Robert B. Kerr

Thermospheric poleward wind surge at midlatitudes during great storm intervals

United States. National Aeronautics and Space Administration (Living with a Star NNX15AB83G)

Periodic variations of geocoronal Balmer‐alpha brightness due to solar‐driven exospheric abundance variations

Measurements of the geocoronal Balmer-alpha (Hα) brightness have been made at the Arecibo Observatory during 11 separate periods since 1983 using both a Fabry-Perot interferometer and a tilting filter photometer. The tilting filter photometer is calibrated for absolute sensitivity using a constant brightness source traceable to National Institute of Standards and Techniques (NIST) standards and is used to cross-calibrate the Fabry-Perot interferometer. Since the observational technique has not changed since 1983, and since the data analyses technique are uniform, these data provide a measure of the solar cycle variation of Hα brightness at Arecibo. Unlike earlier studies, we discern no systematic discrepancy between the Hα brightness and estimates of the solar Lyman-beta flux that pumps the resonant fluorescent Hα emission. Rather, we conclude that geocoronal hydrogen abundance is generally (but not always) larger than models suggest, although not systematically so. The largest Hα emission is measured during solar minimum conditions, when brightness is approximately 50% greater than during solar maximum conditions, for measurements at solar depression angles from 15° to 35°. Above about 40° solar depression (corresponding to an illuminated column with a base at approximately 2000 km), no solar cycle variation is evident, and the brightness there is persistently greater than models. Intricacies of the Lyman-beta radiative transport problem make detection of an early morning maximum of hydrogen density near the exobase difficult using brightness data alone, although the Hα brightness is generally asymmetric with respect to local midnight, with brighter emission in the postmidnight sector. Studies of the semidiurnal (nighttime) variation demonstrate that the Hα brightness can vary by a factor of 2 over a period of days.

Determining exospheric hydrogen density by reconciliation of Hα measurements with radiative transfer theory

The singly scattered resonant fluorescent component of the geocoronal Balmer-alpha (Hα) emission is obtained by subtracting the multiply scattered component of Hα, determined by a radiative-transfer (RT) model, from Hα measurements. Exospheric column abundance then follows a straightforward single-scattering calculation. The reality of that abundance depends upon the validity of the model exosphere used by the RT model, upon the validity of the estimated solar Ly-β flux, and upon the statistical measurement errors. Iterative adjustments of the hydrogen density ([H]) profile input to the RT model, converging to a best fit to Hα observations, generate a unique atmosphere best matching model intensity to measured Hα, brightnesses. Applying this method to ground-based measurements of the geocoronal Hα emission made at Arecibo, Puerto Rico, in 1988, we find that the best estimated multiply and singly scattered components of Hα are about 85 and 60%, respectively, in excess of the initial model values, while the derived [H] is generally in agreement with the initial model [H] at lower altitudes and about 35% higher than the initial model [H] at higher altitudes. Examination of the iteration technique sensitivity indicates that the neutral temperature prescribed by the MSIS-83 model and the solar Ly-β flux estimated by a correlation relation are the parameters with the principal impact on the determined [H]. In addition, the examination shows that these parameters impact the RT-calculated intensity components. Because convergence of the iteration technique is insensitive to the initial model guess for most cases, the technique is capable of accurately determining [H] from Hα measurements, particularly if the temperature is simultaneously measured and the line center solar Ly-β flux is known.

Secular variability of the geocoronal Balmer‐alpha brightness: magnetic activity and possible human influences

Measurements of the geocoronal Balmer-alpha (Hα) brightness taken between 1983 and 1994 at the Arecibo Observatory (18.35° N, 66.75° W) occasionally display aperiodic brightness variations that we attribute to a local response of exospheric hydrogen abundance to geomagnetic activity. Approximately twofold Hα brightness enhancements in the days following moderate storm onset conditions are demonstrated, and the absolute brightness during these events is significantly greater than modeled brightness based on quiet geomagnetic conditions. Although there are no direct measurements of the line center solar Lyman beta (Ly β) flux that pumps Hα during these events, the magnitude of the brightness enhancements and the temporal reproducibility of the phenomena (following onset of all storms in our data) make it likely that the enhanced Hα brightness is due to enhanced column abundances of hydrogen in the exosphere above Arecibo and not to sporadic increases in the solar Ly β flux. Increased hydrogen abundance due to thermal expansion of the midlatitude atmosphere following propagation of the auroral heating event cannot account for the entire enhancement. The data also suggest that Hα. brightness has become systematically stronger from 1984 to 1994, independent of solar cycle variability or variations due to geomagnetic activity. An increase of ∼ 3% per year, may be attributable to real change in the geocoronal hydrogen column abundance during the period, and is possibly a consequence of increasing hydrogenous species concentrations in the lower atmosphere. The enhancement of geocoronal Hα brightness between 1984 and 1994 may be the consequence of increasing methane in the troposphere and may signal that the effects of methane deposition in the lower atmosphere have propagated throughout the atmosphere.

