Pickup Ions Research Articles

Oblique Electromagnetic Instabilities Driven by Pickup Ion Ring-beam Distributions in the Outer Heliosheath. I. Linear Instability Analysis

The energetic neutral atom ribbon observed by the Interstellar Boundary Explorer spacecraft is believed to originate from the pickup ions in the outer heliosheath. The outer heliosheath pickup ions generally have a ring-beam velocity distribution at a certain pickup angle, α, the angle at which these ions are picked up by the interstellar magnetic field. The pickup ion ring-beam distributions can drive unstable waves of different propagation angles with respect to the background interstellar magnetic field, θ. Previous studies of the outer heliosheath pickup ion dynamics were mainly focused on ring-like pickup ion distributions with α ≈ 90° and/or the parallel- and antiparallel-propagating unstable waves (θ = 0° and 180°). The present study carries out linear kinetic instability analysis to investigate both the parallel and oblique unstable modes (0° ≤ θ ≤ 180°) driven by ring-beam pickup ion distributions of different pickup angles between 0° and 90°. Our linear instability analysis reveals that ring-beam pickup ion distributions can excite oblique mirror waves as well as parallel/quasi-parallel and oblique right- and left-helicity waves. The maximum growth rate among all the instabilities belongs to the parallel-propagating left-helicity waves at most pickup angles. Furthermore, the evolution of the unstable mirror waves by varying pickup angle indicates that as the pickup angle increases, the maximum growth rate of the mirror modes increases, while its propagation angle decreases.

Shock-wave heating mechanism of the distant solar wind: Explanation of Voyager-2 data

Aims. One of the important discoveries made by Voyager-2 is the nonadiabatic radial profile of the solar wind proton temperature. This phenomenon has been studied for several decades. The dissipation of turbulence energy has been proposed as the main physical process responsible for the temperature profile. The turbulence is both convected with the solar wind and originated in the solar wind by the compressions and shears in the flows and by pick-up ions. The compression source of the solar wind heating in the outer heliosphere appears due to shock waves, which originated either in the solar corona or in the solar wind itself. The goal of this work is to demonstrate that the shock-wave heating itself is enough to explain the temperature profile obtained by Voyager-2. Methods. The effect of shock-wave heating is demonstrated in the frame of a very simple spherically symmetric high-resolution (in both space and time) gas-dynamical data-driven solar wind model. This data-driven model employs the solar-wind parameters at 1 AU with minute resolution. The data are taken from the NASA OMNIWeb database. It is important to underline that (1) the model captures the shocks traveling and/or originating in the solar wind, and (2) other sources of heating are not taken into account in the model. We extended this simple model to the magnetohydrodynamic (MHD) and two-component models and found very similar results. Results. The results of the numerical modeling with the one-minute OMNI data as the boundary condition show very good agreement with the solar-wind temperature profiles obtained by Voyager-2. It is also noteworthy that the numerical results with daily averaged OMNI data show a very similar temperature profile, while the numerical runs with 27-day-averaged OMNI data demonstrate the adiabatic behavior of the temperature.

A Comprehensive Model for Pickup Ion Formation at the Moon

AbstractThe lunar exosphere is an ensemble of multiple overlapping, noninteracting neutral distributions that reflect the primary physical processes acting on the lunar surface. While previous observations have detected and constrained the behavior of some species, many others have only circumstantial evidence or theoretical modeling suggesting their presence. Many species are so tenuous as to be unobservable by direct neutral sampling, yet in comparison, measurements in their ionized form provide a particularly sensitive method of detection. To better aid the interpretation of past measurements and planning of future observations, we present a model for the production of lunar pickup ions from the Moon consisting of two components: An analytic model for the distributions of 18 neutral species produced by various mechanisms and an analytic model for the ionization and subsequent acceleration of 20 exospheric and surface‐sputtered pickup ion species. The dominant lunar pickup ions in the model are , He+, CO+, 40Ar+, Al+, Na+, K+, Si+, Ca+, and O+ with an asymmetric distribution favoring the positive interplanetary electric field hemisphere of the Moon. We compare the model predictions to statistically averaged pickup ion fluxes around the Moon as observed by the ARTEMIS spacecraft over the past decade. By filtering for interplanetary electric field‐aligned, high‐energy observations, we find that the pickup ion model lacks an additional source of heavy species. We suggest that a dense CO2 exosphere of 3 × 104 − 1 × 105 cm−3 could account for the missing pickup ion flux as part of the recycling of solar wind carbon ions incident to the Moon.

