Pickup Ion Velocity Distribution Research Articles

Mirror Instability Driven by Pickup Ions in the Outer Heliosheath

Abstract The stability of the pickup ions in the outer heliosheath is a vital factor in the generation of the energetic neutral atom (ENA) ribbon observed by the Interstellar Boundary EXplorer according to the secondary ENA mechanism. Most previous studies of the pickup ion stability assumed simple, idealized velocity distributions of the pickup ions and focused on the parallel-propagating modes only. This paper takes a more realistic multicomponent pickup ion velocity distribution given by the global modeling of neutral atoms in the heliosphere and investigates the role of the oblique mirror waves, in addition to the parallel modes. Both linear kinetic instability analysis and hybrid simulations are performed. In contrast to a recent investigation using the same distribution that demonstrated the growth of parallel waves but reported an insignificant contribution of oblique modes, our study reveals substantial growth of the oblique mirror instability. The oblique mirror modes and the parallel/quasi-parallel ion cyclotron waves grow simultaneously with different growth rates. The pickup ion scattering by two types of excited waves together is more pronounced than by either type of wave alone. More importantly, our two-dimensional hybrid simulation results demonstrate that the development of the mirror instability not only produces its own pickup ion scattering, but also leads to the growth of extra ion cyclotron waves (in a quasi-linear manner), which further enhances the pickup ion scattering. The results suggest an important role of the mirror modes that should not be ignored in the stability study of the outer heliosheath pickup ions.

Effect of Rapid Changes of Solar Wind Conditions on the Pickup Ion Velocity Distribution

AbstractWe performed the first systematic analysis of pickup ion (PUI) cutoff speed variations, across compression regions and due to fast fluctuations in solar wind (SW) speed and magnetic field strength. This study is motivated by the need to remove or correct for systematic effects on the determination of the interstellar flow longitude based on the longitudinal variation of the PUI cutoff. Using 2007–2014 STEREO A PLASTIC observations, we identified SW compression regions and accumulated the contained PUI velocity distributions in a superposed epoch analysis. The shift of the cutoff in velocity, interpreted as PUI energization, varies systematically across the compression region and increases approximately linearly with the speed gradient of the compression. Additionally, the shift remains positive into the negative speed gradient at the beginning of the rarefaction region. A similar response is found when PUI distributions are sorted according to the strength of fast fluctuations in SW speed, density, and magnetic field strength. These parameters remain high in the first part of the rarefaction region, suggesting a possible PUI energization through compressive turbulence. Based on these results, we removed the strongest compression regions from the interstellar flow analysis, finding no significant change in direction or uncertainty. Thus, we have revealed the influence of adiabatic compression and compressive turbulence, increasing the PUI cutoff energy, and we have demonstrated that the determination of the interstellar inflow direction via analysis of PUI distributions is robust for a multiyear data set, even in the presence of SW interaction regions.

Challenges in the determination of the interstellar flow longitude from the pickup ion cutoff

Context. The interstellar flow longitude corresponds to the Sun’s direction of movement relative to the local interstellar medium. Thus, it constitutes a fundamental parameter for our understanding of the heliosphere and, in particular, its interaction with its surroundings, which is currently investigated by the Interstellar Boundary EXplorer (IBEX). One possibility to derive this parameter is based on pickup ions (PUIs) that are former neutral ions that have been ionized in the inner heliosphere. The neutrals enter the heliosphere as an interstellar wind from the direction of the Sun’s movement against the partially ionized interstellar medium. PUIs carry information about the spatial variation of their neutral parent population (density and flow vector field) in their velocity distribution function. From the symmetry of the longitudinal flow velocity distribution, the interstellar flow longitude can be derived.Aim. The aim of this paper is to identify and eliminate systematic errors that are connected to this approach of measuring the interstellar flow longitude; we want to minimize any systematic influences on the result of this analysis and give a reasonable estimate for the uncertainty.Methods. We use He+ data measured by the PLAsma and SupraThermal Ion Composition (PLASTIC) sensor on the Solar TErrestrial RElations Observatory Ahead (STEREO A) spacecraft. We analyze a recent approach, identify sources of systematic errors, and propose solutions to eliminate them. Furthermore, a method is introduced to estimate the error associated with this approach. Additionally, we investigate how the selection of interplanetary magnetic field angles, which is closely connected to the pickup ion velocity distribution function, affects the result for the interstellar flow longitude.Results. We find that the revised analysis used to address part of the expected systematic effects obtains significantly different results than presented in the previous study. In particular, the derived uncertainties are considerably larger. Furthermore, an unexpected systematic trend of the resulting interstellar flow longitude with the selection of interplanetary magnetic field orientation is uncovered.

