P. K. Swaminathan

Nitric oxide abundance in the mesosphere/lower thermosphere region : Roles of solar soft X rays, suprathermal N(4S) atoms, and vertical transport

This paper carefully examines the inability of photochemical models to account for the large nitric oxide densities of ∼108 cm−3 at ∼105 km obtained from IR, UV, and microwave measurements. A detailed and up-to-date photochemical model is constructed that incorporates measured YOHKOH soft X ray fluxes, hot N atom chemistry with an energy dependent thermalization cross section and seven reaction sources, and laboratory-constrained N(2D) yields. The resulting model which has well-constrained chemistry compared to past models fails to generate high enough NO densities in comparison with the most reliable measurements of absolute NO concentrations in the lower thermosphere. The sensitivity of the model results and the known uncertainties in the inputs are used to identify where future efforts should be focused. A deficit remains despite an increase in the vertical mixing rates in the lower thermosphere from the very low Kzz profile used in our calculations and/or an increase in the N(2D) yield from electron impact dissociation of N2 from its nominal value of 0.54 to 0.62. The sensitivity of NO profiles to the nascent energy distributions of the atmospheric sources of suprathermal N atoms is illustrated by including the thermalization of suprathermal N atoms with an updated thermalization cross section. The diurnally averaged NO concentration at 105 km is enhanced by factors of 1.2 and 2.6 when the energy distributions of the N atoms from electron impact dissociation of N2 are chosen with peaks near 0.6 eV or 3–4 eV, but deficits of factors of ∼7 and ∼3, respectively, remain. There is higher sensitivity to vertical transport than to variations of chemistry within known uncertainties.

observation of auroral emissions induced by artificial plasma jets

In this paper we present ultraviolet to near infrared spectrographic observations of high-speed artificial plasma jet interactions with the ionosphere. The plasma jets were injected quasi-parallel to the magnetic field at an altitude of 140 km during the Fluxus-1 and -2 experiments. The jets contained aluminum ions and were generated using a shaped-charge device known as an Explosive Type Generator (ETG). Satellite-based spectrographic observations of the plasma jet show typical auroral emission features associated with electron impact excitation. The auroral features include emission at 135.6 nm (OI) and 557.7 nm (OI). The 135.6 nm emission was prompt while the 557.7 nm was observed for 5 seconds. The most likely source of these auroral emissions are ionospheric and magnetospheric electrons that neutralize the plasma jet.

The APEX north star experiment: observations of high-speed plasma jets injected perpendicular to the magnetic field

Abstract Initial results from the Active Plasma EXperiment North Star experiment are presented. The North Star active experiment included two separate plasma jet injections, both perpendicular to the Earths magnetic field. The objective of the experiment is to investigate the interaction of high-speed (7–40 km/s) plasma jets with the ionospheric plasma and the coupling to the magnetosphere and lower ionosphere. The plasma jets were produced using an Explosive Type Generator device. This device is a shaped-charge device that vaporizes porous aluminum inside the generator and forces the vaporized products out of a nozzle, resulting in the production of a high-speed, partially ionized aluminum plasma jet. Instrumentation on three different payloads was used to obtain multi-point observations of the plasma jet properties, optical radiation from the jets, and ionospheric perturbations. Imagery, high-speed photometry, and spectrographic imagery using ground- and space-based sensors were used to monitor the dynamics and spectral content of the plasma jet. This paper describes the experiment and summarizes the initial results from the North Star experiment.

ATMOS/ATLAS 1 measurements of thermospheric and mesospheric nitric oxide

The atmospheric trace molecule spectroscopy (ATMOS) instrument obtained solar occultation spectra of the terrestrial atmosphere during the Atmospheric Laboratory for Applications and Science (ATLAS 1) mission of March 26–April 4, 1992. During this time, Ap varied between 11 and 18 while the F10.7 index was near 192. The analyses of the 5.3-μm spectral data to derive nitric oxide densities in the lower thermosphere and mesosphere are described here. The results show that a peak NO density of 1.0×108 cm−3 occurs at 105±2.5 km for the latitude range 38°N–58°S. The density values are higher than previously reported using UV measurements. These measurements worsen the discrepancy with photochemical models at low latitudes where models already underpredict nitric oxide.

