We present a novel Bayesian model and a corresponding robust, probabilistic calibration procedure for the CORSAIR polarimeter that can be applied to other polarimeters. Our calibration procedure combines existing Mueller matrix representations of polarimeters with Bayesian methods, and computes the posterior distribution of the parameters by collecting data from the polarimeter at different states. We show that the algorithm is able to converge and recover a well-constrained posterior of the free parameters with a credible interval that is consistent with the ground truth values. Posterior predictive checks indicate that our generative model with inferred parameters can reproduce the calibration data within the predictive uncertainty, and captures the dominant systematic effects of the calibration procedure. We further show that we can propagate calibration uncertainties in the distributions to downstream reconstructions of Stokes measurements and magnetic-field estimates. We find that the contribution of calibration uncertainty towards the reconstructed results is minimal relative to that of the photon noise uncertainty, indicating that estimates using our Bayesian calibration algorithm can achieve photon noise-limited measurements in the magnetic-field parameters. Finally, we test the Bayesian calibration algorithm on a lab prototype of the CORSAIR polarimeter, and show that it converges and closely recovers theoretical estimates of the free parameters from real-world measurements.

Abstract. Strategies for mitigating methane emissions rely on understanding the underlying drivers of methane losses to the atmosphere. Observations of methane plumes emerging from point sources, combined with correct statistical interpretation, can provide key information. In this work, we examine a critical parameter, the probability of detection of a plume. For a given observing system, probability of detection is affected by the properties of the sensor, plume detection algorithm, observing conditions, and emission rate of the source. We parameterize relevant aspects of remotely sensed scenes containing plumes using a nondimensional observability parameter that predicts probability of detection. Our probability of detection model is trained using simulated plumes to capture natural variability in different meteorological conditions, and validated with data from controlled release experiments. We model probability of detection for two airborne imaging spectrometer systems, MethaneAIR and Insight M LeakSurveyorTM, and one high resolution satellite system, MethaneSAT. Monte Carlo simulations of emissions distributions implied by data from the extensive 2023 MAIRX campaign of MethaneAIR demonstrate the importance of an accurate probability of detection model, due to the heavy tailed emission distribution found in most oil and gas basins.

On 2024 April 17, the third successful Hi-C sounding rocket flight, Hi-C Flare, recorded coronal images in Fe XXI 129 Å emission from 11 MK plasma during the postmaximum phase of an M1.6-class solar flare, achieving unprecedented spatial (∼300 km) and temporal (1.3 s) resolutions. The flare started at 21:55 UT, peaked at 22:08 UT, and lasted ∼40 minutes. Hi-C observed for over 5 minutes (22:15:45–22:21:25), starting roughly 8 minutes after flare maximum. A sudden compact bright burst—875 ± 25 km wide, lasting 90 ± 1.3 s, exhibiting a proper motion of ∼ 50 km s ^−1 , and splitting into two toward the end—occurs near the foot of some postflare loops. Its size and brightness are reminiscent of flare-ribbon kernels during a flare’s rapid rise phase, kernels marking sites of sudden heating and hot plasma upflow, making its occurrence during the late phase surprising. Such isolated brightenings in a flare’s postmaximum phase are rare and have not been previously reported. The kernel was detected in all Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly channels. Its 1600 Å light curve peaked ∼50 s earlier than its 131 Å light curve, similar to that of flare-ribbon kernels, albeit with a smaller delay of ∼25 s, during the impulsive phase of the flare. In SDO/Helioseismic Magnetic Imager magnetograms, the kernel sits in unipolar positive magnetic flux near an embedded clump of negative flux. Although localized magnetic reconnection within the kernel (a microflare) cannot be ruled out for its cause, the observations favor the localized brightening being an isolated, exceptionally late flare-ribbon kernel, resulting from an exceptionally late burst of the flare’s coronal reconnection.

We report on a further analysis of the Airborne Infrared Spectrometer (AIR-Spec) observations from the 2019 July 2 total eclipse and present the identifications of three new coronal lines from Fe x , Si xi , and Si vi in AIR-Spec’s 1.87–1.99 μ m band. The new lines were predicted to be observable but were not observed previously in the corona. The identifications are confirmed on the basis of observed and expected line wavelengths and radiances. The observed wavelengths are very accurate, as confirmed by neutral H and He lines measured during the same observation. The predicted wavelengths are assessed by reviewing previous experimental data used to estimate the energies of these ions. We find discrepancies with several previous measurements. A reanalysis of Solar Ultraviolet Measurement of Emitted Radiation data corrects previous measurements and provides excellent agreement with the AIR-Spec values for Si xi . AIR-Spec was developed to survey the near-infrared (NIR) coronal spectrum from above Earth’s atmosphere, a critical first step in designing future balloonborne and spaceborne instrumentation to observe coronal plasma and magnetic fields using NIR emission lines. The reported results are also of interest for ground-based observatories, as the precise measurements of line wavelength enable observers to understand the nearby telluric spectrum and determine which lines can be used from the ground.

