Interstellar Boundary Explorer Mission Research Articles

Transport Equation of Kappa Distributions in the Heliosphere

Abstract In this paper, we develop the transport equation of kappa, the fundamental thermodynamic parameter that labels kappa distributions of particle velocities. Using the recently developed concept of entropy defect, we are able to formulate the transport equation of kappa as a function of a general, positive or negative, rate of entropy change. Then, we derive the particular case of exchanging plasma ions with low-dimensionality, newly born pickup protons, which interact and decrease the entropy of the flow of otherwise kappa-distributed plasma protons. Finally, we apply the transport equation of kappa to the solar wind plasma protons, which leads to the radial profile of kappa values, as well as the evolution of the kappa distributions through the heliosphere. The results show that the solar wind kappa decreases with increasing heliocentric distance, corresponding to plasmas residing in stationary states far from classical thermal equilibrium. Moreover, in the outer heliosphere and the heliosheath, kappa reaches its lowest values and is spread across the far-equilibrium region of 1.5 < κ < 2.5, which coincides with independent observations provided by NASA’s Interstellar Boundary Explorer mission.

Improved ion optics for low energy neutral atom imaging with high sensitivity

An ion-optical system suitable for imaging neutral atom fluxes with energies <∼1 keV has been developed by means of computer simulations. Although conceptually similar to the IBEX-Lo sensor flown on the interstellar boundary explorer mission “IBEX”, the new ion optics presented here exhibits up to 29-fold higher sensitivity with similar angular resolution. This improvement is achieved through a substantial increase in the geometric factor and ion collection–transmission following conversion to negative ions. Energy resolution can be dynamically adjusted during operations by changing a single electrode voltage.

IBEX's Enigmatic Ribbon in the sky and its many possible sources

Launched in October 2008, NASA's Interstellar Boundary Explorer (IBEX) has provided the first-ever energy-resolved all-sky maps of energetic neutral atom (ENA) emissions from the boundary of the heliosphere with the local interstellar medium (LISM). The maps reveal an enigmatic “Ribbon” of enhanced emissions superposed on a smoothly varying global distribution of ENA emissions. The Ribbon lies between the locations where the two Voyagers crossed the termination shock and was not sampled by them on their trajectories out through the heliosphere. IBEX's discovery of the Ribbon—a feature completely unpredicted by theory and numerical simulations—requires a new paradigm in our understanding of the heliosphere/LISM interaction. The Ribbon appears to be ordered by the interstellar magnetic field and the solar wind's latitudinal structure. The Ribbon's origin, whether from processes occurring in the heliosphere or beyond in the LISM, is unknown and the subject of intense investigation. Continued observations over the first 3 years of the IBEX mission have characterized the Ribbon, including its location, energy spectra, fine structure, and time variations. Here we review and compare the various hypotheses that have been proposed to explain the Ribbon, assess their ability to account for the features that have emerged from 3 years of careful analysis of the IBEX sky maps, and discuss how evolving theories and observations during IBEX's extended mission and by the National Research Council's recommended follow-on mission (the Interstellar Mapping and Acceleration Probe) will be used to finally determine the true source of the Ribbon.

A NEW DETECTION OF LYα ABSORPTION FROM THE HELIOTAIL

We present new Hubble Space Telescope observations of H I Lyα absorption toward the F8 V star HD 35296. This line of sight is only a few degrees from the downwind direction of the local interstellar medium flow vector. As a consequence, Lyα absorption from the heliotail is detected in the spectrum, consistent with three previous downwind detections of heliotail absorption. The clustering of the heliotail absorption detections around the downwind direction demonstrates that the heliotail is pointed close to that direction, limiting the extent to which the interstellar magnetic field might be distorting and deflecting the heliotail. We explore this issue further using three-dimensional MHD models of the global heliosphere. The three computed models represent the first three-dimensional MHD models with both a kinetic treatment of neutrals and an extended grid in the tail direction, both of which are necessary to model Lyα absorption downwind. The models indicate only modest heliotail asymmetries and deflections, which are not large enough to be inconsistent with the clustering of heliotail absorption detections around the downwind direction. The models are reasonably successful at reproducing the observed absorption, but they do overpredict the Lyα opacity by a factor of 2-3. We discuss implications ofmore » these results in light of observations of the heliotail region from the Interstellar Boundary Explorer mission.« less

Evidence of direct detection of interstellar deuterium in the local interstellar medium by IBEX

