Atmospheric Shielding Research Articles

MCADAM: A Continuous Paleomagnetic Dipole Moment Model for at Least 3.7 Billion Years

AbstractUnderstanding the evolution of Earth's magnetic field can provide insights into core processes and can constrain plate tectonics and atmospheric shielding. The absolute paleointensity database PINT provides a curated repository of site mean (i.e., cooling unit), estimates of the strength of the magnetic field. We present a minor update to the PINT database to version 8.1.1 by adding 295 records from 34 studies. The PINT database is used to define a continuous model of the dipole field, using an approach combining non‐parametric and Monte Carlo (MC) resampling termed Monte Carlo Axial Dipole Average Model (MCADAM). Three dipole field strength models spanning 50 ka to 3.7–4.2 Ga (MCADAM.1a‐c) are presented, reflecting three tiers of increasingly more stringent data selection. The MCADAM models allow for the estimation of the magnetic standoff distance, constraining the shielding of Earth's atmosphere against solar wind erosion provided by the geodynamo.

Upgrading the prediction model for Mn content in submerged arc welded metal via CALPHAD technology: Case study into typical acidic and basic fluxes

Fluxes (the granular materials containing oxides and CaF2) serve vital functions with respect to atmospheric shielding, arc stabilization, and weld metal composition control during the submerged arc welding process. Mn content must be carefully controlled since it dictates the toughness of the weldment. The slag-metal equilibrium model has been applied for decades to predict the Mn content in submerged arc welded metal, although there are several limitations regarding its application. With the aim of compensating for the limitations of the slag-metal equilibrium model, this study has been undertaken to upgrade the slag-metal equilibrium model to the gas-slag-metal equilibrium model via CALPHAD technology. Then, the Mn content in the weld metal is predicted by using both slag-metal and gas-slag-metal equilibrium models for typical acidic and basic fluxes. The capabilities of the models to predict Mn content is evaluated from the perspective of thermodynamics. It is concluded that the gas-slag-metal equilibrium model, which is independent of the basicity index model, possesses higher prediction accuracy than the slag-metal equilibrium model. Furthermore, the transfer direction of Mn, as well as Mn content are predictable even when no MnO is contained in the initial flux. Based on the measured and thermodynamic data, the mechanisms pertinent to Mn transfer in the welding process have been interpreted, and the limitations of the gas-slag-metal equilibrium model have been summarized.

Parametric model-order reduction for radiation transport using multi-resolution proper orthogonal decomposition

For parametric high-fidelity simulations, it is often desirable to utilize a reduced-order model (ROM) to emulate, at a reduced computational cost, parametric solutions of the governing partial differential equations (PDEs) for unseen parameter values. One commonly employed option is to utilize a data-driven, projection-based ROM supplemented with subspace identification via proper orthogonal decomposition (POD). POD discovers the ROM subspace by computing the singular value decomposition (SVD) of a set of training data from the full-order model (FOM). In streaming-dominated radiation transport simulations with localized sources, solutions often greatly vary over the spatial domain by many orders of magnitude. In such cases, machine-precision arithmetic can be insufficient to obtain an accurate SVD, resulting in a poorly performing ROM. We present a method called multiresolution POD (mrPOD) that mitigates these inaccuracies. The mrPOD method works by decomposing the spatial domain into regions and performing proper orthogonal decomposition on the training dataset separately in each region. mrPOD is tested on single energy group and multigroup atmospheric shielding transport problems and is shown to outperform classic POD.

Crater production on Titan and surface chronology

Context. Impact crater counts on the Saturnian satellites are a key element for estimating their surface ages and placing constraints on their impactor population. The Cassini mission radar observations allowed crater counts to be made on the surface of Titan, revealing an unexpected scarcity of impact craters that show high levels of degradation. Aims. Following previous studies on impact cratering rates on the Saturnian satellites, we modeled the cratering process on Titan to constrain its surface chronology and to assess the role of centaur objects as its main impactors. Methods. A theoretical model previously developed was used to calculate the crater production on Titan, considering the centaur objects as the main impactors and including two different slopes for the size-frequency distribution (SFD) of the smaller members of their source population. A simple model for the atmospheric shielding effects is considered within the cratering process and our results are then compared with other synthetic crater distributions and updated observational crater counts. This comparison is then used to compute Titan’s crater retention age for each crater diameter. Results. The cumulative crater distribution produced by the SFD with a differential index of s2 = 3.5 is found to consistently predict large craters (D > 50km) on the surface of Titan, while it overestimates the number of smaller craters. As both the modeled and observed distributions flatten for craters with D ≲ 25 km due to atmospheric shielding, the difference between the theoretical results and the observational crater counts can be considered as a proxy for the scale to which erosion processes have acted on the surface of Titan throughout the Solar System age. Our results for the surface chronology of Titan indicate that craters with D > 50 km can prevail over the Solar System age, whereas smaller craters may be completely obliterated due to erosion processes acting globally.