Anisotropy of the He+, C+, N+, O+, and Ne+pickup ion velocity distribution functions

Context. Interstellar and inner-source pickup ions (PUIs) are produced by the ionization of neutral atoms that originate either outside or inside the heliosphere. Just after ionization, the singly charged ions are picked up by the magnetized solar wind plasma and develop strong anisotropic toroidal features in their velocity distribution functions (VDF). As the plasma parcel moves outwards with the solar wind, the pickup ion VDF gets more and more affected by resonant wave-particle interactions, changing heliospheric conditions, and plasma drifts, which lead to a gradual isotropization of the pickup ion VDF. Past investigations of the pickup ion torus distribution were limited to He+ pickup ions at 1 astronomical unit (AU).Aims. The aim of this study is to quantify the state of anisotropy of the He+ , C+ , N+ , O+ , and Ne+ pickup ion VDF at 1 AU. Changes between the state of anisotropy between PUIs of different mass-per-charges can be used to estimate the significance of resonant wave-particle interactions for the isotropization of their VDF, and to investigate the numerous simplifications that are generally made for the description of the phase-space transport of PUIs.Methods. Pulse height analysis data by the PLAsma and SupraThermal Ion Composition instrument (PLASTIC) on board the Solar Terrestrial RElations Observatory Ahead (STEREO A) is used to obtain velocity-spectra of He+ , C+ , N+ , O+ , and Ne+ relative to the solar wind, f (w sw ). The w sw -spectra are sorted by two different configurations of the local magnetic field – one in which the torus distribution lies within the instrument’s aperture, φ ⊥ , and one in which the torus distribution lies exclusively outside the instrument’s field of view, φ ∥ . The ratio of the PUI spectra between φ ⊥ and φ ∥ is used to determine the degree of anisotropy of the PUI VDF.Results. The data shows that the formation of a torus distribution at 1 AU is significantly more prominent for O+ (and N+ ) than for He+ (and Ne+ ). This cannot be explained by resonant wave-particle interactions as the sole mechanism for the isotropization of the PUI VDF. The anisotropy of the O+ VDF compared to He+ is highly fluctuating but consistently higher over an observation period of six years and therefore unlikely to be related to either specific heliospheric conditions or solar activity variations. To our surprise, we also found a clear signature of a C+ torus distribution at 1 AU very similar to the one of He+ , although as an inner-source PUI, C+ should have a considerably different spectral and spatial injection pattern than interstellar PUIs.

The light ([formula omitted]) and heavy ([formula omitted]) pickup ion dynamics in the lunar-like plasma environment: 3D hybrid kinetic modeling

In this report we discuss the self-consistent dynamics of pickup ions in the solar wind flow around the lunar-like object. In our model the solar wind and pickup ions are considered as a particles, whereas the electrons are described as a fluid. Inhomogeneous photoionization, electron-impact ionization and charge exchange are included in our model. The Moon will be chosen as a basic object for our modeling. The current modeling shows that mass loading by pickup ions H+,H2+,He+, and Na+ may be very important in the global dynamics of the solar wind around the Moon. In our hybrid modeling we use exponential profiles for the exospheric components. The Moon is considered as a weakly conducting body. Special attention will be paid to comparing the modeling pickup ion velocity distribution with ARTEMIS observations. Our modeling shows an asymmetry of the Mach cone due to mass loading, the upstream flow density distribution and the magnetic field. The pickup ions form an asymmetrical plasma tails that may disturb the lunar plasma wake.

Effects of [formula omitted] and [formula omitted] pickup ions on the lunar-like plasma environment: 3D hybrid modeling

In this report we discuss the self-consistent dynamics of pickup ions in the solar wind flow around the lunar-like object. In our model the solar wind and pickup ions are considered as a particles, whereas the electrons are described as a fluid. Inhomogeneous photoionization, electron-impact ionization and charge exchange are included in our model. The Moon will be chosen as a basic object for our modeling. The current modeling shows that mass loading by pickup ions Na+ and He+ may be very important in the global dynamics of the solar wind around the Moon. In our hybrid modeling we use exponential profiles for the exospheric components. The Moon is considered as a weakly conducting body. Special attention will be paid to comparing the modeling pickup ion velocity distribution with ARTEMIS observations. Our modeling shows an asymmetry of the Mach cone due to mass loading, the upstream flow density distribution and the magnetic field. The pickup ions form an asymmetrical plasma tails that may disturb the lunar plasma wake.

