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A recently developed time-dependent fractional Parker transport equation is solved to investigate the parallel and momentum superdiffusion of energetic charged particles in an inner heliospheric region containing dynamic small-scale flux ropes (SMFRs). Both types of superdiffusive transport are investigated with fractional transport terms containing a fractional time integral combined with normal spatial or momentum derivatives. Just as for normal diffusion, accelerated particles form spatial peaks with a maximum amplification factor that increases with particle energy. Instead of growth of the spatial peaks until a steady state is reached as for normal diffusion, parallel superdiffusion causes the peaks to dissipate into plateaus followed by a rollover at late times. The peaks dissipate at a faster rate when parallel transport is more superdiffusive. Furthermore, the accelerated particle spectral distribution function inevitably becomes an f 0 ∝ p −3 spectrum at late times in the test particle limit near the particle source despite the potential for spectral steepening from other transport terms. All this is a product of the growing domination of parallel spatial and especially momentum superdiffusion over other transport terms with time. Such extreme late time effects can be avoided by a transition to a normal diffusive state. Finally, fitting spatial peaks observed during SMFR acceleration events with the solution of the fractional Parker transport equation can potentially be used as a diagnostic for estimating the level of spatial and momentum superdiffusion in these events and how the levels of superdiffusion vary with distance from the Sun.

It has been suggested before that small-scale magnetic flux rope (SMFR) structures in the solar wind can temporarily trap energetic charged particles. We present the derivation of a new fractional Parker equation for energetic-particle interaction with SMFRs from our pitch-angle-dependent fractional diffusion-advection equation that can account for such trapping effects. The latter was derived previously in le Roux & Zank from the first principles starting with the standard focused transport equation. The new equation features anomalous advection and diffusion terms. It suggests that energetic-particle parallel transport occurs with a decaying efficiency of advection effects as parallel superdiffusion becomes more dominant at late times. Parallel superdiffusion can be linked back to underlying anomalous pitch-angle transport, which might be subdiffusive during interaction with quasi-helical coherent SMFRs. We apply the new equation to time-dependent superdiffusive shock acceleration at a parallel shock. The results show that the superdiffusive-shock-acceleration timescale is fractional, the net fractional differential particle flux is conserved across the shock ignoring particle injection at the shock, and the accelerated particle spectrum at the shock converges to the familiar power-law spectrum predicted by standard steady-state diffusive-shock-acceleration theory at late times. Upstream, as parallel superdiffusion progressively dominates the advection of energetic particles, their spatial distributions decay on spatial scales that grow with time. Furthermore, superdiffusive parallel shock acceleration is found to be less efficient if parallel anomalous diffusion is more superdiffusive, while perpendicular particle escape from the shock, thought to be subdiffusive during SMFR interaction, is reduced when increasingly subdiffusive.

Case studies show that some energetic particle flux enhancements up to MeV/nuc. observed at 1 AU cannot be treated as a consequence of particle acceleration at shocks or during flares. Atypical energetic particle events (AEPEs) are often detected during crossings of magnetic cavities formed by strong current sheets of various origins in the solar wind. Such cavities confine small-scale magnetic islands (SMIs) produced by magnetic reconnection. SMIs, in turn, trap and re-accelerate energetic particles according to predictions based on the theory of Zank et al. describing stochastic particle energization in the supersonic solar wind via numerous dynamically interacting SMIs. AEPEs possess energies that overlap SEP events and can be an important component in understanding space weather.

Small-scale magnetic flux ropes, in the solar wind, have been studied for decades via the approach of both simulation and observation. Statistical analysis utilizing various in-situ spacecraft measurements is the main observational approach. In this study, we extend the automated detection of small-scale flux ropes based on the Grad-Shafranov reconstruction to the complete dataset of \emph{Ulysses} spacecraft in-situ measurements. We first discuss the temporal variation of the bulk properties of 22,719 flux ropes found through our approach, namely, the average magnetic field and plasma parameters, etc., as functions of the heliographical latitudes and heliocentric radial distances. We then categorize all identified events into three groups based on event distributions in different latitudes separated at 30$^{\circ}$, at different radial distances, and under different solar activities, respectively. By the detailed statistical analysis, we conclude as follows. (1) The properties of flux ropes, such as the duration, scale size, etc., follow the power-law distributions, but with different slope indices, especially for distributions at different radial distances. (2) Also, they are affected by the solar wind speed which has different distributions under the different solar activities, which is manifested as the latitudinal effect. (3) The main difference in flux rope properties between the low and high latitudes is attributed to possible Alfv\'enic structures or waves. (4) Flux ropes with longer duration and larger scale sizes occur more often at larger radial distances. (5) With more strict Wal\'en slope threshold, more events are excluded at higher latitudes. The entire database is published online at \url{http://www.fluxrope.info}., Comment: submitted to ApJ (1 May)

