Interplanetary Shocks Research Articles

Dynamics of Large‐Scale Solar‐Wind Streams Obtained by the Double Superposed Epoch Analysis: 4. Helium Abundance

AbstractThis work is a continuation of our previous articles (Yermolaev et al., 2015, https://doi.org/10.1002/2015JA021274; 2017, https://doi.org/10.1007/s11207-017-1205-1; 2018, https://doi.org/10.1007/s11207-018-1310-9), which describe the average temporal profiles of interplanetary plasma and field parameters in large‐scale solar‐wind (SW) streams: corotating interaction regions (CIRs), interplanetary coronal mass ejections (ICMEs including both magnetic clouds (MCs), and ejecta), and sheaths as well as interplanetary shocks (ISs). In this work, we analyze the average profile of helium abundance, Nα/Np, for 1976–2016. Our results confirm the main results that were obtained in earlier studies: Nα/Np is higher in quasi‐stationary fast streams than in slow ones; it slowly changes in compression regions CIRs and sheaths from values in undisturbed SW to values in the corresponding fast stream type pushing like a piston, in high‐speed stream (HSS) flow for CIRs, or in ICME for sheaths; in ejecta, it is close to the abundance observed in undisturbed streams, and it is maximal in MCs. For the first time, the results show that Nα/Np correlates with the proton β‐parameter in compression regions CIRs and sheath and anticorrelates in ICMEs. The Nα/Np versus β dependence is stronger in MCs than in ejecta and may be used as an indicator of conditions at the place on the Sun where CMEs are formed.

The Streamer Blowout Origin of a Flux Rope and Energetic Particle Event Observed by Parker Solar Probe at 0.5 au

Abstract The distribution of spacecraft in the inner heliosphere during 2019 March enabled comprehensive observations of an interplanetary coronal mass ejection (ICME) that encountered Parker Solar Probe (PSP) at 0.547 au from the Sun. This ICME originated as a slow (∼311 km s−1) streamer blowout (SBO) on the Sun as measured by the white-light coronagraphs on board the Solar TErrestrial RElations Observatory-A and the Solar and Heliospheric Observatory. Despite its low initial speed, the passage of the ICME at PSP was preceded by an anisotropic, energetic (≲100 keV/n) ion enhancement and by two interplanetary shocks. The ICME was embedded between slow (∼300 km s−1) solar wind and a following, relatively high-speed (∼500 km s−1), stream that most likely was responsible for the unexpectedly short (based on the SBO speed) ICME transit time of less than ∼56 hr between the Sun and PSP, and for the formation of the preceding shocks. By assuming a graduated cylindrical shell (GCS) model for the SBO that expands self-similarly with time, we estimate the propagation direction and morphology of the SBO near the Sun. We reconstruct the flux-rope structure of the in situ ICME assuming an elliptic-cylindrical topology and compare it with the portion of the 3D flux-rope GCS morphology intercepted by PSP. ADAPT-WSA-ENLIL-Cone magnetohydrodynamic simulations are used to illustrate the ICME propagation in a structured background solar wind and estimate the time when PSP established magnetic connection with the compressed region that formed in front of the ICME. This time is consistent with the arrival at PSP of energetic particles accelerated upstream of the ICME.

Suprathermal Proton Spectra at Interplanetary Shocks in 3D Hybrid Simulations

Abstract Interplanetary shocks are one of the proposed sources of suprathermal ion populations (i.e., ions with energies of a few times the solar-wind energy). Here we present results from a series of 3D hybrid simulations of collisionless shocks in the solar wind. We focus on the influence of the shock-normal angle, θ Bn , and the shock speed, V s , on producing protons with energies a few to hundreds of times the thermal energy of the upstream plasma. The combined effects of θ Bn and V s result in shocks with Alfvén Mach numbers in the range 3.0–6.0 and fast magnetosonic Mach numbers in the range 2.5–5.0, representing moderate to strong interplanetary shocks. We find that θ Bn largely organizes the shape of proton energy spectra, while shock speed controls acceleration efficiency. All shocks accelerate protons at the shock front, but the spectral evolution depends on θ Bn . Shocks with θ Bn ≥ 60° produce isolated bursts of suprathermal protons at the shock front, while shocks with θ Bn ≤ 45° create suprathermal beams upstream of the shock. Downstream proton energy spectra have exponential or smoothed broken power-law forms when θ Bn ≥ 45° and a single power-law form when θ Bn ≤ 30°. Protons downstream of the strongest shocks have energies at least 100 times the upstream thermal energy, with θ Bn ≤ 30° shocks producing the highest-energy protons and θ Bn ≥ 60° shocks producing the largest number of protons with energies at least a few times the thermal energy.

