Pickup Protons Research Articles

Galileo plasma wave observations of iogenic hydrogen

The Galileo plasma wave instrument has detected intense electromagnetic wave emissions approximately centered on the second and fourth harmonics of the local proton gyrofrequency during the close equatorial flyby of Io on 7 December 1995. Their frequencies suggest these emissions are likely generated locally by an instability driven by non thermal protons. Given that this process occurs close to Io, we suggest that hydrogen-bearing compounds, escaping from Io, are broken up/ionized near this moon, thereby releasing protons. Newly-created protons are thus injected in the Jovian corotating plasma with the corotation velocity, leading to the formation of a ring in velocity space. Several electromagnetic wave–particle instabilities can be driven by a ring of newborn protons. Given that the corotating plasma is sub-Alfvénic relative to Io, the magnetosonic mode cannot be destabilized by this proton ring. The full dispersion relation is studied using the WHAMP program ( Rönmark, 1982. Rep. 179. Kiruna Geophys. Inst., Kiruna, Sweden) as well as a new algorithm that allows us to fit the distribution function of newborn protons in a more realistic way. This improvement in the ring model is necessary to explain the relative narrowness of the observed spectral peaks. The measured E/ B ratio is also used to identify the relevant instability and wave mode: this mode results from the coupling between the ion Bernstein and the ion cyclotron mode (IBCW). To our knowledge this mode has not yet been studied. From the instability threshold an estimate of the density of newborn protons around Io is thus given; at about 2 Io radii from the surface and 40°W longitude from the sub-Jupiter meridian, this density is found to be ≥0.5% of the local plasma density (∼4000 cm −3), namely ≥20 cm −3. Assuming a stationary pickup process and a r − n distribution of pickup protons within several Io radii of Io’s wake, this implies that more than 10 26 protons/s are created around Io. The ultimate origin of these protons is an open issue.

Evolution of a strong shock in the distant heliosphere

The 1991 global merged interaction region (GMIR) shock is a strong forward shock observed from Voyager 2 on day 146 of 1991 at 34.6 AU at the leading edge of the 1991 GMIR. This shock is the strongest shock ever identified from the Voyager data in the distant heliosphere. In this paper we study the evolution and propagation of the 1991 GMIR shock outside 34.6 AU in the upwind direction taking into account the influence of interstellar pickup protons. We calculate the time, location, and various variables of this shock in the subsequent 400 days. We use the shock interaction model to develop a R,t‐simulation code to study this problem. The model treats the shock as a surface of discontinuity with zero thickness and uses the method of characteristics to study the evolution of the solar wind in the disturbed region. This model can accurately calculate the propagation speed and the strength of the shock. The numerical solution shows that as the GMIR shock propagates from 35 AU to 150 AU, the shock speed decreases monotonically from 543 km/s to 478 km/s and the density ratio decreases from 2.55 to 2.09. The solar wind proton temperature and the pickup proton temperature on two sides of the shock decrease monotonically with increasing heliocentric distance. Across the shock the solar wind proton temperature ratio is high, namely, ∼22 at 35 AU and ∼11.5 at 150 AU, while the pickup proton temperature ratio is an order of magnitude smaller: ∼2.1 at 35 AU and ∼1.8 at 150 AU.

Shocks in the distant heliosphere

Voyager data are used to study two aspects of pickup proton shocks in the distant heliosphere: (1) Two shocks having similar speed jump of ∼50 km/s observed at 5 AU and at 43 AU demonstrate the effects of pickup protons on weakening interplanetary shocks at increasing heliocentric distance. (2) On the basis of three shocks identified outside 34 AU from high‐resolution plasma and magnetic field data we develop an empirical model to study the jump conditions of pickup protons across solar wind shocks. Making use of the empirical model one can study the postshock conditions of pickup protons for the global merged interaction region (GMIR) shock and the termination shock; this will enable us to calculate more accurately the propagation of the GMIR shock and the interaction of GMIR shock with the termination shock. We also find that the variation of the pickup proton temperature TI across the shock is very close to the adiabatic relation that TI is proportional to the magnetic field strength B.