Small-scale magnetic flux ropes, most with duration ≤ 1 hour at 1 AU, are found to be ubiquitous in the solar wind from in-situ spacecraft measurements. We have built an event database (fluxrope.info) for these structures identified and summarized their main physical properties. Quantitative analysis provided strong evidence in support of the view of their generation through turbulence cascade processes in space plasmas. We extend such analysis to spacecraft measurements at larger radial distances, mainly those from the Ulysses spacecraft mission. We perform comparison of relevant properties of these structures and reveal their radial evolution near the ecliptic plane. Meanwhile we also try to derive, quantitatively, the parameters important for the underlying processes including magnetic reconnection, intrinsic to turbulence cascade, in order to aid in relevant theoretical investigations. We also supplement the largely statistical analysis results with individual case studies to illustrate the process of flux rope merging and associated particle energization signatures through an observational approach. We speculate on the implications of our results and foresee future investigations that can help improve our current understanding of the origin and evolution of these small-scale magnetic flux ropes throughout the heliosphere.

Recent studies of particle acceleration in the heliosphere have revealed a new mechanism that can locally energize particles up to several MeV nucleon–1. Stream–stream interactions, as well as the heliospheric current sheet (CS)—stream interactions, lead to formation of large magnetic cavities, bordered by strong CSs, which in turn produce secondary CSs and dynamical small-scale magnetic islands (SMIs) of ~0.01 au or less owing to magnetic reconnection. It has been shown that particle acceleration or reacceleration occurs via stochastic magnetic reconnection in dynamical SMIs confined inside magnetic cavities observed at 1 au. The study links the occurrence of CSs and SMIs with characteristics of intermittent turbulence and observations of energetic particles of keV–MeV nucleon–1 energies at ~5.3 au. We analyze selected samples of different plasmas observed by Ulysses during a widely discussed event, which was characterized by a series of high-speed streams of various origins that interacted beyond Earth's orbit in 2005 January. The interactions formed complex conglomerates of merged interplanetary coronal mass ejections, stream/corotating interaction regions, and magnetic cavities. We study properties of turbulence and associated structures of various scales. We confirm the importance of intermittent turbulence and magnetic reconnection in modulating solar energetic particle flux and even local particle acceleration. Coherent structures, including CSs and SMIs, play a significant role in the development of secondary stochastic particle acceleration, which changes the observed energetic particle flux time–intensity profiles and increases the final energy level to which energetic particles can be accelerated in the solar wind

In August of 2012 the Voyager 1 space probe has left the solar-wind bubble of ionized gas we call the heliosphere and entered the denser and colder environment of the interstellar cloud surrounding the solar system. Energetic charged particles underwent dramatic changes past the heliopause: the heliospheric ions disappeared completely, while the galactic cosmic rays were for the first time measured in their unmodulated state. The interstellar medium turned out to be almost entirely devoid of turbulent magnetic fluctuations, therefore the transport of cosmic rays is governed by a large-scale geometry of the magnetic field. We discuss observations of heliospheric ions, including anomalous cosmic rays, near the heliopause transition, and propose interpretations of the measured intensities and pitch-angle distributions based on gradient drift in a weakly nonuniform magnetic field. The heliopause transition appears to be permeated by magnetic flux tubes connected to the interstellar space and facilitating particle escape. These flux tubes may be a product of interchange instability driven by a plasma pressure gradient across the heliopause. The curvature of magnetic field lines and the anti-sunward gradient in plasma kinetic pressure provide conditions favorable for an interchange. The two flux tube crossings by the spacecraft allowed an indirect measurement of the plasma radial velocity near the heliopause.