MMS Observations of Accelerated Interstellar Pickup He+ Ions at an Interplanetary Shock

Abstract Interstellar pickup ions (PUI) are interstellar neutrals that have been ionized in transit through the heliosphere via charge exchange or photoionization. These new PUIs then “freeze” into the solar wind (SW) and move with the bulk SW velocity (V SW). They also begin to gyrate around the local magnetic field with V SW, resulting in a maximum PUI velocity of 2 ∗ V SW. Understanding how He+ PUIs are accelerated at shocks in space provides valuable insights into shock dynamics and shock acceleration mechanisms. On 2018 January 8, while the Magnetospheric Multiscale (MMS) Mission was in the SW, it observed the forward shock of a corotating interaction region. In this work, we analyze the upstream and downstream velocity distributions of He+ for this quasiperpendicular, marginally supercritical (θ Bn = 67°, M A = 2.8) interplanetary shock. We derive average two-dimensional pitch-angle distributions in the field-aligned SW frame, as well as reduced one-dimensional velocity distributions for selected upstream and downstream intervals. We find that the shock accelerates He+ largely perpendicular to the magnetic field, consistent with shock reflection. Furthermore, we derive a measured ratio of accelerated He+ ions by estimating a maximum speed in the downstream frame, above which we assume that He+ ions have been accelerated by their interaction with the shock. The measured acceleration ratio, R Measured = 0.14, is then compared to a theoretical ratio derived from a simple model of the upstream distribution, R Theoretical = 0.06.

Interaction of interplanetary shock wave with the solar wind

Among the physical processes in the solar wind (SW) plasma, the processes taking place in the vicinity of the front of the interplanetary shock wave and on the front itself are of particular interest. In this paper, we present an analytical description of such processes which is based on the solution of a system of nonlinear equations of magnetohydrodynamics in a thin layer surrounding the front of the interplanetary shock wave and also on the use of the Rankine-Hugoniot conditions in the frontal zone. Two models of a shock wave are considered: one which does not takes into account the thickness of the front (the surface of discontinuity) and another one which takes into account the finite thickness and the fine structure of the front. The influence of both models on the solar wind parameters are compared with each other. Explicit formulas describing the changes of the main solar wind parameters: particle flow velocities, pressure, and the accompanying magnetic field in the vicinity of the front have been derived for both models. Two important facts have been established in the interaction of a SW with a shock wave. The magnetic energy of the shock front is absorbed by the SW and increases its speed. In the case of model II, this process is much more intensive than with model I. The kinetic energy of the shock front, in the case of model I, on the contrary, takes energy from the SW, which leads to a decrease of the velocity of the SW particles. For model II, this effect is practically absent. These results of this work are analytically proven and evaluated quantitatively.

Electromagnetic instabilities of low-beta alpha/proton beams in space plasmas

Relative drifts between different species or particle populations are characteristic to solar plasma outflows, e.g., in the fast streams of the solar winds, coronal mass ejections and interplanetary shocks. This paper characterizes the dispersion and stability of the low-beta alpha/proton drifts in the absence of any intrinsic thermal anisotropies, which are usually invoked in order to stimulate various instabilities. The dispersion relations derived here describe the full spectrum of instabilities and their variations with the angle of propagation and plasma parameters. The results unveil a potential competition between instabilities of the electromagnetic proton cyclotron and alpha cyclotron modes. For conditions specific to a low-beta solar wind, e.g., at low heliocentric distances in the outer corona, the instability operates on the alpha cyclotron branch. The growth rates of the alpha cyclotron mode are systematically stimulated by the (parallel) plasma beta and/or the alpha-proton temperature ratio. One can therefore expect that this instability develops even in the absence of temperature anisotropies, with potential to contribute to a self-consistent regulation of the observed drift of alpha particles.

