Interstellar Pickup Protons Research Articles

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

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

Hybrid Simulations of Interstellar Pickup Protons Accelerated at the Solar-wind Termination Shock at Multiple Locations

Abstract We estimate the intensity of interstellar pickup protons accelerated to ∼50 keV at various locations along the solar-wind termination shock, using two-dimensional hybrid simulations. Parameters for the solar wind, interstellar pickup ions (PUIs), and magnetic field just upstream of the termination shock at one flank of the heliosphere, and at the location in the downwind (or tail-ward) direction are based on a solar-wind/pickup-ion/turbulence model. The parameters upstream of the shock where Voyager 2 crossed are based on observations. The simulation is limited in size, and therefore cannot accurately model the distribution to energies much beyond ∼50 keV. This is sufficient to study the origin of the high-energy tail of the distribution, which is the low-energy portion of the anomalous cosmic-ray spectrum. We also extrapolate our results to other locations along the termination shock, such as the other flank, and the poles of the heliosphere. We find that the intensity of ∼10–50 keV accelerated pickup protons is remarkably similar at all three locations we simulated, suggesting that particles in this energy range are relatively uniformly distributed along the termination shock, and are likely quite uniform throughout the entire heliosheath. In addition, we find significant differences in the distribution in the 0.5–1 keV energy range for energetic neutral atoms coming from the tail region of the heliosphere compared to that at the nose or flank look directions. This is because the peak in the PUI distribution is at a higher energy there.

The role of charge-exchange cross-section for pickup protons and neutrals in the inner heliosheath

The process of deceleration of the solar wind downstream of the termination shock is studied on the basis of a one-dimensional multi-component model. It is assumed that the solar wind consists of thermal protons, electrons and interstellar pickup protons. The protons interact with interstellar hydrogen atoms by charge-exchange. Two cases are considered. In the first one, the charge-exchange cross-section for thermal protons and hydrogen atoms is the same as for pickup protons and atoms. Under this condition, there is a strong dependence of the solar wind velocity on the downstream temperature of pickup protons. When the proton temperature is close to 10 keV, the change in the velocity with the distance from the termination shock is similar to that measured on the Voyager 1 spacecraft: linear velocity decrease is accompanied by an extended transition region with near-zero velocity. However, with a more careful approach to the choice of the charge-exchange cross-section, the situation changes dramatically. The strong dependence of the solar wind speed on the pickup proton temperature disappears and the transition region in the heliosheath disappears as well, at least at reasonable distances from the TS.

Listing of 502 Times When the Ulysses Magnetic Fields Instrument Observed Waves Due to Newborn Interstellar Pickup Protons

Abstract In two earlier publications we analyzed 502 intervals of magnetic waves excited by newborn interstellar pickup protons that were observed by the Ulysses spacecraft. Due to the considerable effort required in identifying these events, we provide a list of the times for the 502 wave event intervals previously identified. In the process, we provide a brief description of how the waves were found and what their properties are. We also remind the reader of the conditions that permit the waves to reach observable levels and explain why the waves are not seen more often.

MULTIPLE CURRENT SHEET SYSTEMS IN THE OUTER HELIOSPHERE: ENERGY RELEASE AND TURBULENCE

ABSTRACT In the outer heliosphere, beyond the solar wind termination shock, it is expected that the warped heliospheric current sheet forms a region of closely packed, multiple, thin current sheets. Such a system may be subject to the ion-kinetic tearing instability, and hence may generate magnetic islands and hot populations of ions associated with magnetic reconnection. Reconnection processes in this environment have important implications for local particle transport, and for particle acceleration at reconnection sites and in turbulence. We study this complex environment by means of three-dimensional hybrid simulations over long timescales, in order to capture the evolution from linear growth of the tearing instability to a fully developed turbulent state at late times. The final state develops from the highly ordered initial state via both forward and inverse cascades. Component and spectral anisotropy in the magnetic fluctuations is present when a guide field is included. The inclusion of a population of newborn interstellar pickup protons does not strongly affect these results. Finally, we conclude that reconnection between multiple current sheets can act as an important source of turbulence in the outer heliosphere, with implications for energetic particle acceleration and propagation.