Interaction of pickup ions with quasi‐parallel shocks

We have performed a number of numerical experiments to study the interaction of interstellar pickup protons and helium ions with quasi‐parallel stationary (bow) shocks and interplanetary traveling shocks. The collisionless shocks are modeled by the hybrid simulation method which treats the ions as macroparticles and the electrons as a massless background fluid. Solar wind protons, alpha particles, pickup H+ and He+ are included self‐consistently. The pickup ion distributions are modeled as spherical shells in velocity space convected with the solar wind. Pickup ions are rather efficiently reflected at both types of shocks. The reflection coefficient, measured as the ratio of incident to reflected ions, is more than one order of magnitude larger than the reflection coefficients for either solar wind protons or alpha particles, and is of the order of 30%. We have found that at high Mach number shocks pickup ions can reach many tens to over hundred times the plasma ramming energy in essentially one shock encounter. The reflected pickup ion distributions have a pancake‐type shape in velocity space.

The acceleration of pickup ions at shock waves: Test particle‐mesh simulations

A mechanism for the acceleration of pickup ions by repeated reflection from the electrostatic cross‐shock potential of a quasi‐perpendicular shock was proposed independently by Zank et al. [1996b] and Lee et al. [1996]. The acceleration mechanism, known variously as Multiply Reflected Ion (MRI) acceleration or “shock surfing,” was studied by these authors in the limit of an idealized shock, which possessed neither fine‐scale structure (distinct foot, ramp, or overshoot) nor pickup ion scattering turbulence. Here the acceleration of pickup ions at cometary and interplanetary shocks and at the termination shock is studied in the “test” particle limit on the basis of a particle‐mesh simulation. All simulations assume a shell distribution for the pickup ions (either pickup protons or helium), and the dynamics of pickup ions propagating in a fixed electromagnetic field profile are investigated. The effect of a shock foot, ramp and overshoot on the acceleration of pickup ions at perpendicular and oblique shocks is described. The acceleration of pickup ions in the presence of strong turbulence inside the foreshock region is also addressed. For quasi‐perpendicular shocks with structure, we obtained the following results. First, the accelerated H+ and He+ spectrum is a very hard power law ∝ E−k, k = 0.92–1.2, which is much harder than that predicted by diffusive shock acceleration. Also, as θbn, the angle between the upstream magnetic field and shock normal, decreases from 90°, the accelerated pickup ion spectrum flattens until MRI acceleration ceases. Second, the fine structure of the shock is found to reduce slightly the maximum energy gain for an accelerated pickup ion compared to that gained at an unstructured shock. Third, a flat turbulence spectrum is found to lead to an increase in the maximum energy for transmitted pickup ions, whereas a power law turbulence spectrum leads to a modest reduction in the maximum pickup ion energy gain. However, the basic MRI acceleration mechanism continues to operate in the presence of turbulent magnetic fluctuations and the characteristic hard spectra are preserved. Fourth, MRI acceleration is found to work only for those shocks for which θbn>60°–70°. The results from the test particle simulations described here are also used to interpret particle acceleration in self‐consistent hybrid simulations.

Solar wind in the distant heliosphere

This paper presents a quantitative MHD model to study the effects of pickup protons on the solar wind. The model includes the kinetic theory solution of interstellar neutral hydrogen, the ionization process for the production of pickup protons, the spiral interplanetary magnetic field, and the full MHD equations. In order to study the effects of pickup protons, we first obtain the kinetic theory solutions of interstellar neutral hydrogen. This solution is then used to calculate the flow of interstellar hydrogen and the production of pickup protons. We study a one‐dimensional, steady state model of the pickup proton solar wind in the upwind direction. The solar wind proton parameters predicted by this solution agree reasonably well with observational data from Voyager 2. Throughout the heliosphere the temperature of pickup protons is higher than the temperature of the solar wind protons by two orders of magnitude. Outside the ionization radius, the solar wind temperature and the fast speed begin to increase and the bulk speed and the Mach number of the wind begin to decrease. The component of the equation of motion along the streamline direction can be written in the form of a Bernoulli equation; it can clearly explain the deceleration of the solar wind. In the distant heliosphere the sum of the kinetic energy and the thermal energy decreases with the heliocentric distance due to the charge exchange process; at the same time, the thermal energy of the solar wind plasma increases due to heating by the pickup process. The combined action causes a significant radial decrease in the kinetic energy of the wind.

