Energetic Neutral Atom (ENA) Imaging of the Heliosheath: Spectral Characteristics Across the Sky and Implications for Heliosheath Structure from Observations by the Neutral Particle Detector (NPD) on board Venus Express (VEX)
Here we report on the first three years of spectral observations across the sky of ENAs in the ∼0.3-3 keV range generated in the heliosheath observed by NPD on board Venus Express. The observed spectral shapes are consistent with the expected ENA spectra derived from the Voyager-1 and 2 observations, but with an additional "ledge" starting at around 0.5 keV extending to a drop at a couple 1.0 keV. We interpret this additional feature as the enhancement of ions picked-up by the solar wind and convected out into the heliosheath. Furthermore, plasma and energetic particle measurements during the termination shock crossings by the Voyagers reveal that most of the solar wind flow energy is tranferred into the particles in the pick-up ion energy range (∼0.1-10 keV) [Richardson, Nature, July, 2008] - a range that was not covered by the Voyager instrumentation, but is covered by the ASPERA-4/NPD measurements. We seek to estimate if the observed ENA spectra are consistent with the required energization at these energies. To date the statistical coverage of the NPD observations are sufficient to obtain statistically significant spectra in 60x60 deg angular bins (higher resolution close to the ecliptic equator), covering all ecliptic longitudes and up to about 60 deg northern and southern ecliptic latitudes. High-resolution measurements of the entire sky are soon expected from the Interstellar Boundary Explorer (IBEX), which is the only mission so far dedicated to imaging the heliosheath and was launched October 2008. While the IBEX data is currently being analyzed, several other, non-dedicated instruments on board various missions (Venus Express, Cassini, IMAGE, Mars Express, SOHO, STEREO) are seeking to provide meaningful measurements of the structure and spectral characteristics of the heliosheath. These continuing efforts will be particularly important to study the possible temporal variations of the heliosheath ENA signals, which would be a important constraint on the spatial extent and dynamics of the heliosheath. NPD belongs to the ASPERA-4 experiment suite on board VEX and is a time-of-flight (TOF) instrument using a coincidence technique to identify ENAs (and is therefore not sensitive to, for example X-rays) and has an instantaneous field-of-view (FOV) of 5°×180° with six sectors of each 5°×30° FOV. A scanning platform allows the FOV to scan across half of the entire sky in one scan.
Modeling Neutral Hydrogen in the Heliospheric Interface
Observational data of neutral atoms provides us with a 1 AU picture of the neutral atom flux in the heliosphere. The large mean free paths of neutrals allow us to infer properties of their distant source, as well as the properties of the intermediary medium. Energetic neutral hydrogen, for example, travels on almost straight trajectories, so that the particles observed coming from a particular direction were created from energetic protons along that line of sight. Similarly, low energy interstellar atoms are attenuated and deflected as they enter the heliosphere, and this deflection tells us something about the structure of the heliospheric interface. Of course, to infer quantitative features of the global heliosphere from neutral atom observations at 1 AU, we need accurate models that capture the 3D structure of the heliosphere. We will present an advanced MHD-neutral model of the heliosphere which is 3D, employs kinetic neutral Hydrogen, and incorporates a suprathermal tail on the solar wind proton distribution to approximate pick-up ions. We will demonstrate that with the help of such a model, we can test various hypotheses regarding the heliospheric boundary via forward modeling and comparison with data.
Global Heliospheric Sheath Imaging in Fluxes of Energetic Neutral Atoms (ENAs) from a Sun-Pointed Earth-Orbiting Spinning Spacecraft
A sun-pointed Earth-orbiting spinning spacecraft offers a convenient cost-efficient platform for global imaging the heliosphere in fluxes of energetic neutral atoms (ENAs). A single-pixel ENA instrument pointed normally to the spin axis images a full swath in the sky perpendicular to the ecliptic plane during each spacecraft rotation. As the Earth revolves around the Sun, the spacecraft spin axis is precessed to maintain its sun-pointed direction. Consequently, the instrument achieves the full 4π coverage of the sky in six months. The recently launched NASA's Interstellar Boundary Explorer (IBEX) mission implements such an observational approach to remotely probe the solar wind and pickup proton populations beyond the termination shock in the heliospheric sheath. The opposing forces of the sun's gravity and solar radiation pressure determine the energy change and survival probabilities of heliospheric ENAs along their trajectory to an observation point near 1 AU. ENA imaging simulations usually consider energetic neutral atoms approaching the Sun on radial trajectories. The observation geometry that is perpendicular to the sun-pointed spinning axis is unique because, for atomic hydrogen, the instrument detects H ENAs at their distance of the closest approach to the Sun. The images obtained under such observational conditions would differ markedly from those obtained for radial ENA trajectories. In addition, the 30-km/sec velocity of the observational platform with respect to the sun leads to velocity aberration effects and energy shifts that increase with the decreasing ENA energies. These effects of non-radial observational geometry are predictable. We present here a theoretical and modeling approach to account for specifics of ENA global imaging from a sun-pointed spinning spacecraft. We focus on the energy range of one ENA sensor on IBEX, IBEX-Hi, from 200-6000 eV. Quantitative analysis shows that the image-forming aberration effects are typically confined within the instrument imaging pixel (6 deg) while the combined modifications of ENA energies due to spacecraft motion and inward transport from the heliospheric sheath are generally small and easily corrected, even at the lowest energies measured by IBEX-Hi.