A Statistical Analysis of the Fluctuations in the Upstream and Downstream Plasmas of 109 Strong‐Compression Interplanetary Shocks at 1 AU

AbstractThe upstream and downstream plasmas of 109 strong‐compression forward interplanetary shocks are statistically analyzed using 3‐s measurements from the WIND spacecraft. The goal is a comparison of the fluctuation properties of downstream plasmas in comparison with the fluctuation properties of upstream plasmas in the inertial range of frequencies and the magnetic‐structure range of spatial scales. The shocks all have density compression rations of ~2 or more. When possible, each shock is categorized according to the type of solar wind plasma it propagates through: 15 shocks are in coronal‐hole‐origin plasma, 42 shocks are in streamer‐belt‐origin plasma, 36 shocks are in sector‐reversal‐region plasmas, and 11 shocks are in ejecta plasma. The statistical study examines magnetic field and velocity spectral indices, the Alfvénicity, the fluctuation amplitudes, Alfvén ratios, the degree of plasma inhomogeneity, and Taylor microscales, looking in particular at (1) fluctuation values downstream that are related to fluctuation values upstream and (2) systematic differences in fluctuation values associated with the type of plasma. It is argued that inhomogeneity of the downstream plasma can be caused by spatial variations in the shock normal angle θBn caused by field direction variations in the upstream magnetic structure. The importance of determining the type of plasma that the shock propagates through is established.

On the Emission Region of Type II Radio Bursts in Interplanetary Shock Fronts

We analyze Type II radio emissions observed by the Radio and Plasma Waves instrument onboard Wind to determine the emission region in shock fronts, using the blast wave reconstruction technique. Our results suggest that the processes of emission of a Type II radio burst occur not only in the upstream region of the shock, i.e. the region that precedes the collision, but also that there must be an emission of the material that has been compressed, as well as of the material that is in the compression region within the structure of the shock itself.

Counterstreaming Strahls and Heat Flux Dropouts as Possible Signatures of Local Particle Acceleration in the Solar Wind

Abstract Suprathermal electrons with energies of ∼70 eV and above are observed at 1 au as dispersionless halo electrons and magnetic field-aligned beams of strahls. For a long time, it has been thought that both populations originate only from the solar corona, and that the only active process impacting their properties in the solar wind is scattering. This view has consequently impacted the interpretation of typical patterns of pitch-angle distributions (PADs) of suprathermal electrons. Meanwhile, recent observational studies supported by numerical simulations have shown that there is an unaccounted population of electrons accelerated to suprathermal energies at reconnecting current sheets (RCSs) and 3D dynamical plasmoids (or 2D magnetic islands (MIs)) directly in the heliosphere. We present multispacecraft observations of counterstreaming strahls and heat flux dropouts in PADs within a region filled with plasmoids and RCSs unaffected by interplanetary shocks, comparing observed PAD features with those predicted by particle-in-cell simulations. We show typical PAD patterns determined by local acceleration of thermal-core electrons up to hundreds of electron volts. Resulting PAD views depend on properties and topology of particular RCSs, MIs, and plasma/magnetic field parameters. Our study suggests that solar wind-borne suprathermal electrons coexist with those of solar origin. Therefore, some of heat flux dropout and bidirectional strahl events can be explained by local dynamical processes involving magnetic reconnection. Possible implications of the results for the interpretation of the actively debated decrease in the strahl/halo relative density with heliocentric distance and puzzling features of suprathermal electrons observed at crossings of the heliospheric current sheet and cometary comas are also discussed.

Electron Energy Partition across Interplanetary Shocks. III. Analysis

Abstract An analysis of model fit results of 15,210 electron velocity distribution functions (VDFs), observed within ±2 hr of 52 interplanetary (IP) shocks by the Wind spacecraft near 1 au, is presented as the third and final part on electron VDFs near IP shocks. The core electrons and protons dominate in the magnitude and change in the partial-to-total thermal pressure ratio, with the core electrons often gaining as much or more than the protons. Only a moderate positive correlation is observed between the electron temperature and the kinetic energy change across the shock, while weaker, if any, correlations were found with any other macroscopic shock parameter. No VDF parameter correlated with the shock normal angle. The electron VDF evolves from a narrowly peaked core with flaring suprathermal tails in the upstream to either a slightly hotter core with steeper tails or much hotter flattop core with even steeper tails downstream of the weaker and strongest shocks, respectively. Both quasi-static and fluctuating fields are examined as possible mechanisms modifying the VDF, but neither is sufficient alone. For instance, flattop VDFs can be generated by nonlinear ion acoustic wave stochastic acceleration (i.e., inelastic collisions), while other work suggested they result from the combination of quasi-static and fluctuating fields. This three-part study shows that not only are these systems not thermodynamic in nature; even kinetic models may require modification to include things like inelastic collision operators to properly model electron VDF evolution across shocks or in the solar wind.

