Paul Henry Janzen

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.

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.

Pick-Up Ions in the Outer Heliosheath: A Possible Mechanism for the Interstellar Boundary EXplorer Ribbon

First data from NASAs Interstellar Boundary EXplorer (IBEX) mission show a striking ribbon feature of enhanced energetic neutral atom (ENA) emission. The enhancement in flux is between 2 and 3 times greater than adjacent regions of the sky. Yet the spectral index of ENAs appears to be the same both inside and outside the ribbon. While the ribbon itself was not predicted by any models of the heliospheric interface, its geometry appears to be related to the predicted interstellar magnetic field (ISMF) outside the heliopause (HP). In this Letter, we examine a process of ENA emission from the outer heliosheath, based on a source population of non-isotropic pick-up ions that themselves originate as ENAs from inside the HP. We find that our simplistic approach yields a ribbon of enhanced ENA fluxes as viewed from the inner heliosphere with a spatial location and ENA flux similar to the IBEX measurements, with the provision that the ions retain a partial shell distribution long enough for the ions to be neutralized. As a corollary, our idealized simulation of this mechanism suggests that ISMF is likely oriented close to the center of the observed ribbon.

SEPARATION OF THE INTERSTELLAR BOUNDARY EXPLORER RIBBON FROM GLOBALLY DISTRIBUTED ENERGETIC NEUTRAL ATOM FLUX

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.

THE FIRST THREE YEARS OF IBEX OBSERVATIONS AND OUR EVOLVING HELIOSPHERE

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.

Energetic neutral atoms from the Earth's subsolar magnetopause

The shocked solar wind in the Earths magnetosheath becomes nearly stationary at the subsolar magnetopause. At this location, solar wind protons are neutralized by charge exchange with neutral hydrogen atoms at the extreme limits of the Earths tenuous exosphere. The resulting Energetic Neutral Atoms (ENAs) propagate away from the subsolar region in nearly all directions. Simultaneous observations of hydrogen ENAs from the Interstellar Boundary Explorer (IBEX) and proton distributions in the magnetosheath from the Cluster spacecraft are used to quantify this charge exchange process. By combining these observations with a relatively simple model, estimates are obtained for the ratio of ENA to shocked solar wind flux (about 10−4) and the exospheric density at distances greater than 10 Earth Radii (RE) upstream from the Earth (about 8 cm−3).

HELIOSPHERIC NEUTRAL ATOM SPECTRA BETWEEN 0.01 AND 6 keV FROM IBEX

Since 2008 December, the Interstellar Boundary Explorer (IBEX) has been making detailed observations of neutrals from the boundaries of the heliosphere using two neutral atom cameras with overlapping energy ranges. The unexpected, yet defining feature discovered by IBEX is a Ribbon that extends over the energy range from about 0.2 to 6 keV. This Ribbon is superposed on a more uniform, globally distributed heliospheric neutral population. With some important exceptions, the focus of early IBEX studies has been on neutral atoms with energies greater than ∼0.5 keV. With nearly three years of science observations, enough low-energy neutral atom measurements have been accumulated to extend IBEX observations to energies less than ∼0.5 keV. Using the energy overlap of the sensors to identify and remove backgrounds, energy spectra over the entire IBEX energy range are produced. However, contributions by interstellar neutrals to the energy spectrum below 0.2 keV may not be completely removed. Compared with spectra at higher energies, neutral atom spectra at lower energies do not vary much from locationtolocationinthesky,includinginthedirectionoftheIBEXRibbon.Neutralfluxesareusedtoshowthatlow energy ions contribute approximately the same thermal pressure as higher energy ions in the heliosheath. However, contributions to the dynamic pressure are very high unless there is, for example, turbulence in the heliosheath with fluctuations of the order of 50‐100 km s −1 .

