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The acceleration of protons and electrons to high (sometimes GeV/nucleon) energies by solar phenomena is a key component of space weather. These solar energetic particle (SEP) events can damage spacecraft and communications, as well as present radiation hazards to humans. In-depth particle acceleration simulations have been performed for idealized magnetic fields for diffusive acceleration and particle propagation, and at the same time the quality of MHD simulations of coronal mass ejections (CMEs) has improved significantly. However, to date these two pieces of the same puzzle have remained largely decoupled. Such structures may contain not just a shock but also sizable sheath and pileup compression regions behind it, and may vary considerably with longitude and latitude based on the underlying coronal conditions. In this work, we have coupled results from a detailed global three-dimensional MHD time-dependent CME simulation to a global proton acceleration and transport model, in order to study time-dependent effects of SEP acceleration between 1.8 and 8 solar radii in the 2005 May 13 CME. We find that the source population is accelerated to at least 100 MeV, with distributions enhanced up to six orders of magnitude. Acceleration efficiency varies strongly along field lines probing different regions of the dynamically evolving CME, whose dynamics is influenced by the large-scale coronal magnetic field structure. We observe strong acceleration in sheath regions immediately behind the shock., Comment: 28 pages, 8 figures; Published in The Astrophysical Journal

Recent combined observations from the first three years of Interstellar Boundary Explorer (IBEX) data allow us to examine the heliosphere's downwind region—the heliotail—for the first time. In contrast to a preliminary identification of a narrow "offset heliotail" structure, we find a broad slow solar wind plasma sheet crossing essentially the entire downwind side of the heliosphere at low to mid-latitudes, with fast wind tail regions to the north and south. The slow wind plasma sheet exhibits the steepest ENA spectra in the IBEX sky maps, appears as a two-lobed structure (lobes on the port and starboard sides), and is twisted in the sense of (but at a smaller angle than) the external magnetic field. The overall heliotail structure clearly demonstrates the intermediate nature of the heliosphere's interstellar interaction, where both the external dynamic and magnetic pressures strongly affect the heliosphere.

We examined solar wind plasma and interplanetary magnetic field (IMF) observations from Ulysses' first and third orbits to study hemispheric differences in the properties of the solar wind and IMF originating from the Sun's large polar coronal holes (PCHs) during the declining and minimum phase of solar cycles 22 and 23. We identified hemispheric asymmetries in several parameters, most notably ~15%-30% south-to-north differences in averages for the solar wind density, mass flux, dynamic pressure, and energy flux and the radial and total IMF magnitudes. These differences were driven by relatively larger, more variable solar wind density and radial IMF between ~36°S-60°S during the declining phase of solar cycles 22 and 23. These observations indicate either a hemispheric asymmetry in the PCH output during the declining and minimum phase of solar cycles 22 and 23 with the southern hemisphere being more active than its northern counterpart, or a solar cycle effect where the PCH output in both hemispheres is enhanced during periods of higher solar activity. We also report a strong linear correlation between these solar wind and IMF parameters, including the periods of enhanced PCH output, that highlight the connection between the solar wind mass and energy output and the Sun's magnetic field. That these enhancements were not matched by similar sized variations in solar wind speed points to the mass and energy responsible for these increases being added to the solar wind while its flow was subsonic.

This study provides, for the first time, complete and validated observations from the first three years (2009-2011) of the Interstellar Boundary Explorer (IBEX) mission. Energetic neutral atom (ENA) fluxes are corrected for both the time-variable cosmic ray background and for orbit-by-orbit variations in their probability of surviving en route from the outer heliosphere in to 1 AU where IBEX observes them. In addition to showing all six six-month maps, we introduce new annual ram and anti-ram maps, which can be produced without the need for algorithm-dependent Compton-Getting corrections. Together, the ENA maps, data, and supporting documentation presented here support the full release of these data to the broader scientific community and provide the citable reference for them. In addition, we show that heliospheric ENA emissions have been decreasing over the epoch from 2009 to 2011 with the IBEX Ribbon decreasing by the largest fraction and only the heliotail (which is offset from the down wind direction by the interstellar magnetic field) showing essentially no reduction and actually some increase. Finally, we show how the much more complete observations provided here strongly indicate a quite direct and latitude-dependent solar wind source of the Ribbon.

We investigated the first-order flow anisotropies in the solar wind frame of ~0.06-0.95 MeV nucleon–1 He ions during three corotating interaction region (CIR)-associated particle intensity enhancements observed at 1 AU by Wind, examining CIRs with (1) a reverse shock, (2) a well-formed compression region, and (3) a weak compression region. We identified anti-sunward flows in the compression region downstream of the CIR trailing edge in the events with a reverse shock and well-formed compression that transitioned to sunward flows across and upstream of this boundary. These observations suggest that the trailing edge is organizing the He ion flows and is a local source for the particles in these events, this source being inside 1 AU prior to the trailing edge arrival and beyond 1 AU after its passage. The event with the weak compression region had predominantly sunward flows throughout, indicating that the source of the He ions was beyond 1 AU. These observations provide compelling evidence that He ions at 1 AU can be accelerated to suprathermal energies at the CIR trailing edge in events where the compression region is well formed.