Unusual Location of the Geotail Magnetopause Near Lunar Orbit: A Case Study

AbstractThe Earth's magnetopause is highly variable in location and shape and is modulated by solar wind conditions. On 8 March 2012, the ARTEMIS probes were located near the tail current sheet when an interplanetary shock arrived under northward interplanetary magnetic field conditions and recorded an abrupt tail compression at ∼(‐60, 0, ‐5) RE in Geocentric Solar Ecliptic coordinate in the deep magnetotail. Approximately 10 minutes later, the probes crossed the magnetopause many times within an hour after the oblique interplanetary shock passed by. The solar wind velocity vector downstream from the shock was not directed along the Sun‐Earth line but had a significant Y component. We propose that the compressed tail was pushed aside by the appreciable solar wind flow in the Y direction. Using a virtual spacecraft in a global magnetohydrodynamic (MHD) simulation, we reproduce the sequence of magnetopause crossings in the X‐Y plane observed by ARTEMIS under oblique shock conditions, demonstrating that the compressed magnetopause is sharply deflected at lunar distances in response to the shock and solar wind VY effects. The results from two different global MHD simulations show that the shocked magnetotail at lunar distances is mainly controlled by the solar wind direction with a timescale of about a quarter hour, which appears to be consistent with the windsock effect. The results also provide some references for investigating interactions between the solar wind/magnetosheath and lunar nearside surface during full moon time intervals, which should not happen in general.

Influence of He++and Shock Geometry on Interplanetary Shocks in the Solar Wind: 2D Hybrid Simulations

AbstractAfter protons, alpha particles (He++) are the most important ion species in the solar wind, constituting typically about 5% of the total ion number density. Due to their different charge‐to‐mass ratio, protons and He++particles are accelerated differently when they cross the electrostatic potential in a collisionless shock. This behavior can produce changes in the velocity distribution function (VDF) for both species generating anisotropy in the temperature, which is considered to be the energy source for various phenomena such as ion cyclotron and mirror mode waves. How these changes in temperature anisotropy and shock structure depend on the percentage of He++particles and the geometry of the shock is not completely understood. In this paper, we have performed various 2D local hybrid simulations (particle ions, massless fluid electrons) with similar characteristics (e.g., Mach number) to interplanetary shocks for both quasi‐parallel and quasi‐perpendicular geometries self‐consistently including different percentages of He++particles. We have found changes in the shock transition behavior as well as in the temperature anisotropy as functions of both the shock geometry and He++particle abundance: The change of the initialθBnleads to variations of the efficiency with which particles can escape to the upstream region facilitating or not the formation of compressive structures in the magnetic field that will produce increments in perpendicular temperature. The regions where both temperature anisotropy and compressive fluctuations appear tend to be more extended and reach higher values as the He++content in the simulations increases.

Extreme Magnetosphere‐Ionosphere‐Thermosphere Responses to the 5 April 2010 Supersubstorm

AbstractThe extreme substorm event on 5 April 2010 (THEMIS AL = −2,700 nT, called supersubstorm) was investigated to examine its driving processes, the aurora current system responsible for the supersubstorm, and the magnetosphere‐ionosphere‐thermosphere (M‐I‐T) responses. An interplanetary shock created shock aurora, but the shock was not a direct driver of the supersubstorm onset. Instead, the shock with a large southward IMF strengthened the growth phase with substantially larger ionosphere currents, more rapid equatorward motion of the auroral oval, larger ionosphere conductance, and more elevated magnetotail pressure than those for the growth phase of classical substorms. The auroral brightening at the supersubstorm onset was small, but the expansion phase had multistep enhancements of unusually large auroral brightenings and electrojets. The largest activity was an extremely large poleward boundary intensification (PBI) and subsequent auroral streamer, which started ~20 min after the substorm auroral onset during a steady southward IMF Bz and elevated dynamic pressure. Those were associated with a substorm current wedge (SCW), plasma sheet flow, relativistic particle injection and precipitation down to the D‐region, total electron content (TEC), conductance, and neutral wind in the thermosphere, all of which were unusually large compared to classical substorms. The SCW did not extend over the entire nightside auroral activity but was localized azimuthally to a few 100 km in the ionosphere around the PBI and streamer. These results reveal the importance of localized magnetotail reconnection for releasing large energy accumulation that can affect geosynchronous satellites and produce the extreme M‐I‐T responses.

