Hanscom Air Force Base

Dust aerosols often create hazardous air quality conditions that affect human health, visibility, agriculture, and communication in various parts of the world. While substantial progress has been made in dust-event simulation and hazard mitigation over the last several decades, accurately forecasting the spatial and temporal variability of dust emissions continues to be a challenge. This report documents an analysis of atmospheric conditions for a series of dust events in North Africa. The researchers highlight four analyzed events that occurred between January 2016 to present in the following locations: (1) the western Sahara Desert; (2) East Algeria and the Iberian Peninsula; (3) Chad-Bodélé Depression; (4) Algeria and Morocco. For each event, the researchers developed an overview of the general synoptic, mesoscale, and local environmental forcing conditions that controlled the event evolution and used a combination of available lidar data, surface weather observations, upper-air soundings, aerosol optical depth, and satellite imagery to characterize the dust conditions. These assessments will support downstream forecast model evaluation and sensitivity testing; however, the researchers also encourage broader use of these assessments as reference case studies for dust transport, air quality modeling, remote sensing, soil erosion, and land management research applications.

<strong class="journal-contentHeaderColor">Abstract.</strong> We report the recovery and re-calibration of an extensive dataset of vertical profile measurements of the 1963/64 stratospheric aerosol layer measured from a two-site searchlight measurement facility at White Sands missile base and Sacramento Peak observatory, in New Mexico, US. The recovered dataset comprises 105 profiles of 550 nm aerosol extinction (&beta;<em>_p</em> (<em>z</em>)) and is part of a longer program of measurements with the US Air Force Cambridge Research Laboratories (AFCRL) searchlight facility that began in February 1963. The recovered series of &beta;<em>_p</em> (<em>z</em>) profiles span the 13-month period December 1963 to December 1964 and provide a unique record of the altitude and vertical extent of the Northern Hemisphere dispersed portion of the aerosol cloud from the March 1963 Agung volcanic eruption. The data recovery methodology involved first re-digitizing the 105 original &beta;<em>_p</em> (<em>z</em>) profiles (&beta;<em>_p^Orig</em> (<em>z</em>)) from individual Figures within an AFCRL research report (Elterman, 1966a). The re-calibration involves inverting the original equation used to compute &beta;<em>_p</em> (<em>z</em>) in Elterman (1966a; 1966b) to retrieve a normalized detector response ((<em>E_rp (z)</em>)/(<em>E_rp</em> (35))) profile for each of the 105 re-digitized &beta;<em>_p^Orig</em> (<em>z</em>) profiles. An iterative procedure was then used to compute the re-calibration &beta;<em>_p^Orig</em> (<em>z</em>) profiles (&beta;<em>_p^Recal (z)</em>), with the molecular extinction profile calculated with the corresponding daily molecular extinction profile calculated from local soundings, rather than the US Standard Atmosphere 1962 in the original dataset. Two-way molecular and aerosol transmittance corrections are applied using the MODTRAN (MODerate resolution atmospheric TRANsmission) code in transmission mode, applying a best-estimate aerosol phase function calculated from measurements, applied for the entire 2.76 to 35.2 km column. For the tropospheric aerosol transmittance, the AERONET aerosol phase function from White Sands High Energy Laser Systems Test Facility (HELSTF) was applied (2.76 to 10.7 km), a separate stratospheric phase function applied between 11.2 and 35.2 km, calculated from a set of particle size distributions measured by the U-2 high-altitude aircraft over a US region in the vicinity of White Sands in early 1964. Errors were estimated taking as a reference the errors determined in the computation of &beta;<em>_p^Orig</em> (<em>z</em>). Using available tabulated data from the original procedure the errors in the re-digitalization of &beta;<em>_p^Orig</em> (<em>z</em>) and in the retrieval of the (<em>E_rp (z)</em>)/(<em>E_rp</em> (35)) procedures were calculated and later added to the original estimates. Both the &beta;<em>_p^Recal (z)</em> and the stratospheric aerosol optical depth magnitudes showed higher magnitudes than &beta;<em>_p^Orig</em> (<em>z</em>) and the original stratospheric aerosol optical depth, however their magnitudes show a reasonable agreement with other contemporary observations. Both the original and re-calibration datasets are being submitted to PANGAEA data repository for its storage and public access.

As the Fermi level in semi-insulating InP is close to the iron acceptor level, both charge states-Fe/sup 2+/ as well as Fe/sup 3+/ corresponding to the acceptor level occupied/unoccupied by an electron-are present. W. Meier et al. (1991) presented a method based on absorption measurements in the near-bandgap region of InP to determine the concentration of both charge states separately. In the present work, the authors compare iron concentrations obtained by this method with the results from intracenter absorption, DLTS (deep level transient spectroscopy), electron paramagnetic resonance, Hall effect measurements, and glow discharge mass spectroscopy. They present a new calibration for the optical absorption cross sections. >

This chapter contains sections titled: Introduction Observations Summary and Discussion

The first effort at computer simulating the behavior of the inner magnetosphere during a substorm-type event on 19 September 1976 was completed. The computer model simulates many aspects of the behavior of the closed-field-line portion of the earth's magnetosphere, and the auroral and subauroral ionosphere. For these regions, the program self-consistently computes electric fields, electric currents, hot-plasma densities, plasma flow velocities and other parameters. Highlights of the results of our event simulation are presented. Predicted electric fields for several times during the event agree reasonably well with corresponding data from satellite S3-2. Detailed discussion is presented for a case of rapid subauroral flow that was observed on one S3-2 pass and is predicted by the computer runs. The computed global distribution of Birkeland current agrees reasonably well with the observations of Iijima and Potemra.