We report the first in situ measurements of neutral deuterium originating in the local interstellar medium (LISM) in Earth’s orbit. These measurements were performed with the IBEX-Lo camera on NASA’s interstellar boundary explorer (IBEX) satellite. All data from the spring observation periods of 2009 through 2011 have been analysed. In the three years of the IBEX mission time, the observation geometry and orbit allowed for a total observation time of 115.3 days for the LISM. However, the effects of the spinning spacecraft and the stepping through 8 energy channels mean that we are only observing the interstellar wind for a total time of 1.44 days, in which 2 counts for interstellar deuterium were collected. We report here a conservative number, because a possibility of systematic error or additional noise, though eliminated in our analysis to the best of our knowledge, only supports detection at a 1-sigma level. From these observations, we derive a ratio D/H = � 5.8 ± 4.4 � × 10 −4 at 1 AU. After modelling the transport and loss of D and H from the termination shock to Earth’s orbit, we find that our result of D/HLISM = � 1.6 ± 1.2 � × 10 −5 agrees with D/HLIC = � 1.6 ± 0.4 � × 10 −5 for the local interstellar cloud. This weak interstellar signal is extracted from a strong terrestrial background signal consisting of sputter products from the sensor’s conversion surface. As reference, we accurately measure the terrestrial D/H ratio in these sputtered products and then discriminate this terrestrial background source. Because of the diminishing D and H signal at Earth’s orbit during the rising solar activity due to photoionisation losses and increased photon pressure, our result demonstrates that in situ measurements of interstellar deuterium in the inner heliosphere are only possible during solar minimum conditions.

Dynamics of Orbits Near 3:1 Resonance in the Earth-Moon System

The Interstellar Boundary Explorer (IBEX) spacecraft is currently in a highly elliptical orbit around Earth with a period near 3:1 resonance with the Moon. Its orbit is oriented so that apogee does not approach the Moon. Simulations show this orbit to be remarkably stable over the next 20 years. This article examines the dynamics of such orbits in the Circular Restricted 3-Body Problem (CR3BP). We look at three types of periodic orbits, each exhibiting a type of symmetry of the CR3BP. For each of the orbit types, we assess the local stability using Floquet analysis. Although not all of the periodic solutions are stable in the mathematical sense, any divergence is so slow as to produce practical stability over several decades. We use Poincare maps with twenty-year propagations to assess the nonlinear stability of the orbits, where the perturbation magnitudes are related to the orbit uncertainty for the IBEX mission. Finally we show that these orbits belong to a family of orbits connected in a bifurcation diagram that exhibits exchange of stability. The analysis of these families of period orbits provides a valuable starting point for a mission orbit trade study.

Comparison of Sigma-Point and Extended Kalman Filters on a Realistic Orbit Determination Scenario

Sigma-point filters have received a lot of attention in recent years as a better alternative to extended Kalman filters for highly nonlinear problems. In this paper, we compare the performance of the additive divided difference Sigma-point filter to the extended Kalman filter when applied to orbit determination of a realistic operational scenario based on the Interstellar Boundary Explorer mission. For the scenario studied, both filters provided equivalent results. The performance of each is discussed in detail.

Does the space environment affect the ecosphere?

The Sun is now emerging from a deep and protracted solar minimum, when the power, pressure, flux, and magnetic flux of solar wind were at their lowest levels [McComas et al., 2008; Schwadron and McComas, 2008;Connick et al., 2011]. Because of an anomalously weak heliospheric magnetic field and low solar wind pressure, galactic cosmic rays (GCRs)—protons, electrons, and ionized nuclei of elements accelerated to high energies—achieved the highest fluxes observed in the space age (Figure 1) [Mewaldt et al., 2010]. Related observations have shown remarkably rapid changes in the fluxes of energetic neutral atoms (ENAs) used by NASA's Interstellar Boundary Explorer mission to image the global heliosphere surrounding the solar system [McComas et al., 2010]. These changes in ENAs are caused by decreasing solar wind pressure. Does the recent anomalous deep solar minimum hint at larger changes in store? And how do changing GCR fluxes and conditions on the Sun influence Earth's ecosphere? Given the fact that GCR radiation can damage living tissue, causing cellular mutagenesis, the changing state of the Sun may have serious implications for life on the planet.