Atmospheric Escape Processes and Planetary Atmospheric Evolution

AbstractThe habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, chemical, and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our solar system: It is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H‐dominated thermosphere for astrophysicists or the Sun‐like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth's atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so‐called Xenon paradox.

Distinct Responses to Light in Plants.

The development of almost every living organism is, to some extent, regulated by light. When discussing light regulation on biological systems, one is referring to the sun that has long been positioned in the center of the solar system. Through light regulation, all life forms have evolved around the presence of the sun. As soon our planet started to develop an atmospheric shield against most of the detrimental solar UV rays, life invaded land, and in the presence of water, it thrived. Especially for plants, light (solar radiation) is the source of energy that controls a high number of developmental aspects of growth, a process called photomorphogenesis. Once hypocotyls reach soil′s surface, its elongation deaccelerates, and the photosynthetic apparatus is established for an autotrophic growth due to the presence of light. Plants can sense light intensities, light quality, light direction, and light duration through photoreceptors that accurately detect alterations in the spectral composition (UV-B to far-red) and are located throughout the plant. The most well-known mechanism promoted by light occurring on plants is photosynthesis, which converts light energy into carbohydrates. Plants also use light to signal the beginning/end of key developmental processes such as the transition to flowering and dormancy. These two processes are particularly important for plant´s yield, since transition to flowering reduces the duration of the vegetative stage, and for plants growing under temperate or boreal climates, dormancy leads to a complete growth arrest. Understanding how light affects these processes enables plant breeders to produce crops which are able to retard the transition to flowering and avoid dormancy, increasing the yield of the plant.

First Steps Toward the Verification of Models for the Assessment of the Radiation Exposure at Aviation Altitudes During Quiet Space Weather Conditions

AbstractSpace weather is an important driver of the exposure of aircrew and passengers to cosmic rays at flight altitudes, which has been a matter of concern for several decades. The assessment of the corresponding radiation doses can be realized by measurements or model calculations that cover the whole range of the radiation field in terms of geomagnetic shielding, atmospheric shielding, and the effects of space weather. Since the radiation field at aviation altitudes is very complex in terms of particle composition and energy distribution, the accurate experimental determination of doses at aviation altitudes is still a challenging task. Accordingly, the amount of data with comparatively small uncertainties is scarce. The Community Coordinated Modeling Center invited the Federal Aviation Administration, the German Aerospace Center, and the National Aeronautics and Space Administration to make their radiation models for aviation CARI‐7A, PANDOCA, and NAIRAS available for interested users via the Community Coordinated Modeling Center web site. A concomitant comparison of model calculations with measuring data provided information on the predicting capabilities and the uncertainties of the current versions of these models under quiet space weather conditions.

Dependence of the Martian radiation environment on atmospheric depth: Modeling and measurement

AbstractThe energetic particle environment on the Martian surface is influenced by solar and heliospheric modulation and changes in the local atmospheric pressure (or column depth). The Radiation Assessment Detector (RAD) on board the Mars Science Laboratory rover Curiosity on the surface of Mars has been measuring this effect for over four Earth years (about two Martian years). The anticorrelation between the recorded surface Galactic Cosmic Ray‐induced dose rates and pressure changes has been investigated by Rafkin et al. (2014) and the long‐term solar modulation has also been empirically analyzed and modeled by Guo et al. (2015). This paper employs the newly updated HZETRN2015 code to model the Martian atmospheric shielding effect on the accumulated dose rates and the change of this effect under different solar modulation and atmospheric conditions. The modeled results are compared with the most up‐to‐date (from 14 August 2012 to 29 June 2016) observations of the RAD instrument on the surface of Mars. Both model and measurements agree reasonably well and show the atmospheric shielding effect under weak solar modulation conditions and the decline of this effect as solar modulation becomes stronger. This result is important for better risk estimations of future human explorations to Mars under different heliospheric and Martian atmospheric conditions.

