Pickup Ion Population Research Articles

Properties and sources of the dayside martian magnetosphere

This study is based on measurements of the energy-mass analyzer Suprathermal and Thermal Ion Composition instrument (STATIC, McFadden et al., 2015) and magnetometer (MAG, Connerney et al., 2015) on the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft (Jakosky et al., 2015). 48 passes of MAVEN at SZA < 70° in the northern hemisphere on the day side of Mars (in order to avoid the influence of remnant magnetic field) were selected with condition to have continuous observations of STATIC from ionosphere to magnetosheath (“complete” observation crossings). It was found that in the vast majority of such “complete” crossings there is a layer between the ionosphere and magnetosheath, where two oxygen ion populations are observed: (1) heated ionospheric ions and (2) higher energy oxygen ions. The heated ionospheric ions are located just above the cold ionospheric ions and is identified by the increase of energy and temperature and decrease of number density. The second ion population has a wide energy spread from a few eV to hundreds of eV and has a velocity distribution typical to pick-up ions. The quite similar characteristics of the pick-ions just upstream of the magnetopause and characteristics of the second ions component in the layer between magnetosheath and ionosphere suggests their common origin. Pick-up ions in this layer have energies from a few to hundred electron volts have gyroradii comparable or significantly larger (from ~80 to ~570 km) than the thickness of this layer (~90 km).Pick-up ions from magnetosheath penetrate into upper ionosphere and interact with ionospheric ions and upper atmosphere. The population of pick-up ions provides additional energy source to the ionosphere and the upper atmosphere of Mars.The layer between dayside magnetosheath and ionosphere is not sufficiently studied and has many different names (see Espley, 2018). The physical state and properties of this layer (formed by combination of hydrodynamic and kinetic processes) are very much different from magnetosheath and ionosphere. This is defined as the dayside magnetosphere of Mars (Vaisberg et al., 2018).

Nonstationary Quasiperpendicular Shock and Ion Reflection at Mars

AbstractCollisionless shocks in space plasma are regions of heating and acceleration of charged particles and dissipation of kinetic energy. These accelerated particles are the source of electromagnetic emissions from supernova remnants and other astrophysical structures. At high Mach numbers, shocks can be inherently nonstationary and exhibit modulated energy transfer and recurring plasma compression areas in the form of reformation. We use data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft to study reformation of the Martian bow shock, which has a relatively high curvature compared to that at Earth, and the upstream solar wind is often mass loaded with a population of pickup ions. We show evidence of ion reflection effects in reformation of a supercritical quasiperpendicular shock.

Ion pickup observed at comet 67P with the Rosetta Plasma Consortium (RPC) particle sensors: similarities with previous observations and AMPTE releases, and effects of increasing activity

Rosetta's unique trajectory is allowing exciting measurements of the development of cometary activity between ∼3.6 and 1.2 AU for the first time. For a few months following Rosetta's arrival at comet 67P in August 2014, data from the Rosetta Plasma Consortium (RPC) particle instruments (the Ion and Electron Spectrometer (IES) and the Ion Composition Analyser (ICA)), have shown that the low activity cometary environment was initially dominated by the solar wind. This was expected in the early stages of the mission. In addition to the solar wind and related He+ populations, a low energy pickup ion population is seen intermittently in the early phase of the mission near the comet. The population is very time dependent, but at times reaches higher energy approaching the solar wind energy. During these intervals, ICA data indicate that the composition is mainly water group ions. The rising energy signatures of these ions observed at times indicate that they are in the early phases of the pickup process, initially accelerated by the electric field (‘early phase pickup’). Here, we compare these exciting pickup ion measurements with Giotto measurements at the relatively weak (compared to Halley) comet Grigg-Skjellerup, where early phase pickup was seen including non-gyrotropic cometary ions, and with the AMPTE lithium and barium releases. Our results reveal some striking similarities with the AMPTE releases, particularly the rising energy signature related to early pickup, and a momentum balance between the pickup ions and the deflected solar wind. There is also evidence for momentum transfer between the pickup ions and the solar wind, with less velocity change seen in the solar wind alpha particles compared to the protons; this was also observed in an AMPTE lithium release. We discuss the effects of increasing activity observed between 3.6 to 1.8 AU, including the increasing dominance and energisation of pickup ions, increasing ionospheric effects and the decreasing effect of the solar wind.

