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We have measured the 30 and 100 eV far ultraviolet (FUV) emission cross sections of the optically allowed Fourth Positive Group (4PG) band system (A1Π → X1Σ+) of CO and the optically forbidden O (5So → 3P) 135.6 nm atomic transition by electron‐impact‐induced‐fluorescence of CO and CO2. We present a model excitation cross section from threshold to high energy for the A1Π state, including cascade by electron impact on CO. The A1Π state is perturbed by triplet states leading to an extended FUV glow from electron excitation of CO. We derive a model FUV spectrum of the 4PG band system from dissociative excitation of CO2, an important process observed on Mars and Venus. Our unique experimental setup consists of a large vacuum chamber housing an electron gun system and the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission Imaging Ultraviolet Spectrograph optical engineering unit, operating in the FUV (110–170 nm). The determination of the total O i (5So) at 135.6 nm emission cross section is accomplished by measuring the cylindrical glow pattern of the metastable emission from electron impact by imaging the glow intensity about the electron beam from nominally zero to ~400 mm distance from the electron beam. The study of the glow pattern of O i (135.6 nm) from dissociative excitation of CO and CO2 indicates that the O i (5So) state has a kinetic energy of ~1 eV by modeling the radial glow pattern with the published lifetime of 180 μs for the O i (5So) state. Plain Language Summary: Both Mars and Venus have upper atmospheres that are similar in composition: mostly CO2, CO, and N2 are dominant molecular gases, with nearly identical UV spectra. The modeling studies of atmospheric UV emissions cannot presently be accurately conducted to the same accuracy as the planetary UV measurements, which avail themselves with state‐of‐the‐art calibrated spectrographs. This dichotomy in accuracy between planetary observation and model occurs because the atomic and molecular emission cross sections with uncertainties of certain transitions are greater than 100%. Furthermore, the analysis is complicated by the spectral blending of the various emissions of the low‐resolution spectral spaceborne instruments. We present in this paper a UV laboratory instrument unique in the world at the University of Colorado that can measure for the first time the excitation mechanisms with accurate emission cross sections of both allowed and optically forbidden transitions that are occurring in a planetary atmosphere. Key Points: We measured 30 and 100 eV emission cross sections of Fourth Positive band system of CO and O i (135.6 nm) from electron impact on CO and CO2We conducted a laboratory experiment measuring single‐scattering electron‐impact‐induced‐fluorescence FUV spectra from excitation of CO and CO2Fragment kinetic energy measurement for O i (5So) atoms found to be around ~1 eV from both CO and CO2 molecular dissociation [ABSTRACT FROM AUTHOR]

Abstract: We report the response of the Martian upper atmosphere to a strong X‐class flare on 10 September 2017 as observed by the Imaging Ultraviolet Spectrograph (IUVS) instrument aboard the Mars Atmosphere Volatile EvolutioN (MAVEN) mission. The solar flare peaked at 16:24 hr UT, and IUVS dayglow observations were taken about an hour after the flare peak. Retrieved temperatures from IUVS dayglow observations show a significant increase during the flare orbit, with a mean value of ∼270 K and a maximum value of ∼310 K. The retrieved temperatures during the flare orbit also show a strong latitudinal gradient, indicating that the flare‐induced heating is limited between low and middle latitudes. During this event IUVS observed an ∼70% increase in the observed brightness of CO 2 + ultraviolet doublet and CO Cameron band emission at 90 km, where high‐energy photons (< 10 nm) deposit most of their energy. [ABSTRACT FROM AUTHOR]

Abstract: Mars has lost a large fraction of its water to space, with the H component of this loss thought to occur mainly as a result of thermal (Jeans) escape from the upper atmosphere. Constraints on H loss have historically been made using hydrogen Lyman alpha (121.6 nm) light scattered in the planet's extended upper atmosphere or corona. Here we employ observations from the Mars Atmosphere and Volatile Evolution (MAVEN) mission's Imaging Ultraviolet Spectrograph (IUVS) to constrain H escape in December 2014 and August 2016, when MAVEN observed the dayside corona at low latitude. To obtain adequate fits and address systematic sources of uncertainty including instrument calibration, we fit in exobase number density and escape rate instead of density and temperature, employing Markov Chain Monte Carlo techniques. This produces better model fits to data than most previous analyses. When we assume a single population of H atoms, we obtain H temperatures inconsistent with expected trends and a shape mismatch between observed and modeled profiles, similar to previous studies. Introducing either a second population of H (at a distinct temperature and density) or adding deuterium to the corona allows for essentially perfect fits. Despite this model ambiguity, derived loss rates for both periods are within a factor of four, 3.3–8.8×108cm−2/s in December 2014 (Ls∼250) and 0.6–2.3×108cm−2/s in August 2016 (Ls∼200). These rates are similar to those found in prior studies and confirm the known seasonal trend—doing so while incorporating the substantial uncertainty in absolute calibration insufficiently explored by previous studies. [ABSTRACT FROM AUTHOR]