The resurfacing history of Venus: Constraints from buffered crater densities

Because of atmospheric shielding and endogenic resurfacing, the population of impact craters on Venus is small (about a thousand) and consists of large craters. This population has been used in numerous studies with the goal of deciphering the geologic and geodynamic history of Venus, but the nearly spatially random nature of the crater population has complicated efforts to understand this history. Here we utilize the recent 1:15M-scale global geological map of Venus (Ivanov, M.A., Head, J.W. [2011]. Planet. Space Sci. 59, 1559–1600) to help address this problem. The global geological map provides a stratigraphic sequence of units, and known areas where each unit is exposed on the planet. For each crater on Venus we identify the specific geological units predating and postdating the crater. We perform a statistical analysis of this set of observations with a buffered crater density approach, which rigorously and consistently takes into account the large size of craters and the fact that many craters are known to predate and/or postdate more than one unit. In this analysis we consider crater emplacement as random and resurfacing history as determined (although unknown). We obtain formal confidence intervals for the mean ages of geological units and the mean age differences between the pairs of units at the unit boundaries. We find that (1) size–frequency distributions of craters superposed on each unit are consistent with each other; (2) regional plains and stratigraphically older units have similar crater retention ages; (3) stratigraphically younger units have a mean crater retention age significantly younger than the regional plains. These findings are readily and consistently explained by global resurfacing scenarios and are difficult to reconcile with equilibrium resurfacing scenarios. Our analysis also shows that the latest recorded part of intensive resurfacing period lasted on the order of 10% of the mean surface age (tens of millions of years). The termination of intensive resurfacing may or may not be synchronous over the planet. Our results also indicate that there are extended deposits associated with large craters that are almost indiscernible in the radar images, but obscure radar contrasts between predating lava flows. We do not see evidence for any significant and prolonged change of atmospheric pressure following the termination of the intensive resurfacing epoch.

Diurnal variations of energetic particle radiation at the surface of Mars as observed by the Mars Science Laboratory Radiation Assessment Detector

AbstractThe Radiation Assessment Detector onboard the Mars Science Laboratory rover Curiosity is detecting the energetic particle radiation at the surface of Mars. Data collected over the first 350 Martian days of the nominal surface mission show a pronounced diurnal cycle in both the total dose rate and the neutral particle count rate. The diurnal variations detected by the Radiation Assessment Detector were neither anticipated nor previously considered in the literature. These cyclic variations in dose rate and count rate are shown to be the result of changes in atmospheric column mass driven by the atmospheric thermal tide that is characterized through pressure measurements obtained by the Rover Environmental Monitoring Station, also onboard the rover. In addition to bulk changes in the radiation environment, changes in atmospheric shielding forced by the thermal tide are shown to disproportionately affect heavy ions compared to H and He nuclei.

Radiation exposure in the moon environment

During a stay on the moon humans are exposed to elevated radiation levels due to the lack of substantial atmospheric and magnetic shielding compared to the Earth's surface. The absence of magnetic and atmospheric shielding allows cosmic rays of all energies to impinge on the lunar surface. Beside the continuous exposure to galactic cosmic rays (GCR), which increases the risk of cancer mortality, exposure through particles emitted in sudden nonpredictable solar particle events (SPE) may occur. SPEs show an enormous variability in particle flux and energy spectra and have the potential to expose space crew to life threatening doses. On Earth, the contribution to the annual terrestrial dose of natural ionizing radiation of 2.4mSv by cosmic radiation is about 1/6, whereas the annual exposure caused by GCR on the lunar surface is roughly 380mSv (solar minimum) and 110mSv (solar maximum). The analysis of worst case scenarios has indicated that SPE may lead to an exposure of about 1Sv. The only efficient measure to reduce radiation exposure is the provision of radiation shelters.Measurements on the lunar surface performed during the Apollo missions cover only a small energy band for thermal neutrons and are not sufficient to estimate the exposure. Very recently some data were added by the Radiation Dose Monitoring (RADOM) instrument operated during the Indian Chandrayaan Mission and the Cosmic Ray Telescope (CRaTER) instrument of the NASA LRO (Lunar Reconnaisance Orbiter) mission. These measurements need to be complemented by surface measurements.Models and simulations that exist describe the approximate radiation exposure in space and on the lunar surface. The knowledge on the radiation exposure at the lunar surface is exclusively based on calculations applying radiation transport codes in combination with environmental models.Own calculations are presented using Monte-Carlo simulations to calculate the radiation environment on the moon and organ doses on the surface of the moon for an astronaut in an EVA suit and are compared with measurements. Since it is necessary to verify/validate such calculations with measurement on the lunar surface, a description is given of a radiation detector for future detailed surface measurements. This device is proposed for the ESA Lunar Lander Mission and is capable to characterize the radiation field concerning particle fluencies, dose rates and energy transfer spectra for ionizing particles and to measure the dose contribution of secondary neutrons.

