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A Polar magnetosonic wave (MSW) study was conducted using 1 year of 1996–1997 data (during solar minimum). Waves at and inside the plasmasphere were detected at all local times with a slight preference for occurrence in the midnight-postmidnight sector. Wave occurrence (and intensities) peaked within~±5° of the magnetic equator, with half maxima at ~±10°. However, MSWs were also detected as far from the equator as +20° and 60° MLAT but with lower intensities. An extreme MSW intensity event of amplitude Bw = ~± 1 nT and Ew = ~± 25 mV/m was detected. This event occurred near local midnight, at the plasmapause, at the magnetic equator, during an intense substorm event, e.g., a perfect occurrence. These results support the idea of generation by protons injected from the plasma sheet into the midnight sector magnetosphere by substorm electric fields. MSWs were also detected near noon (1259 MLT) during relative geomagnetic quiet (low AE). A possible generation mechanism is a recovering/expanding plasmasphere engulfing preexisting energetic ions, in turn leading to ion instability. The wave magnetic field components are aligned along the ambient magnetic field direction, with the wave electric components orthogonal, indicating linear wave polarization. The MSW amplitudes decreased at locations further from the magnetic equator, while transverse whistler mode wave amplitudes (hiss) increased. We argue that intense MSWs are always present somewhere in the magnetosphere during strong substorm/convection events. We thus suggest that modelers use dynamic particle tracing codes and the maximum (rather than average) wave amplitudes to simulate wave-particle interactions.

Cosmic rays of solar and galactic origin at energies >100 MeV/n charge and induce spurious forces on free-floating test masses on board interferometers devoted to gravitational wave detection in space. LISA Pathfinder (LISA-PF), the technology testing mission for eLISA/NGO, will carry radiation monitors for on board test-mass charging monitoring. We present here the results of a simulation of radiation monitor performance during the evolution of solar energetic particle (SEP) events of different intensity. This simulation was carried out with the Fluka Monte Carlo package by taking into account for the first time both energy and spatial distributions of solar protons for the SEP events of 23 February 1956, 15 November 1960 and 7 May 1978. Input data for the Monte Carlo simulations was inferred from neutron monitor measurements. Conversely, for the SEP event of 13 December 2006 observed by the PAMELA experiment in space, we used the proton pitch angle distribution (PAD) computed from the Particle Acceleration and Transport in the Heliosphere (PATH) code. We plan to adopt this approach at the time of LISA-PF data analysis in order to optimize the correlation between radiation monitor observations and test-mass charging. The results of this work can be extended to the future space interferometers and other space missions carrying instruments for SEP detection.

We compare the efficiency of proton acceleration at oblique strong CME shocks at radial heliocentric distances from 0.13 to 2.5 AU. We use the PATH code to obtain the diffusion coefficients and maximum achievable particle energies at shocks with 15°, 45° and 75° shock angles. In computing the maximum energy, we use the total diffusion coefficient which is a function of the parallel diffusion coefficient and the perpendicular diffusion coefficient. The parallel diffusion coefficient is calculated from the wave intensity of Alfven waves generated by streaming protons in the shock vicinity [1, 2]. The perpendicular diffusion is described in the NLGC approach and depends on the parallel diffusion coefficient and pre-existing turbulence in the solar wind [3, 4, 6]. It is shown that efficiency of particle acceleration at a fast CME-driven shock depends on the shock angle, injection energy and radial heliocentric distance. Quasi-perpendicular strong shocks are more efficient in accelerating higher-energy seed particles, especially at larger heliocentric distances and close to the Sun. By contrast, quasi-parallel strong shocks can accelerate particles more efficiently at distances about 1 AU.