Explanation of Heliospheric Energetic Neutral Atom Fluxes Observed by the Interstellar Boundary Explorer

Interstellar neutral atoms propagating into the heliosphere experience charge exchange with the supersonic solar wind (SW) plasma, generating ions that are picked up by the SW. These pickup ions (PUIs) constitute ∼25% of the proton number density by the time they reach the heliospheric termination shock (HTS). Preferential acceleration of PUIs at the HTS leads to a suprathermal, kappa-like PUI distribution in the heliosheath, which may be further heated in the heliosheath by traveling shocks or pressure waves. In this study, we utilize a dynamic, 3D magnetohydrodynamic model of the heliosphere to show that dynamic heating of PUIs at the HTS and in the inner heliosheath (IHS), as well as a background source of energetic neutral atoms (ENAs) from outside the heliopause, can explain the heliospheric ENA signal observed by the Interstellar Boundary Explorer (IBEX) in the Voyager 2 direction. We show that the PUI heating process at the HTS is characterized by a polytropic index larger than 5/3, likely ranging between γ ∼ 2.3 and 2.7, depending on the time in solar cycle 24 and SW conditions. The ENA fluxes at energies >1.5 keV show large-scale behavior in time with the solar cycle and SW dynamic pressure, whereas ENAs < 1.5 keV primarily exhibit random-like fluctuations associated with SW transients affecting the IHS. We find that ≲20% of the ENAs observed at ∼0.5–6 keV come from other sources, likely from outside the heliopause as secondary ENAs. This study offers the first model replication of the intensity and evolution of IBEX-Hi ENA observations from the outer heliosphere.

Ion‐Ion Cross‐Field Instability of Lower Hybrid Waves in the Inner Coma of Comet 67P

AbstractWe show that an ion‐ion cross‐field streaming instability between cold newborn cometary ions and heated heavy ions that were picked up upstream is likely a contributing source of observed lower hybrid (LH) waves in the inner coma of comet 67P/Churyumov‐Gerasimenko. Electric field oscillations in the LH frequency range are common here, and have previously been attributed mainly to the lower‐hybrid drift instability, driven by gradients associated with observed local density fluctuations. However, the observed wave activity is not confined to such gradients, nor is it always strongest there. Thus, other instabilities are likely needed as well to explain the observed wave activity. Several previous works have shown the existence of multiple populations of cometary ions in the inner coma of 67P, distinguished by differences in mass, energy and/or flow direction. We here examine two selected time intervals in October and November 2015, with substantial wave activity in the LH frequency range, where we identify two distinct cometary ion populations: a bulk population of locally produced, predominantly radially outflowing ions, and a more tenuous population picked up further upstream and accelerated back toward the comet by the solar wind electric field. These two populations exhibit strong relative drifts (∼20 km/s, or about five times the pickup ion thermal velocity), and we perform an electrostatic dispersion analysis showing that conditions should be favorable for LH wave generation through the ion‐ion cross‐field instability.

First High-resolution Observations of Interstellar Pickup Ion Mediated Shocks in the Outer Heliosphere

