Acceleration Of Pickup Ions Research Articles

Injection Process of Pickup Ion Acceleration at an Oblique Heliospheric Termination Shock

The injection process of pickup ion acceleration at a heliospheric termination shock is investigated. Using two-dimensional fully kinetic particle-in-cell simulation, accelerated pickup ions are self-consistently reproduced by tracking long time evolution of shocks with an unprecedentedly large system size in the shock normal direction. Reflected pickup ions drive upstream large-amplitude waves through resonant instabilities. Convection of the large-amplitude waves causes shock surface reformation and alters the downstream electromagnetic structure. A part of pickup ions are accelerated to tens of upstream flow energy in the timescale of ∼100 times inverse ion gyrofrequency. The initial acceleration occurs through the shock surfing acceleration (SSA) mechanism followed by the shock drift acceleration mechanism. Large electrostatic potential accompanied by the upstream waves enables the SSA to occur.

Probing the Length of the Heliospheric Tail with Energetic Neutral Atoms (ENAs) from 0.52 to 80 keV

The shape of the heliosphere is currently under active debate. Energetic neutral atoms (ENAs) offer the best method for investigating the global structure of the heliosphere. To date, the Interstellar Boundary Explorer (IBEX) and the Ion and Neutral Camera (INCA) that was on board Cassini provide the only global ENA observations of the heliosphere. While extensive modeling has been done at IBEX-Hi energies (0.52–6 keV), no global ENA modeling has been conducted for INCA energies (5.2–55 keV). Here, we use an ENA model of the heliosphere based on hybrid results that capture the heating and acceleration of pickup ions (PUIs) at the termination shock to compare modeled global ENA results with IBEX-Hi and INCA observations using both a long- and short-tail model of the heliosphere. We find that the modeled ENA results for the two heliotail configurations produce similar results from the IBEX-Hi through the INCA energies. We conclude from our modeled ENAs, which only include PUI acceleration at the termination shock, that ENA observations in currently available energy ranges are insufficient for probing the shape and length of the heliotail. However, as a prediction for the future IMAP-Ultra mission (3–300 keV) we present modeled ENA maps at 80 keV, where the cooling length (∼600 au) is greater than the distance where the long- and short-heliotail models differ (∼400 au), and find that IMAP-Ultra should be able to identify the shape of the heliotail, predicting differences in the north lobe to downwind flux ratio between the models at 48%.

Signature of a Heliotail Organized by the Solar Magnetic Field and the Role of Nonideal Processes in Modeled IBEX ENA Maps: A Comparison of the BU and Moscow MHD Models

Abstract Energetic neutral atom (ENA) models typically require post-processing routines to convert the distributions of plasma and H atoms into ENA maps. Here we investigate how two kinetic-MHD models of the heliosphere (the BU and Moscow models) manifest in modeled ENA maps using the same prescription and how they compare with Interstellar Boundary Explorer (IBEX) observations. Both MHD models treat the solar wind as a single-ion plasma for protons, which include thermal solar wind ions, pick-up ions (PUIs), and electrons. Our ENA prescription partitions the plasma into three distinct ion populations (thermal solar wind, PUIs transmitted and ones energized at the termination shock) and models the populations with Maxwellian distributions. Both kinetic-MHD heliospheric models produce a heliotail with heliosheath plasma that is organized by the solar magnetic field into two distinct north and south columns that become lobes of high mass flux flowing down the heliotail; however, in the BU model, the ISM flows between the two lobes at distances in the heliotail larger than 300 au. While our prescription produces similar ENA maps for the two different plasma and H atom solutions at the IBEX-Hi energy range (0.5–6 keV), the modeled ENA maps require a scaling factor of ∼2 to be in agreement with the data. This problem is present in other ENA models with the Maxwellian approximation of multiple ion species and indicates that either a higher neutral density or some acceleration of PUIs in the heliosheath is required.

Heliospheric Structure as Revealed by the 3–88 keV H ENA Spectra

Energetic neutral atoms (ENA) are an important tool for investigating the structure of the heliosphere. Recently, it was observed that fluxes of ENAs (with energy $\le$ 55 keV) coming from the upwind and downwind regions of the heliosphere are similar in strength. This led the authors of these observations to hypothesize that the heliosphere is bubble-like rather than comet-like, meaning that it has no extended tail. We investigate the directional distribution of the ENA flux for a wide energy range (3--88 keV) including the observations from IBEX (Interstellar Boundary Explorer), INCA (Ion and Neutral Camera, on board Cassini), and HSTOF (High energy Suprathermal Time Of Flight sensor, on board SOHO, Solar and Heliospheric Observatory). An essential element is the model of pickup ion acceleration at the termination shock (TS) proposed by Zank. We use state of the art models of the global heliosphere, interstellar neutral gas density, and pickup ion distributions. The results, based on the "comet-like" model of the heliosphere, are close in flux magnitude to ENA observations by IBEX, HSTOF and partly by INCA (except for the 5.2-13.5 keV energy channel). We find that the ENA flux from the tail dominates at high energy (in agreement with HSTOF, but not INCA). At low energy, our comet-like model produces the similar strengths of the ENA fluxes from the upwind and downwind directions, which, therefore, removes this as a compelling argument for a bubble-like heliosphere.