A review of the Solar Probe Plus dust protection approach

The Solar Probe Plus (SPP) spacecraft will go closer to the Sun than any manmade object has gone before, which has required the development of new thermal and micrometeoroid protection technologies. During the 24 solar orbits of the mission, the spacecraft will encounter a thermal environment that is 50 times more severe than any previous spacecraft. It will also travel through a dust environment previously unexplored, and be subject to particle hypervelocity impacts (HVI) at velocities much larger than anything previously encountered. New analytical methodologies and designs have been developed to meet this environments extreme micrometeoroid protection challenge while also fulfilling the missions low mass requirement. These new analytical capabilities and protection system concepts could produce similar benefits if applied to Earth orbiting and deep space missions. The SPP dust study was developed to overcome the velocity limitations in the existing micrometeoroid and orbital debris (MMOD) analysis capability. In this study, we developed the hydrocode modeling techniques needed to characterize the material behaviors for a high-shock particle impact event. An additional novel development was an algorithm to calculate the particle flux on specific spacecraft surfaces. Our approach predicts particle impacts for a given spacecraft geometry, including the aforementioned effects. In addition, our approach introduces a size-velocity particle correlation, which lowers the shielding needed for a given protection level. This paper covers the new analytical capabilities developed for the SPP dust environment and how they dramatically lower the mass of the protective systems. The paper also discusses the application of these new analytical capabilities to spacecraft protection in the LEO debris field.

North Star Plasma-Jet Space Experiment

The objective of the Active Plasma Experiment North Star mission was to study the interaction of artificially produced aluminum ion plasma jets with the space environment. Two separate plasma jets were injected almost perpendicular to the local magnetic field during the North Star experiment. The jets were created using an explosivetype generator designed to produce a high-speed (7‐42-km/s) aluminum ion plasma jet with plasma densities exceeding 10 9 cm −3 at a distance 170 m from the plasma-jet source. The first plasma-jet injection occurred at an altitude of 360 km and was preceded by the release of an artificial air cloud. The second injection occurred at an altitude of 280 km and did not include the air cloud. Interactions of the plasma jet with the local space environment and artificial air cloud were monitored using instrumentation on three diagnostic payloads, ground-based optical sensors, and space-based optical sensors. An overview is provided of the experiment, along with a summary of the principal results from the mission.

Effects of refraction on photochemical calculations

A general method for the calculation of refracted ray paths through the atmosphere is developed and adapted for photochemical modeling applications. The adapted method is then used to study the effect of refraction on the twilight radiation field and stratospheric photochemistry. We show that refraction has the effect of increasing the amount of solar flux available for photodissociation in the atmosphere. This is caused by both the reduction of the optical path of the direct solar beam and a significant lengthening of the sunlit day. It is also shown that for polar conditions, concentrations of O(3P), NO3, and NO are significantly affected by refraction.

Use of hydrocode modeling to develop advanced MMOD shielding designs

A multi-physics computations-based methodology for space debris hypervelocity impact (HVI) damage mitigation is presented. Specifically, improved debris mitigation through development of innovative, lightweight structural designs is described. The methodology has been applied to the design of the Solar Probe Plus (SPP) spacecraft to mitigate extreme solar microdust hypervelocity impacts (50-300 km/s) by the Johns Hopkins University Applied Physics Laboratory (JHU/APL). The methodology combines hydrocode computations of the complex, early-time transient material and structural responses with experimental hypervelocity impact data to directly obtain end-state damage predictions for the requisite hypervelocities that are in excess of available test capabilities (~10 km/s). The computations are validated in the low-velocity regime (<;10 km/s) by direct HVI testing and verified in the high-velocity regime (50-300 km/s) by comparisons with bounding energy calculations and extrapolations of Ballistic Limit Equations (BLEs). In addition to hydrocode computations, HVI experimental data and supporting structural/solid mechanics analyses are used to define the eventual damage. In addition to being able to treat realistic hypervelocities and spacecraft materials in layered and Whipple configurations systematically, the methodology provides the margin of safety for any design. Sample lightweight design calculations involving state-of-art and innovative protective materials are presented to demonstrate the methodology and its benefits.

Modeling of Gas Cloud Expansion at High Altitude with Radiation Transport

Time-accurate simulations of the expansion of a spherically symmetric aluminum gas cloud produced by an explosive-type generator (ETG) are performed using a newly developed radiative-gasdynamic algorithm. The method utilizes a kinetic e ux-splitting scheme, extended to account for dense gases, in conjunction with coupled radiation transport. Results obtained for typical ETG detonation conditions are characterized by extremely high expansion velocities, as well as the transition of radiation transport from an optically thick to an optically thin regime. The e ow structures and radiative power output histories observed in these simulations are presented and discussed.

Simulations of gas cloud expansion using a multi-temperature gas dynamics model

Simulations are performed using a multiple temperature gas model to investigate translational non-equilibrium effects in a rapid expansion of a high temperature argon gas cloud into a rarefied atmosphere. A set of continuum conservation equations based on kinetic theory, which includes anisotropic forms for the temperature, pressure and speed ratio, are solved numerically using a flux splitting scheme for the inviscid fluxes and a central difference scheme for the viscous fluxes in a time accurate manner. Results obtained for the initial expansion of the spherical gas cloud from a high density source condition show that translational non-equilibrium exists in the shock front region which propagates into the ambient atmosphere. For a lower density source condition, translational non-equilibrium not only exists in the shock front but also in the inner gas cloud region where the temperature normal to the radial direction freezes at a value just below the initial source temperature.