This study reports the first high-time-resolution observations of interstellar pickup ions (PUIs) in the outer heliosphere, including the first high-resolution observations of PUIs mediating shocks collected anywhere. These new data were enabled by a clever flight software reprogramming of the Solar Wind Around Pluto (SWAP) instrument on New Horizons to provide ∼30 minutes resolution as compared to the previous ∼24 hr time resolution. This time resolution is sufficient to resolve the shock structures and quantify the particle heating across these shocks. In the ∼10 months of initial data, we observed seven relatively small shocks, including one reverse shock. We find that the PUIs are preferentially compressed and heated across the shocks, indicating compression ratios from ∼1.2–1.8, with little heating for values less than ∼1.5 and progressively more PUI heating for larger compression ratios. In contrast, core solar wind properties did not show consistent changes across the shocks, indicating that these particles (1) participate little in the large-scale fluid-like interactions of the outer heliosphere’s combined solar wind and PUI plasma and (2) cannot be used to characterize PUI-mediated shocks as prior studies sought to do. All six forward shock crossings showed gradual increases in PUI pressure over shock widths of ∼0.05–0.13 au, which is roughly three decades larger than characteristic particle scales such as the PUI gyroradii. The new high-resolution observations and results described here are important for understanding shocks in the outer heliosphere, the termination shock, and more broadly for PUI-mediated shocks across many astrophysical systems.

Dynamical Coupling between Anomalous Cosmic Rays and Solar Wind in Outer Heliosphere

Voyager 2 (V2) observed that pickup ions (PUIs) and anomalous cosmic rays (ACRs) have a significant influence on the solar wind structures in the outer heliosphere. In particular, the maxima in energetic particle intensities often lagged behind the shock front, while the flow velocity in some cases featured a precursor in front of the shock. These two effects are believed to be caused by the cease of ACR injection from PUIs at large heliocentric distances, and the backward diffusion of ACRs, respectively. This paper investigates the dynamical coupling between the ACRs and the solar wind in the outer heliosphere by means of a time-dependent numerical MHD simulation, in which the ACRs are treated as a massless fluid that only contributes its pressure to the system. Two types of inner boundary conditions were used, namely a synthetic short-term shock event and an extended period of data-driven solar wind variability based on the OMNI database. The lag of the ACR pressure maximum relative to the shock front, and the extended shock precursor were reproduced by the numerical results. The increase rate of the lag is related to the corresponding diffusion coefficient and the injection efficiency from PUIs to ACRs. The model was also applied to the termination shock, where simulations likewise showed that the peak in the ACR distribution can be located a short distance downstream of the shock front, indicating that the time-dependent diffusive shock acceleration mechanism is a candidate to interpret the lag between the ACR pressure peak and the shock front observed by V2.

Cometary ions detected by the Cassini spacecraft 6.5 au downstream of Comet 153P/Ikeya–Zhang

During March–April 2002, while between the orbits of Jupiter and Saturn, the Cassini spacecraft detected a significant enhancement in pickup proton flux. The most likely explanation for this enhancement was the addition of protons to the solar wind by the ionization of neutral hydrogen in the corona of comet 153P/Ikeya–Zhang. This comet passed relatively close to the Sun–Cassini line during that period, allowing pickup ions to be carried to Cassini by the solar wind. This pickup proton flux could have been further modulated by the passage of the interplanetary counterparts of coronal mass ejections past the comet and spacecraft. The radial distance of 6.5 Astronomical Units (au) traveled by the pickup protons, and the implied total tail length of >7.5 au make this cometary ion tail the longest yet measured.

Generation and motion of pickup ions in the upstream regionof Mars

Generation and motion of pickup ions in the upstream regionof Mars

Bulk Properties of Pickup Ions Derived from the Ulysses Solar Wind Ion Composition Spectrometer Data

Nonthermal, pickup ions (PUIs) represent an energetic component of the solar wind (SW). While a number of theoretical models have been proposed to describe the PUI flow, of major importance are in situ measurements providing us with the vital source of model validation. The Solar Wind Ion Composition Spectrometer (SWICS) instrument on board the Ulysses spacecraft was specifically designed for this purpose. Zhang et al. proposed a new, accurate method for the derivation of ion velocity distribution function in the SW frame on the basis of count rates collected by SWICS. We calculate the moments of these distribution functions for protons (H+) and He+ ions along the Ulysses trajectory for a period of 2 months including the Halloween 2003 solar storm. This gives us the time distributions of PUI density and temperature. We compare these with the results obtained earlier for the same interval of time, in which the ion spectra are converted to the SW frame using the narrow-beam approximation. Substantial differences are identified, which are of importance for the interpretation of PUI distributions in the 3D, time-dependent heliosphere. We also choose one of the shocks crossed by Ulysses during this time interval and analyze the distribution functions and PUI bulk properties in front of and behind it. The results are compared with the test-particle calculations and diffusive shock acceleration theory.