Investigation of the Cooling Behavior of Interstellar Pickup Helium and Its Effect on the Determination of Neutral Density Profiles and Ionization Rates
The shape of pickup ion velocity distributions in the solar wind is typically used to infer the radial profile of the neutral source distributions sunward of the observing instrument. Since this radial profile is a direct function of the ionization rate relative to the neutral flow time, this information can also determine the ionization rate. However, this analysis also makes the significant assumption that the pickup distribution cools from the point of ionization in the expanding solar wind as an isotropic adiabatic gas. This isotropic-adiabatic assumption allows a simple mapping of the pickup intensity in velocity space to the ion production rate in radial position. However, the finding of substantial anisotropies of pickup ion distributions, in particular under radial interplanetary magnetic field conditions, and of large variations of pickup ion distributions that are correlated with magnetic field conditions, suggests that pitch-angle scattering of these particles is substantially inhibited in contrast to the simple theory. These observations prompt a re-evaluation of the existing models. Here, we explore the effect of different cooling functions on the shape of interstellar pickup He distributions under perpendicular magnetic field conditions. The shape also depends on the ionization rate through the radial neutral density profile. We show that pickup He observations over a solar activity cycle, during which the ionization rate will vary by factors of 3 - 4, can strongly constrain the actual cooling behavior of the pickup distribution and provide information on the extent of isotropization.
Heliospheric Phenomena due to Ion-atom Interactions
We review physical phenomena occurring in the outer heliosphere due to the solar wind (SW) interaction with the partially-ionized circum-heliospheric interstellar medium (CHISM) illustrating them by observational data and numerical simulations. In particular, we compare Voyager 1 (V1) and Voyager 2 (V2) observations with the results of our 3D, MHD-neutral model of the SW-CHISM interaction. It is shown that the increase in the interstellar magnetic field (ISMF) strength, although increasing the difference in the heliocentric distance at which V1 and V2 crossed the termination shock (TS), also increases the discrepancy in the SW velocity distributions measured in the spacecraft frames. We show that the gradual decrease in the SW dynamic pressure being observed by Ulysses for more than 3 years is a likely explanation of the above-mentioned 10 AU difference. We discuss the implications of different scenarios resulting in the 2-3 kHz radio emission and their possible consequences for the distribution of radio emission sources. The effect of the solar cycle on the 3D, time-dependent distribution of neutral atoms is analyzed, while taking into account variations in the latitudinal extent of the slow SW and the angle between the Sun's rotation and magnetic axes.
The interaction of pickup ions at the termination shock and implications for neutral atoms
Observations by the Voyager 2 spacecraft of the structure of the heliospheric termination shock revealed a quasi-perpendicular structure that possessed many of the characteristics that are familiar to us from perpendicular shocks observed in the inner heliosphere. However, a surprise was the relative lack of heating experienced by thermal solar wind ions (and electrons), leading to the suggestion that the pickup ions were primarily heated at the heliospheric termination shock. Because the Voyager plasma instrument cannot measure the interstellar pickup ions directly, their behavior at the termination shock could not be gauged directly. Nonetheless, it had already been predicted by Zank et al. 1996 that the primary dissipation mechanism for the quasi-perpendicular termination shock would be the reflection of pickup ions rather than solar wind ions; this because of the shell-like structure of the pickup ion distribution function. We discuss the effect of pickup ions including their trajectories within the structure of the shock observed by Voyager 2. A model for the shock structure is constructed on the basis of preferentially reflected pickup ions, and some solutions are discussed. We estimate the degree of heating for the various components and discuss how this will influence the character of the energetic neutral atoms created in the inner heliosheath.
Upper Limits on Heliosheath ENA Intensities (4-70 keV) from STEREO/STE
An upper limit on heliosheath ENA intensities provides a crucial limitation on the dynamic range for ENA detection and simulation. Here, we present an upper limit estimate for heliosheath ENAs at ∼4-70 keV based on measurements by STEREO/STE (SupraThermal Electron) sensors during March 2007 - March 2009. STE has recently demonstrated its capability to detect ENAs with images from the Earth's magnetosphere. From March 2007 to March 2009, the 8 detectors of STE (four on STEREO A and four on B) performed in total almost 16 full scans of the heliosphere in longitude. However, STE didn't detect any significant ENA enhancements above the background (mainly interplanetary electrons) in directions outside the heliosphere nose region (220-300° ecliptic longitude), where STE is contaminated by X-rays from galactic sources. In this study, we assume that heliosheath ENAs don't vary with time. Then we obtain the typical lowest flux level f0 in STE at ∼4-70 keV as a function of the source direction excluding the nose. The preliminary results show f0 (cm-2 s-1 str-1 eV-1)= 0.9-3.2×106 [E(eV)]-2.3. If heliosheath ENAs are spatially uniform, f0 would be a good estimate of their upper limit from the heliosphere flank or tail direction. If they are localized with an angular width <20-30° in longitude, then they would show high fluxes only in one detector since the other three detectors simultaneously look in other directions. The absence of such anisotropy suggests that for a localized ENA source, the upper limit would be less than 20%- 30% of f0.