The Arcus Probe mission addresses a wide range of Astro2020 Decadal and NASA Science Mission Directorate Priority science areas, and is designed to explore astrophysical feedback across all mass scales. Arcus' three baseline science goals include: (i) Characterizing the drivers of accretion-powered feedback in supermassive black holes, (ii) Quantifying how feedback at all scales drives galaxy evolution and large-scale structure, including the tenuous cosmic web, and (iii) Analyzing stellar feedback from exoplanetary to galactic scales, including its effects on exoplanet environments targeted by current and future NASA missions. These science goals, along with a robust General Observer program, will be achieved using a mission that provides a high-sensitivity soft (10-60 angstrom) X-ray spectrometer (XRS), working simultaneously with a co-aligned UV spectrometer (UVS; 970-1580 angstrom). Arcus enables compelling baseline science and provides the broader astronomy community a revolutionary tool to characterize the full ionization range of warm and hot plasmas - including hydrogen, helium, and all abundant metals - in the Universe, from the halos of galaxies and clusters to the coronae of stars.

Reducing methane (CH4) emissions from the oil and gas (O&G) sector is crucial for mitigating climate change in the near term. MethaneSAT is an upcoming satellite mission designed to monitor basin-wide O&G emissions globally, providing estimates of emission rates and helping identify the underlying processes leading to methane release in the atmosphere. MethaneSAT data will support advocacy and policy efforts by helping to track methane reduction commitments and targets set by countries and industries. Here, we introduce a CH4 retrieval algorithm for MethaneSAT based on the CO2 proxy method. We apply the algorithm to observations from the maiden campaign of MethaneAIR, an airborne precursor to the satellite that has similar instrument specifications. The campaign was conducted during winter 2019 and summer 2021 over three major US oil and gas basins. Analysis of MethaneAIR data shows that measurement precision is typically better than 2 % at a 20 x 20 m(2) pixel resolution, exhibiting no strong dependence on geophysical variables, e.g., surface reflectance. We show that detector focus drifts over the course of each flight, likely due to thermal gradients that develop across the optical bench. The impacts of this drift on retrieved CH4 can mostly be mitigated by including a parameter that squeezes the laboratory-derived, tabulated instrument spectral response function (ISRF) in the spectral fit. Validation against coincident EM27/SUN retrievals shows that MethaneAIR values are generally within 1 % of the retrievals. MethaneAIR retrievals were also intercompared with retrievals from the TROPOspheric Monitoring Instrument (TROPOMI). We estimate that the mean bias between the instruments is 2.5 ppb, and the latitudinal gradients for the two data sets are in good agreement. We evaluate the accuracy of MethaneAIR estimates of point-source emissions using observations recorded over the Permian Basin, an O&G basin, based on the integrated-mass-enhancement approach coupled with a plume-masking algorithm that uses total variational denoising. We estimate that the median point-source detection threshold is 100-150 kg h-1 at the aircraft's nominal above-surface observation altitude of 12 km. This estimate is based on an ensemble of Weather Research and Forecasting (WRF) large-eddy simulations used to mimic the campaign's conditions, with the threshold for quantification set at approximately twice the detection threshold. Retrievals from repeated basin surveys indicate the presence of both persistent and intermittent sources, and we highlight an example from each case. For the persistent source, we infer emissions from a large O&G processing facility and estimate a leak rate between 1.6 % and 2.1 %, higher than any previously reported emission levels from a facility of its size. We also identify a ruptured pipeline that could increase total basin emissions by 2 % if left unrepaired; this pipeline was discovered 2 weeks before it was found by its operator, highlighting the importance of regular monitoring by future satellite missions. The results showcase MethaneAIR's capability to make highly accurate, precise measurements of methane dry-air mole fractions in the atmosphere, with a fine spatial resolution (similar to 20 x 20 m(2)) mapped over large swaths (similar to 100 x 100 km(2)) in a single flight. The results provide confidence that MethaneSAT can make such measurements at unprecedentedly fine scales from space (similar to 130 x 400 m(2) pixel size over a target area measuring similar to 200 x 200 km(2)), thereby delivering quantitative data on basin-wide methane emissions.