Observations of Modulation of Ion Flux in the Coma of Comet 67P/Churyumov‐Gerasimenko

On 6–8 June 2015, the Ion and Electron Sensor on board Rosetta observed keV-range water-group pickup ions arriving from the solar direction. Based on magnetic field intensification and variations, the appearance of the ions was likely to have been caused by a coronal mass ejection. During the 3-day period when Rosetta was 200 km from the comet, peak ion energy/charge (E/q) varied over a range from 50 eV to 1 keV in concert with neutral gas density variations caused by the rotation of the comet and its variable solar illumination. Thermal ion densities showed the same variations. The neutral density variations provided a unique opportunity to observe the repeated slowing of the solar wind by mass loading caused by charge exchange between energetic water-group ions and thermal water-group molecules. Such solar wind slowing was observed previously only by flyby missions that provided single events.

On the Energization of Pickup Ions Downstream of the Heliospheric Termination Shock by Comparing 0.52–55 keV Observed Energetic Neutral Atom Spectra to Ones Inferred from Proton Hybrid Simulations

We present an unprecedented comparison of ∼0.52–55 keV energetic neutral atom (ENA) heliosheath measurements, remotely sensed by the Interstellar Boundary Explorer (IBEX) mission and the Ion and Neutral Camera (INCA) on the Cassini mission, with modeled ENAs inferred from interstellar pickup protons that have been accelerated at the termination shock, using hybrid simulations, to assess the pickup ion energetics within the heliosheath. This is the first study to use hybrid simulations that are able to accurately model the acceleration of ions to tens of keV energies, which is essential in order to model ENA fluxes in the heliosheath, covering the full energy range observed by IBEX and CASSINI/INCA. The observed ENA intensities are an average value over the time period from 2009 to the end of 2012, along the Voyager 2 (V2) trajectory. The hybrid simulations upstream of the termination shock, where V2 crossed, are constrained by observations. We report an energy-dependent discrepancy between observed and simulated ENA fluxes, with the observed ENA fluxes being persistently higher than the simulated ones. Our analysis reveals that the termination shock may not accelerate pickup ions to sufficient energies to account for the observed ENA fluxes. We, thus, suggest that the further acceleration of these pickup ions is most likely occurring within the heliosheath, via additional physical processes like turbulence or magnetic reconnection. However, the redistribution of energy inside the heliosheath remains an open question.

Water-Group Pickup Ions From Europa-Genic Neutrals Orbiting Jupiter.

Water‐group gas continuously escapes from Jupiter's icy moons to form co‐orbiting populations of particles or neutral toroidal clouds. These clouds provide insights into their source moons as they reveal loss processes and compositions of their parent bodies, alter local plasma composition, and act as sources and sinks for magnetospheric particles. We report the first observations of H2 + pickup ions in Jupiter's magnetosphere from 13 to 18 Jovian radii and find a density ratio of H2 +/H+ = 8 ± 4%, confirming the presence of a neutral H2 toroidal cloud. Pickup ion densities monotonically decrease radially beyond 13 R J consistent with an advecting Europa‐genic toroidal cloud source. From these observations, we derive a total H2 neutral loss rate from Europa of 1.2 ± 0.7 kg s−1. This provides the most direct estimate of Europa's H2 neutral loss rate to date and underscores the importance of both ion composition and neutral toroidal clouds in understanding satellite‐magnetosphere interactions.