This paper focuses on the analysis and significance of the spectral curvature of energetic neutral atoms (ENAs) detected by the Interstellar Boundary Explorer. The flux versus energy spectrum is analytically expressed in terms of the source proton distributions, namely: (1) the solar wind kappa distribution of protons and (2) the coexisting filled spherical shell distribution of pick-up ions (PUIs). The influence of PUIs on the spectral index and curvature is modeled and investigated in detail. It is analytically shown that (1) the PUI speed upper limit is restricted by the Earthward PUI velocity vector, (2) the PUI distribution causes a positive spectral curvature, and (3) the exact expressions of the spectral index and curvature can be used to extract information about the governing parameters of the parent proton distributions. The sky maps of the spectral curvature reveal a possible band-like configuration of positive spectral curvature that is missing in the original flux sky maps. This band can be roughly separated into the north/south polar regions and two ecliptic meridional “columns” located around the ecliptic longitudes ∼5 ◦ and ∼150 ◦ . The geometric locus between the two cones with noseward axis, and apertures ∼60 ◦ and ∼120 ◦ , configures the band-like region of (1) the positive curvature and (2) the maximum values of PUI distribution. Indeed, the observed curvature band is highly correlated with PUI distributions, and is possibly caused by the influence of PUIs on bending the spectrum from linear (log‐log scale) to concave upward, thus increasing its spectral curvature.

The Interstellar Boundary Explorer (IBEX) has observed energetic neutral atom (ENA) hydrogen emissions from the edge of the solar system for more than three years. The observations span energies from 0.01 to 6?keV FWHM. At energies greater than 0.5-6?keV, and for a travel distance of ~100?AU, the travel time difference between the slowest and the fastest ENA is more than a year. Therefore, we construct spectra including the effect that slower ENAs left the source at an earlier time than faster ones. If the source produces a steady rate of ENAs and the extinction does not vary, then we expect that the spectral shape would be time independent. However, while the extinction of ENAs has been fairly constant during the first two and a half years, the source appears to have changed, and thus the spectra at a single time may not represent the conditions at the source. IBEX's viewing allows continuous sampling of the ecliptic poles where fluxes can be continuously monitored. For a given source distance we construct spectra assuming that the measured ENAs left the source at roughly the same time. To accomplish this construction, we apply time lag corrections to the signal at different ENA energies that take into account the travel time difference. We show that the spectral shape at the poles exhibits a statistically significant change with time.

Energetic neutral atoms (ENAs) observed by the Interstellar Boundary Explorer (IBEX) provide powerful diagnostics about the origin of the progenitor ion populations and the physical mechanisms responsible for their production. Here we survey the fluxes, energy spectra, and energy dependence of the spectral indices of ~0.5-6?keV ENAs measured by IBEX-Hi along the lines of sight of Voyager 1 and 2. We compare the ENA spectra observed at IBEX with predictions of Zank et?al. who modeled the microphysics of the heliospheric termination shock to predict the shape and relative contributions of three distinct heliosheath ion populations. We show that (1) the ENA spectral indices exhibit similar energy dependence along V1 and V2 directions?the spectrum hardens to ? ~ 1 between ~1 and 2?keV and softens to ? ~ 2 below ~1?keV and above ~2?keV, (2) the observed ENA fluxes agree to within ~50% of the Zank et?al. predictions and are unlikely to be produced by core solar wind (SW) ions, and (3) the ENA spectra do not exhibit sharp cutoffs at ~twice the SW speed as is typically observed for shell-like pickup ion (PUI) distributions in the heliosphere. We conclude that ENAs at IBEX are generated by at least two types of ion populations whose relative contributions depend on the ENA energy: transmitted PUIs in the ~0.5-5?keV energy range and reflected PUIs above ~5?keV energy. The ~0.5-5?keV PUI distribution is probably a superposition of Maxwellian or kappa distributions and partially filled shell distributions in velocity space.

We examined the temporal profiles and peak intensities for 73 corotating interaction region (CIR)-associated suprathermal ({approx}0.1-8 MeV nucleon{sup -1}) helium (He) ion enhancements identified at STEREO-A, STEREO-B, and/or Advanced Composition Explorer between 2007 and 2010. We found that in most events the peak He intensity times were well organized by the CIR compression region trailing edge, regardless of whether or not a reverse shock was present. Out of these events, 19% had their 0.193 MeV nucleon{sup -1} He intensities peak within 1 hr and 50% within 4.75 hr of the CIR trailing edge, the distribution having a 1{sigma} value of 7.3 hr. Events with a 0.193 MeV nucleon{sup -1} He intensity peak time within 1{sigma} of the CIR trailing edge showed a positive correlation between the {approx}0.1 and 0.8 MeV nucleon{sup -1} He peak intensities and magnetic compression ratios in events both with and without a reverse shock. The peak intensities in all other events showed little to moderate correlation between these parameters. Our results provide evidence that some fraction of the CIR-associated