A plasma model consisting of warm downward streaming electrons, cool Boltzmann electrons, cold upward streaming ions, and hot Boltzmann ions is considered. It is shown that there is little difference between having a finite temperature in the electron beam and leaving the beam cold. This implies a greater utility for a cold electron beam model, and adds weight to the idea that the smaller amplitude electrostatic shocks seen in conjunction with ion beams are nonlinear ion acoustic modes.

We develop a phase screen model called the Synthetic Aperture Radar (SAR) Scintillation Simulator (SAR‐SS) for predicting the impacts of ionospheric scintillation on SAR image formation. SAR‐SS consists of a phase screen generator and a propagator. The screen generator creates a 2‐D random realization of spatial phase fluctuations resulting from the traversal of small‐scale field‐aligned irregularities in the ionosphere. It accounts for the motion of the radar platform, the drift of the ionospheric irregularities, and the oblique angle of propagation, all of which determine the scale sizes of the irregularities sampled by the radar beam. The propagator solves the 3‐D parabolic wave equation using the split step technique to compute the ionospheric transfer function for two‐way propagation. This ionospheric transfer function is used to modulate the SAR signal due to terrestrial features in order to assess the ionospheric impact on SAR image formation in the small target approximation. We compare simulated and observed PALSAR imagery over Brazil during disturbed ionospheric conditions. We demonstrate that SAR‐SS can reproduce the field‐aligned streaks in PALSAR imagery caused by irregularities in the equatorial ionosphere that have been observed by previous authors. The field‐aligned streaks exhibited a dominant wavelength larger than the Fresnel break scale, which suggests that refractive scatter was dominant over diffraction as the physical mechanism responsible for the scintillation of the radar signal in this case. The spectral index of phase fluctuations in the screen was quite large (9.0), suggesting that these irregularities were possibly associated with bottomside sinudoidal irregularities rather than equatorial plasma bubbles.

This paper presents the design and development of autonomous attitude stabilization, navigation in unstructured, GPS-denied environments, aggressive landing on inclined surfaces, and aerial grippin...

Beat‐wave generation of very low frequency (VLF) waves by two HF heaters in the ionosphere is formulated theoretically and demonstrated experimentally. The heater‐induced differential thermal pressure force and ponderomotive force, which dominate separately in the D and F regions of the ionosphere, drive an electron current for the VLF emission. A comparison, applying appropriate ionospheric parameters shows that the ponderomotive force dominates in beat‐wave generation of VLF waves. Three experiments, one in the nighttime in the absence of D and E layers and two in the daytime in the presence of D and E layers, were performed. X mode HF heaters of slightly different frequencies were transmitted at CW full power. VLF waves at 10 frequencies ranging from 3.5 to 21.5 kHz were generated. The frequency dependencies of the daytime and nighttime radiation intensities are quite similar, but the nighttime radiation is much stronger than the daytime one at the same radiation frequency. The intensity ratio is as large as 9 dB at 11.5 kHz. An experiment directly comparing VLF waves generated by the beat‐wave approach and by the amplitude modulation (AM) approach was also conducted. The results rule out the likely contribution of the AM mechanism acting on the electrojet and indicate that beat‐wave in the VLF range prefers to be generated in the F region of the ionosphere through the ponderomotive nonlinearity, consistent with the theory. In the nighttime experiment, the ionosphere was underdense to the HF heaters, suggesting a likely setting for effective beat‐wave generation of VLF waves by the HF heaters.

[1] The Communications/Navigation Outage Forecasting System (C/NOFS) satellite observed large-scale density depletions at postmidnight and early morning local times in the Northern Hemisphere summer during solar minimum conditions. Using electric field data obtained from the vector electric field instrument (VEFI) as input, the assimilative physics-based model (PBMOD) qualitatively reproduced more than 70% of the large-scale density depletions observed by the Planar Langmuir Probe (PLP) onboard C/NOFS. In contrast, the use of a climatological specification of plasma drifts in the model produces no plasma depletions at night. Results from a one-month statistical study, we found that the large-scale depletion structures most often occur near longitudes of 60°, 140°, and 330°, suggesting that these depletions may be associated with nonmigrating atmospheric tides, although the generation mechanisms of eastward electric fields at postmidnight local times are still uncertain. In this paper, densities obtained from both assimilation and climatology for the entire month of June 2008 are compared with PLP data from C/NOFS and the Challenging Minisatellite Payload (CHAMP), as well as special sensor ionospheric plasma drift/scintillation meter (SSIES) measurements from the Defense Meteorological Satellite Program (DMSP) satellites. Our statistical study has shown that, on average, the densities obtained by the PBMOD when it assimilates VEFI electric fields agree better with observed background densities than when PBMOD uses climatological electric fields.