We have developed a new automated small-scale magnetic flux rope (SSMFR) detection algorithm based on the Grad-Shafranov (GS) reconstruction technique. We have applied this detection algorithm to the Wind spacecraft in-situ measurements during 1996 - 2016, covering two solar cycles, and successfully detected a total number of 74,241 small-scale magnetic flux rope events with duration from 9 to 361 minutes. This large number of small-scale magnetic flux ropes has not been discovered by any other previous studies through this unique approach. We perform statistical analysis of the small-scale magnetic flux rope events based on our newly developed database, and summarize the main findings as follows. (1) The occurrence of small-scale flux ropes has strong solar cycle dependency with a rate of a few hundreds per month on average. (2) The small-scale magnetic flux ropes in the ecliptic plane tend to align along the Parker spiral. (3) In low speed ($, Comment: ApJS, in press; containing electronic spreadsheet of the database

We have quantitatively examined one type of fundamental space plasma structures in the solar wind, the magnetic flux ropes, especially those of relatively small scales. They usually are of durations ranging from a few minutes to a few hours. The main objectives are to reveal the existence in terms of their occurrence and distributions in the solar wind, to quantitatively examine their configurations and properties, and to relate to other relevant processes, involving particle energization and intermittent structures in the solar wind. The technical approach is a combination of time-series analysis methods with the Grad-Shafranov reconstruction technique. This modeling method is capable of characterizing two and a half dimensional cross section of space plasma structures, based on in-situ spacecraft measurements along a single path across. We present the automated detection of flux ropes, construction of an online magnetic flux rope database, and detailed case studies of such structures identified downstream of interplanetary shocks.

The transport of charged energetic particles in the presence of strong intermittent heliospheric turbulence is computationally analyzed based on known properties of the interplanetary magnetic field and solar wind plasma at 1 Astronomical Unit (AU). The turbulence is assumed to be static, composite, and quasi-three-dimensional with a varying energy distribution between a one-dimensional Alfv\'enic (slab) and a structured two-dimensional component. The spatial fluctuations of the turbulent magnetic field are modeled either as homogeneous with a Gaussian probability distribution function (PDF), or as intermittent on large and small scales with a q-Gaussian PDF. Simulations showed that energetic particle diffusion coefficients both parallel and perpendicular to the background magnetic field are significantly affected by intermittency in the turbulence. This effect is especially strong for parallel transport where for large-scale intermittency results show an extended phase of subdiffusive parallel transport during which cross-field transport diffusion dominates. The effects of intermittency are found to depend on particle rigidity and the fraction of slab energy in the turbulence, yielding a perpendicular to parallel mean free path ratio close to 1 for large-scale intermittency. Investigation of higher order transport moments (kurtosis) indicates that non-Gaussian statistical properties of the intermittent turbulent magnetic field are present in the parallel transport, especially for low rigidity particles at all times., Comment: 54 pages, 16 figures, accepted for publication in ApJ

Recent spacecraft observations reveal that energetic tails in ion spectra are present continually in the slow low-latitude solar wind, even in the absence of nearby shocks. To explain this phenomenon we take into account growing evidence that MHD turbulence in the solar wind has a strong two-dimensional (2D) component. 2D MHD turbulence simulations exhibit large-scale field-aligned electric field fluctuations which are also observable in the solar wind. We explore pickup ion propagation and acceleration between the Sun and Earth with a newly developed kinetic numerical model which includes a random distribution of large-scale field-aligned electric field fluctuations consistent with observations in the low-latitude solar wind. The model is found to qualitatively reproduce the accelerated interstellar pickup He+ spectra observed at Earth. Like the observations, the simulated accelerated “tail” is approximately a power law f(υ) ∝ υ−4.5 until it reaches particle speeds about ten times the solar wind speed where a rollover occurs.

We study the physics of locally born interstellar pickup proton acceleration at the nearly perpendicular solar wind termination shock (SWTS) in the presence of a random magnetic field spiral angle using a focused transport model. Guided by Voyager 2 observations, the spiral angle is modeled with a q-Gaussian distribution. The spiral angle fluctuations, which are used to generate the perpendicular diffusion of pickup protons across the SWTS, play a key role in enabling efficient injection and rapid diffusive shock acceleration (DSA) when these particles follow field lines. Our simulations suggest that variation of both the shape (q-value) and the standard deviation (σ-value) of the q-Gaussian distribution significantly affect the injection speed, pitch-angle anisotropy, radial distribution, and the efficiency of the DSA of pickup protons at the SWTS. For example, increasing q and especially reducing σ enhances the DSA rate.