Acceleration of interstellar pickup protons at the heliospheric termination shock:Voyager 1/2energetic proton fluxes in the inner heliosheath

Acceleration of interstellar pickup protons at the heliospheric termination shock:<i>Voyager 1/2</i>energetic proton fluxes in the inner heliosheath

THREE-FLUID, THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC SOLAR WIND MODEL WITH EDDY VISCOSITY AND TURBULENT RESISTIVITY

We have developed a three-fluid, three-dimensional magnetohydrodynamic solar wind model that incorporates turbulence transport, eddy viscosity, turbulent resistivity, and turbulent heating. The solar wind plasma is described as a system of co-moving solar wind protons, electrons, and interstellar pickup protons, with separate energy equations for each species. Numerical steady-state solutions of Reynolds-averaged solar wind equations coupled with turbulence transport equations for turbulence energy, cross helicity, and correlation length are obtained by the time relaxation method in the corotating with the Sun frame of reference in the region from 0.3 to 100 AU (but still inside the termination shock). The model equations include the effects of electron heat conduction, Coulomb collisions, photoionization of interstellar hydrogen atoms and their charge exchange with the solar wind protons, turbulence energy generation by pickup protons, and turbulent heating of solar wind protons and electrons. The turbulence transport model is based on the Reynolds decomposition and turbulence phenomenologies that describe the conversion of fluctuation energy into heat due to a turbulent cascade. In addition to using separate energy equations for the solar wind protons and electrons, a significant improvement over our previous work is that the turbulence model now uses an eddy viscosity approximation for themore » Reynolds stress tensor and the mean turbulent electric field. The approximation allows the turbulence model to account for driving of turbulence by large-scale velocity gradients. Using either a dipole approximation for the solar magnetic field or synoptic solar magnetograms from the Wilcox Solar Observatory for assigning boundary conditions at the coronal base, we apply the model to study the global structure of the solar wind and its three-dimensional properties, including embedded turbulence, heating, and acceleration throughout the heliosphere. The model results are compared with plasma and magnetic field observations on WIND, Ulysses, and Voyager 2 spacecraft.« less

Shock-drift acceleration of interstellar pickup protons at the solar wind termination shock

The numerical model of the shock-drift acceleration of interstellar pick-up protons at the solar wind termination shock is presented taking into account multiple reflections of the particles at and passing through the shock. The processes of the multiple interactions with the shock are possible due to pitch-angle scattering in the upstream and downstream parts of the solar wind flow. The model takes into account variations of the magnetic field direction near the shock front connected with its three-dimensional shape. The main advantage of the model is the self-consistent treatment of protons and interstellar hydrogen atoms, which interact with each other through the resonant charge exchange process. The model gives very simple and natural resolution of so-called injection problem for anomalous cosmic rays. The shock-drift acceleration can explain the fluxes of the anomalous cosmic rays measured at the Voyager 1/2 spacecraft after the termination shock crossings.

SELF-CONSISTENT MODEL OF THE INTERSTELLAR PICKUP PROTONS, ALFVÉNIC TURBULENCE, AND CORE SOLAR WIND IN THE OUTER HELIOSPHERE

A self-consistent model of the interstellar pickup protons, the slab component of the Alfvenic turbulence, and core solar wind (SW) protons is presented for r ≥ 1 along with the initial results of and comparison with the Voyager 2 (V2) observations. Two kinetic equations are used for the pickup proton distribution and Alfvenic power spectral density, and a third equation governs SW temperature including source due to the Alfven wave energy dissipation. A fraction of the pickup proton free energy, fD , which is actually released in the waveform during isotropization, is taken from the quasi-linear consideration without preexisting turbulence, whereas we use observations to specify the strength of the large-scale driving, C sh, for turbulence. The main conclusions of our study can be summarized as follows. (1) For C sh 1-1.5 and fD 0.7-1, the model slab component agrees well with the V2 observations of the total transverse magnetic fluctuations starting from ~8 AU. This indicates that the slab component at low-latitudes makes up a majority of the transverse magnetic fluctuations beyond 8-10 AU. (2) The model core SW temperature agrees well with the V2 observations for r 20 AU if fD 0.7-1. (3) A combined effect of the Wentzel-Kramers-Brillouin attenuation, large-scale driving, and pickup proton generated waves results in the energy sink in the region r 10 AU, while wave energy is pumped in the turbulence beyond 10 AU. Without energy pumping, the nonlinear energy cascade is suppressed for r 10 AU, supplying only a small energy fraction into the k-region of dissipation by the core SW protons. A similar situation takes place for the two-dimensional turbulence. (4) The energy source due to the resonant Alfven wave damping by the core SW protons is small at heliocentric distances r 10 AU for both the slab and the two-dimensional turbulent components. As a result, adiabatic cooling mostly controls the model SW temperature in this region, and the model temperature disagrees with the V2 observations in the region r 20 AU.

THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC MODELING OF THE SOLAR WIND INCLUDING PICKUP PROTONS AND TURBULENCE TRANSPORT

To study the effects of interstellar pickup protons and turbulence on the structure and dynamics of the solar wind, we have developed a fully three-dimensional magnetohydrodynamic solar wind model that treats interstellar pickup protons as a separate fluid and incorporates the transport of turbulence and turbulent heating. The governing system of equations combines the mean-field equations for the solar wind plasma, magnetic field, and pickup protons and the turbulence transport equations for the turbulent energy, normalized cross-helicity, and correlation length. The model equations account for photoionization of interstellar hydrogen atoms and their charge exchange with solar wind protons, energy transfer from pickup protons to solar wind protons, and plasma heating by turbulent dissipation. Separate mass and energy equations are used for the solar wind and pickup protons, though a single momentum equation is employed under the assumption that the pickup protons are comoving with the solar wind protons. We compute the global structure of the solar wind plasma, magnetic field, and turbulence in the region from 0.3 to 100 AU for a source magnetic dipole on the Sun tilted by 0°-90° and compare our results with Voyager 2 observations. The results computed with and without pickup protons are superposed to evaluate quantitatively the deceleration and heating effects of pickup protons, the overall compression of the magnetic field in the outer heliosphere caused by deceleration, and the weakening of corotating interaction regions by the thermal pressure of pickup protons.

TURBULENT HEATING OF THE DISTANT SOLAR WIND BY INTERSTELLAR PICKUP PROTONS IN A DECELERATING FLOW

Previous models of solar wind heating by interstellar pickup proton-driven turbulence have assumed that the wind speed is a constant in heliocentric radial position. However, the same pickup process, which is taken to provide the turbulent energy, must also decelerate the wind. In this paper, we extend our phenomenological turbulence model to include variable wind speed, and then incorporate the deceleration due to interstellar pickup protons into the model. We compare the model results with plasma and field data from Voyager 2, taking this opportunity to present an extended and improved data set of proton core temperature, magnetic field fluctuation intensity, and correlation length along the Voyager trajectory. A particular motivation for including the solar wind deceleration in this model is the expectation that a slower wind would reduce the resulting proton core temperature in the region beyond ~60 AU, where the previous model predictions were higher than the observed values. However, we find instead that the deceleration of the steady-state wind increases the energy input to the turbulence, causing even higher temperatures in that region. The increased heating is shown to result from the larger values of the ratio of Alfven speed to solar wind speed that develop in the decelerating wind.

SCATTER-FREE PICKUP IONS BEYOND THE HELIOPAUSE AS A MODEL FOR THE INTERSTELLAR BOUNDARY EXPLORER RIBBON

We present a new kinetic-gasdynamic model of the solar wind interaction with the local interstellar medium. The model incorporates several processes suggested earlier for the origin of the ribbon—the most prominent feature seen in the all-sky maps of heliospheric energetic neutral atoms (ENAs) discovered by the Interstellar Boundary Explorer (IBEX). The ribbon is a region of enhanced fluxes of ENAs crossing almost the entire sky. Soon after the ribbon's discovery, it was realized that the enhancement of the fluxes could be in the directions where the radial component of the interstellar magnetic field around the heliopause is close to zero. Our model includes secondary charge exchange of the interstellar H atoms with the interstellar pickup protons outside the heliopause. Previously, in the frame of a kinetic-gasdynamic model where pickup protons are treated as a separate kinetic component, it was shown that the interstellar pickup protons outside the heliopause may be a significant source of ENAs at energies above 1 keV. The key difference between the current work and the previous models is in the assumption of no pitch-angle scattering for newly created pickup protons outside the heliopause. We demonstrate that in the limit of no pitch-angle scattering ribbon of enhanced ENA fluxes appears in the model, and this may qualitatively explain the ribbon discovered by IBEX.