Background and pickup ion velocity distribution dynamics in Titan’s plasma environment: 3D hybrid simulation and comparison with CAPS T9 observations

In this report we discuss the ion velocity distribution dynamics from the 3D hybrid simulation. In our model the background, pickup, and ionospheric ions are considered as a particles, whereas the electrons are described as a fluid. Inhomogeneous photoionization, electron-impact ionization and charge exchange are included in our model. We also take into account the collisions between the ions and neutrals. The current simulation shows that mass loading by pickup ions H + , H 2 + , CH 4 + and N 2 + is stronger than in the previous simulations when O + ions are introduced into the background plasma. In our hybrid simulations we use Chamberlain profiles for the atmospheric components. We also include a simple ionosphere model with average mass M = 28 amu ions that were generated inside the ionosphere. The moon is considered as a weakly conducting body. Special attention will be paid to comparing the simulated pickup ion velocity distribution with CAPS T9 observations. Our simulation shows an asymmetry of the ion density distribution and the magnetic field, including the formation of the Alfvén wing-like structures. The simulation also shows that the ring-like velocity distribution for pickup ions relaxes to a Maxwellian core and a shell-like halo.

IMF Direction Derived from Cycloid-Like Ion Distributions Observed by Mars Express

Although the Mars Express (MEX) does not carry a magnetometer, it is in principle possible to derive the interplanetary magnetic field (IMF) orientation from the three dimensional velocity distribution of pick-up ions measured by the Ion Mass Analyser (IMA) on board MEX because pick-up ions' orbits, in velocity phase space, are expected to gyrate around the IMF when the IMF is relatively uniform on a scale larger than the proton gyroradius. During bow shock outbound crossings, MEX often observed cycloid distributions (two dimensional partial ring distributions in velocity phase space) of protons in a narrow channel of the IMA detector (only one azimuth for many polar angles). We show two such examples. Three different methods are used to derive the IMF orientation from the observed cycloid distributions. One method is intuitive (intuitive method), while the others derive the minimum variance direction of the velocity vectors for the observed ring ions. These velocity vectors are selected either manually (manual method) or automatically using simple filters (automatic method). While the intuitive method and the manual method provide similar IMF orientations by which the observed cycloid distribution is well arranged into a partial circle (representing gyration) and constant parallel velocity, the automatic method failed to arrange the data to the degree of the manual method, yielding about a 30° offset in the estimated IMF direction. The uncertainty of the derived IMF orientation is strongly affected by the instrument resolution. The source population for these ring distributions is most likely newly ionized hydrogen atoms, which are picked up by the solar wind.

Reabsorption of self-generated turbulent energy by pick-up protons in the outer heliosphere

It is well known that newly created pick-up ions are first injected into a toroidal distribution in the velocity space which comoves with the solar wind. This distribution is, however, unstable with respect to excitation of resonant MHD waves, and thus by wave-driving some specific amount of the initial kinetic energy of pick-up ions is handed over to the ambient wave fields, while the pick-up ion velocity distribution thereby is rearranged. In this paper we describe the developement of the wavenumber spectrum of pick-up proton-generated turbulence and the pick-up proton energy spectrum resulting from resonant reabsorption of this turbulence, by solving simultaneously the coupled system of two differential equations for the spectral turbulence power and the pick-up proton spectral intensity. We show that the majority of pick-up generated wave energy in the range of wavenumbers above the wavenumber at which the waves are produced is reabsorbed by the pick-up protons themselves.

Models and numerical simulation of velocity distribution of pickup ions at low‐Earth orbit

Large manned spacecraft release and carry up huge amounts of water in their orbital motion. Some of the neutral water molecules undergo charge exchange with the ambient ionospheric plasma and become what are called pickup ions at low‐Earth orbit (LEO) altitudes. In this paper the velocity distribution of the pickup ions is studied on the basis of the analysis of the trajectories of the pickup ions, the influence of thermal movement, and the influence of collisions between the pickup ions and other particles. The integral models are developed to calculate the velocity distribution of the pickup ions in the frame moving with the orbiter. The comparison between the simulation result and actual measurement data from the spherical retarding potential analyzer (SRPA) on mission Tethered Satellite System ‐ 1 Reflight (TSS‐1R) and earlier missions on which the SRPA was flown shows that the models obtained are correct for the special environment of a large space platform at LEO altitudes and are useful in analyzing the data of the ion probes mounted on similar spacecraft. With the models obtained the influence of some factors on the speed distribution of the ions collected by the payload SRPA of a large spacecraft orbiter is analyzed.