The diffusion coefficients, both parallel and perpendicular to the background magnetic field, are calculated using test particle simulations of the transport of energetic particles in the heliosphere. We consider strong static, low-frequency solar wind turbulence. Two models for the magnetic field turbulence are examined: slab turbulence with and without intermittency of the turbulence amplitude. Results show that the diffusion coefficients parallel and perpendicular to the background field are weakly dependent on the power index (q) of the probability distribution function of the intermittency within the chosen interval 1.68 ≤ q ≤ 1.8. The diffusion coefficients however, show a strong dependence on the scale length of the intermittency, and are strongly reduced for smaller scale intermittency.

In recent years, a phenomenological solar wind heating model based on a turbulent energy cascade prescribed by the Kolmogorov theory has produced reasonably good agreement with observations on proton temperatures out to distances around 70 AU, provided the effect of turbulence generation due to pickup ions is included in the model. In a recent study [Ng et al., J. Geophys. Res., 115, A02101 (2010)], we have incorporated in the heating model the energy cascade rate based on Iroshnikov-Kraichnan (IK) scaling. We showed that the IK cascade rate can also produce good agreement with observations, with or without the inclusion of pickup ions. This effect was confirmed both by integrating the model using average boundary conditions at 1 AU, and by applying a method [Smith et al., Astrophys. J., 638, 508 (2006)] that uses directly observed values as boundary conditions. The effects due to pickup ions is found to be less important for the IK spectrum, which is shallower than the Kolmogorov spectrum. In this paper, we will present calculations of the pickup ions effect in more details, and discuss the physical reason why a shallower spectrum generates less waves and turbulence.

The combination of the Interstellar Boundary Explorer (IBEX) all‐sky maps of the energetic neutral atom fluxes with the Voyager in situ measurements gives us a unique opportunity to learn more about the physics governing the solar wind (SW) interaction with the local interstellar medium (LISM). Moreover, since the position of the ribbon of an enhanced ENA flux in the sky strongly depends on the LISM properties, we are able to constrain those by comparing numerical simulations with the IBEX observations. In this paper, we discuss the current status of the Huntsville model of the SW‐LISM interaction, compare numerical results with the IBEX and Voyager observations, and discuss the importance of taking into account time‐dependent phenomena, particularly the solar cycle effects.

Comets provide a wonderful laboratory for studying the ion pickup process. As a comet nears the Sun, neutral atoms and molecules sublime from the nucleus and from ice grains in the coma, providing an extended water‐rich region. Photoionisation and charge exchange provide a source of new pickup ions, which immediately interact with the electric and magnetic field in the surrounding plasma. The missions to comets Halley, Giacobini‐Zinner, Grigg‐Skjellerup and Borrelly provided a wealth of data on pickup ions and their interaction with plasma waves, including observations of ring and bispherical shell distributions and mass loading over significant regions of space. At comet Grigg‐Skjellerup, non‐gyrotropic pickup ions were also seen. Here, we review the observations and interpretation of pickup ions at comets, illustrating what was learned from the cometary missions and following the evolution of the ion distribution. We also discuss the international Rosetta mission which will orbit rendezvous with and the...

The relative motion of the Sun through the Local Interstellar Cloud (LIC) leads to a neutral wind through the heliosphere. Because of its high ionization potential, He remains neutral to well within 1 AU, where it is deflected by the Sun’s gravity and forms a focusing cone on the downwind side. This flow pattern has been studied with UV backscattering, through pickup ions (PUI), and atom imaging. A consolidated set of the physical parameters of He in the LIC has been derived combining all three methods. However, it is still poorly understood why PUI fluxes and velocity distributions vary substantially on temporal scales from hours to many days, which leads among other phenomena to apparent changes in the appearance of the focusing cone, even after averaging over several days. With the combination of PLASTIC on STEREO A and B as well as SWICS on ACE, simultaneous PUI observations over an increasing range of heliospheric longitudes have become possible, for which we have initiated a cross‐calibration effort...

A short survey of work done on the interaction of the interstellar medium with the solar wind in view of the Voyagers data and observations of the Interstellar Boundary Explorer (IBEX) is made. The paper reviews several heliospheric numerical models presented in the last decades. Some philosophical questions concerning current possibilities of modeling are raised, in particular, to what extent it is necessary to complicate models to reproduce the nature.