Thermospheric planetary wave‐type oscillations observed by FPIs over Xinglong and Millstone Hill

Three-year (2010-2013) observations of thermospheric winds (at similar to 250 km) by Fabry-Perot interferometers at Xinglong (XL, 40.2 degrees N, 117.4 degrees E) and Millstone Hill (MH, 42.6 degrees N, 71.5 degrees W) are used to study the climatology of atmospheric planetary wave-type oscillations (PWTOs) with periods of 4-19 days. We find that (1) these PWTOs occur more frequently in the months from May to October. They are consistent with the summertime preference of middle-latitude ionospheric electron density oscillations noted in other studies. (2) The month-to-month variations in PWTOs show phase changes between MH and XL, switching from antiphase to in phase when PWTO periods vary from short to long. (3) Typical PWTOs show annual and semiannual variations. The relative intensity of annual over semiannual components for PWTOs is different between XL and MH. (4) Magnetic storms and substorms have little influences on the annual and semiannual variations of the typical PWTO amplitudes. (5) Meridional wind PWTOs with typical periodicity bands around 5, 8, and 16 days appear to be correlated to both solar wind speed and K-p oscillations, suggesting a possible influence of the solar wind corotating interaction regions on neutral wind dynamics.

Tunable narrow-band filter for LWIR hyperspectral imaging

IR sensing has been a key enabling technology in military systems providing advantages in night vision, surveillance, and ever more accurate targeting. Passive hyperspectral imagin, the ability to gather and process IR spectral information from each pixel of an IR image, can ultimately provide 2D composition maps of a scene under study. FInding applications such as atmospheric, and geophysical remote sensing, camouflaged target recognition, and defence against chemical weapons.

First Palmer and Millstone Hill midlatitude conjugate observation of thermospheric winds

The first midlatitude conjugate thermospheric wind observations in the American sector showed various degrees of conjugacy between Palmer (64°S, 64°W, magnetic latitude (MLAT) 50°S) and Millstone Hill (42.82°N, 71.5°W, MLAT 53°N) under three different geomagnetic conditions (recovery after a substorm, moderately active, and quiet). The agreement with the National Center for Atmospheric Researchs Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations also varies with the geomagnetic activity level. During substorm recovery, the observations at Palmer (PA) and Millstone Hill (MH) both showed strong westward zonal winds, which the standard TIEGCM greatly underestimated. Inadequate ion convection pattern size and lack of effect from Subauroral Polarization Streams (SAPS) may be the cause of the large discrepancy. The TIEGCM with a SAPS model produced stronger westward zonal winds near PA but did not change the zonal wind near MH. The empirical SAPS model needs further refinements. In general, there is better conjugacy with moderate geomagnetic activity levels. The TIEGCM also agrees better with the observations. Under geomagnetically quiet conditions, the meridional winds appear to be less conjugate. The agreement between the observations and model is reasonable. Optical conjugate observations are severely limited by the seasons and weather conditions in the two hemispheres. Yet they are necessary to understanding the thermospheric dynamics in the subauroral region and its relationship with geomagnetic activity levels. The comparisons with TIEGCM are necessary for future model improvements.

Investigation of a resonance Lidar for measurement of thermospheric metastable helium

Abstract Expected signal returns for a ground-based resonance Lidar system used for profiling metastable thermospheric helium are presented, scaleable to specific system configurations. The signal estimate is dependent on the calculation of the effective backscatter cross-section for the He 10 830 A resonant transition as well as an estimate of thermospheric metastable helium densities obtained from a recent model. The peak backscatter cross-section is found to be 2.7(±0.3) × 10 −16 m 2 with an effective backscatter cross-section (assuming a 1 GHz rms laser linewidth centered at a wavelength of 10 830.32 A) of 2.6(±0.3) × 10 −16 m 2 . Measurements using the metastable He 3188 A and 3889 A lines are evaluated. Challenges in experimental design (i.e., laser characteristics, near infrared single photon detection, and background noise), as well as potential operation from a space-borne platform, are also discussed. With current technology, profiles with relatively high temporal and vertical resolution are shown to be attainable under twilight conditions.

Fabry Perot observations of helium 10830 Å emission at Millstone Hill

The He(23S) triplet at 10830 A is observed using a Fabry-Perot interferometer optimized for the near infrared. These observations, made at the zenith at the Millstone Hill Optical Facility during evening twilight, are the first to resolve the individual members of the helium triplet from the surrounding OH(5-2) lines. The introduction of an aeronomically unique germanium detector Noto et al., [1993] was crucial to this achievement. The instrument is configured with a spectral resolution of 1/2 the helium line width at 1000°K, assuring that the line widths accurately reflect the velocity distribution of metastable helium. Although the intensities observed are within a factor of two of helium 10830 A intensities observed by [Tinsley, 1968, Tinsley & Christensen et. al, 1971], the observed temperatures are different from the expected behavior of a neutral species in the upper-thermosphere during twilight.

Novel tunable liquid crystal Fabry-Perot filters for fiber optical system

We propose new tunable liquid crystal Fabry-Perot filters for fiber-optical telecommunication application. The filters have a low insertion loss, fast response time, wide tunable range to cover total c-band, or L-band, or both. They are solid-state filters without moving parts and the tuning voltages are low.