Pick-up Ion Measurements in the Heliosphere – A Review

Measurements of the composition and spatial distribution of pick-up ions inside the heliosphere are reviewed. The first interstellar 4He+pick-up ions were detected with the SULEICA instrument on the AMPTE spacecraft near Earth's orbit. Most data on pick-up ions were taken in the solar-wind and suprathermal energy range of SWICS on Ulysses while the spacecraft cruised from 1.4 to 5.4 AU and explored the high-latitude heliosphere and solar wind from the ecliptic to ± 80° heliolatitude. This includes the discovery of H+, 4He++, 3He+, N+,O+, and Ne+ pick-up ions that originate from the interstellar neutralgas penetrating the heliosphere. From their fluxes properties of the interaction region between the heliosphere and the Local Interstellar Cloud such as the limits on filtration and the strength of the interstellar magnetic field have been revealed. Detailed analysis of the velocity distributions of pick-up ions led to 1) the discovery of a new distinct source, the so-called Inner Source, consisting of atoms released from interstellar and interplanetary dust inside the heliosphere, 2) the determination of pick-up ion transport parameters such as the long mean free path for pitch-angle scattering of order1 AU, and 3) detailed knowledge on the very preferential injection and acceleration of pick-up ions during interplanetary energetic particle events such as Co-rotating Interaction Regions and Coronal Mass Ejections. SWICS measurements have fully confirmed the theory of Fisk, Koslovsky, and Ramaty that pick-up ions derived from the interstellar gas are the dominant source of the Anomalous Cosmic Rays; they are pre-accelerated inside the heliosphere and re-accelerated at the solar-wind Termination Shock according to Pesses, Eichler, and Jokipii. The data indicate that the Inner Source of pick-up ionsis largely responsible for the occurence of C+ in the Anomalous Cosmic Rays. The abundances of recently discovered Inner-Source Mg+ and Si+ are solar-wind like and consistent with their abundances in the energetic particles associated with Co-rotating Interaction Regions. Knowledge on the injection and acceleration processes in Co-rotating Interaction Regions is applied to discuss the current observational evidence for the Interplanetary Focusing Cone of the interstellar neutral gas due to the Sun's gravitational force. The 25–150 keV/amu suprathermal 4He+ pick-up ion fluxes measured by CELIAS/STOF on board SOHO over 360° of ecliptic longitude represent a `local' ionization and acceleration of interstellar atoms at 1 AU or smaller heliocentric distances. Completing the first limited data set of SULEICA/AMPTE on 4He+ pick-up ions they indicate a density enhancement in the Interplanetary Focusing Cone which is confirmed by recent SWICS/ACE data. Clear evidence for signatures in ecliptic longitude are found in the data on energetic neutral H fluxes observed with the CELIAS/HSTOF sensor on board SOHO. These fluxes are enhanced in the upstream and downstream directions of the interstellar wind. Detection of energetic H atoms, which propagate unaffected by the Heliospheric Magnetic Field, provided for the first time a diagnostic tool for observations near Earth to analyze the structure in ecliptic longitude of the interface region between the heliosphere and the Local Interstellar Cloud.

Spatial variation of accelerated pickup ions at co‐rotating interaction regions

The spatial distribution of accelerated particles at corotating interaction regions (CIR's) provides constraints on possible acceleration mechanisms. We point out some previously unrecognized aspects of acceleration at the forward and reverse shocks bounding CIRs, which support the idea that acceleration of pickup ions at these shocks is an important contributor to energetic ions in the inner heliosphere. We find that the acceleration of pickup ions is strongly favored at the reverse shock over the forward shock. Also, the scaling of the pickup‐ion velocity distribution with respect to local solar wind speed is not necessarily useful within the CIR.

Low‐frequency instabilities and the resulting velocity distributions of pickup ions at comet Halley

The interaction between the solar wind and newborn cometary ions is studied using a new analytical theory as well as one‐ and two‐dimensional hybrid simulations. Using the observed parameters upstream of the comet Halley, a detailed study of wave excitation and the resulting particle distributions is presented. Linear theory as well as simulations show that a variety of modes such as the fast magnetosonic mode, high frequency whistlers and obliquely propagating Alfvén ion cyclotron waves can be excited. However, parallel propagating waves are found to be dominant in the wave spectrum and to control the scattering of the pickup ions. Several features of the observed distributions of pickup protons are explained. In particular, it is shown that the observed asymmetric pitch angle distribution for the pickup protons is due to the small saturation amplitude of the waves for the given parameters. Water group associated waves can lead to energy diffusion and further pitch angle scattering of protons. This effect is most likely to be important in the vicinity of the bow shock of comet Halley where the density of water group ions becomes comparable to that of protons. It is shown that the observed increase in the radius of the proton velocity shell just outside the bow shock can be due to water group waves. The nearly isotropic proton pitch angle distribution observed by Neugebauer et al. [1989] just outside the bow shock may, however, be related to the presence of a rotational discontinuity which has been identified in the magnetic field data. Just outside the bow shock, simulations show that parallel propagating water group waves can steepen with attached whistler wave packets. The steepening process at parallel propagation is a transient effect, in an important contrast to the case of steepening at oblique angles. The smaller beam densities at comet Halley appears to be the main reason not only why waves at comet Halley have smaller amplitudes but also why oblique, steepening magnetosonic waves have not been detected at comet Halley, whereas they have been seen at comet Giacobini‐Zinner.