exospheric density at the Hermean surface by solar wind sputtering is about 4×10⁷ m^-3, which is much less than the experimental upper limit of the exospheric density of 10¹² m^-3. The total calculated exospheric density from micro-meteorite impact vaporisation is about 1.6×10⁸ m^-3, also much less than the observed value. We conclude that solar wind sputtering and micro-meteorite impact vaporisation contribute only a small fraction of Mercury’s exosphere, at least close to the surface. Because of the considerably larger scale height of atoms released via sputtering into the exosphere, sputtered atoms start to dominate the exosphere at altitudes exceeding around 1000 km, with the exception of some light and abundant species released thermally, e.g. H₂ and He. Because of Mercury’s strong gravitational field not all particles released by sputtering and micro-meteorite impact escape. Over extended time scales this will lead to an alteration of the surface composition.     

A Hall MHD model to study energetic charged particle events produced during fluctuations in the interplanetary magnetic field intensity

Energetic charged particles, which are often observed in solar active regions, may be also produced in interplanetary space due to the decoupling of ions and electrons in plasma. The Hall term in general Ohm's law is generally thought to be responsible for the decoupling of electrons and ions in plasma during magnetic reconnection. In this paper, a Hall MHD model is developed to study energetic charged particle events produced during fluctuations in the interplanetary magnetic field intensity. Two energetic charged particle events are used to test this model. It is concluded that the Hall effect does not only play the important role in the process of magnetic reconnection, but also in energetic charged particle events produced during fluctuations in the interplanetary magnetic field intensity.     

Auroral zone effects on hydrogen geocorona structure and variability

The instantaneous structure of planetary exospheres is determined by the time history of energy dissipation, chemical, and transport processes operative during a prior time interval set by intrinsic atmospheric time scales. The complex combination of diurnal and magnetospheric activity modulations imposed on the Earth's upper atmosphere no doubt produce an equally complex response, especially in hydrogen, which escapes continuously at exospheric temperatures. Vidal-Madjar and Thomas (1978) have discussed some of the persistent large scale structure which is evident in satellite ultraviolet observations of hydrogen, noting in particular a depletion at high latitudes which is further discussed by Thomas and Vidal-Madjar (1978). The latter authors discussed various causes of the H density depletion, including local neutral temperature enhancements and enhanced escape rates due to polar wind H⁺ plasma flow or high latitude ion heating followed by charge exchange. We have reexamined the enhancement of neutral escape by plasma effects including the recently observed phenomenon of low altitude transverse ion acceleration. We find that, while significant fluxes of neutral H should be produced by this phenomenon in the auroral zone, this process is probably insufficient to account for the observed polar depletion. Instead, the recent exospheric temperature measurements from the Dynamics Explorer-2 spacecraft suggest that neutral heating in and near the high latitude cusp may be the major contributor to depleted atomic hydrogen densities at high latitudes.     

Coronal streamers

The geometrical and dynamical structure of a corona consisting of streamer and interstreamer regions is examined. The present paper is an extension of previous works of this series in that energy transport processes are included in the theoretical framework of the model. Under specified conditions at some reference level above the coronal base, the structure at larger distances is determined by simultaneous integration of the continuity, momentum, and energy equations for each region subject to the condition for a lateral balance of magnetic and gas pressure at all levels. Outward thermal conduction and convection by the solar wind are assumed to be the processes contributing to the energy balance of each region, the magnetic field effectively thermally insulating one region from the other.Numerical results are presented for situations representative of the solar corona. Regions occupied by streamers are found to have higher densities than their surroundings at all distances from the sun. For a given density at the coronal base, the density at the orbit of earth is lower in both the streamer and interstreamer region than that predicted for radial flow. The density enhancement increases outward to a maximum value at a distance of several solar radii. In addition, beyond a distance of a few radii streamers are characterized by higher expansion velocities and lower temperatures than their immediate surroundings. Similar to the case of radial flow, supersonic solutions exist only for base densities below a certain value, which depends upon the specified base temperature and magnetic field distribution. The general features illustrated by these models are expected to persist in the advent of more sophisticated multi-region models.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Organization of Energetic Particles by the Solar Wind Structure During the Declining to Minimum Phase of Solar Cycle 23

