N. A. Schwadron

Composition of quasi‐stationary solar wind flows from Ulysses/Solar Wind Ion Composition Spectrometer

Using improved, self-consistent analysis techniques, we determine the average solar wind charge state and elemental composition of nearly 40 ion species of He, C, N, O, Ne, Mg, Si, S, and Fe observed with the Solar Wind Ion Composition Spectrometer on Ulysses. We compare results obtained during selected time periods, including both slow solar wind and fast streams, concentrating on the quasi-stationary flows away from recurrent or intermittent disturbances such as corotating interaction regions or coronal mass ejections. In the fast streams the charge state distributions are consistent with a single freezing-in temperature for each element, whereas in the slow wind these distributions appear to be composed of contributions from a range of temperatures. The elemental composition shows the well-known first ionization potential (FIP) bias of the solar wind composition with respect to the photosphere. However, it appears that our average enrichment factor of low-FIP elements in the slow wind, not quite a factor of 3, is smaller than that in previous compilations. In fast streams the FIP bias is found to be yet smaller but still significantly above 1, clearly indicating that the FIP fractionation effect is also active beneath coronal holes from where the fast wind originates. This imposes basic requirements upon FIP fractionation models, which should reproduce the stronger and more variable low-FIP bias in the slow wind and a weaker (and perhaps conceptually different) low-FIP bias in fast streams. Taken together, these results firmly establish the fundamental difference between the two quasi-stationary solar wind types.

Global Observations of the Interstellar Interaction from the Interstellar Boundary Explorer (IBEX)

Whats Happening in the Heliosphere The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959, published online 15 October), Fuselier et al. (p. 962, published online 15 October), Funsten et al. (p. 964, published online 15 October), and Möbius et al. (p. 969, published online 15 October) present data taken by NASAs Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966, published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971, published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Suns travel through the galaxy as was previously thought. Observations by the Interstellar Boundary Explorer have revealed surprising features in the interaction between the heliosphere and the interstellar medium. The Sun moves through the local interstellar medium, continuously emitting ionized, supersonic solar wind plasma and carving out a cavity in interstellar space called the heliosphere. The recently launched Interstellar Boundary Explorer (IBEX) spacecraft has completed its first all-sky maps of the interstellar interaction at the edge of the heliosphere by imaging energetic neutral atoms (ENAs) emanating from this region. We found a bright ribbon of ENA emission, unpredicted by prior models or theories, that may be ordered by the local interstellar magnetic field interacting with the heliosphere. This ribbon is superposed on globally distributed flux variations ordered by both the solar wind structure and the direction of motion through the interstellar medium. Our results indicate that the external galactic environment strongly imprints the heliosphere.

The Behavior of the Open Magnetic Field of the Sun

The magnetic field of the Sun can be considered as having two distinct components: open magnetic flux, in which the field lines remain attached to the Sun and are dragged outward into the heliosphere with the solar wind, and closed magnetic flux, in which the field remains entirely attached to the Sun and forms loops and active regions in the solar corona. In this paper a model is presented for the behavior of the Suns open magnetic flux. It is argued that the total open flux should tend to be constant in time since it can be destroyed only if open flux of opposite polarity reconnects, a process that may be unlikely since the open flux is ordered into large-scale regions of uniform polarity. The behavior of open flux is thus governed by its motion on the solar surface. The motion may be primarily due to a diffusive process that results from open field lines reconnecting with randomly oriented closed loops, and also to the usual convective motions on the solar surface, such as differential rotation. The diffusion process needs to be described by a diffusion equation appropriate for transport by an external medium, which is different from the usual diffusion equation used in energetic particle transport. The loops required for the diffusion have been identified in recent observations of the Sun and have properties, of both size and composition, consistent with their use in the model. The diffusive motions can account for the concentrations of open flux at the coronal holes, where the loops are reduced and the diffusion limited. The motions result in the reversal of the polarity of open flux over the solar cycle simply by the rotation of the current sheet that separates regions of single polarity. The diffusive process, in which reconnection occurs between open field lines and loops, is responsible for the input of mass and energy into the solar wind. The motions of the open field lines also influence the configuration of the heliospheric magnetic field.