The Martian Magnetosphere - A Laboratory For Bi-Ion Plasma Investigations

The Martian space gives us an example of the direct interaction of the solar wind dominated by protons and the planetary plasma of heavy ions. Due to the lack of the intrinsic magnetic field the solar wind exposes the extended exosphere/atmosphere of Mars far from the bow shock. Photoionization, electron impact and charge exchange lead to the creation of newly born ions which may affect the incoming solar wind. At large distances the most abundant neutral components are molecular and atomic hydrogen, which could reach very high altitudes as the consequence of a weak gravitational attraction of Mars. It was found that pickup protons modify significantly the structure of the ion foreshock and, probably, the bow shock itself due to their effective reflection at the bow shock (Dubinin et al., 1994; Dubinin et al., 1995). Fast oxygen atoms originated in the process of the dissociative recombination of molecular oxygen, which dominates at the ionosphere heights additionally contribute to the neutral exosphere. Closer to the planet, the shocked solar wind interacts with a dense oxygen atmosphere, which produces an effective obstacle to the solar wind. In addition to the features related with the extended atmosphere, the importance of the ion gyroradius effects is also unique at Mars. The pick-up gyroradius of O + ions exceeds the characteristic size of the system and the classical MHD description is violated. In such Large Larmour Radius (LLR) configurations a relative streaming of different ion species is admissible and leads to a strong coupling between the solar wind protons and heavy planetary ions. In this paper we focus on effects of heavy ion population on dynamics of the solar wind. In section 2, a description of the MHD approach for two ion species is given. In section 3, we discuss mechanism of coupling between both species moving with different speeds. In section 4, the problem of critical points and shocks in bi-ion flows is considered. In section 5, we briefly discuss phenomenon of the’ obstacle boundary’ near Mars. In section 6 we derive the dispersion relations for low-frequency waves in bi-ion plasma composed of the solar wind as a core plasma and a minority of unmagnetized heavy ions which are in a relative streaming. In section 7, dispersion analysis of whistler waves is made. Application of these results to the Martian space is given. Observations and results of modeling are presented to illustrate our findings.

A self‐consistent determination of the heliospheric termination shock structure in the presence of pickup, anomalous, and galactic cosmic ray protons

A self‐consistent time‐dependent model is used to study the modification of the heliospheric termination shock in the upwind direction under the influence of three suprathermal particle populations, namely pickup, anomalous and galactic cosmic ray protons. By ensuring that the resulting modulated cosmic ray proton spectra are consistent with those observed by the Voyager 2 and Pioneer 10 spacecraft during the solar activity minimum in 1987, two alternative modifications of the shock can be identified. While the first is characterized by a low injection efficiency of pickup protons into the process of diffusive shock acceleration and mainly determined by those particles, the second, resulting from a high injection efficiency, is clearly dominated by anomalous cosmic rays.

The Influence of Pickup, Anomalous, and Galactic Cosmic-Ray Protons on the Structure of the Heliospheric Shock: A Self-Consistent Approach

A self-consistent time-dependent model is used to study the modification of the heliospheric shock in the upwind direction under the influence of three suprathermal particle populations, namely, pickup, anomalous, and Galactic cosmic-ray protons. By ensuring that the resulting modulated cosmic-ray proton spectra are consistent with those observed by the Voyager and Pioneer spacecraft during the solar activity minimum in 1987, two alternative modifications of the heliospheric shock can be identified. While the first is characterized by a low injection efficiency of pickup protons into the process of diffusive shock acceleration and is mainly determined by those particles, the second, resulting from a high injection efficiency, is clearly dominated by anomalous cosmic rays.

Pickup protons in the heliosphere

We calculate the conditions of pickup protons inside the termination shock. Outside 50 AU the partial pressure of pickup protons is greater than the magnetic pressure by a factor of > 10, and greater than the partial pressure of solar wind protons by a factor of > 100. Thus, pickup protons have a significant dynamical influence on the structures of the solar wind in the outer heliosphere.