The Particle Acceleration and Transport in the Heliosphere (PATH) numerical code was developed to understand solar energetic particle (SEP) events in the near-Earth environment. We discuss simulation results for the 13 December 2006 SEP event. The PATH code includes modeling a background solar wind through which a CME-driven oblique shock propagates. The code incorporates a mixed population of both flare and shock-accelerated solar wind suprathermal particles. The shock parameters derived from ACE measurements at 1 AU and observational flare characteristics are used as input into the numerical model. We assume that the diffusive shock acceleration mechanism is responsible for particle energization. We model the subsequent transport of particles originated at the flare site and particles escaping from the shock and propagating in the equatorial plane through the interplanetary medium. We derive spectra for protons, oxygen, and iron ions, together with their time-intensity profiles at 1 AU. Our modeling results show reasonable agreement with in situ measurements by ACE, STEREO, GOES, and SAMPEX for this event. We numerically estimate the Fe/O abundance ratio and discuss the physics underlying a mixed SEP event. We point out that the flare population is as important as shock geometry changes during shock propagation for modeling time-intensity profiles and spectra at 1 AU. The combined effects of seed population and shock geometry will be examined in the framework of an extended PATH code in future modeling efforts.

The largest solar energetic particle (SEP) events are thought to be due to particle acceleration at a shock driven by a fast coronal mass ejection (CME). We investigate the efficiency of this process by comparing the total energy content of energetic particles with the kinetic energy of the associated CMEs. The energy content of 23 large SEP events from 1998 through 2003 is estimated based on data from ACE, GOES, and SAMPEX, and interpreted using the results of particle transport simulations and inferred longitude distributions. CME data for these events are obtained from SOHO. When compared to the estimated kinetic energy of the associated coronal mass ejections (CMEs), it is found that large SEP events can extract ~10% or more of the CME kinetic energy. The largest SEP events appear to require massive, very energetic CMEs.

Using data from the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE) mission, heavy ion composition measurements have been made in 26^3He-rich solar energetic particle (SEP) events that occurred between 1998 and 2004. Relative abundances of 13 elements from C through Ni have been investigated, as have the isotopic compositions of the elements Ne and Mg. We find a general tendency for the abundances to follow trends similar to those found in gradual SEP events, in which fractionation can be represented in the form of a power-law in Q/M. However several deviations from this pattern are noted that may provide useful diagnostics of the acceleration process occurring in solar flares.

We have surveyed He-Fe spectra for 41 Corotating Interaction Regions (CIRs) from 1998–2007 observed on ACE. The spectra are similar for all species, and have the form of broken power laws with the spectral break occurring at a few MeV/nucleon. Except for overabundances of He and Ne, the abundances are close to those of the solar wind. We find the rare isotope ^3He is enhanced in ~40% of the events. In individual CIRs the Fe/O ratio correlates strongly with the solar wind Fe/O ratio measured 2–4 days prior to the CIR passage. Taken together with previously reported observations of pick-up He^+ in CIRs, these observations provide evidence that CIRs are accelerated out of a suprathermal ion pool of heated solar wind ions, pick-up ions, and remnant suprathermal ions from impulsive solar energetic particle (SEP) events.

The acceleration of interstellar pick-up ions as well as solar wind species has been observed at a multitude of interplanetary (IP) shocks by different spacecraft. This paper expands upon previous work modeling the phase space distributions of accelerated ions associated with the shock event encountered on day 292 of 1991 by the Ulysses mission at 4.5 AU. A kinetic Monte Carlo simulation is employed here to model the diffusive acceleration process. This exposition presents recent developments pertaining to the incorporation into the simulation of the diffusive characteristics incurred by field line wandering (FLW), according to the work of Giacalone and Jokipii. For a pure field-line wandering construct, it is determined that the upstream spatial ramp scales are too short to accommodate the HI-SCALE flux increases for 200 keV protons, and that the distribution function for H+ somewhat underpopulates the combined SWICS/HI-SCALE spectra at the shock. This contrasts our earlier theory/data comparison where it was demonstrated that diffusive transport in highly turbulent fields according to kinetic theory can successfully account for both the proton distributions and upstream ramp scales, using a single turbulence parameter. The principal conclusion here is that, in a FLW scenario, the transport of ions across the mean magnetic field is slightly less efficient than is required to effectively trap energetic ions within a few Larmor radii of the shock layer and thereby precipitate efficient acceleration. This highlights the contrast between ion transport in highly turbulent shock environs and remote, less-disturbed interplanetary regions., 6 pages, 1 embedded figure, to appear in Proc. of the 7th IGPP International Astrophysics Conference "Particle Acceleration and Transport in the Heliosphere and Beyond" (2008), eds. G. Li, et al. (AIP Conf. Proc., New York)