The Heliosphere’s Interstellar Interaction: No Bow Shock

No Shock Ahead of the Sun The boundary of the heliosphere is the region where the solar wind interacts with interstellar space, and it marks the edge of our solar system. Based on observations from NASAs Interstellar Boundary Explorer, McComas et al. (p. 1291, published online 10 May; see the Perspective by Redfield) determined values for local interstellar parameters—such as speed, direction, and temperature—and show that these and other recent constraints are not consistent with a bow shock ahead of the heliosphere, as previously believed. Observations from the Interstellar Boundary Explorer are not consistent with a bow shock ahead of the heliosphere. As the Sun moves through the local interstellar medium, its supersonic, ionized solar wind carves out a cavity called the heliosphere. Recent observations from the Interstellar Boundary Explorer (IBEX) spacecraft show that the relative motion of the Sun with respect to the interstellar medium is slower and in a somewhat different direction than previously thought. Here, we provide combined consensus values for this velocity vector and show that they have important implications for the global interstellar interaction. In particular, the velocity is almost certainly slower than the fast magnetosonic speed, with no bow shock forming ahead of the heliosphere, as was widely expected in the past.

Comparison of Interstellar Boundary Explorer Observations with 3D Global Heliospheric Models

Whats Happening in the Heliosphere The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959, published online 15 October), Fuselier et al. (p. 962, published online 15 October), Funsten et al. (p. 964, published online 15 October), and Möbius et al. (p. 969, published online 15 October) present data taken by NASAs Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966, published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971, published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Suns travel through the galaxy as was previously thought. Observations by the Interstellar Boundary Explorer have revealed surprising features in the interaction between the heliosphere and the interstellar medium. Simulations of energetic neutral atom (ENA) maps predict flux magnitudes that are, in some cases, similar to those observed by the Interstellar Boundary Explorer (IBEX) spacecraft, but they miss the ribbon. Our model of the heliosphere indicates that the local interstellar medium (LISM) magnetic field (BLISM) is transverse to the line of sight (LOS) along the ribbon, suggesting that the ribbon may carry its imprint. The force-per-unit area on the heliopause from field line draping and the LISM ram pressure is comparable with the ribbon pressure if the LOS ~ 30 to 60 astronomical units and BLISM ~ 2.5 microgauss. Although various models have advantages in accounting for some of the observations, no model can explain all the dominant features, which probably requires a substantial change in our understanding of the processes that shape our heliosphere.

Structures and Spectral Variations of the Outer Heliosphere in IBEX Energetic Neutral Atom Maps

Whats Happening in the Heliosphere The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959, published online 15 October), Fuselier et al. (p. 962, published online 15 October), Funsten et al. (p. 964, published online 15 October), and Möbius et al. (p. 969, published online 15 October) present data taken by NASAs Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966, published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971, published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Suns travel through the galaxy as was previously thought. Observations by the Interstellar Boundary Explorer have revealed surprising features in the interaction between the heliosphere and the interstellar medium The Interstellar Boundary Explorer (IBEX) has obtained all-sky images of energetic neutral atoms emitted from the heliosheath, located between the solar wind termination shock and the local interstellar medium (LISM). These flux maps reveal distinct nonthermal (0.2 to 6 kilo–electron volts) heliosheath proton populations with spectral signatures ordered predominantly by ecliptic latitude. The maps show a globally distributed population of termination-shock–heated protons and a superimposed ribbonlike feature that forms a circular arc in the sky centered on ecliptic coordinate (longitude λ, latitude β) = (221°, 39°), probably near the direction of the LISM magnetic field. Over the IBEX energy range, the ribbon’s nonthermal ion pressure multiplied by its radial thickness is in the range of 70 to 100 picodynes per square centimeter AU (AU, astronomical unit), which is significantly larger than the 30 to 60 picodynes per square centimeter AU of the globally distributed population.