Transmission of galactic cosmic rays through Mars atmosphere

For human operations on the surface of Mars, methods of estimating radiation exposures from galactic cosmic rays (GCRs) are needed. To facilitate making estimates of human radiation exposures for crew operations in the Martian atmosphere, lookup tables have been generated that provide doses for critical body organs and effective doses for exposures from galactic cosmic rays anywhere on the surface of Mars. The organ doses and effective doses are tabulated for carbon dioxide atmospheric shielding areal densities ranging from 0 to 300 g cm−2 followed by aluminum spacecraft or habitat shield areal densities ranging from 0 to 100 g cm−2. The Badhwar‐O'Neill GCR model for interplanetary magnetic field potentials, ranging from the most probable solar minimum (currently 417 MV) to solar maximum conditions (1800 MV) in the solar cycle, is used as input into the calculations. This model is the standard one used for space operations at the Space Radiation Analysis Group at NASA Johnson Space Center. Use of the tables is illustrated for an environment consisting of the current galactic cosmic radiation spectrum impinging on an aluminum habitat on the surface of Mars.

Titan's young surface: Initial impact crater survey by Cassini RADAR and model comparison

Cassini's Radar instrument has mapped over 10% of Titan's surface through the T18 flyby in August 2006 and has detected remarkably few impact craters. Three impact craters have been conclusively identified and named, and a handful of unconfirmed candidate impact structures are documented. These results indicate that the overall crater density is a factor of ∼5 higher than Earth, and has a size‐distribution slope very similar to Earth's indicating rapid obliteration of smaller structures by erosion and burial. The paucity of small craters was expected from atmospheric shielding considerations, but the rarity of medium (20–100 km diameter) craters – some 30–300 times less abundant than on nearby Saturnian satellites – implies rapid resurfacing. Erosion and burial appear to be the dominant means of crater obliteration, with viscous relaxation playing a lesser role. Models of the 20–100 km impact crater production rate suggest an overall crater retention age of 100 Myr–1 Gyr, the same as the corresponding age for the Earth.

Characterisation of bubble detectors for aircrew and space radiation exposure

The Earth's atmosphere acts as a natural radiation shield which protects terrestrial dwellers from the radiation environment encountered in space. In general, the intensity of this radiation field increases with distance from the ground owing to a decrease in the amount of atmospheric shielding. Neutrons form an important component of the radiation field to which the aircrew and spacecrew are exposed. In light of this, the neutron-sensitive bubble detector may be ideal as a portable personal dosemeter at jet altitudes and in space. This paper describes the ground-based characterisation of the bubble detector and the application of the bubble detector for the measurement of aircrew and spacecrew radiation exposure.

Visualization of particle flux in the human body on the surface of Mars.

For a given galactic cosmic ray (GCR) environment, information on the particle flux of protons, alpha particles, and heavy ions, that varies with respect to the topographical altitude on the Martian surface, are needed for planning exploration missions to Mars. The Mars Global Surveyor (MGS) mission with its Mars Orbiter Laser Altimeter (MOLA) instrument has been providing precise topographical surface map of the Mars. With this topographical data, the particle flux at the Martian surface level through the CO2 atmospheric shielding for solar minimum and solar maximum conditions are calculated. These particle flux calculations are then transported first through an anticipated shielding of a conceptual shelter with several water equivalent shield values (up to 50 g/cm2 of water in steps of 5 g/cm2) considered to represent a surface habitat, and then into the human body. Model calculations are accomplished utilizing the HZETRN, QMSFRG, and SUM-MARS codes. Particle flux calculations for 12 different locations in the human body were considered from skin depth to the internal organs including the blood-forming organs (BFO). Visualization of particle flux in the human body at different altitudes on the Martian surface behind a known shielding is anticipated to provide guidance for assessing radiation environment risk on the Martian surface for future human missions.