We present numerical simulations modeling acceleration of ACRs at a blunt termination shock with the spiral field lines intersecting the shock multiple times. The 2‐D model includes parallel and perpendicular diffusion. Our results confirm the qualitative expectations: the nose segment of the blunt shock is inefficient to accelerate particles to ACR energies, which results in a continuing increase of the ACR flux and a gradual unfolding of the ACR spectrum behind the shock, as seen by Voyager‐1. We also present a simple analytically tractable test‐case to illustrate physical effects at a 2‐D shock with a structure along the shock‐front.

Using high-resolution mass spectrometers on board the Advanced Composition Explorer (ACE), we surveyed the event-averaged ~0.1-60 MeV/nuc heavy ion elemental composition in 64 large solar energetic particle (LSEP) events of cycle 23. Our results show the following: (1) The rare isotope ^3He is greatly enhanced over the corona or the solar wind values in 46% of the events. (2) The Fe/O ratio decreases with increasing energy up to ~10 MeV/nuc in ~92% of the events and up to ~60 MeV/nuc in ~64% of the events. (3) Heavy ion abundances from C-Fe exhibit systematic M/g-dependent enhancements that are remarkably similar to those seen in ^3He-rich SEP events and CME-driven interplanetary (IP) shock events. Taken together, these results confirm the role of shocks in energizing particles up to ~60 MeV/nuc in the majority of large SEP events of cycle 23, but also show that the seed population is not dominated by ions originating from the ambient corona or the thermal solar wind, as previously believed. Rather, it appears that the source material for CME-associated large SEP events originates predominantly from a suprathermal population with a heavy ion enrichment pattern that is organized according to the ion's mass-per-charge ratio. These new results indicate that current LSEP models must include the routine production of this dynamic suprathermal seed population as a critical pre-cursor to the CME shock acceleration process.

A brief narrative description of the phenomenon of dynamical complexity is presented. Intermittency characteristics of such phenomena are considered based on the rank—ordered multifractal spectrum, a method recently introduced by Chang and Wu. Analyses of data from MHD simulations and in‐situ solar wind observations are used to illustrate the basic ideas.

We discuss the influence of magnetic fields on the distribution of parameters in the outer heliosphere. The physical model acknowledges the crucial importance of charge exchange and collisions between neutral and charged particles in the partially ionized plasma and takes into account both the interstellar and interplanetary magnetic fields (ISMF and IMF). We summarize the results obtained for different ISMF orientations and discuss possible consequences of increasing the ISMF magnitude for both the 2–3 kHz radio emission and the deflection of the flow of neutral hydrogen in the inner heliosheath from its original direction in the unperturbed interstellar medium.

The two basic types of particle acceleration at the Sun—a flare‐resident process (or processes) and acceleration at coronal shock waves—were first identified in solar metric radio emissions through their associated type III bursts and type II bursts, respectively. A key question for solar energetic particle (SEP) physics today concerns the relative contributions of flares and shocks to large gradual SEP events, particularly at >30 MeV energies. We address this question by: (1) comparing low‐frequency (∼1 MHz) radio emissions for samples of the largest gradual and impulsive SEP events from cycle 23; and (2) determining SEP associations for a sample of large favorably‐located low‐frequency type III radio bursts. Our results indicate that a strong shock, as commonly manifested by a low‐frequency ( 30 MeV proton event. We propose a revision to the standard two‐class paradigm for SEP events in which we subdivide the current gradual ...

