Jeffrey M. Holmes

Artificial ionospheric modification: The Metal Oxide Space Cloud experiment

Clouds of vaporized samarium (Sm) were released during sounding rocket flights from the Reagan Test Site, Kwajalein Atoll in May 2013 as part of the Metal Oxide Space Cloud (MOSC) experiment. A network of ground-based sensors observed the resulting clouds from five locations in the Republic of the Marshall Islands. Of primary interest was an examination of the extent to which a tailored radio frequency (RF) propagation environment could be generated through artificial ionospheric modification. The MOSC experiment consisted of launches near dusk on two separate evenings each releasing ~6 kg of Sm vapor at altitudes near 170 km and 180 km. Localized plasma clouds were generated through a combination of photoionization and chemi-ionization (Sm + O → SmO+ + e–) processes producing signatures visible in optical sensors, incoherent scatter radar, and in high-frequency (HF) diagnostics. Here we present an overview of the experiment payloads, document the flight characteristics, and describe the experimental measurements conducted throughout the 2 week launch window. Multi-instrument analysis including incoherent scatter observations, HF soundings, RF beacon measurements, and optical data provided the opportunity for a comprehensive characterization of the physical, spectral, and plasma density composition of the artificial plasma clouds as a function of space and time. A series of companion papers submitted along with this experimental overview provide more detail on the individual elements for interested readers.

Empirical modeling of plasma clouds produced by the Metal Oxide Space Clouds experiment

The Advanced Research Project Agency (ARPA) Long-Range Tracking And Instrumentation Radar (ALTAIR) radar at Kwajalein Atoll was used in incoherent scatter mode to measure plasma densities within two artificial clouds created by the Air Force Research Laboratory (AFRL) Metal Oxide Space Clouds (MOSC) experiment in May 2013. Optical imager, ionosonde, and ALTAIR measurements were combined to create 3-D empirical descriptions of the plasma clouds as a function of time, which match the radar measurements to within 15%. The plasma clouds closely track the location of the optical clouds, and the best fit plasma cloud widths are generally consistent with isotropic neutral diffusion. Cloud plasma densities decreased as a power of time, with exponents between −0.5 and −1.0, or much more slowly than the −1.5 predicted by diffusion. These exponents and estimates of total ion number from integration through the model volume are consistent with a scenario of slow ionization and a gradually increasing total number of ions with time, reaching a net ionization fraction of 20% after approximately half an hour. These robust representations of the plasma density are being used to study impacts of the artificial clouds on the dynamics of the background ionosphere and on RF propagation.

A physics‐based model for the ionization of samarium by the MOSC chemical releases in the upper atmosphere

Atomic samarium has been injected into the neutral atmosphere for production of electron clouds that modify the ionosphere. These electron clouds may be used as high-frequency radio wave reflectors or for control of the electrodynamics of the F region. A self-consistent model for the photochemical reactions of Samarium vapor cloud released into the upper atmosphere has been developed and compared with the Metal Oxide Space Cloud (MOSC) experimental observations. The release initially produces a dense plasma cloud that that is rapidly reduced by dissociative recombination and diffusive expansion. The spectral emissions from the release cover the ultraviolet to the near infrared band with contributions from solar fluorescence of the atomic, molecular, and ionized components of the artificial density cloud. Barium releases in sunlight are more efficient than Samarium releases in sunlight for production of dense ionization clouds. Samarium may be of interest for nighttime releases but the artificial electron cloud is limited by recombination with the samarium oxide ion.

Multisite Optical Imaging of Artificial Ionospheric Plasmas

Artificial ionospheric plasmas are formed on the bottom side of the natural ionospheric F region during high-power high-frequency (HF) heating experiments and descend to altitudes as low as 140 km before disappearing. Optical emissions produced during these events often exhibit bulls-eye structures, where the artificial plasma is thought to form a central spot that diverts or blocks HF waves to form an empty ring of emissions from the natural ionosphere at higher altitudes. We present multisite image data showing that, in some cases, both the spot and ring represent distinct artificial plasma layers.

Altitudinal dependence of meteor radio afterglows measured via optical counterparts

Utilizing the all-sky imaging capabilities of the LWA1 radio telescope along with a host of all-sky optical cameras, we have now observed 44 optical meteor counterparts to radio afterglows. Combining these observations we have determined the geographic positions of all 44 afterglows. Comparing the number of radio detections as a function of altitude above sea level to the number of expected bright meteors we find a strong altitudinal dependence characterized by a cutoff below ∼ 90 km, below which no radio emission occurs, despite the fact that many of the observed optical meteors penetrated well below this altitude. This cutoff suggests that wave damping from electron collisions is an important factor for the evolution of radio afterglows, which agrees with the hypothesis that the emission is the result of electron plasma wave emission.

RF-Induced Airglow Observed Using Composite Multispectral Imaging

Atmospheric airglow emissions accompanying artificial ionospheric plasmas occur when the bottom-side ionospheric F-region is exposed to high-power HF heating. These artificial emissions are spectrally similar to those which occur naturally as airglow and aurora, yet they have spatiotemporal behavior commensurate with the heater beam geometry. Interesting dynamics of both the artificial plasma layers and optical emissions have been observed, namely, the presence of multiple descending plasma layers distributed in altitude, mesoscale (10-100 km) bullseye-type airglow emission patterns, and, finally, small-scale (≲ 10 km) field-aligned filaments. We present visualizations of such features exhibited by the heated region using composite multispectral imaging.