This paper outlines the present status of the kinetic theory of turbulence in magnetized plasmas as being developed by the present author. The systematic program to formulate the theory of turbulence starting from the Vlasov‐Klimontovich formalism began with the works by pioneers of modern plasma physics in the 1960s and 1970s. However, early efforts adopted the heuristic semi‐classical method instead of the statistical mechanical formulation, which is necessary for a quantitative analysis. Recently, the present author picked up where the early pioneers left, and began to reformulate the kinetic turbulence theory of turbulence in magnetized plasmas from statistical mechanical formalism. This paper is a brief outline of the progress to date.

The particle instrument CELIAS/HSTOF onboard SOHO is observing the energetic neutral atom (ENA) flux in the ecliptic plane since 1996. During quiet time periods, the ENA flux is well separated from the energetic ion flux, originating in the solar corona and/ or heliosphere. The instrument is capable to measure the energetic neutral hydrogen (ENH) and helium (ENHe) fluxes along the line‐of‐sight of the instrument field‐of‐view. We will discuss the neutral energetic flux measurements of hydrogen and helium in the 28 to 88 keV/n energy/mass range of CELIAS/HSTOF, focusing on the frequently increased fluxes from the tail direction of the heliosphere. Potential source regions might be co‐rotating interaction regions (CIRs) and the helium cone in the inner heliosphere, solar energetic particle events, accelerated pick‐up ions in the outer heliosphere and energetic neutral particles originating from the heliosheath region and beyond.

Ions created from the neutral comae of active comets are picked up by the solar wind and carried downstream to form induced magnetotails. These are observed remotely as comets’ ion or plasma tails. Here, we review the magnetic field structure of these tails, and summarize remote and in situ observations that have helped our understanding of them. Magnetometer measurements by spacecraft have confirmed the predicted two‐lobe magnetic field structure, but this is complicated by variations in the heliospheric magnetic field, HMF, local to the comet. The tails formed at comets close to the Sun appear to be surrounded by a region of the solar wind where the HMF has draped around the magnetotail structure itself. Candidate weakened shock structures on either side of magnetotail traversals indicate that the regions affected by an active comet’s presence often occupies a significant portion of the inner heliosphere.

Interstellar pickup ions, in scattering to isotropy, should generate waves in particular frequency ranges, but observations of such waves are very rare. To date, only proton‐generated waves at Ulysses have been reported, and only sporadically during times of quasi‐radial field. Here, we present two sets of wave observations. The first set are waves due to pickup H+ seen by the Ulysses spacecraft as part of a new study to gather and build a data base of observations that can better be questioned regarding the fundamental physics of wave generation by interstellar pickup ions. There we demonstrate that the waves may have small signatures in the power spectrum while showing strong evidence of wave‐particle dynamics in the polarization spectra. The second set is a spectral enhancement measured at the Voyager 2 spacecraft during 4.5 hours on DOY 7, 1979 at a heliocentric radial position of 4.5 AU. The large‐scale IMF was nearly radial, and the frequency range and polarization of the enhanced fluctuations are c...

Two of the most recent outer heliospheric kHz emissions detected by the University of Iowa plasma wave detector on Voyager 1 started at about 2004.64 and 2006.39, respectively. Large interplanetary shocks reached V1 and V2, which are near the heliospheric termination shock, at almost the same time that the two kHz radio emissions turned‐on. So, in fact, the arrivals of these large shocks near the heliospheric termination shock were coincident with the onset of these kHz emissions. These two large shocks were unusual in that they were the two strongest shocks in solar cycle 23 seen in the outer heliosphere at V2. They had developed maximum dynamic pressures ∼4–6 times the average solar wind dynamic pressure for periods >26 days by the time they reached V2.

We show realistic and time‐varying 3D MHD models of the outer heliosphere which satisfy both Voyager 1 (V1) and Voyager 2 (V2) observed crossing times of the termination shock (TS) simultaneously. The short‐term variations found are a) the TS position increases whenever a solar‐wind high‐ram pressure pulse collides with the TS, b) a large amplitude magneto‐sonic pulse is generated downstream of the TS when a solar‐wind high ram pressure pulse collides with the TS, c) the generated pulse propagates outward in the heliosheath and is reflected at the plasma sheet, and d) when the reflected pulse collides with the TS, the TS position decreases. We also present preliminary results of galactic cosmic rays (GCRs) trajectories as they respond to three‐dimensional global electric and magnetic fields in the outer heliosphere. This allows us to investigate (1) how GCRs cross the heliosphere and enter the inner heliosphere, and (2) their long‐term variation. Preliminary GCR distributions in the outer heliosphere are ...