Self‐consistent multifluid MHD simulations of Europa's exospheric interaction with Jupiter's magnetosphere

AbstractThe Jovian moon, Europa, hosts a thin neutral gas atmosphere, which is tightly coupled to Jupiter's magnetosphere. Magnetospheric ions impacting the surface sputter off neutral atoms, which, upon ionization, carry currents that modify the magnetic field around the moon. The magnetic field in the plasma is also affected by Europa's induced magnetic field. In this paper we investigate the environment of Europa using our multifluid MHD model and focus on the effects introduced by both the magnetospheric and the pickup ion populations. The model self‐consistently derives the electron temperature that governs the electron impact ionization process, which is the major source of ionization in this environment. The resulting magnetic field is compared to measurements performed by the Galileo magnetometer, the bulk properties of the modeled thermal plasma population is compared to the Galileo Plasma Subsystem observations, and the modeled surface precipitation fluxes are compared to Galileo Ultraviolet Spectrometer observations. The model shows good agreement with the measured magnetic field and reproduces the basic features of the plasma interaction observed at the moon for both the E4 and the E26 flybys of the Galileo spacecraft. The simulation also produces perturbations asymmetric about the flow direction that account for observed asymmetries.

Plasma Physics of the Very Local Interstellar Medium

Models of the large-scale heliosphere show that the very local interstellar medium (VLISM), bounded by the heliopause and either a bow wave or bow shock, experiences significant heating by the deposition of neutral hydrogen (H) originating from the hot inner heliosheath. As hot neutrals stream into the interstellar medium, they experience charge exchange with the background cooler interstellar protons, creating a population of energetic (~ 1 keV) pickup ions. Similarly, fast neutrals created in the supersonic solar wind stream into the local interstellar medium and create a pickup ion population in the VLISM. The importance of these pickup ions is thought to manifest itself in the creation of the IBEX ribbon. Like the outer regions of the supersonic solar wind and the inner heliosheath, the VLISM is a pickup ion mediated plasma. Here we derive a model of a pickup ion mediated plasma using an approach analogous to a Chapman-Enskog expansion. We derive the anomalous heat flux and obtain a three-fluid model comprising electrons, thermal protons, and pickup ions. We investigate waves in a pickup ion mediated VLISM plasma, comparing the basic properties to those of the better known two-fluid model.

COMET 1P/HALLEY MULTIFLUID MHD MODEL FOR THE GIOTTO FLY-BY

The interaction of comets with the solar wind has been the focus of many studies including numerical modeling. We compare the results of our multifluid MHD simulation of comet 1P/Halley to data obtained during the flyby of the European Space Agency's Giotto spacecraft in 1986. The model solves the full set of MHD equations for the individual fluids representing the solar wind protons, the cometary light and heavy ions, and the electrons. The mass loading, charge-exchange, dissociative ion-electron recombination, and collisional interactions between the fluids are taken into account. The computational domain spans over several million kilometers, and the close vicinity of the comet is resolved to the details of the magnetic cavity. The model is validated by comparison to the corresponding Giotto observations obtained by the Ion Mass Spectrometer, the Neutral Mass Spectrometer, the Giotto magnetometer experiment, and the Johnstone Plasma Analyzer instrument. The model shows the formation of the bow shock, the ion pile-up, and the diamagnetic cavity and is able to reproduce the observed temperature differences between the pick-up ion populations and the solar wind protons. We give an overview of the global interaction of the comet with the solar wind and then show the effects of the Lorentz force interaction between the different plasma populations.

Pickup ions and ion cyclotron wave amplitudes upstream of Mars: First results from the 1D hybrid simulation

Observations upstream of Mars show strong and persistent plasma wave activity near the local proton cyclotron frequency. These waves are thought to be generated by unstable populations of newborn ions produced by ionization of the Martian hydrogen exosphere. It is generally believed that the amplitudes of the waves are related to the energy of the local pickup ion population, such that the observed spatial and temporal distribution of the wave amplitudes could be used as a diagnostic of local exospheric structure and loss rate. To understand the relationship between wave amplitudes and ion pickup rate, we carry out a parametric study using the 1D hybrid simulation (kinetic ions, fluid electrons) for varying pickup rates and pickup geometry for the Martian planetary environment. Our results indicate that, contrary to expectations, the observed local ion cyclotron wave amplitudes cannot be easily related to local ion pickup rates because the growth time of the instability is long compared to the transit time of the waves past Mars. Additionally, we find the amount of pickup ion energy lost to wave growth is not a constant over all injection rates and wave amplitudes are likely affected by propagation through regions of spatially non‐uniform ion production. Because the waves in the upstream region are in a state of growth rather than saturated, the simulations suggest the pickup ion distributions are likely only partially scattered.