Voyager observations of the magnetic field, interstellar pickup ions and solar wind in the distant heliosphere

Voyagers 1 and 2 are now observing the latitudinal structure of the heliospheric magnetic field in the distant heliosphere (the region between ≃ 30 AU and the termination shock). Voyager 2 is observing the influence of the interstellar medium on the solar wind. The pressure of the interstellar pickup protons, measured by their contribution to pressure balanced structures, is greater than or equal to the magnetic pressure and much greater than the thermal pressures of the solar wind protons and electrons in the distant heliosphere. The solar wind speed is observed to decrease and the proton temperature increase with increasing distance from the sun. This may result from the production of pickup ions by the charge exchange process with the interstellar neutrals. The introduction of the pickup ions into the dynamics of the magnetized solar wind plasma appears to be an important new process which must be considered in future theoretical studies of the termination shock and boundary with the local interstellar medium.

The abundance of atomic 1H, 4He and 3He in the local interstellar cloud from pickup ion observations with SWICS on Ulysses

Pickup ions measured deep inside the heliosphere open a new way to determine the absolute atomic density of a number of elements and isotopes in the local interstellar cloud (LIC). We derive the atomic abundance of hydrogen and the two isotopes of helium from the velocity and spatial distributions of interstellar pickup protons and ionized helium measured with the Solar Wind Ion Composition Spectrometer (SWICS) on the Ulysses spacecraft between ~2 and ~5 AU. The atomic hydrogen density near the termination shock derived from interstellar pickup ion measurements is 0.115±0.025 cm-3, and the atomic H/He ratio from these observations is found to be 7.7 ± 1.3 in the outer heliosphere. Comparing this value with the standard universal H/He ratio of ~10 we conclude that filtration of hydrogen is small and that the ionization fraction of hydrogen in the LIC is low.

Pickup protons and pressure‐balanced structures from 39 to 43 AU: Voyager 2 observations during 1993 and 1994

The pressure of interstellar pickup protons in the distant heliosphere can be determined by analyzing pressure‐balanced structures, observed on a scale of a few hundredths of an AU. This paper extends the earlier work of L. F. Burlaga et al. (Journal of Geophysical Research, 99, 21,511, 1994) by analyzing pressure‐balanced structures observed by Voyager 2 from 39.3 to 40.6 AU in 1993 and from 42.6 to 43.2 AU during 1994. The pickup proton temperature is high in the region of the distant heliosphere that we considered: (5.4 ± 0.1) × 106K at 39–41 AU and (6.0 ± 0.4) × 106K at ≈43 AU. The density of the pickup protons is (1.6 ± 0.3) × 10−4cm−3at 39–41 AU and (1.2 ± 0.2) × 10−4cm−3at ≈43 AU. The ratio of the pickup proton density to the solar wind proton density (Ni/N) is small, only 0.03 ± 0.01 during both 1993 and 1994. Nevertheless, the pickup proton pressures are relatively high because of their high temperatures. The pickup proton pressure was (11 ± 2) × 10−14erg cm−3at 39–41 AU and (9 ± 1) × 10−14erg cm−3at 43 AU. There is a possible decrease inPiwith increasing distance from the Sun. The pickup proton pressure is an order of magnitude greater than the solar wind proton pressure:Pi/Pswp= 10 ± 2 at 39–41 AU and 8 ± 2 at 43 AU. Our results support the hypothesis of Burlaga et al. that the pickup proton pressure is more important than the solar wind thermal pressure in the dynamics of the distant heliosphere. The ratio of the pickup ion pressure to the magnetic pressure isPi/(B2/8π) = 1.7 ± 0.3 at 39–41 AU and 1.7 ± 0.72 at ≈43 AU. These results are compared with a model.