This work presents the development of the MethaneAIR Level0-Level1B processor, which converts raw L0 data to calibrated and georeferenced L1B data. MethaneAIR is the airborne simulator for MethaneSAT, a new satellite under development by MethaneSAT LLC, a subsidiary of the Environmental Defense Fund (EDF). MethaneSAT's goals are to precisely map over 80 % of the production sources of methane from oil and gas fields across the globe to an accuracy of 2-4 ppb on a 2 km 2 scale. Efficient algorithms have been developed to perform dark corrections, estimate the noise, radiometrically calibrate data, and correct stray light. A forward model integrated into the L0-L1B processor is demonstrated to retrieve wavelength shifts during flight accurately. It is also shown to characterize the instrument spectral response function (ISRF) changes occurring at each sampled spatial footprint. We demonstrate fast and accurate orthorectification of MethaneAIR data in a three-step process: (i) initial orthorectification of all observations using aircraft avionics, a simple camera model, and a medium-resolution digital elevation map; (ii) registration of oxygen (O 2 ) channel grayscale images to reference Multispectral Instrument (MSI) band 11 imagery via Accelerated-KAZE (A-KAZE) feature extraction and linear transformation, with similar co-registration of methane (CH 4 ) channel grayscale images to the registered O 2 channel images; and finally (iii) optimization of the aircraft position and attitude to the registered imagery and calculation of viewing geometry. This co-registration technique accurately orthorectifies each channel to the referenced MSI imagery. However, in the pixel domain, radiance data for each channel are offset by almost 150-200 across-track pixels (rows) and need to be aligned for the full-physics or proxy retrievals where both channels are simultaneously used. We leveraged our orthorectification tool to identify tie points with similar geographic locations in both CH 4 and O 2 images in order to produce shift parameters in the across-track and along-track dimensions. These algorithms described in this article will be implemented into the MethaneSAT L0-L1B processor.

The MethaneSAT satellite instrument and its aircraft precursor, MethaneAIR, are imaging spectrometers designed to measure methane concentrations with wide spatial coverage, fine spatial resolution, and high precision compared to currently deployed remote sensing instruments. At 12 960 m cruise altitude above ground (13 850 m above sea level), MethaneAIR datasets have a 4.5 km swath gridded to 10 m × 10 m pixels with 17–20 ppb standard deviation on a flat scene. MethaneAIR was deployed in the summer of 2021 in the Permian Basin to test the accuracy of the retrieved methane concentrations and emission rates using the algorithms developed for MethaneSAT. We report here point source emissions obtained during a single-blind volume-controlled release experiment, using two methods. (1) The modified integrated mass enhancement (mIME) method estimates emission rates using the total mass enhancement of methane in an observed plume combined with winds obtained from Weather Research Forecast driven by High-Resolution Rapid Refresh meteorological data in Large Eddy Simulations mode (WRF-LES-HRRR). WRF-LES-HRRR simulates winds in stochastic eddy-scale (100–1000 m) variability, which is particularly important for low-wind conditions and informing the error budget. The mIME can estimate emission rates of plumes of any size that are detectable by MethaneAIR. (2) The divergence integral (DI) method applies Gauss's theorem to estimate the flux divergence fields through a series of closed surfaces enclosing the sources. The set of boxes grows from the upwind side of the plume through the core of each plume and downwind. No selection of inflow concentration, as used in the mIME, is required. The DI approach can efficiently determine fluxes from large sources and clusters of sources but cannot resolve small point emissions. These methods account for the effects of eddy-scale variation in different ways: the DI averages across many eddies, whereas the mIME re-samples many eddies from the LES simulation. The DI directly uses HRRR winds, while mIME uses WRF-LES-HRRR wind products. Emissions estimates from both the mIME and DI methods agreed closely with the single-blind volume-controlled experiments (N = 21). The York regression between the estimated emissions and the released emissions has a slope of 0.96 [0.84, 1.08], R = 0.83 and N = 21, with 30 % mean percentage error for the whole dataset, which indicates that MethaneAIR can quantify point sources emitting more than 200 kg h−1 for the mIME and 500 kg h−1 for the DI method. The two methods also agreed on methane emission estimates from various uncontrolled sources in the Permian Basin. The experiment thus demonstrates the powerful potential of the MethaneAIR instrument and suggests that the quantification method should be transferable to MethaneSAT if it meets the design specifications.