PICKUP ION DYNAMICS AT THE HELIOSPHERIC TERMINATION SHOCK OBSERVED BYVOYAGER 2

The recent Voyager 2 (V2) observations of the termination shock (TS) indicate that it is a plasma shock unlike any other in the heliosphere with the dynamics and structure heavily influenced by the presence of an energized population of pickup ions (PUIs). The unexpected finding of cold plasma downstream of the TS in the heliosheath, corresponding to very little heating of the thermal solar wind (SW), suggests that the energy dissipated by the shock is dominated by the energization of PUIs at the TS. We examine the shock surfing mechanism at the test particle level, where multiply reflected ions (MRIs) gain energy from the motional electric field as a consequence of reflection from the cross-shock potential (CSP), for a model of the TS3 (the third TS crossing measured by V2). The energization of PUI filled-shell distributions at a stationary, perpendicular model of the TS3 indicates that shock surfing can provide both substantial PUI acceleration and a dissipation mechanism at the TS. For a sufficiently strong CSP and sufficiently narrow shock ramp MRI acceleration can account for the missing energy of the downstream SW plasma.

A MEASUREMENT OF THE ADIABATIC COOLING INDEX FOR INTERSTELLAR HELIUM PICKUP IONS IN THE INNER HELIOSPHERE

Interstellar neutral gas enters the inner heliosphere where it is ionized and becomes the pickup ion population of the solar wind. It is often assumed that this population will subsequently cool adiabatically, like an expanding ideal gas due, to the divergent flow of the solar wind. Here, we report the first independent measure of the effective adiabatic cooling index in the inner heliosphere from SOHO CELIAS measurements of singly charged helium taken during times of perpendicular interplanetary magnetic field. We use a simple adiabatic transport model of interstellar pickup helium ions, valid for the upwind region of the inner heliosphere. The time averaged velocity spectrum of helium pickup ions measured by CELIAS/CTOF is fit to this model with a single free parameter which indicates an effective cooling rate with a power-law index of gamma = 1.35 +- 0.2. While this average is consistent with the 'ideal-gas' assumption of gamma = 1.5, the analysis indicates that such an assumption will not apply in general, and that due to observational constraints further measurements are necessary to constrain the cooling process. Implications are discussed for understanding the transport processes in the inner heliosphere and improving this measurement technique.

Non‐Maxwellian Proton Velocity Distributions in Nonradiative Shocks

The Balmer line profiles of nonradiative supernova remnant shocks provide the means to measure the postshock proton velocity distribution. While most analyses assume a Maxwellian velocity distribution, this is unlikely to be correct. In particular, neutral atoms that pass through the shock and become ionized downstream form a nonthermal distribution similar to that of pickup ions in the solar wind. We predict the Hα line profiles from the combination of pickup protons and the ordinary shocked protons, and we consider the extent to which this distribution could affect the shock parameters derived from Hα profiles. The Maxwellian assumption could lead to an underestimate of shock speed by up to about 15%. The isotropization of the pickup ion population generates wave energy, and we find that for the most favorable parameters this energy could significantly heat the thermal particles. Sufficiently accurate profiles could constrain the strength and direction of the magnetic field in the shocked plasma, and we discuss the distortions from a Gaussian profile to be expected in Tycho's supernova remnant.