TIME-DEPENDENT ACCELERATION OF INTERSTELLAR PICKUP IONS AT THE HELIOSPHERIC TERMINATION SHOCK USING A FOCUSED TRANSPORT APPROACH

A time-dependent focused transport approach to modeling diffusive shock acceleration of interstellar pickup ions at the termination shock is discussed. By taking into account time variations in the magnetic field angle, and thus by implication in the shock obliquity and injection speed at the termination shock using Voyager 1 observations as guide, we show that unaccelerated core interstellar pickup protons can be accelerated by the nearly perpendicular termination shock at the heliospheric nose. Many features of anomalous cosmic rays, observed by the Voyager spacecraft at energies below the big spectral dip starting at ~3 MeV, can be successfully reproduced with this approach. This includes multiple power-law spectral slopes with stable breaking points, power-law spectral slopes that are harder than predicted by standard diffusive shock acceleration, large pitch-angle anisotropies with strong time variations upstream that converge to steady-state isotropy in the heliosheath, upstream pitch-angle anisotropies that peak at ~1 MeV, episodic strongly anisotropic intensity spikes at the termination shock, and strong spectral volatility upstream that is reduced downstream and almost completely disappears farther downstream.

Interstellar pickup protons and solar wind heating in the outer heliosphere

The ionization of hydrogen atoms that penetrate into the heliosphere from the interstellar medium gives rise to a peculiar population of energetic protons (interstellar pickup protons) in the solar wind. The short-wavelength Alfvenic turbulence in the outer heliosphere is entirely attributable to the source associated with the instability of the initial anisotropic pickup proton velocity distribution. The bulk of the generated turbulent energy is subsequently absorbed by the pickup protons themselves through the cyclotron-resonance particle-wave interaction, and only an insignificant fraction of this energy can be transferred to the solar wind protons and heat them up.

Turbulent Heating of the Solar Wind by Newborn Interstellar Pickup Protons

Spacecraft missions to the outer heliosphere have shown that the thermal protons that make up the solar wind are hotter than simple adiabatic expansion would predict. We examine a theory that describes this heating by using the time-varying 1 AU measurements as input and comparing the predictions with observations in the outer heliosphere. Inside 20 AU wind shear and shocks provide the dominant energy source to drive the turbulence. Outside 20 AU little remains to inject energy into the fluctuations except newborn interstellar pickup protons. The theory is built on a combination of two-dimensional magnetohydrodynamic turbulence concepts and the latest kinetic theory describing the scattering of newborn interstellar pickup protons. We find that application of the theory to the observations produces encouraging agreement at the same time that it illuminates latitudinal effects associated with solar minimum conditions. A remaining challenge is to close the gap of a factor of 2 between observed and predicted proton temperatures beyond 40 AU. For this, we suggest that further structural development of the theory is needed, rather than ad hoc adjustment of the model parameters, which are reasonably well constrained by theory, simulation, and observations.

Heating of the solar wind in the outer heliosphere

Small-scale Alfvturbulence in the outer helio- sphere is mainly determined by a source which is connected with the instability of the initially highly anisotropic velocity distribution of interstellar pick-up protons. The main portion of the generated turbulent energy is subsequently absorbed by the pick-up protons themselves due to the cyclotron- resonant interaction between waves and particles. A small fraction of this energy can be transferred to solar wind pro- tons resulting in their heating. The heating is more efficient in the high-speed solar wind.

Turbulence‐driven Solar Wind Heating and Energization of Pickup Protons in the Outer Heliosphere

We improve and extend the dominant turbulence model of Isenberg et al., which describes the turbulent heating of the distant solar wind by the dissipation of wave energy generated by the isotropization of interstellar pickup protons. We first modify some of the detailed expressions in Isenberg et al. to fully incorporate the effects of wave dispersion in the pitch-angle scattering of the newly ionized pickup protons. The corrected pickup proton distributions have a more involved shape, but the energy given up to the waves is only slightly smaller than that found in Isenberg et al. We then treat the effects of second-order Fermi acceleration of the pickup protons, which occurs through the same resonant cyclotron interaction that controls the pitch-angle scattering. In principle, the pickup proton energization is an energy sink that was not included in Isenberg et al. and so could modify the model results. In addition, since this turbulent-evolution model makes specific predictions of the intensity of resonant waves as a function of heliocentric radius, our quantitative results for the pickup proton evolution are on a firmer footing than possible in previous models. We find that neither of these modifications produces significant changes in the solar wind heating predicted by Isenberg et al. Thus, the results of the dominant-turbulence model appear to be robust in that they are not sensitive to the incorporation of second-order effects.