The Interstellar Boundary Explorer Science Operations Center

The Interstellar Boundary Explorer (IBEX) Science Operations Center is responsible for supporting analysis of IBEX data, generating special payload command procedures, delivering the IBEX data products, and building the global heliospheric maps of energetic neutral atoms (ENAs) in collaboration with the IBEX team. We describe here the data products and flow, the sensor responses to ENA fluxes, the heliospheric transmission of ENAs (from 100 AU to 1 AU), and the process of building global maps of the heliosphere. The vast majority of IBEX Science Operations Center (ISOC) tools are complete, and the ISOC is in a remarkable state of readiness due to extensive reviews, tests, rehearsals, long hours, and support from the payload teams. The software has been designed specifically to support considerable flexibility in the process of building global flux maps. Therefore, as we discover the fundamental properties of the interstellar interaction, the ISOC will iteratively improve its pipeline software, and, subsequently, the heliospheric flux maps that will provide a keystone for our global understanding of the solar wind’s interaction with the interstellar medium. The ISOC looks forward to the next chapter of the IBEX mission, as the tools we have developed will be used in partnership with the IBEX team and the scientific community over the coming years to define our global understanding of the solar wind’s interaction with the local interstellar medium.

IBEX—Interstellar Boundary Explorer

The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October 2008 with the sole, focused science objective to discover the global interaction between the solar wind and the interstellar medium. IBEX is designed to achieve this objective by answering four fundamental science questions: (1) What is the global strength and structure of the termination shock, (2) How are energetic protons accelerated at the termination shock, (3) What are the global properties of the solar wind flow beyond the termination shock and in the heliotail, and (4) How does the interstellar flow interact with the heliosphere beyond the heliopause? The answers to these questions rely on energy-resolved images of energetic neutral atoms (ENAs), which originate beyond the termination shock, in the inner heliosheath. To make these exploratory ENA observations IBEX carries two ultra-high sensitivity ENA cameras on a simple spinning spacecraft. IBEX’s very high apogee Earth orbit was achieved using a new and significantly enhanced method for launching small satellites; this orbit allows viewing of the outer heliosphere from beyond the Earth’s relatively bright magnetospheric ENA emissions. The combination of full-sky imaging and energy spectral measurements of ENAs over the range from ∼10 eV to 6 keV provides the critical information to allow us to achieve our science objective and understand this global interaction for the first time. The IBEX mission was developed to provide the first global views of the Sun’s interstellar boundaries, unveiling the physics of the heliosphere’s interstellar interaction, providing a deeper understanding of the heliosphere and thereby astrospheres throughout the galaxy, and creating the opportunity to make even greater unanticipated discoveries.

Implications of solar wind suprathermal tails for IBEX ENA images of the heliosheath

Decades of interplanetary measurements of the solar wind and other space plasmas have established that the suprathermal ion intensity distributions (j) are non‐Maxwellian and are characterized by high‐energy power law tails (j ∼ E−κ). Recent analysis by Fisk and Gloeckler of suprathermal ion observations between 1–5 AU demonstrates that a particular differential intensity distribution function emerges universally between ∼2–10 times the solar wind speed with κ ∼ 1.5. This power law tail is particularly apparent in downstream distributions beyond reverse shocks associated with corotating interaction regions. Similar power law tails have been observed in the downstream flow beyond the termination shock by the Low Energy Charged Particle instrument on both Voyager 1 and Voyager 2. Using kappa distributions with internal energy, density, and bulk flow derived from large‐scale magnetohydrodynamic models, we calculate the simulated flux of energetic neutral atoms (ENAs) produced in the heliosheath by charge exchange between solar wind protons and interstellar hydrogen. We then produce simulated ENA maps of the heliosheath, such as will be measured by the Interstellar Boundary Explorer Mission (IBEX). We also estimate the expected signal to noise and background ratio for IBEX. The solar wind suprathermal tail significantly increases the ENA flux within the IBEX energy range, ∼0.01–6 keV, by more than an order of magnitude at the highest energies over the estimates using a Maxwellian. It is therefore essential to consider suprathermal tails in the interpretation of IBEX ENA images and theoretical modeling of the heliospheric termination shock.

Negative helium generation upon surface scattering: Application in space science

We report the formation of negatively charged helium ions upon scattering of neutral helium atoms from silicon wafers coated with diamondlike carbon (DLC). The energy of the primary He atoms ranges from 10–1500eV. The negative ion fraction in the scattered He beam ranges from 7×10−5 down to 10−5 with increasing energy. The decreasing negative ion fraction with increasing energy we find for the DLC surfaces is in contrast with earlier results obtained for low work function metallic surfaces. The observed energy dependence cannot be explained by theory because at present there is no theory available that explains the electron transfer between diamond surfaces and scattered atoms. Although the negative ion fraction is low, this ionization technique allows the direct detection of interstellar neutral He with species identification for the first time. This measurement will be performed with the IBEX-Lo sensor on the Interstellar Boundary Explorer mission of NASA.