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The [O.sub.2] atmospheric band transition is observed in the altitude region between ~40 and 200 km in the dayglow and between ~80 and 100 km in the nightglow. Wallace and Hunten (J. Geophys. Res. 73, 4813 (1968)) presented the first detailed analysis of the sources and sinks of this [O.sub.2] airglow emitting state. Because of its extended altitude coverage, bright signal, and spectral and photometric properties, this emission provides an important means to remotely sense the thermal, dynamical, and compositional structures of the upper atmosphere. In this paper we present a photochemical and emission-absorption model that calculates the spectral brightnesses of the four brightest vibrational manifolds of this band system that, for the first time, extends from the mesosphere to the thermosphere. This model incorporates the latest rate constants, cross sections, and spectral parameters relevant to this emission, some of which were not considered in previous remote sensing retrieval applications. The model results are compared with our previous rocket experiments to assess the utility of this emission for upper atmospheric remote sensing and to identify key future measurement challenges. This model, together with improved instrument capabilities, permits us to study the atmosphere from 40 up to 200 km, a region where the strongest coupling between the lower atmosphere and upper atmosphere occurs. PACS Nos.: 07.07.Df, 92.60.hw, 92.60.Vb, 92.60.H Nous observons la bande de transition du [O.sub.2] atmospherique entre les altitudes de ~40 a 200 km dans la lueur diurne et entre ~80 et 100 km dans la lueur nocturne. Wallace et Hunten (J. Geophys. Res. 73, 4813 (1968)) ont presente les premieres analyses sur les sources et les puits d'emission de cette lueur en altitude. A cause du grand domaine en altitude, de la brillance du signal et des proprietes spectrales et photometriques, remission fournit un moyen important pour sonder a distance les structures thermiques, la dynamique et la composition de l'atmosphere superieure. Nous presentons ici un modele de photochimie et d'emission-absorption qui calcule les brillances spectrales des quatre plus brillantes varietes vibrationnelles de ce systeme de bandes qui, pour la premiere fois, s'etend de la mesosphere jusqu'a la thermosphere. Ce modele incorpore les valeurs les plus recentes des constantes de taux, des sections efficaces et des parametres spectraux pertinents au probleme, dont certains n'ont pas ete integres anterieurement dans le processus de sondage a distance. Les resultats du modele sont compares a nos resultats anterieurs obtenus par fusee sonde, afin de valider l'utilisation de cette emission pour le sondage de la haute atmosphere et pour identifier les defis a venir. Ce modele, avec des capacites experimentales ameliorees, nous permet d'etudier l'atmosphere de 40 jusqu'a 200 km d'altitude, une region oh on retrouve le couplage le plus fort entre l'atmosphere basse et l'atmosphere superieure. [Traduit par la Redaction], 1. Introduction The [O.sub.2] atmospheric band emission ([b.sup.1][Σ.sup.+.sub.g]→[X.sup.3][Σ.sup.-.sub.g]) is a bright feature observed in the visible and near infrared region of the airglow spectrum. Since its discovery in the nightglow [...]

As a sharp feature in the sky, the ribbon of enhanced energetic neutral atom (ENA) flux observed by the Interstellar Boundary Explorer (IBEX) mission is a key signature for understanding the interaction of the heliosphere and the interstellar medium through which we are moving. Over five nominal IBEX energy passbands (0.7, 1.1, 1.7, 2.7, and 4.3 keV), the ribbon is extraordinarily circular, with a peak location centered at ecliptic (λRC, βRC) = (219.°2 ± 1.°3, 39.°9 ± 2.°3) and a half cone angle of ϕC = 74.°5 ± 2.°0. A slight elongation of the ribbon, generally perpendicular to the ribbon center-heliospheric nose vector and with eccentricity ∼0.3, is observed over all energies. At 4.3 keV, the ribbon is slightly larger and displaced relative to lower energies. For all ENA energies, a slice of the ribbon flux peak perpendicular to the circular arc is asymmetric and systematically skewed toward the ribbon center. We derive a spatial coherence parameter δC ⩽ 0.014 that characterizes the spatial uniformity of the ribbon over its extent in the sky and is a key constraint for understanding the underlying processes and structure governing the ribbon ENA emission. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) is a NASA Explorer Mission-of-Opportunity to stereoscopically image the Earth’s magnetosphere for the first time. TWINS extends our understanding of magnetospheric structure and processes by providing simultaneous Energetic Neutral Atom (ENA) imaging from two widely separated locations. TWINS observes ENAs from 1–100 keV with high angular (∼4°×4°) and time (∼1-minute) resolution. The TWINS Ly- α monitor measures the geocoronal hydrogen density to aid in ENA analysis while environmental sensors provide contemporaneous measurements of the local charged particle environments. By imaging ENAs with identical instruments from two widely spaced, high-altitude, high-inclination spacecraft, TWINS enables three-dimensional visualization of the large-scale structures and dynamics within the magnetosphere for the first time. This “instrument paper” documents the TWINS design, construction, calibration, and initial results. Finally, the appendix of this paper describes and documents the Southwest Research Institute (SwRI) instrument calibration facility; this facility was used for all TWINS instrument-level calibrations. [ABSTRACT FROM AUTHOR]