On the Coronal Magnetic Field: Consequences of Large-Scale Motions

A model has been introduced for the magnetic field in the heliosphere in which the field lines execute large excursions in heliographic latitude. The excursions result from the interplay between the differential rotation of the photosphere and the nonradial expansion of the solar wind near the Sun. The model accounts for the observed ease with which low-rigidity particles propagate in latitude in the solar wind. In this paper the consequences of this model for the behavior of the coronal magnetic field are explored. It is pointed out that the model provides an explanation for the time evolution and apparent rigid rotation of polar coronal holes and the differences between fast and slow solar wind.

INTERSTELLAR GAS FLOW PARAMETERS DERIVED FROM INTERSTELLAR BOUNDARY EXPLORER-Lo OBSERVATIONS IN 2009 AND 2010: ANALYTICAL ANALYSIS

Neutral atom imaging of the interstellar gas flow in the inner heliosphere provides the most detailed information on physical conditions of the surrounding interstellar medium (ISM) and its interaction with the heliosphere. The Interstellar Boundary Explorer (IBEX) measured neutral H, He, O, and Ne for three years. We compare the He and combined O + Ne flow distributions for two interstellar flow passages in 2009 and 2010 with an analytical calculation, which is simplified because the IBEX orientation provides observations at almost exactly the perihelion of the gas trajectories. This method allows separate determination of the key ISM parameters: inflow speed, longitude, and latitude, as well as temperature. A combined optimization, as in complementary approaches, is thus not necessary. Based on the observed peak position and width in longitude and latitude, inflow speed, latitude, and temperature are found as a function of inflow longitude. The latter is then constrained by the variation of the observed flow latitude as a function of observer longitude and by the ratio of the widths of the distribution in longitude and latitude. Identical results are found for 2009 and 2010: an He flow vector somewhat outside previous determinations (λISM∞ = 79. ◦ 0+3 . 0(−3. ◦ 5), βISM∞ =− 4. 9 ± 0. 2, VISM∞ = 23.5 + 3.0(−2.0) km s −1 , THe = 5000–8200 K), suggesting a larger inflow longitude and lower speed. The O + Ne temperature range, T O+N e = 5300–9000 K, is found to be close to the upper range for He and consistent with an isothermal medium for all species within current uncertainties.

Width and Variation of the ENA Flux Ribbon Observed by the Interstellar Boundary Explorer

Whats Happening in the Heliosphere The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959, published online 15 October), Fuselier et al. (p. 962, published online 15 October), Funsten et al. (p. 964, published online 15 October), and Möbius et al. (p. 969, published online 15 October) present data taken by NASAs Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966, published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971, published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Suns travel through the galaxy as was previously thought. Observations by the Interstellar Boundary Explorer have revealed surprising features in the interaction between the heliosphere and the interstellar medium. The dominant feature in Interstellar Boundary Explorer (IBEX) sky maps of heliospheric energetic neutral atom (ENA) flux is a ribbon of enhanced flux that extends over a broad range of ecliptic latitudes and longitudes. It is narrow (~20° average width) but long (extending over 300° in the sky) and is observed at energies from 0.2 to 6 kilo–electron volts. We demonstrate that the flux in the ribbon is a factor of 2 to 3 times higher than that of the more diffuse, globally distributed heliospheric ENA flux. The ribbon is most pronounced at ~1 kilo–electron volt. The average width of the ribbon is nearly constant, independent of energy. The ribbon is likely the result of an enhancement in the combined solar wind and pickup ion populations in the heliosheath.

Implications of Solar Wind Composition for Cometary X-Rays

Solar wind composition provides a strong source of variability for cometary X-rays. Using recently constrained composition observations from the Solar Wind Ion Composition Spectrometer on the Ulysses spacecraft (SWICS/Ulysses) and information on ion-neutral charge-transfer collisions, we model cometary X-ray emission and its variability. Fast and slow solar wind cases are adopted and compared to observations of X-ray spectra from the Rontgen-Satellit (ROSAT) for comet Levy. The predicted and observed spectra compare most favorably for fast solar wind in optically thin conditions. Solar wind composition can be predicted as a function of the coronal temperature near its source and, consequently, cometary X-ray spectra may be predicted as a function of the solar winds coronal temperature. Implications and additional developments necessary to unravel the compositional information contained in X-ray spectra are explored.