A dispersive analysis of bispherical pickup ion distributions

We investigate the properties of a bispherical shell distribution of pickup ions when dispersion of the resonant waves is taken into account. We also extend the method of Johnstone et al. [1991] and Huddleston and Johnstone [1992] for determining the spectra of the quasi‐linear self‐generated waves to include dispersion. Our specific results assume that all waves propagate along the average magnetic field, and that the waves satisfy the cold electron‐proton dispersion relation. The major effect of including dispersion in the analysis is that the particles in a bispherical distribution retain more of their initial energy than in the nondispersive treatment, so the resulting wave spectra are less intense than predicted when dispersion is neglected. The dispersive analysis also encounters the well‐known gap in the cyclotron resonance with fast‐mode waves which is absent in the nondispersive picture. We present detailed results for pickup protons in the two extreme configurations of magnetic field perpendicular to, and parallel to, the solar wind flow for a range of values of VA/VSW. We find that when pickup protons are scattered to a bispherical distribution with VA/VSW = 0.1, typical of conditions in the solar wind, the discrepancy in the total energy density of the self‐generated waves between the dispersive and nondispersive results amounts to 18% in the perpendicular configuration. This discrepancy falls between 9% and 73% in the parallel configuration, depending on the extent of the particle transport through the resonance gap. These discrepancies are substantially larger for larger values of VA/VSW. These results indicate that dispersive effects can be important in the correct modeling of the quasi‐linear resonant wave‐particle interaction between pickup ions and self‐generated waves in the solar wind.

Locations of the termination shock and the heliopause

The locations of the termination shock and the heliopause are restudied by taking into account the effects of pickup protons. The study uses available plasma and magnetic field data from Voyagers over a 14‐year period (1978–1991) and Voyager observations of the 1992–1993 radio emission event, outside 30 AU, pickup protons have a significant influence on dynamical structures of the outer heliosphere. The solar wind is treated as a mixture of electrons, solar wind protons, and interstellar pickup protons. If the magnitude of the interstellar magnetic field Bint is given, one can quantitatively study the motion and location of the termination shock. We find that the location is anticorrelated with the sunspot number. The absolute mean of the shock speed is 19 km/s, and the quadratic mean of the shock speed is 24 km/s. Because Bint is poorly known, additional information is needed in studying the termination shock. Cummings et al. (1994) have used observations of anomalous cosmic rays to estimate the location of the shock. The observations of the 1991 global merged interaction region (GMIR) and GMIR shock and the 1992–1993 radio emission event provide another handle for the study of the termination shock and the heliopause. After its penetration through the termination shock, the GMIR shock continued to propagate in the subsonic region of the solar wind and eventually interacted with the heliopause. This interaction produced a transmitted shock propagating outward in the interstellar medium and a reflected shock propagating inward toward the Sun in the subsonic solar wind. The plasma frequencies behind the reflected and the transmitted shocks can be responsible for the 2‐ and 3‐kHz radio emissions, respectively. We assume that the impingement of the GMIR shock at the heliopause occurred at the time when Voyagers started receiving the radio emissions. Taking into account the effects of pickup protons, we find that the locations of the termination shock and the heliopause in 1991–1992 are at approximately 66 AU and 150 AU, respectively.

Proton phase space densities (0.5eV<Ep<5MeV) at midlatitudes from Ulysses SWICS/HI-SCALE measurements

Proton phase space densities in the solar wind frame from suprathermal velocities 10 kms-1 to 30,000 km s-1 (0.5 eV–5 MeV) were derived from combined SWICS and HI-SCALE measurements when Ulysses was at 5 AU and -24° heliolatitude. The period (19–23 January 1993) encompasses a forward/reverse shock pair (20 January, 0500 UT and 22 January, 0300 UT). Strong evidence is found for shock acceleration of pickup protons from interstellar hydrogen at all energies measured.

Pickup proton cyclotron turbulence at comet P/Halley

Low‐frequency electromagnetic turbulence near the proton gyrofrequency observed far upstream of comet P/Halley can be excited by a beam instability driven by relative streaming between cometary protons, solar wind protons, and water group ions. For a given solar wind velocity the growth rates peak at a certain optimum frequency shift (from the exact proton gyrofrequency), and the wavelengths involved can be deduced in a self‐consistent way from the dispersion law. Only under ideal conditions when all parameters remain constant would the mode corresponding to the optimum frequency shift grow fastest and might it be possible to observe a nearly constant frequency mode. However, if the solar wind parameters were not constant, then a mode that was in resonance earlier would no longer remain so, and some other mode with a slightly different frequency shift might start to grow fastest, leading to a mixing of many modes. Thus only rarely would one be able to observe a single mode near the proton gyrofrequency, exactly as happens in the observations. Our self‐consistent approach yields resonant instabilities with left‐handed polarizations in the spacecraft frame.