Separation of the Ribbon from globally distributed energetic neutral Atom Flux using the first five Years of Ibex Observations

The Interstellar Boundary Explorer (IBEX) observes the IBEX ribbon, which stretches across much of the sky observed in energetic neutral atoms (ENAs). The ribbon covers a narrow (~20°-50°) region that is believed to be roughly perpendicular to the interstellar magnetic field. Superimposed on the IBEX ribbon is the globally distributed flux that is controlled by the processes and properties of the heliosheath. This is a second study that utilizes a previously developed technique to separate ENA emissions in the ribbon from the globally distributed flux. A transparency mask is applied over the ribbon and regions of high emissions. We then solve for the globally distributed flux using an interpolation scheme. Previously, ribbon separation techniques were applied to the first year of IBEX-Hi data at and above 0.71 keV. Here we extend the separation analysis down to 0.2 keV and to five years of IBEX data enabling first maps of the ribbon and the globally distributed flux across the full sky of ENA emissions. Our analysis shows the broadening of the ribbon peak at energies below 0.71 keV and demonstrates the apparent deformation of the ribbon in the nose and heliotail. We show global asymmetries of the heliosheath, including both deflection of the heliotail and differing widths of the lobes, in context of the direction, draping, and compression of the heliospheric magnetic field. We discuss implications of the ribbon maps for the wide array of concepts that attempt to explain the ribbons origin. Thus, we present the five-year separation of the IBEX ribbon from the globally distributed flux in preparation for a formal IBEX data release of ribbon and globally distributed flux maps to the heliophysics community.

Variations in the Heliospheric Polar Energetic Neutral Atom Flux Observed by the Interstellar Boundary Explorer

The ecliptic poles are observed continuously by the Interstellar Boundary Explorer (IBEX); thus, it is possible to discern temporal variations in the energetic neutral atoms (ENAs) from the outer heliosphere on timescales much shorter than the time it takes for IBEX to generate a full sky map (six months). Observations indicate that the ENA flux from the polar directions incident at Earth has been steadily decreasing for the two-year period from 2008 December through 2011 February. Over the IBEX-Hi energy range, the decrease in flux is energy dependent, varying at the south ecliptic pole from no drop at 0.71 keV, to 70% at 1.1 keV. At higher energies the drop ranges between 10% and 50%. The decline observed at the north ecliptic pole is as high as 48%, also at 1.1 keV. The trend correlates with the steady decline in solar wind dynamic pressure observed at 1 AU between 2005 and 2009, the likely period when solar wind protons that provide the source for ENAs observed by IBEX would have been outbound from the Sun. We propose a method by which the correlation between the 1 AU solar wind dynamic pressure and the ENA-derived pressure within the inner heliosheath (IHS) can be used to estimate the distance to the termination shock and the thickness of the IHS in the direction of the ecliptic poles. Our new analysis based on IBEX data shows the TS distances to be 110 AU and 134 AU at the south and north poles, respectively, and the corresponding IHS thicknesses to be 55 AU and 82 AU. Our analysis is consistent with the notion that the observed ENA fluxes originate in the IHS and their variations are driven by the solar wind as it evolves through the solar cycle.

Low energy neutral atoms from the heliosheath

In the heliosheath beyond the termination shock, low energy (<0.5 keV) neutral atoms are created by charge exchange with interstellar neutrals. Detecting these neutrals from Earths orbit is difficult because their flux is reduced substantially by ionization losses as they propagate from about 100 to 1 AU and because there are a variety of other signals and backgrounds that compete with this weak signal. Observations from IBEX-Lo and -Hi from two opposing vantage points in Earths orbit established a lower energy limit of about 0.1 keV on measurements of energetic neutral atoms (ENAs) from the heliosphere and the form of the energy spectrum from about 0.1 to 6 keV in two directions in the sky. Below 0.1 keV, the detailed ENA spectrum is not known, and IBEX provides only upper limits on the fluxes. However, using some assumptions and taking constraints on the spectrum into account, we find indications that the spectrum turns over at an energy between 0.1 and 0.2 keV.