In this paper we focus on hybrid simulations, which were performed in order to support data analysis and interpretation of Voyager data and most recent IBEX observations. Two‐dimensional multi‐species hybrid simulations for the termination shock have been performed in which we used almost realistic plasma parameters. In these simulations we included self‐consistently solar wind protons and pickup protons. We investigated the change of the turbulence around and at the shock front as well as spatial and temporal evolution of the solar wind proton distribution under the impact of pickup protons. The results of this study show that pickup ions have a significant impact. Pickup ions mediate the shock turbulence, ion reflection, thermalization, and also slow down incoming solar wind. Furthermore, these simulations also show that gyrating solar wind and pickup ions may be a significant source for Energetic Neutral Atoms (ENAs).

Studies of reconnection in the solar wind led by Gosling and collaborators have revealed surprising results that are posing serious challenges to current theoretical understanding of the reconnection process. These include presence of prolonged quasi‐steady reconnection, low magnetic shear angles, and no substantial particle acceleration. Here we put forth the conjecture that many of the solar wind exhaust events may be fossil sites. We explore the viability of this possibility using full particle simulations.

Through ejection from its southern hemisphere, Enceladus is a dominant source of neutral gas and dust in Saturn’s inner magnetosphere. The interaction of the corotating plasma with the gas and dust modifies the plasma environment around Enceladus. We use 3‐D hybrid (kinetic ions, fluid electrons) simulations to examine the effects of gas and dust on the nature of the interaction region and use Cassini observations to constrain their properties.

The structure of the heliosphere is strongly affected by the pickup protons produced by the photoionization of the interstellar neutral hydrogen and by the charge exchange of the hydrogen atoms with solar wind protons. In the distant heliosphere (r>5–10 AU), the thermal pressure of pickup protons is higher than the solar‐wind proton and electron pressures, and the energy and momentum redistribution between solar‐wind and pickup protons leads to a deceleration of the solar wind flow and to an increase of average plasma temperature with heliocentric distance. The deceleration effect, combined with the additional plasma pressure from the pickup protons, acts to weaken the corotating interaction regions (CIRs). It causes, however, an overall compression of the (mostly azimuthal) interplanetary magnetic field. Using a three‐dimensional magnetohydrodynamic solar wind model that describes the pickup protons as a separate fluid, we have simulated the magnetic field and plasma distribution throughout the heliosphe...

We apply test‐particle numerical simulations to model the acceleration of inner‐source C+ by a strong interplanetary shock. The model consists of three distinct parts: (a) the distribution of inner‐source pickup ions; (b) the initial acceleration of pickup ions (to produce seed particles) for a kinematically defined interplanetary shock; and (c) the acceleration to higher energies and the resulting event‐averaged energy spectrum seen by an observer at 1 AU based on the solution to the Parker transport equation for a propagating shock. Our results are compared with observational estimates of the upper bound on the spectrum of C+ ions associated with the Aug. 25, 1998 solar‐energetic particle event. Our calculated spectrum is very close to this upper‐bound estimate, suggesting that the intensity of accelerated C+ is just at the sensitivity of measurement.

The ion pick‐up near the icy Galilean satellites is studied using ion cyclotron waves. Using Galileo magnetometer data, we show evidence for the existence of ion cyclotron waves, which are generated by pick‐up of freshly ionized particles. Near Europa, in the wake various kinds of ions are detected, which were already predicted to be present on the moon. Upstream of the moon there is evidence for water ion pick‐up, which could facilitate the slow down of the plasma flow. Ganymede shows evidence for either water or oxygen pick up on the flanks of the magnetosphere. Callisto shows indication of hydrogen pick‐up from its atmosphere.

The Rankine‐Hugoniot (R‐H) jump conditions at the heliospheric termination shock provide a means of knitting together the in situ measurements from Voyager 2 (VGR2) with the remote sensing of the heliosheath plasma via energetic neutral atom (ENA) imaging by IBEX and Cassini/INCA. The VGR2 instrument suite has a gap (∼1–30 keV) in the ion measurements. While the ENA images (0.2–6 keV and 5–55 keV) fill the VGR2 gap in the pixel containing the VGR2 spacecraft, they do so only in the sense that they provide the ion intensity integrated along the radial line of sight throughout the entire heliosheath. The synthesis we attempt is further complicated by the observational results from all three spacecraft that the non‐thermal component of the ion pressure dominates that of the thermal component. We therefore have developed (and applied) a generalized formulation of the R‐H conditions that does not invoke an equation of state, but rather can directly ingest the instrumentally‐measured non‐thermal spectrum. The r...