Injection and acceleration of interstellar pickup ions at the heliospheric termination shock

We have used a one‐dimensional hybrid code (macro‐ions, massless electron fluid) in order to study the interaction of an interstellar pickup ion distribution with the heliospheric termination shock. The shock is generated by reflecting the solar plasma at a rigid wall. The interstellar pickup ions are additional populations modeled as spheres in velocity space with radii given by the solar wind speed, comoving with the solar wind. The pickup ions are located on the outer shell of the spheres. We have determined the reflection rates of the pickup ions, which have been defined as the ratio of the reflected to the incident pickup ions at the shock. The dependence of these reflection rates on the shock normal angle ΘBn and on an upstream imposed turbulence has been investigated. No backscattered pickup ions are found for ΘBn greater than 70° and the reflection rates decrease with increasing level of upstream imposed turbulence. The dependence of the reflection rate on the ratio of the upstream pickup proton density to the upstream solar wind density has been investigated. Since this ratio is proportional to the heliospheric distance of the termination shock we have been able to investigate the reflection rates of the pickup ions at different heliospheric locations of the termination shock. Based on these hybrid simulations a model for the acceleration of anomalous component of the cosmic rays has been developed which is able to explain the differential flux of anomalous helium at the termination shock needed in modulation calculations to fit observations in the inner heliosphere. In this model we have not included the influence of the anomalous component on the shock structure. From comparison with modulation calculations it is concluded that the location of the termination shock is at distances between 80 and 120 AU.

Mass‐loading asymmetry in upstream region near Mars

The measurements carried out by the electric field probe onboard the PHOBOS‐2 spacecraft reveal that the mass‐loading taking place in the solar wind far upstream of the Martian bow shock is dependent upon the orientation of the interplanetary magnetic field. The asymmetry of mass‐loading is caused by H+ pickup ions originating from the extended cold hydrogen Martian exosphere and reflected from the bow shock. The distribution of pickup protons in the upstream region is analysed in a test particle approach, which seems to be confirmed by the Phobos data.

Pickup protons and pressure‐balanced structures: Voyager 2 observations in merged interaction regions near 35 AU

Five pressure‐balanced structures, each with a scale of the order of a few hundredths of an astronomical unit (AU), were identified in two merged interaction regions (MIRs) near 35 AU in the Voyager 2 plasma and magnetic field data. They include a tangential discontinuity, simple and complex magnetic holes, slow correlated variations among the plasma and magnetic field parameters, and complex uncorrelated variations among the parameters. The changes in the magnetic pressure in these events are balanced by changes in the pressure of interstellar pickup protons. Thus the pickup protons probably play a major role in the dynamics of the MIRs. The solar wind proton and electron pressures are relatively unimportant in the MIRs at 35 AU and beyond. The region near 35 AU is a transition region: the Sun is the source of the magnetic field, but the interstellar medium is the source of pickup protons. Relative to the solar wind proton gyroradius, the thicknesses of the discontinuities and simple magnetic holes observed near 35 AU are at least an order of magnitude greater than those observed at 1 AU. However, the thicknesses of the tangential discontinuity and simple magnetic holes observed near 35 AU (in units of the pickup proton Larmor radius) are comparable to those observed at 1 AU (in units of the solar wind proton gyroradius). Thus the gyroradius of interstellar pickup protons controls the thickness of current sheets near 35 AU. We determine the interstellar pickup proton pressure in the PBSs. Using a model for the pickup proton temperature, we estimate that the average interstellar pickup proton pressure, temperature, and density in the MIRs at 35 AU are (0.53 ± 0.14) × 10−12 erg/cm³, (5.8 ± 0.4) × 106 °K and (7 ± 2) × 10−4 cm−3, respectively.