We investigate the organization of the low energy energetic particles (≤1 MeV) by solar wind structures, in particular corotating interaction regions (CIRs) and shocks driven by interplanetary coronal mass ejections, during the declining-to-minimum phase of Solar Cycle 23 from Carrington rotation 1999 to 2088 (January 2003 to October 2009). Because CIR-associated particles are very prominent during the solar minimum, the unusually long solar minimum period of this current cycle provides an opportunity to examine the overall organization of CIR energetic particles for a much longer period than during any other minimum since the dawn of the Space Age. We find that the particle enhancements associated with CIRs this minimum period recurred for many solar rotations, up to 30 at times, due to several high-speed solar wind streams that persisted. However, very few significant CIR-related energetic particle enhancements were observed towards the end of our study period, reflecting the overall weak high-speed streams that occurred at this time. We also contrast the solar minimum observations with the declining phase when a number of solar energetic particle events occurred, producing a mixed particle population. In addition, we compare the observations from this minimum period with those from the previous solar cycle. One of the main differences we find is the shorter recurrence rate of the high-speed solar wind streams (～10 solar rotations) and the related CIR energetic particle enhancements for the Solar Cycle 22 minimum period. Overall our study provides insight into the coexistence of different populations of energetic particles, as well as an overview of the large-scale organization of the energetic particle populations approaching the beginning of Solar Cycle 24.     

Solar protons on closed magnetospheric field lines after an interplanetary flux decrease

Particle measurements from the low altitude polar-orbiting satellite GRS-A/Azur and from Explorer 41 in the magnetosheath during a time period after the sudden commencement at 14:30 UT on 8 March 1970, have been used in order to study the access mode of solar particles into the closed field line region of the magnetosphere. A particle decrease in the magnetosheath and over the central polar cap but not in the stable trapping region indicates that solar particles are temporarily trapped and can complete several drifts around the Earth. A single loss cone distribution ～2° inside of the stable trapping region cannot be explained by strong pitch angle scattering but is probably due to non-adiabatic particle motion.     

Cosmological considerations regarding Uranus

The cosmogony of Uranus is discussed within the context of a picture in which solid condensed materials accumulate to form a large body, which then acquires significant amounts of gas from the primitive solar nebula. Of prime cosmogonical importance is the tilt of the equatorial plane of the planet and of the plane of tilt of the planet can easily occur as a result of a major collision during the formation process; it seems most likely that the tilt of the satellite orbits requires that they were formed from a gaseous disc rotating about the planet after the tilt of the planetary rotational axis had occurred. Possible methods for tilting this gaseous disc are discussed. A strong early magnetic field may have helped in this and may have played an essential role in showing down the spin of the planet to the present observed value. These processes may have produced significant compositional differences between the satellites of Uranus and those of Jupiter and Saturn.     

Dust in the solar system and in extra-solar planetary systems

Among the observed circumstellar dust envelopes a certain population, planetary debris disks, is ascribed to systems with optically thin dust disks and low gas content. These systems contain planetesimals and possibly planets and are believed to be systems that are most similar to our solar system in an early evolutionary stage. Planetary debris disks have been identified in large numbers by a brightness excess in the near-infrared, mid-infrared and/or submillimetre range of their stellar spectral energy distributions. In some cases, spatially resolved observations are possible and reveal complex spatial structures. Acting forces and physical processes are similar to those in the solar system dust cloud, but the observational approach is obviously quite different: overall spatial distributions for systems of different ages for the planetary debris disks, as opposed to detailed local information in the case of the solar system. Comparison with the processes of dust formation and evolution observed in the solar system therefore helps understand the planetary debris disks. In this paper, we review our present knowledge of observations, acting forces, and major physical interactions of the dust in the solar system and in similar extra-solar planetary systems.     

Effect of Coulomb collisions on the particle acceleration in collapsing magnetic traps

The problem of particle acceleration in collapsing magnetic traps in the solar corona has been solved by taking into account the particle scattering and braking in the high-temperature plasma of solar flares. The Coulomb collisions are shown to be weak in traps with lifetimes t _l < 10 s and strong for t _l > 100 s. In the approximation of strong collisions, collapsing magnetic traps are capable of confining up to 20% of the injected particles in the corona for a long time. In the collisionless approximation, this value exceeds 90%. The question about the observational manifestations of collisions is examined. For collision times comparable to t _l , the electron spectrumat energies above 10 keV is shown to be a double-power-law one. Such spectra were found by the RHESSI satellite in flares.