Acceleration of Interstellar Pickup He+ at Earth's Perpendicular Bow Shock

AbstractOn 05 December 2015 the Magnetospheric MultiScale constellation observed interstellar pickup ion distributions during an inbound crossing of Earth's perpendicular bow shock near the subsolar point, which provides new insights into shock acceleration of pickup ions throughout the heliosphere. In this study we analyze the upstream and downstream velocity distributions of H+ and He+ using data from the Hot Plasma Composition Analyzer on Magnetospheric MultiScale. We derive average two‐dimensional pitch angle distributions in the upstream and downstream field‐aligned bulk plasma frame, as well as integrated one‐dimensional velocity distributions. By comparing the upstream and downstream distributions, we find evidence of accelerated H+ and He+ downstream of the shock. By comparing measured and theoretical reflection ratios for He+, we attribute a significant part of this acceleration to a single reflection at the shock.

Pickup ion acceleration in the successive appearance of corotating interaction regions

AbstractAcceleration of pickup ions (PUIs) in an environment surrounded by a pair of corotating interaction regions (CIRs) was investigated by numerical simulations using a hybrid code. Energetic particles associated with CIRs have been considered to be a result of the acceleration at their shock boundaries, but recent observations identified the ion flux peaks in the sub‐MeV to MeV energy range in the rarefaction region, where two separate CIRs were likely connected by the magnetic field. Our simulation results confirmed these observational features. As the accelerated PUIs repeatedly bounce back and forth along the field lines between the reverse shock of the first CIR and the forward shock of the second one, the energetic population is accumulated in the rarefaction region. It was also verified that PUI acceleration in the dual CIR system had two different stages. First, because PUIs have large gyroradii, multiple shock crossing is possible for several tens of gyroperiods, and there is an energy gain in the component parallel to the magnetic field via shock drift acceleration. Second, as the field rarefaction evolves and the radial magnetic field becomes dominant, Fermi‐type reflection takes place at the shock. The converging nature of two shocks results in a net energy gain. The PUI energy acquired through these processes is close to 0.5 MeV, which may be large enough for further acceleration, possibly resulting in the source of anomalous cosmic rays.

Solar radiation pressure as a mechanism of acceleration of atoms and first ions with low ionization potentials

Calculated results are presented for solar radiation pressure acting on atoms and first ions. For some of these particles, radiation pressure exceeds the gravitational attraction and can accelerate them to large velocities. A comparison of the results with ionization potentials shows that the maxima of radiation pressure on neutral atoms coincide with the minima of the first ionization potentials (FIPs). This relationship is even more apparent for first ions. The minima of the second ionization potentials (SIPs) coincide with the radiation pressure maxima for a number of ions such as Be II, Mg II, Ca II, and the neighboring elements. Thus, radiation pressure may serve as a possible mechanism of acceleration of pickup ions and energetic neutral atoms (ENA) coming from an inner source (zodiacal dust and sungrazing comets). These atoms and ions, which are not typical of the solar wind, are formed as a result of the disintegration of comets or meteor showers near the Sun and can accelerate and reach the Earth’s orbit as part of the solar wind. Doubly ionized atoms have resonance lines in the UV range, where solar radiation pressure has no apparent impact on the particle dynamics; thus, the proposed acceleration mechanism can only be applied to neutral atoms and first ions with low potentials of the subsequent ionization.