Terrestrial in situ cosmogenic nuclides: theory and application

The cosmogenic nuclide exposure history method is undergoing major developments in analytical, theoretical, and applied areas. The capability to routinely measure low concentrations of stable and radioactive cosmogenic nuclides has led to new methods for addressing long-standing geologic questions and has provided insights into rates and styles of surficial processes. The different physical and chemical properties of the six most widely used nuclides: 3He, 10Be, 14C, 21Ne, 26Al, and 36Cl, make it possible to apply the surface exposure dating methods on rock surfaces of virtually any lithology at any latitude and altitude, for exposures ranging from 10 2 to 10 7 years. The terrestrial in situ cosmogenic nuclide method is beginning to revolutionize the manner in which we study landscape evolution. Single or multiple nuclides can be measured in a single rock surface to obtain erosion rates on boulder and bedrock surfaces, fluvial incision rates, denudation rates of individual landforms or entire drainage basins, burial histories of rock surfaces and sediment, scarp retreat, fault slip rates, paleoseismology, and paleoaltimetry. Ages of climatic variations recorded by moraine and alluvium sediments are being directly determined. Advances in our understanding of how cosmic radiation interacts with the geomagnetic field and atmosphere will improve numerical simulations of cosmic-ray interactions over any exposure duration and complement additional empirical measurements of nuclide production rates. The total uncertainty in the exposure ages is continually improving. This article presents the theory necessary for interpreting cosmogenic nuclide data, reviews estimates of parameters, describes strategies and practical considerations in field applications, and assesses sources of error in interpreting cosmogenic nuclide measurements. TABLE OF CONTENTS 1. Introduction 1.1. Development of the TCN methods 1.2. Applications of TCN Exposure methods 1.3. Previous reviews 2. Glossary 2.1. Terminology 2.2. Notation 3. Principles 3.1. Introduction 3.1.1. Source of the primary radiation 3.1.2. Effects of the geomagnetic field on GCR 3.1.3. Trajectory models and models of secondary nuclide production rates 3.1.4. Recent numerical models of GCR particle production 3.1.5. Nuclide production from primary GCR 3.1.6. TCN production by energetic nucleons 3.1.7. TCN production by low-energy neutron 3.1.8. TCN production by muons 3.1.9. Factors limiting TCN applications 3.2. Numerical simulation of low-energy neutron behavior 3.3. Analytical equations for TCN production 3.3.1. Fast neutron (Spallation) production 3.3.2. Production by epithermal neutrons 3.3.3. Production by thermal neutrons 3.3.4. Production by muons and muon-derived neutrons 3.3.5. Total nuclide production 3.4. Energetic neutron attenuation length 3.5. Temporal variations in production rates 3.5.1. Variations in the primary GCR flux 3.5.2. Variations due to solar modulation of the magnetic field 3.5.3. Effects of the geomagnetic field 3.5.4. Variations in atmospheric shielding 3.5.5. Other sources of temporal variations in production 3.6. Estimation of production rates 3.6.1. Helium-3 3.6.2. Beryllium-10 3.6.3. Carbon-14 3.6.4. Neon-21 3.6.5. Aluminum-26 3.6.6. Chlorine-36 3.7. Scaling and correction factors for production rates 3.7.1. Spatial scaling 3.7.2. Topographic shielding 3.7.3. Surface coverage 3.7.4. Sample thickness 3.7.5. Thermal neutron leakage 3.8. Exposure dating with a single TCN 3.9. Exposure dating with multiple nuclides 3.10. Nuclide-specific considerations 3.10.1. Helium-3 3.10.2. Beryllium-10 3.10.3. Carbon-14 3.10.4. Neon-21 3.10.5. Aluminum-26 3.10.6. Chlorine-36 3.11. TCN dating of sediment 4. Sampling strategies 4.1. Field sampling considerations 4.1.1. Sample description 4.1.2. Sampling methodology 4.2. Other lithological considerations 4.3. How much sample is needed? 4.4. Strategies for concentration-depth profiles 5. Sample preparation and experimental data interpretation 5.1. TCN sample preparation 5.1.1. Preparation time 5.1.2. Physical and chemical pretreatment 5.1.3. Isotopic extraction 5.2. Experimental data interpretation 6. Uncertainty and sources of error 6.1. Sources of error 6.1.1. Sample characteristics 6.1.2. Sample preparation and elemental analyses 6.1.3. Mass spectrometry 6.1.4. Systematic errors 6.2. Reporting the uncertainty 6.2.1. Error propagation 6.2.2. Evaluating accuracy by intercomparison 6.2.3. Multiple sample measurements 6.2.4. Sensitivity analysis 7. Directions of future contributions Acknowledgements References