Abstract: In this work we compare ionospheric measurements made at several ionosonde stations with parameters inferred from airglow measurements made by the Global Ultraviolet Imager (GUVI) instrument aboard the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) spacecraft. The purpose of this comparison is twofold. First, our study provides a validation of the technique of using TIMED/GUVI airglow measurements to infer nighttime electron densities in F region of the ionosphere. Second, we use the global characterization of the ionosphere provided by TIMED/GUVI to identify potential biases in individual ionosonde station records. We have found for peak electron densities over the range that the GUVI measurements are consistent with the ionosonde observations. We have also found some station to station and regional variations that warrant further study. [Copyright &y& Elsevier]

The circular ribbon of enhanced energetic neutral atom (ENA) emission observed by the Interstellar Boundary Explorer (IBEX) mission remains a critical signature for understanding the interaction between the heliosphere and the interstellar medium. We study the symmetry of the ribbon flux and find strong, spectrally dependent reflection symmetry throughout the energy range 0.7-4.3 keV. The distribution of ENA flux around the ribbon is predominantly unimodal at 0.7 and 1.1 keV, distinctly bimodal at 2.7 and 4.3 keV, and a mixture of both at 1.7 keV. The bimodal flux distribution consists of partially opposing bilateral flux lobes, located at highest and lowest heliographic latitude extents of the ribbon. The vector between the ribbon center and heliospheric nose (which defines the so-called BV plane) appears to play an organizing role in the spectral dependence of the symmetry axis locations as well as asymmetric contributions to the ribbon flux. The symmetry planes at 2.7 and 4.3 keV, derived by projecting the symmetry axes to a great circle in the sky, are equivalent to tilting the heliographic equatorial plane to the ribbon center, suggesting a global heliospheric ordering. The presence and energy dependence of symmetric unilateral and bilateral flux distributions suggest strong spectral filtration from processes encountered by an ion along its journey from the source plasma to its eventual detection at IBEX. [ABSTRACT FROM AUTHOR]

The Interstellar Boundary Explorer (IBEX) mission continues to measure energetic neutral atom (ENA) emissions produced by charge exchange between solar wind (SW) protons and interstellar neutrals at the edge of our heliosphere. Using the first 3 yr of IBEX-Hi ENA measurements (2009-2011), we examined the spectral evolution of ∼0.5-6 keV ENAs at the polar regions (above 60°). We found the following: (1) pixels with a characteristic “ankle” spectra (lower spectral index at higher energies) increase by ∼5% in 2010 and ∼10% in 2011 compared to 2009. (2) The averaged spectral index in 2011 is smaller than that of 2009. (3) The slope of the ENA spectrum above ∼1.7 keV is more variable than the slope below ∼1.7 keV. The lower spectral index at higher energies of the spectrum does not appear to be caused by an increase of the ENA production at these energies, but rather from a consistent decrease at lower energies. (4) The decrease in polar ENA fluxes does not correlate significantly with the averaged SW dynamic pressure, back-traced in time to 1 AU along the flow streamlines (originating between 10° and 30° for slow SW, and 60° and 80° for fast SW), assuming these are the respective conditions of ENA progenitors back in time. These results provide insights into the complexity of relating the slow and fast SW contributions to polar ENAs and shed light on how the solar output and the resulting change in the global heliospheric structure possibly affect the heliosheath (HS) populations. [ABSTRACT FROM AUTHOR]