We study the energy dependence of ~0.5-6 keV energetic neutral atom (ENA) spectra in the southern heliospheric polar region obtained during five six-month sky maps measured by IBEX-Hi. We calculate the spectral slopes in the south pole in four different energy bands, namely, ~0.7-1.1 keV, ~1.1-1.7 keV, ~1.7-2.7 keV, and ~2.7-4.3 keV. We show (1) a persistent flattening of the ENA spectrum between ~1 and 2 keV, (2) significantly different modes (2.31, 1.58, 0.97, and 1.44) for the distributions of the slopes in the four different energy bands, and (3) a general decrease with increasing energy in the widths (FWHM) and mode fluctuations (their spread) of the slope distributions. We also compare the averaged ENA spectra measured at the south pole and at mid-latitudes. We conclude that the flattening between ~1 and 2 keV in the polar spectrum (spectral break) is produced by an enhancement of ENAs created by charge exchange between interstellar neutrals and pick-up ions in the fast solar wind.

We study the spectral properties of different regions and structures in the energetic neutral atom (ENA) maps at energies from {approx}0.5 keV to {approx}6 keV from the Interstellar Boundary Explorer (IBEX) mission. We find that (1) an ankle-shaped break (spectrum hardens) between {approx}1 keV and {approx}2 keV characterizes the polar spectra and the right flank, while a knee-shaped break (spectrum softens) describes the ribbon, nose, and the front region spectra; (2) the spectral indices across full latitudinal range (tail and poles) comprise a dependence reflecting a knee break at mid latitudes and an ankle break at high latitudes. This latitudinal evolution has inflection points at {approx}40 deg. S and {approx}36 deg. N, and is strongly correlated with the solar wind speed structure obtained by the Ulysses/SWOOPS instrument during its fast latitude scan in 2007. Our study confirms that the ecliptic latitude predominantly orders the spectral signatures of ENA distributions. This ordering may reflect the average solar wind properties that vary characteristically with latitude around solar minimum. We report on the spectral analyses of six regions and two structures in the IBEX maps. We also discuss the spectral asymmetries between the north and the south polar regions, their correlation with solarmore » wind measurements, and the implications of these observations. Thus, we show detailed connections between the IBEX energy spectra and latitudinal properties of solar wind.« less

Analysis of the IBEX-Hi Energetic Neutral Atom spectra reveals, for the first time, the sky map of the source ion temperatures. The solar wind exists in non-equilibrium stationary states and can be described by kappa distributions. The high-energy asymptotic behavior of kappa distributions leads to a power law of the flux versus energy spectrum, while its specific formulation derives the temperature and kappa index that govern these distributions. We find that the observed temperature in most directions is about a million degrees, in agreement with most heliospheric models for the inner heliosheath. Thus, the termination shock is stronger in most places than revealed by the Voyagers' observations at their two unique crossing points. The global sky maps indicate low-temperature regions in various directions, including toward Voyager 2 where the temperature is an order of magnitude lower, consistent with in situ Voyager observations. Interestingly, the vast majority of measured kappa indices are between {approx}1.5 and {approx}2.5, consistent with the far-equilibrium 'cavity' of minimum entropy discovered by Livadiotis and McComas. The sky maps suggest that the Ribbon is a string of localized regions, while its thermodynamical behavior differentiates it from the global distributed flux. A simple model is developed to derive themore » density, heliosheath thickness, and thermal pressure. We find that the Ribbon thermal pressure is {approx}3.5 pDyn cm{sup -2}, roughly equal to the mechanical pressure exerted by the Local Interstellar Medium.« less

The Interstellar Boundary Explorer (IBEX) observes a remarkable feature, the IBEX ribbon, which has energetic neutral atom (ENA) flux over a narrow region ~20° wide, a factor of 2-3 higher than the more globally distributed ENA flux. Here, we separate ENA emissions in the ribbon from the distributed flux by applying a transparency mask over the ribbon and regions of high emissions, and then solve for the distributed flux using an interpolation scheme. Our analysis shows that the energy spectrum and spatial distribution of the ribbon are distinct from the surrounding globally distributed flux. The ribbon energy spectrum shows a knee between ~1 and 4 keV, and the angular distribution is approximately independent of energy. In contrast, the distributed flux does not show a clear knee and more closely conforms to a power law over much of the sky. Consistent with previous analyses, the slope of the power law steepens from the nose to tail, suggesting a weaker termination shock toward the tail as compared to the nose. The knee in the energy spectrum of the ribbon suggests that its source plasma population is generated via a distinct physical process. Both the slope in the energy distribution of the distributed flux and the knee in the energy distribution of the ribbon are ordered by latitude. The heliotail may be identified in maps of globally distributed flux as a broad region of low flux centered ~44°W of the interstellar downwind direction, suggesting heliotail deflection by the interstellar magnetic field.