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Armstrong, J. C.

Publications and source records attributed to Armstrong, J. C..

11 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2014

Effects of Extreme Obliquity Variations on the Habitability of Exoplanets

We explore the impact of obliquity variations on planetary habitability in hypothetical systems with high mutual inclination. We show that large-amplitude, high-frequency obliquity oscillations on Earth-like exoplanets can suppress the ice-albedo feedback, increasing the outer edge of the habitable zone. We restricted our exploration to hypothetical systems consisting of a solar-mass star, an Earth-mass planet at 1 AU, and 1 or 2 larger planets. We verified that these systems are stable for 108 years with N-body simulations and calculated the obliquity variations induced by the orbital evolution of the Earth-mass planet and a torque from the host star. We ran a simplified energy balance model on the terrestrial planet to assess surface temperature and ice coverage on the planet's surface, and we calculated differences in the outer edge of the habitable zone for planets with rapid obliquity variations. For each hypothetical system, we calculated the outer edge of habitability for two conditions: (1) the full evolution of the planetary spin and orbit and (2) the eccentricity and obliquity fixed at their average values. We recovered previous results that higher values of fixed obliquity and eccentricity expand the habitable zone, but we also found that obliquity oscillations further expand habitable orbits in all cases. Terrestrial planets near the outer edge of the habitable zone may be more likely to support life in systems that induce rapid obliquity oscillations as opposed to fixed-spin planets. Such planets may be the easiest to directly characterize with space-borne telescopes.

habitability of exoplanets↗
NASA NTRS2004

The Compact Microimaging Spectrometer (CMIS): A New Tool for In-Situ Planetary Science

In-situ identification of trace minerals, ices, or organics in planetary samples may be difficult with panchromatic microscopic imagery and spot spectroscopy. The panchromatic imagery acquired by a microscopic imager provides morphological information and albedo, but these are generally insufficient for unambiguous identification. The spatially-averaged spectra acquired by a nonimaging ( point- or spot- ) spectrometer may enable identification of the major components but identification of unknown trace components is difficult at best. With our Compact Micro-Imaging Spectrometer (CMIS), however, we acquire spectroscopic data in an imaging format at microscopic scales. The distinct spectra of individual grains, provided by our approach, make detection and identification possible even for trace components in regolith or heterogeneous samples.

Armstrong, J. C.↗
NASA NTRS1973

The photoelectron-spectrometer experiment on Atmosphere Explorer.

The Atmosphere Explorer (AE) photoelectron spectrometer experiment consists of electrostatic deflection-type electron spectrometers designed to measure the low-energy electron flux expected to be encountered along the AE orbit. The system has been designed to provide general information on the intensity, angular distribution, energy spectrum, and net flows along field lines as well as sufficient energy resolution to allow the fine details of the electron-energy spectrum to be studied. The instrumentation consists of two separate electron-spectrometer detector heads connected to a common electronics system for data processing and telemetry interfacing. The experiment will measure the electron-energy spectrum from 2 to 500 eV (and possibly to less than 0.1 eV).

Doering, J. P.↗
NASA NTRS1971

Ordinary-mode electromagnetic instability in colliding plasma streams.

The instability of the electromagnetic linearly polarized mode propagating perpendicularly to the magnetic field is studied for a system composed of two colliding plasma streams, in each of which the electrons and ions are streaming at the same velocity. Using linearized Vlasov-Maxwell equations and allowing for anisotropic temperatures, it is found that in the presence of streaming ions the instability can occur in very low-beta plasmas. The plasma is increasingly susceptible to the electromagnetic instability with increasing values of beta, streaming velocity, temperature ratio of parallel to perpendicular electrons, and temperature ratio of perpendicular electrons to perpendicular ions.

Lee, K. F.↗