Simulations of pickup ion mediated quasi‐perpendicular shocks: Implications for the heliospheric termination shock

The microstructure of the heliospheric termination shock and the accompanied local acceleration processes of both ions and electrons are investigated by utilizing one‐dimensional full particle‐in‐cell simulations for a variety of parameters. The relative pickup ion density is assumed to be 20–30%. The magnetic field and the shock potential profiles exhibit significant differences, since the former mostly reflects the dynamics of solar wind ions, whereas the latter is mainly sustained by the bulk motion of the reflected pickup ions in the extended foot. The discrepancy between the magnetic field profile and the potential profile increases with Alfvén Mach number. Most of the downstream thermal energy is gained by the pickup ions, while some heating of the solar wind ions and electrons occurs through the modified two‐stream instability excited in the extended foot. Self‐reformation can occur when the relative pickup ion density is 20% but is blurred when it becomes as large as 30%. Reformation is also suppressed if the local solar wind ion temperature in the extended foot is high, which can either be due to heating by the modified two‐stream instability or is already determined by the solar wind temperature far upstream. In all runs presented in this study no evidence for shock surfing acceleration of pickup ions could be found. Nonthermal particle acceleration occurs for oblique shocks. Electron (pickup ion) shock drift acceleration is evidenced when the shock angle is below 80° (60°).

Spatial variation of the pickup-proton-injection rate into the ACR regime at the 3D-heliospheric termination shock

Context. Anomalous cosmic rays are thought to be generated from pick-up ions that undergo Fermi-1 acceleration in the region close to the solar wind termination shock. This selective acceleration process depends on the efficiency at which upstream pick-up ions are injected into the first reflection mode, from where acceleration into higher energy anomalous cosmic rays starts. Aims. The injection efficiency is highly sensitive to the local inclination of the upstream magnetic field with respect to the shock normal. We determine the probability of pick-up ions being reflected at the shock depending on the magnetic inclination angle, which varies characteristically with latitude and longitude. This reflection is the relevant initial step in the acceleration of pickup ions to anomalous cosmic rays. Methods. We point out that the most dramatic change in the magnetic tilt angle occurs whenever two consecutive solar wind sector structures with different magnetic polarities, pass over the shock. We treat this effect by modeling the 3D- variation of the injection efficiency and start from calculations of the pick up ion reflection probability as function of the magnetic tilt angle. We first derive the magnetic tilt angle as function of latitude and longitude, representing the 3D-shape of the termination shock by a triaxial or prolate ellipsoid adaptable for conditions of different solar cycles, respectively. This allows us to semi-analytically express the 3D-variation of the injection efficiency and rate over a solar cycle. Results. We find a substantial variation in the injection efficiency by 5 orders of magnitude when passing from the upwind parts to the flanks of the termination shock or from low to high latitudes (except at the poles). Folding this with the normalized pickup ion density gives a normalized injection rate which increases from 0% to 30% from upwind to crosswind directions and analogously decreases from crosswind into downwind direction. The maximal value of the injection rate highly depends on the underlying sector structure of the heliospheric magnetic field. During solar minimum there is almost no injection in high latitudes, except at the pole itself, while the injection rate is everywhere fairly good during solar maximum. Conclusions. Our model indicates that the injection into the anomalous cosmic ray acceleration mainly takes place in crosswind directions e.g. in the flanks of the termination shock.

Pickup Ion Acceleration at the Termination Shock and in the Heliosheath

We discuss pickup ion acceleration and transport near the solar wind termination shock from the perspective of their spectral, spatial, and pitch-angle distributions. Our study is performed in the framework of a recently developed anisotropic transport model based on a Legendre polynomial expansion technique. Voyager 1 LECP angular distributions of 1 MeV protons, represented in the form of an expansion in spherical harmonics in the frame aligned with the measured interplanetary magnetic field, are used as benchmarks for our theory. We find the observed distributions consistent with our model predictions for particle acceleration and reflection at a highly oblique shock wave. It is shown that first-order (field aligned) anisotropy is a measure of shock obliquity while the second-order (transverse) anisotropy reflects the energy dependence of the particle scattering mean free path. We also discuss the role of enhanced scattering and momentum diffusion on the spectral properties of energetic charged particles.

Longitudinal variation of the pickup-proton-injection efficiency and rate at the heliospheric termination shock

Context. Pickup ions are considered to be the seed population of the anomalous cosmic rays. The acceleration of the pickup ions at the solar wind termination shock depends on the efficiency with which they are injected into the first reflection mode, from where acceleration into high energy anomalous cosmic rays begins. In this letter, we study the longitudinal variation of this efficiency. Aims. We consider the specific MHD conditions at the longitudinal position of the termination shock, which are highly sensitive to the local inclination angle between the magnetic field and the shock normal. We determine the probability for pickup-ions to become reflected which depends on how the inclination angle varies with longitude. This is the first and relevant step in the acceleration of pickup ions to anomalous cosmic rays. Methods. The most dramatic change in the magnetic tilt angle occurs in the current sheet between two consecutive solar wind sector structures of different polarity, when they pass over the shock. We consider this effect by modeling the longitudinal variation in the injection efficiency. Starting from calculations of the pickup-ion reflectance probability as a function of the magnetic tilt angle with respect to the normal, we find an analytic expression for the injection efficiency: to do this we need the magnetic tilt angle as a function of longitude, which we obtain from an analytic representation of the termination shock shape. This enables us to determine the longitudinal dependence of the injection efficiency. Results. We find that a substantial variation in the injection efficiency of a factor of 3 occurs when passing from the upwind part to the flanks of the termination shock. Folding this with the longitudinal dependence of the pickup ion density gives an analytic expression the injection rate which varieis by a factor of 2 between up- and ecliptical crosswind direction and a factor of 5 between cross- and downwind direction. Conclusions. Our model indicates that injection into the anomalous cosmic ray acceleration predominantly occurs in the ecliptical crosswind direction e.g. in the flanks of the termination shock.