Non-thermal escape of the martian CO[formula omitted] atmosphere over time: Constrained by Ar isotopes

The ion escape of Mars’ CO2 atmosphere caused by its dissociation products C and O atoms is simulated from present time to ∼4.1 billion years ago (Ga) by numerical models of the upper atmosphere and its interaction with the solar wind. The planetward-scattered pick-up ions are used for sputtering estimates of exospheric particles including 36Ar and 38Ar isotopes. Total ion escape, sputtering and photochemical escape rates are compared. For solar EUV fluxes ≥3 times that of today’s Sun (earlier than ∼2.6 Ga) ion escape becomes the dominant atmospheric non-thermal loss process until thermal escape takes over during the pre-Noachian eon (earlier than ∼4.0–4.1 Ga). If we extrapolate the total escape of CO2-related dissociation products back in time until ∼4.1 Ga we obtain a maximum theoretical equivalent to CO2 partial pressure of more than ∼0.4 bar through non-thermal escape during quiet solar wind conditions. However, surface–atmosphere interaction and/or extreme solar events such as frequent CMEs could have increased this value even further. By including the surface as a sink, up to 0.9bar, or even up to 1.8bar in case of hidden carbonate reservoirs, could have been present at 4.1Ga The fractionation of 36Ar/38Ar isotopes through sputtering and volcanic outgassing from its initial chondritic value of 5.3, as measured in the 4.1 billion years old Mars meteorite ALH 84001, until the present day, however, can be reproduced for assumed CO2 partial pressures of ∼0.01–0.4bar without, and ∼0.4–1.8bar including surface sinks, and depending on the cessation time of the Martian dynamo (assumed between 3.6 and 4.0 Ga) — if atmospheric sputtering of Ar started afterwards.

Development of a cometosheath at comet 67P/Churyumov-Gerasimenko

Context. The ionosphere of a comet is known to deflect the solar wind through mass loading, but the interaction is dependent on cometary activity. We investigate the details of this process at comet 67P using the Rosetta Ion Composition Analyzer. Aims. This study aims to compare the interaction of the solar wind and cometary ions during two different time periods in the Rosetta mission. Methods. We compared both the integrated ion moments (density, velocity, and momentum flux) and the velocity distribution functions for two days, four months apart. The velocity distribution functions were projected into a coordinate system dependent on the magnetic field direction and averaged over three hours. Results. The first case shows highly scattered H+ in both ion moments and velocity distribution function. The He2+ ions are somewhat scattered, but less so, and appear more like those of H2O+ pickup ions. The second case shows characteristic evidence of mass-loading, where the solar wind species are deflected, but the velocity distribution function is not significantly changed. Conclusions. The distributions of H+ in the first case, when compared to He2+ and H2O+ pickup ions, are indicative of a narrow cometosheath on the scale of the H+ gyroradius. Thus, He2+ and H2O+, with larger gyroradii, are largely able to pass through this cometosheath. An examination of the momentum flux tensor suggests that all species in the first case have a significant non-gyrotropic momentum flux component that is higher than that of the second mass-loaded case. Mass loading is not a sufficient explanation for the distribution functions and momentum flux tensor in the first case, and so we assume this is evidence of bow shock formation.

Particle-in-cell simulations of high-frequency waves driven by pickup ion ring-beam distributions in the outer heliosheath

ABSTRACT Scattering of pickup ion ring-beam distributions in the outer heliosheath is a fundamental element in the spatial retention scenario of the energetic neutral atom (ENA) ribbon observed by the Interstellar Boundary EXplorer (IBEX). According to our earlier linear instability analysis, pickup ion ring-beam distributions trigger magnetic field-aligned, right-hand polarized unstable waves in two separate frequency ranges which are near and far above the proton cyclotron frequency, respectively. We have performed hybrid simulations to study the unstable waves near the proton cyclotron frequency. However, the high-frequency waves well above the proton cyclotron frequency are beyond the reach of hybrid simulations. In this paper, particle-in-cell simulations are carried out to study the parallel- and antiparallel-propagating high-frequency waves excited by the outer heliosheath pickup ions at different pickup angles as well as the scattering of the pickup ions by the waves excited. In the early stages of the simulations, the results confirm the excitation of the parallel-propagating, right-hand polarized high-frequency waves as predicted by the earlier linear analysis. Later in the simulations, enhanced antiparallel-propagating modes also emerge. Furthermore, the evolution of the pickup ion ring-beam distributions of the selected pickup angles reveals that the high-frequency waves do not significantly contribute to the pickup ion scattering. These results are favourable regarding the plausibility of the spatial retention scenario of the IBEX ENA ribbon.