The magnetic field in the local interstellar medium (ISM) provides a key indicator of the galactic environment of the Sun and influences the shape of the heliosphere. We have studied the interstellar magnetic field (ISMF) in the solar vicinity using polarized starlight for stars within 40 pc of the Sun and 90° of the heliosphere nose. In Frisch et al. (Paper I), we developed a method for determining the local ISMF direction by finding the best match to a group of interstellar polarization position angles obtained toward nearby stars, based on the assumption that the polarization is parallel to the ISMF. In this paper, we extend the analysis by utilizing weighted fits to the position angles and by including new observations acquired for this study. We find that the local ISMF is pointed toward the galactic coordinates ℓ, b =47° ± 20°, 25° ± 20°. This direction is close to the direction of the ISMF that shapes the heliosphere, ℓ, b =33° ± 4°, 55° ± 4°, as traced by the center of the “Ribbon” of energetic neutral atoms discovered by the Interstellar Boundary Explorer (IBEX) mission. Both the magnetic field direction and the kinematics of the local ISM are consistent with a scenario where the local ISM is a fragment of the Loop I superbubble. A nearby ordered component of the local ISMF has been identified in the region ℓ ≈0° → 80° and b ≈0° → 30°, where PlanetPol data show a distance-dependent increase of polarization strength. The ordered component extends to within 8 pc of the Sun and implies a weak curvature in the nearby ISMF of ∼0.°25 pc–1. This conclusion is conditioned on the small sample of stars available for defining this rotation. Variations from the ordered component suggest a turbulent component of ∼23°. The ordered component and standard relations between polarization, color excess, and Ho column density predict a reasonable increase of N(H) with distance in the local ISM. The similarity of the ISMF directions traced by the polarizations, the IBEX Ribbon, and pulsars inside the Local Bubble in the third galactic quadrant suggest that the ISMF is relatively uniform over spatial scales of 8-200 pc and is more similar to interarm than spiral-arm magnetic fields. The ISMF direction from the polarization data is also consistent with small-scale spatial asymmetries detected in GeV-TeV cosmic rays with a galactic origin. The peculiar geometrical relation found earlier between the cosmic microwave background dipole moment, the heliosphere nose, and the ISMF direction is supported by this study. The interstellar radiation field at ∼975 Å does not appear to play a role in grain alignment for the low-density ISM studied here. [ABSTRACT FROM AUTHOR]

The Europa Imaging System (EIS) consists of a Narrow-Angle Camera (NAC) and a Wide-Angle Camera (WAC) that are designed to work together to address high-priority science objectives regarding Europa's geology, composition, and the nature of its ice shell. EIS accommodates variable geometry and illumination during rapid, low-altitude flybys with both framing and pushbroom imaging capability using rapid-readout, 8-megapixel (4k × 2k) detectors. Color observations are acquired using pushbroom imaging with up to six broadband filters. The data processing units (DPUs) perform digital time delay integration (TDI) to enhance signal-to-noise ratios and use readout strategies to measure and correct spacecraft jitter. The NAC has a 2.3° × 1.2° field of view (FOV) with a 10-μrad instantaneous FOV (IFOV), thus achieving 0.5-m pixel scale over a swath that is 2 km wide and several km long from a range of 50 km. The NAC is mounted on a 2-axis gimbal, ±30° cross- and along-track, that enables independent targeting and near-global (≥90%) mapping of Europa at ≤100-m pixel scale (to date, only ∼15% of Europa has been imaged at ≤900 m/pixel), as well as stereo imaging from as close as 50-km altitude to generate digital terrain models (DTMs) with ≤4-m ground sample distance (GSD) and ≤0.5-m vertical precision. The NAC will also perform observations at long range to search for potential erupting plumes, achieving 10-km pixel scale at a distance of one million kilometers. The WAC has a 48° × 24° FOV with a 218-μrad IFOV, achieving 11-m pixel scale at the center of a 44-km-wide swath from a range of 50 km, and generating DTMs with 32-m GSD and ≤4-m vertical precision. The WAC is designed to acquire three-line pushbroom stereo and color swaths along flyby ground-tracks., Competing Interests: Competing InterestsThe authors have no competing interests to declare that are relevant to the content of this article., (© The Author(s) 2024.)