The Sun's proximity offers us a unique opportunity to study in detail the physical processes on a star's surface; however, the highly dynamic nature of the stellar surface -- in particular, energetic eruptions such as flares and coronal mass ejections -- presents tremendous observational challenges. Spectroscopy probes the physical state of the solar atmosphere, but conventional scanning spectrographs and spectrometers are unable to capture the full evolutionary history of these dynamic events with a sufficiently wide field of view and high spatial, spectral, and temporal resolution. Resolving the physics of the dynamic sun requires gathering simultaneous spectra across a contiguous area over the full duration of these events, a goal now tantalizingly close to achievable with continued investment in developing powerful new Integral Field Spectrographs to serve as the foundation of both future ground- and space-based missions. This technology promises to revolutionize our ability to study solar flares and CMEs, addressing NASA's strategic objective to "understand the Sun, solar system, and universe." Since such events generate electromagnetic radiation and high-energy particles that disrupt terrestrial electric infrastructure, this investment not only advances humanity's scientific endeavors but also enhances our space weather forecasting capability to protect against threats to our technology-reliant civilization.

The Airborne Infrared Spectrometer (AIR-Spec) offers an unprecedented opportunity to explore the Near Infra-Red (NIR) wavelength range. It has been flown at two total solar eclipses, in 2017 and 2019. The wavelength range of the much improved instrument on the second flight (July 2, 2019) was shifted to cover two density sensitive lines from S XI. In this paper we study detailed diagnostics for temperature, electron density and elemental abundances by comparing results from AIR-Spec slit positions above the east and the west limb with those from Hinode/EIS, the PolarCam detector and SDO/AIA. We find very good agreement in the electron densities obtained from the EIS EUV line ratios, those from the NIR S XI ratio and those obtained from the polarized brightness PolarCam measurements. Electron densities ranged from Log Ne [cm$^{-3}]$ = 8.4 near the limb, falling to 7.2 at $R_0=1.3$. EIS spectra indicate that the temperature distribution above the west limb is near-isothermal at around 1.3 MK, while that on the east has an additional higher-T component. The AIR-Spec radiances in Si X and S XI as well as the AIA data in the 171, 193, and 211 Angstroms bands are consistent with the EIS results. EIS and AIR-Spec data indicate that the sulphur abundance (relative to silicon) is photospheric in both regions, confirming our previous results of the 2017 eclipse. The AIA data also indicate that the absolute iron abundance is photospheric. Our analysis confirms the importance of the diagnostic potential of the NIR wavelength range, and that this important wavelength range can be used reliably and independently to determine coronal plasma parameters.

Calculations S1.1 mIME Effective Wind SpeedsThe effective wind speeds used in the mIME calculation came from the relationship proposed by Varon et al. (2018), U ef f = α • log(10 m wind) + 0.6 where α is between 0.9-1.1.We took advantage of the LES runs by introducing the LES-specific effective wind U adaptive, ef f for each time step.U adaptive, ef f is defined as Q•L IM E .Because we know all the terms in this equation from the LES, we can calculate the true U adaptive, ef f that gives the best estimate of Q.By calculating the U adaptive, ef f for all the time steps of interest, we end up with multiple pairs of U adaptive, ef f and U , which can be fitted into a linear regression function of the form U ef f = a • log(10 m wind) + b with unique a and b coefficients.However, the LES-specific relationship can introduce overfitting results when the data points are limited and wind speeds are low.Since the wind speeds during the controlled release experiments were lower than 5 m/s for most of the time, we decided to use the coefficients in (Varon et al., 2018) to avoid overfitting.