Ionospheric response to the June 2015 geomagnetic storm in the South American region

The ionospheric dynamics in the South America (SA) sector during geomagnetic disturbed period from 21 to 24 June 2015 is investigated through ground ionosonde stations and Global Navigation Satellite System (GNSS) receivers, supported by Very Low Frequency (VLF) and magnetometer data. These disturbances were caused by 3 interplanetary shocks (IS) derived from 3 consecutives coronal mass ejections (CME) from the same solar active region; the first two CME were caused by filament eruptions, and the third was a much larger full halo CME, associated with a M2.6 solar flare. The first 2 shocks were compressive and did not cause an immediate response to the ionosphere in the analyzed region, while the third shock increased considerably the electron density from low to high-latitudes, triggering the second strongest geomagnetic storm of the 24th solar cycle. It was possible to observe the expansion of the crest of equatorial ionospheric anomaly (EIA) at midlatitudes and high-latitudes mainly due to prompt penetration electric field (PPEF) during the main phase and the recovery phase of the geomagnetic storm during the day.

Global Prompt Proton Sensor Network: Monitoring Solar Energetic Protons Based on GPS Satellite Constellation

AbstractEnergetic particle instruments on board Global Positioning System (GPS) satellites form a powerful global prompt proton sensor network (GPPSn) that provides an unprecedented opportunity to monitor and characterize solar energetic protons targeting the Earth. The medium‐Earth orbits of the GPS constellation have the unique advantage of allowing solar energetic protons to be simultaneously measured from multiple points in both open‐ and closed‐field line regions. Examining two example intervals of solar proton events, we showcase in this study how GPS proton data are prepared, calibrated, and utilized to reveal important features of solar protons, including their source, acceleration/scattering by interplanetary shocks, the relative position of Earth when impinged by these shocks, the shape of solar particle fronts, the access of solar protons inside the dynamic geomagnetic field, and temporally varying proton distributions in both energy and space. By comparing to Van Allen Probes data, GPS proton observations are further demonstrated not only to be useful for qualitatively monitoring the dynamics of solar protons but also for quantitative scientific research including determining cutoff L‐shells. Our results establish that this GPPSn can join forces with other existing solar proton monitors and contribute to observing, warning, understanding, and ultimately forecasting the incoming solar energetic proton events.

Shock Drift Acceleration of Ions in an Interplanetary Shock Observed by MMS

Abstract An interplanetary (IP) shock wave was recorded crossing the Magnetospheric Multiscale constellation on 2018 January 8. Plasma measurements upstream of the shock indicate efficient proton acceleration in the IP shock ramp: 2–7 keV protons are observed upstream for about three minutes (∼8000 km) ahead of the IP shock ramp, outrunning the upstream waves. The differential energy flux of 2–7 keV protons decays slowly with distance from the ramp toward the upstream region (dropping by about half within 8 Earth radii from the ramp) and is lessened by a factor of about four in the downstream compared to the ramp (within a distance comparable to the gyroradius of ∼keV protons). Comparison with test-particle simulations has confirmed that the mechanism accelerating the solar wind protons and injecting them upstream is classical Shock Drift Acceleration (SDA). This example of observed proton acceleration by a low-Mach, quasi-perpendicular shock may be applicable to astrophysical contexts, such as supernova remnants or the acceleration of cosmic rays.

Event Studies of High‐Latitude FACs With Inverse and Assimilative Analysis of AMPERE Magnetometer Data

AbstractWe present examples of high‐latitude field‐aligned current (FAC) and toroidal magnetic potential patterns in both hemispheres reconstructed at a 2‐min cadence using an updated optimal interpolation (OI) method that ingests magnetic perturbation data provided by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) program. A solstice and an equinoctial event are studied to demonstrate the reconstructed patterns and to provide scientific insights into FAC response to different solar wind drivers. For the 14 June 2011 high‐speed stream event with mostly northward Bz driving, we found persistently stronger FACs in the Northern Hemisphere. Extreme interhemispheric asymmetry is associated with the interplanetary magnetic field (IMF) direction and large dipole tilt, consistent with earlier studies. FAC asymmetries seen during an isolated substorm can be attributed to dipole tilt. During relatively low geomagnetic activity, the FAC response to IMF Bx changes is identified. For the 17–18 March 2013 period, we provide global snapshots of rapid FAC changes related to an interplanetary shock passage. We further present comparisons between instantaneous and mean behaviors of FAC for the solar wind sheath passage and interplanetary coronal mass ejection southward Bz interval and northward Bz intervals. We show that (1) sheath passage results in strong FAC and high variation in the dayside polar cap region and pre‐midnight region, different from the typical R1/R2 currents during prolonged southward Bz; (2) four‐cell reverse patterns appear during northward Bz but are not stable; and (3) persistent dawn‐dusk asymmetry is seen throughout the storm, especially during an extreme substorm, likely associated with a dawnside current wedge.