Particle acceleration in Supernova Remnant shocks has been known since the discovery of synchrotron emission from young SNRs. However, it has been difficult to pin down the acceleration efficiency even for the electrons observed. This paper summarizes the observational constraints on electron acceleration, then discusses constraints on the ratio of cosmic ray to gas pressure based on compression and electron temperature, and finally discusses evidence for shock wave precursors and constraints on the cosmic ray diffusion coefficient.

In this short review we report about recent findings related to two fundamental points in the study of solar wind turbulence: a) the verification of the equivalent of the −4/5 law in the solar wind and b) the estimate of the energy cascade along the spectrum and its comparison with the heating rate necessary to heat the solar wind during its expansion as deduced from in‐situ measurements. As a matter of fact, a Yaglom‐like scaling relation has recently been found in both high‐latitude and in‐ecliptic data samples. However, analogous scaling law, suitably modified to take into account compressible fluctuations, has been observed in a much more extended fraction of the same data set recorded at high latitude. Thus, it seems that large scale density fluctuations, despite their low amplitude, play a major role in the basic scaling properties of turbulence. The compressive turbulent cascade, moreover, seems to be able to supply the energy needed to account for the local heating of the non‐adiabatic solar wind.

Observations of hard X‐ray/γ‐ray continuum and γ‐ray line emission show that electrons are accelerated to >∼100 s of MeV and ions up to GeV energies, respectively, in large solar flares. The flare‐accelerated electrons above ∼20 keV and ions above a few MeV often contain >∼10–50% or more of the total energy released, indicating that the particle acceleration is intimately related to the energy release mechanism. RHESSI observations show strong evidence that both the ion and electron acceleration are associated with the process of magnetic reconnection. Direct in situ observations of solar energetic particles (SEPs) near 1 AU indicate that shock waves driven by fast (>∼1000 km/s) coronal mass ejections (CMEs) accelerate ions and electrons to similarly high energies, at altitudes of ∼2–40 solar radii. Both CMEs and large flares involve the transient release of up to ∼1032–1033 ergs. Frequent acceleration of electrons to ∼10 keV is observed in smaller flares and even in microflares. Radio type III bursts ind...

Radio synchrotron observations have long shown that electrons are accelerated to mildly relativistic energies in supernova remnants, but only in the last few years has it been conclusively demonstrated that particle acceleration in supernova remnants can reach energies of at least 1014 eV. The key observation here has been the detection of a number of young supernova remnants at TeV energies by the HESS (High Energy Stereoscopic System) collaboration. There is however still debate as to whether the dominant signal comes from electrons or protons. The current observational and theoretical situation will be discussed. Some surprising recent observations by the MILAGRO consortium are also discussed.

It is discussed how a time‐dependent focused transport model, using a time series of shock obliquities at the termination shock based on Voyager 1 observations to model magnetic field‐line random walk, can reproduce observational features of energetic ions at the termination shock and in the heliosheath which is beyond the scope of standard cosmic‐ray transport models.

Type II radio bursts, solar energetic particle (SEP) events, and interplanetary (IP) shocks all have a common cause, viz., fast and wide (speed ⩾900 km/s and width ⩾60°)) coronal mass ejections (CMEs). Deviations from this general picture are observed as (i) lack of type II bursts during many fast and wide CMEs and IP shocks, (ii) slow CMEs associated with type II bursts and SEP events, and (iii) lack of SEP events during many type II bursts. I examine the reasons for these deviations. I also show that ground level enhancement (GLE) events are consistent with shock acceleration because a type II burst is present in every event well before the release of GLE particles and SEPs at the Sun.