Pickup oxygen ion velocity space and spatial distribution around Mars

We report a newly created highly parallelized global test particle model for resolving the pickup oxygen ion distribution around Mars. The background magnetic and convection electric fields are calculated using a three‐dimensional multispecies magnetohydrodynamic model, which includes the effect of the Martian crustal magnetic field. In addition to photo‐ionization, charge exchange collisions and solar wind electron impact ionization are included for the pickup ion generation. The most novel feature of our model is that more than one billion test particles are launched in the simulation domain in total. This corresponds to a profound enhancement by at least 3 orders of magnitude in the total number, compared to all existing test particle models. This substantial improvement enables an unprecedented examination of the pickup ion flux distribution in velocity space, which is not achievable in previous simulation studies due to the insufficient statistics arising from the limited number of test particles. Using the velocity space distribution of pickup O+ ions as a tool, the Mars‐solar wind interaction can be investigated in a unique way. It is shown that the velocity space distribution is highly non‐Maxwellian, exhibiting non‐gyrotropic and non‐symmetric distributions, including many beam‐like features. In the tail region, pickup ions have a prominent outflowing component in the whole energy range. The energy examination of particles traveling across the tail region shows that the acceleration highly depends on the source region where the particles originate. The strong convection electric field in the magnetosheath region is favorable to the pickup ion acceleration.

A Focused Transport Approach to Pickup Ion Shock Acceleration: Implications for the Termination Shock

The discovery of low-energy suprathermal ions coming directly from the solar wind termination shock by the Voyager 1 spacecraft revealed strong beaming of these particles along the direction of the large-scale magnetic field. Since such large anisotropies exclude the use of standard cosmic-ray transport theory for nearly isotropic particle distributions in modeling the acceleration of these particles at the termination shock, we developed a shock acceleration model based on the numerical solution of the standard focused kinetic transport equation. After investigating the appropriateness of the physical content of this equation for shock acceleration, and applying the model to shocks of varying shock obliquity, we find that (1) this approach is a viable alternative to more sophisticated particle codes such as hybrid codes for simulating the shock acceleration of pickup ions and (2) that the model has the potential to reproduce many of the interesting features of energetic ions observed by the Voyager spacecraft at the termination shock.

Effect of the motional electric field on the Venus nightside ionopause

The purpose of this paper is to clarify how the average structure of the Venus nightside ionopause for solar zenith angles (SZA) greater than 90° depends on (1) the direction and (2) the magnitude of the motional electric field of the solar wind. Plasma density structure in the Venus nightside ionosphere has been investigated by using data sets of the Pioneer Venus Orbiter observations. It is found that the distribution of the nightside ionopause locations is asymmetric with respect to the direction of the solar wind electric field, leaning opposite to the electric field vector. It is also found that the asymmetry is increasingly prominent as the magnitude of the motional electric field increases, while not so prominent for small field magnitude. This result suggests that the asymmetric ionopause location in the nightside is related to the acceleration of pickup ions.

On radial evolution of stochastic acceleration in a dissipative solar wind turbulence with spatially distributed sources

The spatial behavior of stochastic acceleration of particles in the expanding solar wind plasma is considered. For this, a standard model is used representing a radially evolving, driven solar wind turbulence. On the basis of a turbulence correlation timescale approach, an analytical model is developed describing the radial dependence of the correlation length of the turbulence driven by stream interactions and the production of pickup ions. This analytical model, which might also be beneficial for other interesting problems such as solar modulation of cosmic rays, is used to consider the radial variation of Fermi‐II acceleration of pickup ions in a dissipative, fully driven solar wind turbulence on their way to the heliospheric termination shock. This might be of particular interest for the injection of pickup ions into the process of diffusive shock acceleration and their subsequent conversion to anomalous cosmic rays.