Interhemispheric Asymmetries in the Ground Magnetic Response to Interplanetary Shocks: The Role of Shock Impact Angle

AbstractInterplanetary (IP) shocks drive magnetosphere‐ionosphere (MI) current systems that in turn are associated with ground magnetic perturbations. Recent work has shown that IP shock impact angle plays a significant role in controlling the subsequent geomagnetic activity and magnetic perturbations; for example, highly inclined shocks drive asymmetric MI responses due to interhemispherical asymmetric magnetospheric compressions, while almost head‐on shocks drive more symmetric MI responses. However, there are few observations confirming that inclined shocks drive such asymmetries in the high‐latitude ground magnetic response. We use data from a chain of Antarctic magnetometers, combined with magnetically conjugate stations on the west coast of Greenland, to test these model predictions (Oliveira & Raeder, 2015, https://doi.org/10.1002/2015JA021147; Oliveira, 2017, https://doi.org/10.1007/s13538-016-0472-x). We calculate the time derivative of the magnetic field ( ) in each hemisphere separately. Next, we examine the ratio of Northern to Southern Hemisphere intensities and the time differences between the maximum immediately following the impact of IP shocks. We order these results according to shock impact angles obtained from a recently published database with over 500 events and discuss how shock impact angles affect north‐south hemisphere asymmetries in the ground magnetic response. We find that the hemisphere the shock strikes first usually has (1) the first response in and (2) the most intense response in . Additionally, we show that highly inclined shocks can generate high‐latitude ground magnetic responses that differ significantly from predictions based on models that assume symmetric driving conditions.

A computational study of the properties of the quasi-perpendicular fast forward shock event during solar maximum

MHD shock waves and discontinuities are often observed in the solar wind. We studied a strong shock wave event on 7 June 2014, during a solar maximum. The properties of forward shock are investigated and all physical parameters of interplanetary shock are analyzed. The upstream parameters, such as Alfven velocity and sound speed are calculated, and the angle ( $${\theta }$$ ) between the upstream magnetic field direction and the shock normal is estimated. To determine the propagation speed of the shock, a full numerical solution of shock adiabatic equation is carried out. The thermal pressure for electrons and ions are calculated and also the pressure ratio and the entropy change across the shock are estimated. The results show that the shock strength is about 3.88 with $${\theta }\approx \text{72 }^{\circ }$$ (a quasi-perpendicular case), and shock propagation velocity is about 678 km/s. The increase in the specific entropy is also evaluated.

Energy Power Spectra Measured at an Interplanetary Shock by the New Horizon's SWAP Experiment: 1D Full Particle Simulations versus Observations

Abstract One-dimensional particle-in-cell (PIC) simulations are used to analyze the energy spectra measured by the New Horizons’ Solar Wind Around Pluto (SWAP) instrument in the upstream region of an interplanetary shock observed at a distance of ∼34 au from the Sun. The use of individual populations simulating the different solar wind ion and pick-up ion (PUI) populations allows us to clearly identify the contribution of each population to the global energy spectra. The important role of shock front obliquity is stressed in the formation of PUIs streaming back along the magnetic field into the upstream region far from the front. Energy spectra measured by the SWAP experiment are well reproduced in the present simulations. A detailed analysis shows that (1) the highest-energy part of the spectrum is formed primarily by both backstreaming PUI–H+ and PUI–He+; (2) the middle-energy part of the energy spectrum is composed of both solar wind SW–H+ and SW–He2+ incoming ions that are superimposed on the PUI–H+ population; and (3) the low-energy range is composed of incoming PUI–H+. The agreement between experimental and simulation results is improved by using an initially filled-shell distribution for the PUI–H+ population (instead of a zero-thickness shell), as this affects the low-energy part of the spectrum strongly. This means that PUI–H+ ions have sufficient time to diffuse onto and fill out a shell distribution after their initial pick-up in the heliosphere, indicating that the subsequent cooling has an important impact on the global energy spectrum.