Characteristics of Scatter‐free Behavior of Heliospheric Pickup Ions

Theoretical analyses of the global heliospheric pickup ion population generally involve the solution of forms of the Boltzmann or transport equation. Like their cosmic-ray counterparts, these approaches often presume a simplified geometry for the interplanetary magnetic field and that diffusive processes dominate the already picked up particles' behavior. At the opposite extreme lies the test particle picture of pickup ions, wherein their motion is tracked from the birth of the ions through their outward travel in the Parker spiral magnetic field. This approach has the advantage that it can include details of the source distribution, the initial pickup by the solar wind convection electric field at the site of ion production, and the combined effects of the radially evolving electric and magnetic fields experienced by the ions. Here test particles representing O+ pickup ions are used to examine the influences of the source characteristics, including the heliocentric radius of their birth and the velocities of their parent neutrals. The consequences for ion populations from interstellar, interplanetary dust, and Jovian sources are considered. The results suggest some potentially observable features of general pickup ion distributions that could aid in the interpretation of their different sources.

The Injection Problem for Anomalous Cosmic Rays

The precise mechanism by which some interstellar pickup ions are selected to be accelerated up to anomalous cosmic-ray (ACR) energies at the heliospheric termination shock is a central puzzle in the physics of the outer heliosphere. Observations by Gloeckler et al. of the mass dependence of the efficiency with which low-energy pickup ions are accelerated at an interplanetary shock is in the opposite sense of related ACR observations made by Cummings and Stone. Furthermore, the rapidity with which ions must be accelerated at the termination shock requires that the scattering of these ions be weak, an effect which requires that relatively energetic ions exist initially. It is shown here that these three sets of observations can be reconciled by using a multiply reflected ion acceleration mechanism to provide a preaccelerated population of pickup ions at the termination shock. The precise fraction of preenergized pickup ions that can subsequently be diffusively shock accelerated depends on the ion scattering strength at the termination shock. It is shown that the assumption of weak scattering yields an anomalously low injection efficiency for pickup H+ compared to those of He+, O+, and Ne+, and those inferred for C+ and N+. The predicted injection efficiencies of C+, N+, O+, and Ne+ are clustered together closely whereas He+ is a little less efficiently injected. Computed ACR termination shock and modulated fluxes compare well to those observed and inferred by Cummings and Stone. Finally, strong scattering of ions at the termination shock is found to eliminate the mass dependence of the ACR injection efficiency.

Shock surfing vs. shock drift acceleration

We numerically investigate the pre‐injection mechanism by which pickup ions approaching quasi‐perpendicular shocks may be accelerated above the threshold velocity required to begin diffusive shock acceleration. We demonstrate that three distinct mechanisms contribute to the acceleration and that they operate on disjoint subsets of the pickup ion population. We quantitatively compare the efficiency of each mechanism as a function of shock Mach number, pickup ion mass and shock ramp width and show that shock surfing predominates below a critical ramp width.

Return to Io by the Galileo spacecraft: Plasma observations

On October 11, 1999, a series of high‐resolution plasma measurements were obtained during the second close flyby of Io of the Galileo mission. The closest approach to Io occurred at an altitude of 617 km along the flank of Jupiter's torus plasma flows past this moon. Energy/charge (E/Q) and mass/charge (M/Q) measurements with the plasma analyzer were used to identify the primary ions. At closest approach the ion number densities increased to their maximum values of 1200 cm−3 as calculated from the plasma moments. These ions were dominated by two thermal populations of torus ions with M/Q = 16, one with density and temperature of 800 cm−3 and kT= 40 eV and the other with 200 cm−3 and 160 eV. On the basis of the previous Voyager 1 remote measurements with a spectrometer, these two thermal ion plasmas are identified as O+ and S++, respectively. A substantial population of pickup ions was also observed, with densities of about 100 cm−3 each for S+ and SO+. The plasma bulk flows were strongly deflected around the body of Io, and the speed of the bulk flow increased to a factor of 1.3 greater than that for rigid corotational flow. The torus ion densities just outside of lo's orbit were about 800 cm−3. These densities at the immediate position of Io are smaller by a factor of about 4 relative to those during the first flyby on December 7, 1995, but should not be taken as indicative of a similar decrease in the bulk of the plasma torus. Two major plasma regimes were encountered before the closest approach to Io. The first was a region of hot ions located inside of lo's orbit which was previously identified as the “ribbon” of enhanced plasma densities with Voyager 1 plasma measurements and with ground‐based imagery. The ion composition in the ribbon included O+ and S++. The second region, closer to Io, exhibited a hot torus plasma which was mixed with pickup hydrogen ions. These hydrogen ions were detected at distances beginning at 7.8 RIo from this moon, which were generally inside the Hill (Lagrange) “sphere” for Io's atmosphere.