A collimator design for monitoring heliospheric energetic neutral atoms at 1 AU

SOHO's detection of heliospheric energetic H and He atoms in the energy range ∼55–280 keV has led to simulations of the production of these particles in the heliosheath and inner heliosphere that could be compared with observation. Improved and extended observation of heliospheric energetic neutral atoms (ENA) in this energy range should be useful in at least three related areas of study. These are: (1) the acceleration of pickup ions in the inner heliosphere, then again at the termination shock to become anomalous cosmic rays, and their subsequent propagation in the heliosheath; (2) the anomalous cosmic ray spectrum at the termination shock; and (3) the latitudinal distribution of heliospheric shock structures. Future advances in this field will require a new generation of ENA spectrometers. Essential to all ENA spectrometers is an effective ion-rejecting collimator. Taking advantage of the lessons learned from the first generation of ENA instruments and new computer simulations, we report on a preliminary design for an ion-rejecting collimator suitable for second-generation instrument to monitor ENA of both inner and outer heliospheric origins at the readily accessible L1 location at 1 AU.

Pickup ion acceleration by turbulent field‐aligned electric fields in the slow low‐latitude solar wind

Recent spacecraft observations reveal the continuous presence of energetic tails in solar wind and pickup ion spectra in the quiet, slow low‐latitude solar wind. To explain this phenomenon, we take into account growing evidence from observations, theory, and simulations that MHD turbulence in the solar wind has a dominant two‐dimensional (2‐D) component. Of special significance is that 2‐D MHD turbulence simulations feature turbulent field‐aligned electric fields due to turbulent convection of 2‐D magnetic field fluctuations. Such electric fields have been observed in the low‐latitude solar wind. We explore pickup ion propagation and acceleration between the Sun and Earth with a newly developed kinetic numerical model. It includes a random distribution of large‐scale field‐aligned electric field fluctuations consistent with observations in the low‐latitude solar wind. The main result is the qualitative reproduction of observed accelerated interstellar pickup He+ spectra at Earth. We find that a spectral “knee” connects the pickup ion core with the accelerated “tail.” Like observations, the simulated tail is approximately a power law f(v) ∝ v−4.5 until it reaches particle speeds ∼10 times the solar wind speed, where a rollover occurs. The energetic tail particles originate closer to the Sun, where acceleration is most efficient. The knee forms because the local pickup ion source particles are abundant but weakly accelerated. Further predictions, such as large knees for pickup ion species formed from interstellar neutrals with large ionization cavities, and the absence of knees in accelerated spectra of inner source pickup ions are also discussed.

Pickup ion acceleration by turbulent electric fields in the slow solar wind

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.

Self‐consistent injection and acceleration of pickup ions at the solar wind termination shock

A unified description of pickup and anomalous cosmic ray proton transport in the near ecliptic regions of the upwind heliosphere is presented. Multiply reflected ion acceleration theory is used for the injection of pickup protons into the diffusive shock acceleration mechanism, so yielding the anomalous cosmic ray component at the quasi‐perpendicular termination shock. The results show that both the multiply reflected and simultaneously accelerated pickup protons and the anomalous protons significantly modify the termination shock. Although the shock mediation lowers the cut‐off speed of the accelerated pickup protons, a sufficient large population of anomalous protons is accelerated at the termination shock to give a favorable comparison of the modulated anomalous proton spectra up‐stream with observations at large heliocentric distances.

Acceleration of Pick-up Ions at the Solar Wind Termination Shock

It is generally accepted that pick-up ions act as a seed population for anomalous cosmic rays originating at the solar wind termination shock. We believe that the ion pre-acceleration process operating in the heliosphere up to the termination shock can be very important to inject the ions into the shock acceleration process. The pick-up ions pre-accelerated by solar wind turbulences have already a pronounced high energy tail when they reach the shock. Some fraction of these ions can experience further acceleration up to energies of anomalous cosmic rays by means of shock drift and diffusive acceleration. In the present paper the shock drift acceleration of pick-up ions suffering multiple reflection due to abrupt changes in both the strength and direction of the magnetic field through the shock is considered. The reflection process operates for high velocity particles different from the reflection by the electric cross-shock potential. During the first reflection the mean kinetic energy of pick-up ions increases by approximately a factor of 10. Reflected particles have highly anisotropic velocity distribution. Subsequent excursion of the particles in the turbulent upstream flow leads to diffusion in pitch-angle space and, as a result, the particles can return to the shock again suffering, thus, multiple encounters. In order to describe the motion of particles in the upstream and down streamparts of the flow we solve the Fokker-Plank transport equation for anisotropic velocity distribution function.