We report on Voyager 1 and 2 observations of anomalous cosmic rays in the outer heliosphere. The energy spectrum of anomalous cosmic ray helium as each spacecraft crossed the solar wind termination shock into the heliosheath remained modulated. Assuming the intensity gradient between the two spacecraft is purely radial, we find that radial gradients in the heliosheath of He with 11.6–22.3 MeV/nuc and with ∼61–73MeV/nuc∼61–73 MeV/nuc are 4.9±1.2%/AU4.9±1.2%/AU and 0.0±0.5%/AU,0.0±0.5%/AU, respectively. Strong temporal variations of the 11.6–22.3 MeV/nuc He intensity at both spacecraft were observed in 2005 just after Voyager 1 crossed the termination shock and while Voyager 2 was upstream. After 2006.0, the intensity variations are more moderate and likely due to a combination of spatial and temporal variations. As of early 2008, the anomalous cosmic ray He energy spectrum has unfolded to what may be a source spectrum. The spectrum at Voyager 2 remains modulated. We examine three recent models of the origin of anomalous cosmic rays in light of these observations.

Spacecraft observations (ACE, Wind, Ulysses, Voyager1/2) of energetic particles during the just‐completed Solar Cycle 23 (1996–2007) have provided multiple case studies in different regions throughout the heliosphere where weak scattering seems to have dominated the local transport process for energetic ions and electrons over a wide range of energies. This paper analyzes in detail two diverse phenomena observed near Earth: (1) approximately “scatter‐free” (r 1 AU) propagation of near‐relativistic electrons (rigidities R∼0.5 MV) in impulsive solar energetic particle (SEP) events along magnetic field lines connecting the coronal injection region to spacecraft at 1 AU; and (2) the anisotropy of galactic cosmic rays (rigidities R∼1–10 GV) as measured by the diurnal variation detected by ground‐based neutron monitors. In the scatter‐free limit, weak scattering can be mathematically described by a functional equation, rather than by a linear partial differential equation (as in foc...

A magnificent phenomenon in major solar eruptions is the creation of a long current sheet connecting solar flares to coronal mass ejections (CMEs). The sheet is important as the site where the magnetic energy is converted efficiently into heat and bulk kinetic energy and where particles are accelerated to high energy. With the development made recently in studying the CME/flare current sheet itself and particle accelerations in both theories and observations, our knowledge about the current sheet, its interior structures, and various processes occurring inside the sheet has been improved significantly. We are weaving various manifestations of solar eruptive processes related to the reconnecting current sheets, and looking into physical mechanisms that connect them to one another in the present work. Implications of these manifestations and the corresponding mechanisms to particle accelerations in the sheet are discussed, and some results we obtained recently for the particle acceleration are also displayed.

Upstream electron plasma oscillations and broadband bursts of electrostatic waves have been observed in the vicinity of the solar wind termination shock by the Voyager 2 plasma wave instrument. The upstream electron plasma oscillations were first detected on August 1, 2007, at a radial distance of 83.4 astronomical units (AU). These oscillations continued sporadically for about a month until, starting on August 31, three well‐defined bursts of broadband electrostatic waves, similar to those observed at planetary bow shocks, were observed at a heliocentric radial distance of about 83.7 AU. Two of these broadband bursts corresponded to shock crossings identified in the magnetometer and plasma data, and one did not. During the crossings labeled TS‐3 and TS‐4 by the magnetometer and plasma teams, the broadband electrostatic bursts corresponded almost exactly with steep ramps in the magnetic field strength. By scaling the frequencies by the upstream electron plasma frequency and the spectral densities by the u...

We consider the acceleration of charged particles and anomalous cosmic rays (ACR) at propagating shocks and especially at the heliospheric termination shock, when upstream variations cause spatial variations along the shock face, or, alternatively, if the global spatial geometry causes the shock to vary. Here we present the results a new analytic solution for the case of a shock in which the injection low‐energy particles varies with position along the shock face. We find that, downstream of the shock, positive spatial gradients together with modulation‐like and multiple‐power‐law spectral features are naturally produced by plausible spatial variations at the shock. Similar effects also result from large‐scale or global spatial structure. These various effects show promising qualitative agreement with observations at and near the heliospheric termination shock, suggesting that they may play an important role.