Global Structure and Dominant Particle Acceleration Mechanism of the Heliosheath: Definitive Conclusions

Abstract During its exploration of the heliosheath, the region that lies between the termination shock of the solar wind and the heliopause that separates the solar wind from the local interstellar medium, the Voyager 1 spacecraft (V1) in 2012 encountered an apparent boundary where there was a precipitous decrease in energetic particles accelerated in the heliosheath, the so-called anomalous cosmic rays (ACRs), and from the occasional plasma density measurements on V1, a density comparable to the expected density in the interstellar medium. In 2013, the Voyager principal investigators announced that this apparent boundary was the heliopause and that V1 had entered the interstellar medium. In 2014, Fisk &amp; Gloeckler presented a detailed model that demonstrated that the apparent boundary was simply an internal surface within the heliosheath, across which compressed solar wind flows and will continue to flow until it encounters the actual heliopause. There is compelling observational evidence that the model of Fisk &amp; Gloeckler for the nose region of the heliosheath is correct: V1 did not cross the heliopause in 2012 and is not now in the interstellar medium. There is also compelling observational evidence that the ACRs are accelerated in the heliosheath by the pump acceleration mechanism of Fisk &amp; Gloeckler. The success of the models of Fisk &amp; Gloeckler confirms that the plasma in the nose region of the heliosheath consists of two separate components, the pickup ions and ACRs, and the thermal solar wind, and as a unique plasma is worthy of more study and, if possible, more exploration.

A New 3D Solar Wind Speed and Density Model Based on Interplanetary Scintillation

Abstract The solar wind (SW) is an outflow of the solar coronal plasma, which expands supersonically throughout the heliosphere. SW particles interact by charge exchange with interstellar neutral atoms; on the one hand, they modify the distribution of this gas in interplanetary space, and, on the other hand, they are the seed populations for heliospheric pickup ions and energetic neutral atoms (ENAs). The heliolatitudinal profiles of the SW’s speed and density evolve during the solar activity cycle. A model of the evolution of the SW’s speed and density is needed to interpret observations of ENAs, pickup ions, the heliospheric backscatter glow, etc. We derive the Warsaw Heliospheric Ionization Model 3DSW—WawHelIon 3DSW—based on interplanetary scintillation (IPS) tomography maps of the SW speed. We use the IPS tomography data from 1985 to 2020, compiled by Tokumaru et al. We derive a novel statistical method of filtering these data against outliers; we present a flexible analytic formula for the latitudinal profiles of the SW speed, based on Legendre polynomials of varying order with additional restraining conditions at the poles; fit this formula to the yearly filtered data; and calculate yearly SW density profiles using the latitudinally invariant SW energy flux observed in the ecliptic plane. Despite the application of a refined IPS data set, a more sophisticated data filtering method, and a more flexible analytic model, the present results mostly agree with those obtained previously, demonstrating the robustness of IPS studies of the SW’s structure.

Time dependent hydrogen ENAs fluxes and structure of the heliosphere in the solar magneto-gravitational field

Energetic neutral atoms (ENAs) consisting mainly of hydrogen with energies of several keV and coming from the outer heliosphere have been detected by various spacecraft missions. The characteristics of the energy spectrum of ENAs include a dependence on the proton distribution at the heliospheric boundaries where the solar wind interaction with the local interstellar medium (LISM) occurs. We determine here the time-dependent differential flux for the neutral hydrogen atoms resulting from the kinetic exchange of energy in the outer heliosphere under the solar magnetic field. With the proton flow along the magnetic field lines, the mean kinetic energy of the particles is shown to increases due to the magnetic field line compression at the outer heliosphere, causing the acceleration of the solar wind protons to the observed range of speeds. Also, the toroidal field driven compression and expansion of the magnetic field lines corresponds to the temperature variations observed in the INCA line-of-sight observations of the ENAs in the inner heliosheath, where as the pickup ions retain their accelerated speed inside the heliosphere in the subsequent motion up to ∼1au.

Characterizing Heliospheric and Interstellar Interactions Using Pickup Ion Measurements

Characterizing Heliospheric and Interstellar Interactions Using Pickup Ion Measurements