Robert P. McCoy

An optical remote sensing technique for determining nighttime F region electron density

We present a technique for using the measured variations of ultraviolet emissions produced by radiative recombination at 911 and 1356 A to determine the nighttime altitude distribution of F region O+ ions and electrons. The algorithm uses an iterative scheme based on discrete inverse theory to determine the best fit to the data. We present the results of simulations that demonstrate the convergence properties of the algorithm and the fidelity with which it reproduces the input ionosphere. The algorithm was tested against more realistic simulated “data” generated using the international reference ionosphere (IRI-90) [Bilitza, 1990]. The algorithm accurately retrieved the nighttime F region electron density at midlatitudes (±25°–65°N) over a wide range of solar and geomagnetic activity and local time.

Investigation of ionospheric O+ remote sensing using the 834‐Å airglow

We have studied the feasibility of ionospheric O+ remote sensing through measurements of the 834-A airglow. Our approach uses discrete inverse theory (DIT) to retrieve O+ number density profiles from the airglow. Our tests of this method assume observations by a limb-scanning system on an orbiting satellite at an altitude of 850 km. The scans cover the range of 10°–26.5° below horizontal, consistent with future multiyear missions. To provide a baseline assessment, we represent the synthetic ground truth (“true”) O+ distribution as a generalized Chapman-type profile with three or more parameters, based on our recent analysis of topside incoherent scattering radar data and standard ionospheric models (International Reference Ionosphere 1990 (IRI-90) and the parameterized ionospheric model (PIM)). The DIT method proves to be robust, converging to an accurate solution for a wide variation in ionospheric profiles. Using a detailed statistical error analysis of synthetic limb intensity data derived from the IRI-90 and PIM models, we work a difficult test case following from recent comments on the concept of 834–A remote sensing of ionospheric O+. We find that the DIT method can correctly distinguish between distinctly different F layers that produce nearly identical intensity profiles, consistent with instrument specifications for future missions.

Tomographic studies of aeronomic phenomena using radio and UV techniques

Tomographic characterization of ionospheric and thermospheric structures using integrated line-of-sight measurements provides a unifying paradigm for the investigation of various aeronomic phenomena. In radio tomography, measurements of the total electron content (TEC) obtained using a chain of ground receivers and a transit satellite are inverted to reconstruct a two-dimensional electron density pro;le. Similarly, prominent optically thin UV emissions, such as 911 and 1356 = A produced by radiative recombination of O + , provide the means to obtain F-region electron densities from space-based spectroscopic measurements. The existence of a number of UV sensors in orbit and in planning stage provide the means to carry out such tomographic remote sensing investigations on global scales. The inherent non-ideal acquisition geometry of such remote sensing observations, however, results in limited-angle tomographic inverse problems that are both ill-posed and ill-conditioned. Furthermore, the intrinsic presence of noise, especially in the case of UV measurements, imposes challenges on conventional reconstruction methods. To overcome these limitations, we approach the solution of these inverse problems from a regularization standpoint. In particular, we apply regularization by incorporating appropriate edge-preserving regularizing functionals that enforce piecewise smoothness of the solution. This paper describes these techniques, investigates associated inversion issues, and demonstrates their applicability through a case study. c � 2002 Published by Elsevier Science Ltd.

Ionospheric topography maps using multiple-wavelength all-sky images

We outline a technique to create three-dimensional topographic maps of the Earths ionosphere. Using all-sky images at 630.0 and 777.4 nm taken with the Cornell All-Sky Imager (CASI) while located at the Arecibo Observatory, we can estimate the maximum density (Nm) and the height (Hm) of the F layer over a 1000 × 1000 km area. This is possible because to first order, the 777.4 nm emission, produced by radiative recombination, is proportional to the integral of the square of the plasma density, whereas the 630.0 nm line, produced by charge exchange and dissociative recombination, depends on both plasma height and density. Using the neutral atmosphere given by the Mass Spectrometer Incoherent Scatter (MSIS-86) model and electron densities from the international reference ionosphere 1995 (IRI-95) model, we show that the estimates given by these maps are good to within 5% of the values used as input into the models. These errors are slightly larger (10%) when extreme gradients in the height of the F layer are present. We apply our technique to two different nights in 1999. In one example these maps show a steeply rising ridge of ionization stretching equatorward of the Caribbean site, punctuated by a series of parallel ridges and valleys. We compare these observations with previous work at Arecibo during very high magnetic activity. In our case we find no evidence for particle precipitation and agree with Sahai et al. [1981a] that spatial variations may have affected the earlier study. