Production, Outflow, Velocity, and Radial Distribution of H2O and OH in the Coma of Comet C/1995 O1 (Hale-Bopp) from Wide-field Imaging of OH

Observations of OH are a useful proxy of the water production rate (Q H2O) and outflow velocity (VH2O) in comets. From wide-field images taken on 1997 March 28 and April 8 that capture the entire scale length of the OH coma of comet C/1995 O1 (Hale-Bopp), we obtain Q OH from the model-independent method of aperture summation and Q H2O from the OH photochemical branching ratio, BROH. Using an adaptive ring summation algorithm, we extract the radial brightness distribution of OH 0-0 band emission out to cometocentric distances of up to 10 to the sixth power km, both as azimuthal averages and in quadrants covering different position angles relative to the comet-Sun line. These profiles are fitted using both fixed and variable velocity two-component spherical expansion models to estimate VOH with increasing distance from the nucleus. The OH coma of Hale-Bopp was more spatially extended than those of previous comets, and this extension is best matched by a variable acceleration of H 2O and OH that acted across the entire coma, but was strongest within 1-2 × 104 km from the nucleus. Our models indicate that VOH at the edge of our detectable field of view (10 to the sixth power km) was ∼2-3 times greater in Hale-Bopp than for 1P/Halley class comet at 1 AU, which is consistent with the results of more sophisticated gas-kinetic models, extrapolation from previous observations of OH in comets with QH2O \u3e 10 to the twenty-ninth power s superscript -1, and direct radio measurements of the outer coma Hale-Bopp OH velocity. The likely source of this acceleration is thermalization of the excess energy of dissociation of H2O and OH over an extended collisional coma. When the coma is broken down by quadrants in position angle, we find an azimuthal asymmetry in the radial distribution that is characterized by an increase in the spatial extent of OH in the region between the orbit-trailing and anti-Sunward directions. Model fits specific to this area and comparison with radio OH measurements suggest greater acceleration here, with VOH ∼ 1.5 times greater at a 10 to the sixth power km cometocentric distance than elsewhere in the coma. We discuss several mechanisms that may have acted within the coma to produce the observed effect

Proton Aurora on Mars: A Dayside Phenomenon Pervasive in Southern Summer

We present observations of proton aurora at Mars made using the Imaging UltraViolet Spectrograph (IUVS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Martian proton aurora display a prominent intensity enhancement in the hydrogen Lyman‐alpha (121.6 nm) emission between ~110 and 150 km altitude. Using altitude‐intensity profiles from periapsis limb scan data spanning nearly two Martian years, we create a comprehensive database of proton aurora and characterize their phenomenology. Due to Mars\u27 lack of a global dipole magnetic field, Martian proton aurora are expected to form on the dayside via electron stripping and charge exchange between solar wind protons and the neutral corona. We observe proton aurora in ~14% of dayside periapsis profiles (with notable seasonal variability), making proton aurora the most commonly observed type of aurora at Mars. We determine that the primary factors influencing proton aurora occurrence rates are solar zenith angle and season. The highest proton aurora occurrence rates are at low solar zenith angles on the Mars dayside, consistent with known formation processes. Proton aurora have highest emission enhancements, peak intensities, peak altitudes, and occurrence rates (nearing 100%) around southern summer solstice. This time period corresponds with the seasonal inflation of the neutral lower atmosphere, the onset of Martian dust storm season, seasonally increased coronal hydrogen column densities, and higher atmospheric temperature and solar wind flux following perihelion. The results of our study provide a new understanding of the primary factors influencing proton aurora, and the long‐term variability of these phenomena as observed over multiple Mars years

Advancing Our Understanding of Martian Proton Aurora through a Coordinated Multi-Model Comparison Campaign

Proton aurora are the most commonly observed yet least studied type of aurora at Mars. In order to better understand the physics and driving processes of Martian proton aurora, we undertake a multi-model comparison campaign. We compare results from four different proton/hydrogen precipitation models with unique abilities to represent Martian proton aurora: Jolitz model (3-D Monte Carlo), Kallio model (3-D Monte Carlo), Bisikalo/Shematovich et al. model (1-D kinetic Monte Carlo), and Gronoff et al. model (1-D kinetic). This campaign is divided into two steps: an inter-model comparison and a data-model comparison. The inter-model comparison entails modeling five different representative cases using similar constraints in order to better understand the capabilities and limitations of each of the models. Through this step we find that the two primary variables affecting proton aurora are the incident solar wind particle flux and velocity. In the data-model comparison, we assess the robustness of each model based on its ability to reproduce a MAVEN/IUVS proton aurora observation. All models are able to effectively simulate the data. Variations in modeled intensity and peak altitude can be attributed to differences in model capabilities/solving techniques and input assumptions (e.g., cross sections, 3-D versus 1-D solvers, and implementation of the relevant physics and processes). The good match between the observations and multiple models gives a measure of confidence that the appropriate physical processes and their associated parameters have been correctly identified and provides insight into the key physics that should be incorporated in future models

Interhemispheric ionosphere-plasmasphere system shows a high sensitivity to the exospheric neutral hydrogen density: a caution of the global reference atmospheric model hydrogen density

This study explores the impact of the exosphere hydrogen (H) density on the ionosphere-plasmasphere system using a model whose key inputs are constrained by ionosphere observations at both ends of the magnetic field line with an L-value of 1.75 in the American longitudinal sector during a period with low solar and magnetic activities. This study is the first to be validated by ground-based and satellite data in the plasmasphere and both hemispheres. The main finding is that the entire ionosphere-plasmasphere system is very sensitive to the neutral hydrogen density in the lower exosphere. It was found that an increase in the H density by a factor of 2.75 from the commonly accepted values was necessary to bring the simulated plasma density into satisfactory agreement with Arase satellite measurements in the plasmasphere and also with DMSP satellite measurements in the topside ionospheres of the northern and southern hemispheres. A factor of 2.75 increase in the H density increases the simulated plasma density in the afternoon plasmasphere up to ∼80% and in the nighttime topside ionosphere up to ∼100%. These results indicate prominently that using the commonly accepted empirical model of the H density causes unacceptable errors in the simulated plasma density of the near-Earth plasma shells. We alert the space science community of this problem

A Relative Measurement of the Longitudinal Neutron Flux Produced in High-Energy Hadronic Showering in Iron

Neutron flux and energy spectrum measurements are of central importance in design considerations for detectors to be built at the SSC. An experiment (E-821) employing four liquid scintillation counters (BC-501A) and seven Bonner spheres was run at FNAL. The longitudinal neutron flux and energy spectrum, produced in high-energy hadronic showering occurring in iron, was measured. A pulse shape discrimination technique was used to discriminate neutrons from gamma rays. The hadronic showering was initiated by a beam of negative pions (25-150 GeV) incident on 1 to 15 interaction lengths of iron. The relative number of neutrons per incident pion, as obtained with the liquid scintillation counters, is presented in this thesis as a function of incident beam energy and interaction length. The technique of pulse shape discrimination is covered in detail

Fabry-Perot Observations of the Hydrogen Geocorona

Within the framework of this dissertation, the Fabry-Perot annular summing optical system was completely redesigned and constructed in order to minimize vignetting and to facilitate improved intensity, wavelength, and linewidth calibration for the study of geocoronal Balmer α. A signal to noise ratio of approximately 50 was obtained for a typical geocoronal Balmer α intensity in a ten minute integration, covering a 75 km/s velocity interval with 3.75 km/s velocity resolution, from a 1.5° beam on the sky.This newly designed Fabry-Perot annular summing spectrometer was operated for two years (2000–2001) at the University of Wisconsin\u27s Pine Bluff Observatory (PBO). An extensive geocoronal Balmer α data set of approximately 1500 spectra over 71 nights was obtained; this represents the highest quality geocoronal Balmer α line profile data set to date.This dissertation reviews past geocoronal observations, the atomic physics associated with geocoronal Balmer α emission, and Fabry-Perot annular summing spectroscopy. The design, calibration, and performance of the PBO Fabry-Perot are discussed at length. The full 2000–2001 PBO geocoronal Balmer α data set is presented including: Balmer α intensities, Doppler widths, and data regarding cascade contributions to the emission. A diurnal signal is clearly observed in the line intensity, but not in the line width. A significant decrease in Balmer α Doppler width with increasing shadow altitude was detected every night in which a wide range of shadow altitudes was observable. Cascade contributions to the Balmer α emission were found to be approximately 5%, consistent with recent estimates. Preliminary applications of the nonisothermal radiative transport code lyao_rt to a subset of the PBO data indicates good general agreement both with regard to trends in the emission line intensity and to line width, indicating that a new level of understanding of the geocorona is likely to emerge through forward-modeling analysis

New Constraints on the Escape of Sodium and Potassium From the Moon

Recent measurements and models of the lunar alkali exosphere enable us to better understand the effect of the Sun on the escape of the Moon\u27s weak atmosphere. Sodium and potassium gases are important tracers of microphysical processes on the Moon because they can be easily observed from Earth as well as from lunar orbiters. Comparing exospheric transport models to recent high-resolution spectroscopic measurements from Earth , we may significantly constrain the velocity distribution of alkalis liberated from the lunar surface and the surface release processes. We find that a systematic increase in Doppler width towards full Moon phases is consistent with photon stimulated desorption with a high speed tail. Additionally, residual Doppler widths correlate to the solar wind flux measured by ARTEMIS, a finding that suggests non-negligible contributions by sputtering

A Coordinated Multi-Line Investigation Aimed At Deriving Hydrogen Densities in the Upper Atmosphere

The Global Ultraviolet Imager (GUVI) on-board the NASA TIMED satellite has been producing a continuous database of limb and disk hydrogen Lyman alpha airglow intensities since early 2002. GUVI data are often coincident with Balmer alpha and Balmer beta intensity measurements routinely made from the ground-based Wisconsin H-alpha Mapper (WHAM) and Pine Bluff Observatory (PBO) Fabry-Perot Spectrometers. The intrinsic value of the GUVI Lyman alpha data, and the retrieval of thermospheric + exospheric atomic hydrogen profiles from that data, is substantially enhanced by these concurrent WHAM/PBO ground-based observations.The approach we are developing is the coupled analysis of existing TIMED/GUVI and groundbased data sets by forward radiative transport (RT) modeling of these multi-line observations. We will utilize the differing transport properties of hydrogen Lyman alpha, Lyman beta (primarily responsible for Balmer alpha), and Lyman gamma (primarily responsible for Balmer beta) in the terrestrial atmosphere, and the forward-model/data comparisons of the variation of these emissions with solar depression angle and viewing geometry, to constrain forwardmodelretrieved hydrogen density profiles. In this poster we will discuss these data sets, our parametric data-model comparison search procedures, and recent results

Production, Outflow Velocity, and Radial Distribution of H2O and OH in the Coma of Comet C/1995 O1 [Hale-Bopp] from Wide Field Imaging of OH

Observations of OH are a useful proxy of the water production rate (Q(sub H2O)) and outflow velocity (V(sub out)) in comets. We use wide field images taken on 03/28/1997 and 04/08/1997 that capture the entire scale length of the OH coma of comet C/1995O1 (Hale-Bopp) to obtain Q(sub H2O) from the model-independent method of aperture summation. We also extract the radial brightness profile of OH 3080 angstroms out to cometocentric distances of up to 10(exp 6) km using an adaptive ring summation algorithm. Radial profiles are obtained as azimuthal averages and in quadrants covering different position angles relative to the comet-Sun line. These profiles are fit using both fixed and variable velocity two-component spherical expansion models to determine VOH with increasing distance from the nucleus. The OH coma of Hale-Bopp was more spatially extended than in previous comets, and this extension is best matched by a variable acceleration of H2O and OH that acted across the entire coma, but was strongest within 1-2 x 10(exp 4) km from the nucleus. This acceleration led to VOH at 10(exp 6) km that was 2-3 times greater than that obtained from a 1P/Halleytype comet at 1 AU, a result that is consistent with gas-kinetic models, extrapolation from previous observations of OH in comets with Q(sub H2O) > 10(exp 29)/s, and radio measurements of the outer coma Hale-Bopp OH velocity profile. When the coma is broken down by quadrant, we find an azimuthal asymmetry in the radial distribution that is characterized by an increase in the spatial extent of OH in the region between the orbit-trailing and anti-sunward directions. Model fits to this area and comparison with radio OH measurements suggest greater acceleration in this region, with VOH UP to 1.5 times greater at 10(exp 6) km radial distance than elsewhere in the coma

Production, Outflow, Velocity, and Radial Distribution of H2O and OH in the Coma of Comet C/1995 O1 (Hale-Bopp) from Wide-field Imaging of OH

Observations of OH are a useful proxy of the water production rate (Q H2O) and outflow velocity (VH2O) in comets. From wide-field images taken on 1997 March 28 and April 8 that capture the entire scale length of the OH coma of comet C/1995 O1 (Hale-Bopp), we obtain Q OH from the model-independent method of aperture summation and Q H2O from the OH photochemical branching ratio, BROH. Using an adaptive ring summation algorithm, we extract the radial brightness distribution of OH 0-0 band emission out to cometocentric distances of up to 10 to the sixth power km, both as azimuthal averages and in quadrants covering different position angles relative to the comet-Sun line. These profiles are fitted using both fixed and variable velocity two-component spherical expansion models to estimate VOH with increasing distance from the nucleus. The OH coma of Hale-Bopp was more spatially extended than those of previous comets, and this extension is best matched by a variable acceleration of H 2O and OH that acted across the entire coma, but was strongest within 1-2 × 104 km from the nucleus. Our models indicate that VOH at the edge of our detectable field of view (10 to the sixth power km) was ∼2-3 times greater in Hale-Bopp than for 1P/Halley class comet at 1 AU, which is consistent with the results of more sophisticated gas-kinetic models, extrapolation from previous observations of OH in comets with QH2O \u3e 10 to the twenty-ninth power s superscript -1, and direct radio measurements of the outer coma Hale-Bopp OH velocity. The likely source of this acceleration is thermalization of the excess energy of dissociation of H2O and OH over an extended collisional coma. When the coma is broken down by quadrants in position angle, we find an azimuthal asymmetry in the radial distribution that is characterized by an increase in the spatial extent of OH in the region between the orbit-trailing and anti-Sunward directions. Model fits specific to this area and comparison with radio OH measurements suggest greater acceleration here, with VOH ∼ 1.5 times greater at a 10 to the sixth power km cometocentric distance than elsewhere in the coma. We discuss several mechanisms that may have acted within the coma to produce the observed effect