We present results on the recurrence of High Energy Ions (HEIs) in the V2 data about 90 days after the initial sunward‐moving termination shock (TS) crossings of V2 in Aug.–Sept. 2007. We associate the HEIs in Nov.–Dec. 2007 with the outward motion of the TS or with a ripple in the TS so that the TS is again near V2. Comparisons of the timings of the recurrence of the HEI detections and the simultaneous V2 convective plasma, energetic particle, and magnetic field data indicate that the variations in all these V2 data sets are consistent with the TS re‐approach to V2 in Nov.–Dec. 2007. We use our three‐dimensional (3D) kinematic HAFSS model to investigate whether the timing of the arrival at V2 of the increase in solar wind dynamic pressure associated with the December 2006 solar events could have been responsible for the dynamic pressure pulse that moved the TS outward toward V2 ∼90 days after the initial TS crossings of V2. This explanation or, alternatively, a 3D solar wind inhomogeneity‐caused TS ripple (on the scale of ∼1 AU) could be viable scenarios to explain the recurrence of the HEIs in the V2 data in Nov.–Dec. 2007.

Nearly all the pickup ions begin their lives as interstellar neutrals flowing into the heliosphere. Their flux and their interactions with the outer heliosphere depend on the nature of the interstellar medium (ISM) that surrounds heliosphere. This very local ISM is part of the Local Interstellar Cloud, which is believed to surround the Solar System. While we have more information about this cloud than any other region of the ISM, many uncertainties remain. I summarize our current knowledge about the cloud including the uncertainties and their possible impact on models for the heliosphere and pickup ions.

Inner source pickup ions are thought to originate from the interaction of solar wind ions with interplanetary dust grains in the inner heliosphere. Processes which produce inner source pickup ions, and which have been considered so far are implantation of solar wind on grains and subsequent desorption, charge exchange of solar wind ions during transit through submicron dust grains, sputtering and backscattering of ions. A large fraction if not all of the dust crossing the sphere of the Earth’s orbit must end up as pickup ions as is evidenced from the comparable order of magnitude of dust flux inward and pickup ion flux outward at 1 AU. This suggests that the ultimate fate for a large fraction of small interplanetary dust particles after evaporation or sputtering is conversion into pickup ions. Sputtering becomes particularly efficient when dust particles—after fragmentation by collisions with each other—have diminished to sizes comparable to the range of solar wind ions in dust material. The sputter produ...

Interstellar pick‐up ion and solar wind spectra were measured with New Horizons/SWAP between 11 and 12 AU. We report on detailed fitting of these spectra. To perform the fitting, we have constructed an empirical model that includes interstellar pick‐up ions as well as solar wind H+, solar wind He2+, and solar wind O6+. A fifth population is needed to complete the fitting; we use inner source pick‐up protons. We find a correlation between the densities of the solar wind H+, and all other populations. The two strongest correlations are with the interstellar pick‐up ions and the inner source pick‐up ions. We discuss the correlation between solar wind and interstellar pick‐up ion densities.

Maps of energetic neutral atoms (ENA) of the heliosphere from Cassini [1] have been constructed spanning the energy range ∼5≤E≤55 keV, and show a “Belt” in the sky of ∼100° FWHM. Similarly, maps >6 keV have been obtained by the IBEX mission [2] and show a “Ribbon” that is narrower than the Belt and inclined to it in both ecliptic latitude (∼25°) and longitude (∼30°). The overlap in energy between Voyager ions [3] and Cassini ENA intensities (averaged over the ENA line of sight) enables us to deduce ion fluxes in the heliosheath, thus providing a continuous spectrum 5≤E≤4000 keV. These measurements are then used to estimate the local partial pressure over this energy range (∼0.1 pPa), suggesting β>25 [4] and the thickness of the heliosheath (∼50 AU). Using a simulated PUI distribution [5], we estimate the E