We are in the process of building a data base of energetic particle and magnetic field measurements in the vicinity of interplanetary shocks in order to test several existing and emerging theories for particle acceleration and wave excitation in these environs. At present we have over 40 such intervals analyzed and the set is growing. We show here that there exist clear examples of shock acceleration to ∼200 keV while in the presence of an intense energetic proton (SEP) population where the seed ions for shock acceleration come from the cold background protons that make up the thermal plasma. This happens in spite of the existence of a potential seed population made up of pre‐energized ions. We also show that at higher energies the seed ions are formed from the pre‐energized background, indicating that both sources are possible.

Voyager 2 (V2) crossed the termination shock in August 2007 and is now making the first plasma measurements of the heliosheath. We describe the plasma data near the termination shock and in the heliosheath. The shock was quasi‐perpendicular with a shock speed of about 100 km/s. The solar wind speed decreased before the shock crossing, from 400 to 300 km/s. Most of the solar wind flow energy did not go into the thermal plasma; the average proton temperature in the heliosheath was 105 K, well below the predicted temperature of 106 K. Most the the flow energy seems to go into the pickup ions. The heliosheath is highly variable, with plasma flow speeds of 100–170 km/s. The crossing of the shock by Voyager 2 at 84 AU compared to the 94 AU distance observed by Voyager 1 (V1) shows that the heliosphere is asymmetric, since only about 2–3 AU of this difference can be ascribed to solar wind pressure changes.

A brief discussion on the transport and acceleration of anomalous cosmic rays in the inner heliosheath is given with emphasis on stochastic acceleration and adiabatic heating of particles. Results from a numerical model is shown and compared to observations. We show that these acceleration processes play a major role in explaining Voyager 1 anomalous cosmic ray observations at the termination shock and in the inner heliosheath.

Protons and Ions are accelerated in flares. Some of them can escape and be detected as so‐called impulsive SEPs, but most of our knowledge of these particles comes from the secondary neutral radiation they emit when they interact in the low corona or photosphere. The γ‐ray and neutron emissions sample different parts of these populations, both in energy and composition. We review the wide variety of population behavior and nature that is revealed via the neutral emissions. These include rapid acceleration, prolonged high‐energy acceleration, a sometimes rapidly evolving spectrum, transport effects, the relationship with electrons and unusual compositions. We review highlights of missions including SMM, CGRO and RHESSI and ground based measurements. We also look ahead to future measurements of γ rays and neutrons on Solar Orbiter and Solar Sentinels and what may be discovered.

During solar flares a large amount of energy is suddenly released. Flares are accompanied with a strong enhancement of emissions of electromagnetic radiation from the radio up to the γ‐ray range and of fluxes of energetic particles indicating the Sun as a giant particle accelerator. The energetic electrons play an important role, since they carry a substantial part of the energy released during flares and are responsible for the nonthermal solar radio and X‐ray radiation. The phenomena of energetic particles and nonthermal radio and X‐ray emissions are demonstrated at the example of the solar event on October 28, 2003. The electron acceleration is dicussed at the shock established in the outflow region of the reconnection site.

Particle acceleration at co‐rotating forward shocks is investigated using Ulysses magnetic field and energetic ion measurements obtained near 5 AU in 2003–2004. The specific objective is the relation between the energetic accelerated particles and upstream magnetic field fluctuations predicted by shock drift acceleration theory that requires the particles to repeatedly scatter from fluctuations upstream and downstream of the shock. Six sample shocks and accelerated particles are analyzed using the intensities of ≈100 keV ions and magnetic field fluctuations upstream and the shock properties. The principal result is a correlation between the ion intensities and the mean square field variations as predicted. However, the field variations exhibit f−5/3 frequency spectra characteristic of the solar wind. Upstream waves generated by the particles predicted to be present nearer the shock are not unambiguously identified but are not ruled out by this preliminary analysis.