Models for space weather forecasting will never be complete/valid without accounting for interhemispheric asymmetries in Earth's magnetosphere, ionosphere and thermosphere.This whitepaper is a strategic vision for understanding these asymmetries from a global perspective of geospace research and space weather monitoring.It includes both observation and modeling studies related to the topic.The importance of investigating inter-hemispheric asymmetries space physics research needs to be shown in the decadal survey.It is crucial to understand how each source of asymmetry interacts with each other and how asymmetric structures incorporate this information into real-time monitoring and predictive models of space weather impacts globally.Future challenges of inter-hemispheric asymmetries for Heliophysics investigations are discussed and so are potential solutions.Recently, interhemispheric asymmetry has gained significant interest with the community, both major funding agencies officially supporting relevant programs: the NSF Geospace Environment Modeling (GEM) is currently hosting a focus group "Interhemispheric Approaches to Understanding M-I Coupling (IHMIC)" for the years 2018-2023.A similar topic is also a focused science topic in NASA's Living With a Star (LWS) program, "Causes and Consequences of Hemispherical Asymmetries in the Magnetosphere-Ionosphere-Thermosphere System".The NASA's Drive Center program once hosted a center called "Center for Unified Studies for Interhemispheric Asymmetries (CUSIA)".The European agency, International Space Science Institute (ISSI), funded two international teams carrying out research focusing on the asymmetries (2014 and current).The community consensus, however, is that there are still extensive data gaps in the southern hemisphere and the current models do not sufficiently address the asymmetric features of the geospace environments.

On 2017 August 21, the Airborne Infrared Spectrometer (AIR-Spec) observed the total solar eclipse at an altitude of 14 km from aboard the NSF/NCAR Gulfstream V research aircraft. The instrument successfully observed the five coronal emission lines that it was designed to measure: Si x 1.431 μm, S xi 1.921 μm, Fe ix 2.853 μm, Mg viii 3.028 μm, and Si ix 3.935 μm. Characterizing these magnetically sensitive emission lines is an important first step in designing future instruments to monitor the coronal magnetic field, which drives space weather events, as well as coronal heating, structure, and dynamics. The AIR-Spec instrument includes an image stabilization system, feed telescope, grating spectrometer, and slit-jaw imager. This paper details the instrument design, optical alignment method, image processing, and data calibration approach. The eclipse observations are described and the available data are summarized.

Arcus, a Medium Explorer (MIDEX) mission, was selected by NASA for a Phase A study in August 2017. The observatory provides high-resolution soft X-ray spectroscopy in the 12-50Å bandpass with unprecedented sensitivity: effective areas of >450 cm2 and spectral resolution >2500. The Arcus key science goals are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, groups, and clusters, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback and (3) to explore how stars, circumstellar disks and exoplanet atmospheres form and evolve. Arcus relies upon the same 12m focal length grazing-incidence silicon pore X-ray optics (SPO) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings, and the results are imaged with flight-proven CCD detectors and electronics. The power and telemetry requirements on the spacecraft are modest. Mission operations are straightforward, as most observations will be long (~100 ksec), uninterrupted, and pre-planned, although there will be capabilities to observe sources such as tidal disruption events or supernovae with a ~3 day turnaround. Following the 2nd year of operation, Arcus will transition to a proposal-driven guest observatory facility.

The Airborne Infrared Spectrometer (AIR-Spec) was commissioned during the 2017 total solar eclipse, when it observed five infrared coronal emission lines from a Gulfstream V research jet owned by the National Science Foundation and operated by the National Center for Atmospheric Research. The second AIR-Spec research flight took place during the 2019 July 2 total solar eclipse across the south Pacific. The 2019 eclipse flight resulted in seven minutes of observations, during which the instrument measured all four of its target emission lines: S xi 1.393 mu m, Si x 1.431 mu m, S xi 1.921 mu m, and Fe ix 2.853 mu m. The 1.393 mu m S xi line was detected for the first time, and probable first detections were made of Si xi 1.934 mu m and Fe x 1.947 mu m. The 2017 AIR-Spec detection of Fe ix was confirmed and the first observations were made of the Fe ix line intensity as a function of solar radius. Telluric absorption features were used to calibrate the wavelength mapping, instrumental broadening, and throughput of the instrument. AIR-Spec underwent significant upgrades in preparation for the 2019 eclipse observation. The thermal background was reduced by a factor of 30, providing a 5.5x improvement in signal-to-noise ratio, and the postprocessed pointing stability was improved by a factor of 5 to <10 '' rms. In addition, two imaging artifacts were identified and resolved, improving the spectral resolution and making the 2019 data easier to interpret.