The energetic neutral atoms (ENAs) that the Interstellar Boundary Explorer (IBEX) is currently studying are messengers from the termination shock and beyond. Ultraviolet spectra from the Hubble Space Telescope (HST) provide another way to study these ENAs, which are capable of producing detectable absorption signatures in HST Lyman‐α spectra of nearby stars. This broad, shallow absorption is only observed within 20° of the downwind direction. Only the lengthy downwind lines of sight through the long heliotail build up enough column density of ENAs to yield detectable absorption. The absorption therefore represents the first real observational detection of the heliotail. We try to connect ENA fluxes observed by IBEX with the Lyman‐α absorption observed by HST. In the downwind direction, IBEX observes ENA fluxes that increase towards lower energies, at least to 0.2 keV, but consistency with the HST measurements seems to require that the ENA fluxes at least flatten if not decrease below 0.2 keV. The “ribbon” of ENAs detected by IBEX is not detected in Lyman‐α absorption, which may be a problem for any explanation of the ribbon that proposes a source beyond our heliosphere.

Here we use theoretical considerations to compute the ion‐electron temperature equilibration rate in Saturn’s inner magnetosphere. We find that the timescale for electrons to equilibrate to the observed temperature with the resident ions at 8 Rs (Rs = one Saturn radius ∼60300 km) via Coulomb collisions alone is ∼70 hours. We estimate the rate of electron production and find that it is very slow, a maximum of ∼0.19 electrons cm−3hr−1 at 8 Rs. The reported electron density at 8 Rs is ∼10 cc−1, implying that the resident electrons are stable against transport for on the order of 53 hours (the time taken to accumulate 10 electrons per cm3 neglecting transport in or out of the region) and are therefore stationary long enough to heat to the observed temperature via this mechanism. Our results are consistent with a simple scenario wherein pick up electrons equilibrate to close to the local pick up proton temperature in situ via Coulomb collisions implying that the cold electron component in Saturn’s inner magnet...

Hot oxygen ions produced at Mars due to dissociative recombination can be picked up by the solar wind electric field and removed from the planet. An account is provided here of early pioneering observations of pickup O+ ions at Mars during the Phobos‐2 mission and of how their identity was confirmed by modeling. Also it is shown that the interaction of the solar wind with the planet compositely involves the physics underlying three classical interaction archetypes in which the obstacle presented to the solar wind is: comet like (resulting in erosion and mass loading); Venus like (characterized by ionospheric thermal pressure) and Earth like (resulting from the presence of a magnetic field).

NASA’s Interstellar Boundary Explorer (IBEX) mission observed an unexpected band of enhanced energetic neutral atom (ENA) flux in its first all‐sky maps. In this paper we discuss a possible mechanism that may explain this effect, and quantitatively test it using a 3D simulation of the global heliosphere. We describe the mechanism, and how the concept of a partial shell distribution can be used to approximate weak wave‐particle scattering by PUIs that form the parent population of ENAs in the model ribbon. We also introduce a simple approximation to a solar cycle for primary ENAs born in the supersonic solar wind, and find that this has the effect of spreading out a bump in the model ENA spectrum centered on the solar wind energy.

Fast supernova remnant shocks in partially neutral gas should produce a population of pickup ions from neutrals that pass through the shock. In favorable cases, these pickup ions may be detectable as a departure from a Gaussian profile in the Hα line. This paper discusses models of the profiles, along with the Hα profile of a shock in Tycho’s Supernova Remnant. The distortion of that profile could be interpreted as an indication of a pickup ion population, but several other interpretations are possible.

During its first year of operations the IBEX satellite discovered a nearly circular ribbon of enhanced fluxes of energetic neutral atoms (ENAs) with energies from 0.2 keV to ∼3 keV. One recent model proposes that the ribbon consists of secondary ENAs produced in the outer heliosheath by charge exchange of PUIs from heliosheath and solar wind ENAs with interstellar neutral hydrogen. This mechanism relies on a long lifespan (>1 year) of the PUI partial shell, something that is difficult to justify in view of the expected instability of such distributions. We analyze stability properties of PUI ring‐beams produced from solar wind ENAs using a simple linear Vlasov theory and more sophisticated hybrid simulations that trace nonlinear interactions of ions with the self‐excited waves. Dispersion analyses produce exponential growth rates of resonant waves of the order of the ion gyrofrequency. We show that many kinds of PUI rings (narrow, broad, and wide) are intrinsically unstable, and that scattering isotropizes them on timescales of days.