Cross‐helicity and residual energy in solar wind turbulence: Radial evolution and latitudinal dependence in the region from 1 to 5 AU

Solar wind plasma and magnetic field measurements by Ulysses have been used to study magnetohydrodynamic turbulence in different heliospheric regions. Four intervals of six solar rotations have been analyzed. Two of them are on the ecliptic around 2 and 5 AU, respectively, one is at midlatitude near 5 AU, and the last one is at high latitude around 3 AU. Conditions on the ecliptic are those typical of high solar activity periods. The midlatitude interval is characterized by very strong gradients in the wind speed, due to an intermittent appearance of the wind coming from the polar coronal hole. In the high‐latitude interval, fully inside the polar wind, the speed is steadily high. We investigated at three different scales (1, 4, and 12 hours) the level of correlation between velocity and magnetic field fluctuations, as given by the normalized cross‐helicity, and the sharing of the fluctuation energy between its kinetic and magnetic component, as measured by the normalized residual energy. The observations on the ecliptic, while confirming previous findings based on Voyagers data, clearly indicate that the normalized cross‐helicity is well different from zero also at distances as large as 5 AU. The midlatitude turbulence, when compared to that at low and high heliographic latitudes, appears much more evolved, with a remarkably lower normalized cross‐helicity (in absolute value). This unambiguously highlights that processes at velocity gradients are an important factor in the turbulence evolution. For all the analyzed intervals the residual energy values indicate an imbalance in favor of magnetic fluctuations, in agreement with previous results. The strongest imbalance is observed for the high‐latitude sample, where the turbulence is comparatively the least evolved. This is a quite unexpected result, probably related to the presence of interstellar pickup ion populations. In conclusion, our analysis indicates that (1) velocity gradients play a dominant role in driving the turbulence evolution in the solar wind and (2) pickup ion effects might be significant.

Observations of the solar wind and interstellar pick-up ion populations in the heliosphere with Ulysses

Observations of the solar wind and interstellar pick-up ion populations in the heliosphere with Ulysses

Correlated plasma wave, magnetic field, and energetic ion observations in the ion pickup region of comet Giacobini‐Zinner

Relationships between simultaneous plasma wave, magnetic field, and energetic heavy ion data obtained by the International Cometary Explorer (ICE) spacecraft in the large‐scale solar wind particle pickup region surrounding comet Giacobini‐Zinner are examined. In particular, we investigate the conditions under which electrostatic emissions at frequencies of a few kilohertz and electromagnetic waves at a few tens of hertz are observed. It is shown that the data are consistent with the view that the kilohertz electrostatic emissions result from a beam‐type instability excited by the pickup photoelectron population when the angle α between the magnetic field and the plasma velocity vectors is not too large (α &lt; 60°). The data also suggest that the few tens of hertz electromagnetic waves may be excited by a ring‐type instability associated with the pickup ion population, which occurs when the magnetic field is near to orthogonality with the flow (α &gt; 60°).

Voyager observations of lower hybrid noise in the Io plasma torus and anomalous plasma heating rates

A study of Voyager 1 electric field measurements obtained by the plasma wave instrument in the Io plasma torus has been carried out. A survey of the data has revealed the presence of persistent peaks in electric field spectra in the frequency range 100-600 Hz consistent with their identification as lower hybrid noise for a heavy-ion plasma of sulfur and oxygen. Typical wave intensities are 0.1 mV/m, and the spectra also show significant Doppler broadening, Delta omega/omega approximately 1. A theoretical analysis of lower hybrid wave generation by a bump-on-tail ring distribution of ions is given. The model is appropriate for plasmas with a superthermal pickup ion population present. A general methodology is used to demonstrate that the maximum plasma heating rate possible through anomalous wave-particle heat exchange is less than approximately 10 to the -14th ergs per cu cm per s. Although insufficient to meet the power requirement of the EUV-emitting warm torus, the heating rate is large enough to maintain a low-density (0.01-0.1 percent) superthermal electron population of keV electrons, which may lead to a small but significant anomalous ionization effect.