The MethaneSAT satellite mission aims at quantifying anthropogenic methane emissions by measuring reflected solar radiation in two spectral windows in the short wave infrared range. In the 1,249 nm - 1,305 nm spectral range the sensor will measure the oxygen (O2) singlet Delta band with a full width at half maximum (FWHM) of 0.17 nm. Additionally, the instrument will observe most of the 2ν3 absorption band of methane (CH4) and the P-branch of a 3ν1+ν3 carbon dioxide (CO2) band in the spectral range 1,605 nm - 1,683 nm at a spectral resolution of 0.23 nm at FWHM. Clouds and aerosols can introduce biases into the inversion of methane column concentrations from solar backscatter measurements and we therefore develop retrieval processors to filter out contaminated scenes. One processor makes use of surface pressure retrievals to infer the presence of scattering particles in the field of view of the sensor. These retrievals specifically take into account O2 airglow emission following the approach of Sun et al. (2018). Surface pressure is retrieved by fitting spectra in the O2 band, assuming a non-scattering atmosphere. We study thresholds in the variations between retrieved surface pressure and a priori meteorological databases which are suitable to screen for clouds and aerosols. A complementary algorithm takes advantage of differences in the atmospheric light path between the two spectral windows of MethaneSAT in the presence of aerosols and clouds. Here, we retrieve water vapor (H2O) column concentrations from the 1.3 µm and 1.6 µm windows under the assumption of the geometric lightpath. The ratio of the H2O retrievals from the two windows is used to construct a filter for aerosol and cloud contaminated scenes. We simulate MethaneSAT measurements with various cloud and aerosol loads to derive the retrieval configurations with highest sensitivity to scattering events. Both filtering approaches are applied to measurements of the MethaneAIR instrument (Staebell et al., 2021) to demonstrate their capacity in screening for clear scenes. Finally, we discuss our on-going efforts in developing a filter for observations affected by cloud shadows. References Staebell, C., Sun, K., Samra, J., Franklin, J., Chan Miller, C., Liu, X., Conway, E., Chance, K., Milligan, S., and Wofsy, S.: Spectral calibration of the MethaneAIR instrument, Atmospheric Measurement Techniques, 14, 3737–3753, https://doi.org/10.5194/amt-14-3737-2021, 2021. Sun, K., Gordon, I. E., Sioris, C. E., Liu, X., Chance, K., and Wofsy, S. C.: Reevaluating the use of O2 a1Δg band in spaceborne remote sensing of greenhouse gases, Geophysical Research Letters, 45, 5779–5787, https://doi.org/10.1029/2018GL077823, 2018.

MethaneAIR is the airborne simulator of MethaneSAT, an area-mapping satellite currently under development with the goal of locating and quantifying large anthropogenic CH4 point sources as well as diffuse emissions at the spatial scale of an oil and gas basin. Built to closely replicate the forthcoming satellite, MethaneAIR consists of two imaging spectrometers. One detects CH4 and CO2 absorption around 1.65 and 1.61 µm, respectively, while the other constrains the optical path in the atmosphere by detecting O2 absorption near 1.27 µm. The high spectral resolution and stringent retrieval accuracy requirements of greenhouse gas remote sensing in this spectral range necessitate a reliable spectral calibration. To this end, on-ground laboratory measurements were used to derive the spectral calibration of MethaneAIR, serving as a pathfinder for the future calibration of MethaneSAT. Stray light was characterized and corrected for through fast-Fourier-transform-based Van Cittert deconvolution. Wavelength registration was examined and found to be best described by a linear relationship for both bands with a precision of ∼ 0.02 spectral pixel. The instrument spectral spread function (ISSF), measured with fine wavelength steps of 0.005 nm near a series of central wavelengths across each band, was oversampled to construct the instrument spectral response function (ISRF) at each central wavelength and spatial pixel. The ISRFs were smoothed with a Savitzky–Golay filter for use in a lookup table in the retrieval algorithm. The MethaneAIR spectral calibration was evaluated through application to radiance spectra from an instrument flight over the Colorado Front Range.

Arcus is an innovative MIDEX-class photon-counting X-ray spectroscopy mission. Due to the nature of the sources that Arcus will focus on, observations can be many tens of kiloseconds (ks) long. The resulting spectral images are reconstructed on the ground to remove measured pointing and instrument deflection effects that take place over that time, achieving a higher resolution than would be possible without removing these effects. Arcus’s 12 m focal length grazing incidence optics are separated from the detectors by a 10.8 m long by Ø1.85 m, onorbit deployable boom. This paper describes an implementation of an internal aspect sensor that uses flight tested commercial off the shelf (COTS) components to measure linear deflection from one end of that boom to the other to achieve a better than 22 micron resolution (3σ) correction for that motion, meeting the required performance that Arcus needs to maintain its achieve its imaging resolution.