New views of the Moon: Improved understanding through data integration

Understanding the Moon is crucial to future exploration of the solar system. The Moon preserves a record of the first billion years of the Earth‐Moon systems history, including evidence of the Moons origin as accumulated debris from a giant impact into early Earth. Lunar rocks provide evidence of early differentiation and extraction of a crust.Lacking an atmospheric shield, the Moon's regolith retains a record of the activity of solar wind over the past 4 billion years. It also holds a complete record of impact cratering, and analysis of samples has allowed calibration of ages, and thus dating of other planetary surfaces. And because of its proximity to Earth, it's low gravity well, and stable surface, the Moon's resources will be useful both in establishing lunar habitations and as fuel for exploration beyond the Moon.

Atmospheric entry of large meteoroids: implication to Titan

The preparation for the Cassini-Huygens mission gives an opportunity to revise the problem of an atmospheric entry and breakup of cratering meteoroids. The numerical modeling of the meteoroid's flight through the atmosphere is presented in comparison with more simple models. The simulation takes into account the brittle/ductile properties of the meteoroid material: the Grady-Kipp-Melosh model of tensile failure is accompanied with a simple model of the shear failure. The main difference with previously published models consists in the treating of the post-failure deformation of the damaged material as a flow of a cohesionless media with a dry friction. Numerical results are used to make a parameterization of a simple Grigorian-like model, which finally is applied to predict the atmospheric shielding effect on Titan. For a modern atmosphere of Titan and mostly ice projectiles the observable deficiency of impact craters due to atmospheric shielding would be in the range of 6–8 km, where the number of craters would be two times smaller than for the airless Titan.

An explanation of the east–west asymmetry of Io's sodium cloud

Since the discovery of a cloud of neutral sodium atoms accompanying the jovian satellite, Io, in its orbit1, asymmetries of part of the cloud (region B)2 have often been noted3–6. Brightness variations with orbital longitude3,4 and asymmetries found by comparing images taken at orbital longitudes differing by 180° (ref. 5) have been explained as effects of solar radiation pressure7. Given that the vast majority of neutrals precede Io in its orbit, and that sputtering of Io is the principal source mechanism, Matson et al.8 postulated a hemispherical source centred 60° towards the sub-Jupiter point from the centre of the leading hemisphere to explain the general cloud shape. However, soft collisions with plasma ions may be important and call for a reassessment9. I show here that, given reasonable assumptions, the east–west brightness asymmetry can be explained by temporal changes in the number of emitting atoms in the cloud caused by atmospheric shielding of the surface.

X-ray scanning of overhead aurorae from rockets

As part of Project Aurorozone I, conducted at Poker Flat Research Range, Alaska (65.1°N, 147.5° W) in September 1976, two Nike Tomahawk rocket payloads were launched into energetic auroral events to investigate their structure and effects on the atmosphere. The instrument complement included X-ray scintillation detectors with energy discrimination in four ranges (5–10, 10–20, 20–40, >40 keV) to measure the deposition of bremsstrahlung produced X-rays within the stratosphere and mesosphere. For this purpose, each instrument was designed for wide angle viewing; however, properties of the rocket motion have permitted coarse observation of distinct spatial X-ray structure. The detectors were mounted at a 45 angle with respect to the payload axis to permit scanning of the upper hemisphere, with rocket spin rates near 5 c/s during the upleg portion of each flight. Here, atmospheric shielding reduced energetic particle contamination effects to insignificant values below 65–75 km. Iterative computer techniques were used to reconstruct X-ray source maps at 100 km, taking atmospheric absorption effects into account. Payload 18.178 was launched on 21 September (0302 LMT) into an aurora observed to have two distinct azimuthal regions of optical brightness. The same features were detected with the X-ray scanner; overlays of the X-ray source maps on all-sky photographs show spatial coincidence of the X-ray with optical features at the lower energies but not above 40 keV. Payload 18.179 (23 September, 0147 LMT) was launched into an aurora of more diffuse character. In this case, some optical structure was observed to occur but did not coincide as well with the measured X-ray structure. The presence of a two component spectrum is indicated for each event with the hard component originating in the more diffuse, optically faint regions.