We discuss some theoretical aspects of pickup ion acceleration at the solar wind termination shock and beyond in the heliosheath. Our model of anisotropic transport permits a wide range of acceleration mechanisms, including (a) adiabatic compression by the shock of PUI suprathermal tails, (b) standard diffusive acceleration at the shock, (c) mirroring by the magnetic field kink at a quasi‐perpendicular shock, and (d) momentum diffusion in the solar wind and the heliosheath. It is shown that momentum diffusion in the heliosheath is an efficient particle accelerator at low energies. We also investigate the processing of a pre‐accelerated pickup ion spectra by the termination shock and analyze the long‐term averaged anisotropies of the combined distributions near a shock whose obliquity varies with time.

Simple predictions of diffusive shock acceleration for stationary planar configurations including upstream ion escape are illustrated for a seed population including both solar wind ions and remnant suprathermal ions. Current challenges to the theory of diffusive shock acceleration are discussed, including the appropriate shock compression ratio, solar wind injection rates, temporal variations, and proton‐excited wave enhancements.

Our Sun is about 4.6 billion years old. Richard C. Carrington made the first flare observations in the 19th century, and we have thus observed solar flares for 10−8 part of its age.Solar flares could be an important factor in the evolution of the solar system. I therefore attempt to paint an evolutionary picture of solar flares based on existing solar observations, original observations of young suns at the beginning of the main sequence and stellar flares. My primary focus will be on deriving the sizes of solar flare plasma loops when the Sun was young. I will also briefly discuss the implications of this picture to particle acceleration and transport in the heliosphere and beyond. a

This paper studies some wave properties of nonlinear rising tone chorus emissions in the outer regions of the dayside equatorial magnetosphere at ∼(6.3,–4.7, 0.7R E) in GSE coordinates. We analyze data obtained with the PWI and WFC receivers on GEOTAIL associated with a substorm on April 29, 1993. Fine structure of the chorus elements and inter-element spacings are shown. Directions of propagation of the chorus elements relative to the local magnetic field lines are analyzed. Wave polarizations, intensities and spectral properties of chorus in the equatorial Earth’s magnetosphere are discussed.

We compare Voyager 1 (V1) and 2 (V2) low‐energy (≈0.03–4.0 MeV) ion intensities averaged over selected periods before and after encounters with the termination shock (TS). Comparisons made during three successive 78‐day periods immediately behind the TS show marked evolution of intensities and spectral indices that differ at V1 and V2. By the third 78‐day period, absolute intensities and spectral forms are comparable at V1 and V2, and the differential intensities are well fit by power laws with indices −1.53±0.03 and −1.41±0.04 respectively.

Characteristics of both interplanetary (IP) shocks and associated energetic particle distributions (e.g., time‐intensity profiles, peak intensities, energy spectra indices, etc.) observed near 1 AU during solar active periods are analyzed. From February 1998 through October 2003, the ACE spacecraft (at 1 AU) detected 298 IP shocks (i.e., occurrence rate ∼50 IP shocks/year). A subset of 191 IP shocks was identified as transient, fast‐forward shocks. We summarize the findings of the statistical survey. Of the 191 sample shocks, 123 (64%) were associated with intensity increases in 47–68 keV ions, 62 (32%) with increases in 1.9–4.8 MeV ions, and 39 (20%) with increases in 38–53 keV electrons, all measured by the EPAM instrument onboard ACE. It is noteworthy that even in the relatively low‐energy ion range 47–68 keV, 68 (36%) of the 191 shocks did not produce an observable intensity increase. The 191 shocks were classified according to the temporal evolution of their time‐intensity profiles and of their energy spectra indices. Ion spectra indices in the immediate post‐shock region of the shocks were compared to spectral indices expected from steady‐state diffusive shock‐acceleration (DSA) theory. The majority of the measured spectral indices do not agree with the DSA‐predicted indices, which depend only upon shock compression ratio. Ion spectra measured at the shock are often softer than the ion spectra measured well upstream of the shock. This suggests that shock interactions of >47 keV ions are weak at 1 AU; consequently, the strong interaction (i.e., multiple shock crossings) description may not apply to most shock events at 1 AU.