Turbulent Heating of the Distant Solar Wind by Interstellar Pickup Protons

The solar wind heating observed in the outer heliosphere can be attributed to the turbulent dissipation of wave energy generated by the isotropization of interstellar pickup protons. We calculate the fluctuation energy provided by the pickup protons, assuming that the ambient fluctuations are dominated by turbulent processes. This assumption is in contrast to previous estimates of this energy, which obtained a bispherical result in the absence of turbulent interactions. We find that when turbulent processes dominate the wave generation, a more isotropic distribution of pickup protons is produced and less energy is injected into the solar wind fluctuations. When this form of the pickup proton energy source is used in a phenomenological model of turbulent evolution in the solar wind, we find good agreement with the solar wind temperature measurements by the Voyager 2 spacecraft available at present out to almost 70 AU. The efficient spectral smoothing by the turbulent transport may also explain the absence of observable pickup proton–generated waves in the outer heliosphere. Subject headings: solar wind — turbulence Neutral atoms that flow slowly into the heliosphere from the surrounding interstellar medium may become ionized by solar ultraviolet radiation or by charge exchange with ions in the outflowing solar wind. At that moment, the new ions stream toward the Sun with respect to the reference frame of the plasma with a speed close to the solar wind speed Vsw. These suddenly energetic ions, termed ‘‘ pickup ions,’’ then enter into an involved interaction with the solar wind particles and fields that is still not completely determined. In broad outline this interaction consists of several steps, which take place on distinct timescales when the solar wind

On ion injection at quasiparallel shocks

A large number of numerical experiments has been performed in order to study the interaction of interstellar pickup protons and helium ions with quasiparallel collisionless shocks. The shocks are modeled by a one-dimensional hybrid simulation method which treats the ions as macroparticles and the electrons as a massless fluid. Solar wind alpha particles and pickup protons are included self-consistently. In addition, the particle splitting method is used for the solar wind ions so that the distribution function can be followed over more than 10 orders of magnitude. A large part of the pickup ion distribution is reflected; the reflection efficiency is very high, and can reach in cases where the pickup ion density is low as much as 50%–60%. The reflection efficiency is almost independent of magnetic field–shock normal angle. This indicates that magnetic mirroring is unimportant and does not lead to larger reflection efficiencies. The reflection efficiency of pickup protons rapidly decreases when the pickup ion density exceeds a few percent of the solar wind density. An addition of 25% pickup protons decreases the reflection coefficient for these ions to ∼10%. This represents the fact that a quasiparallel shock cannot be considered as being uncoupled from the upstream region: at high additions of pickup ions the shock structure is changed in such a way as to reflect less pickup ions. The intensity of diffuse ions upstream of a quasiparallel shock does not depend on the temperature of the core distribution. Within the framework of the present model even solar wind distributions with a hard power law tail do not produce higher intensities of diffuse ions. It is argued that this can be understood by the fact that the intrinsic self-consistency between the processes in the upstream region and at the shock transition determines the injection and reflection properties of the core solar wind distribution.

The Bastille day Shock and Merged Interaction Region at 63 au: Voyager 2 Observations

A transient flow system containing several streams and shocks associated with the Bastille Day 2000 solar event was observed by the WIND and ACE spacecraft at 1 AU. Voyager 2 (V2) at 63 AU observed this flow system after it moved through the interplanetary medium and into the distant heliosphere, where the interstellar pickup protons strongly influence the MHD structures and flow dynamics. We discuss the Voyager 2 magnetic and plasma observations of this event. Increases in the magnetic field strength B, density N, temperature T and speed V were observed at the front of a stream at V2, consistent with presence of a shock related to the Bastille Day shock at 1 AU. However, the jumps occurred in a 16.9-hour data gap, so that the shock was not observed directly, and the properties of the candidate shock cannot be determined precisely. The candidate shock was followed by a merged interaction region (MIR) that moved past V2 for at least 10 days. The first part of this MIR contains a structure that might be a magnetic cloud. Just ahead of the shock there was an abrupt increase in density associated with a decrease in temperature such that the solar wind thermal pressure was constant across it. Just behind the shock there was an abrupt decrease in density associated with a net increase in magnetic field strength. This appears to be a pressure balanced structure in which the interstellar pickup protons make a significant contribution.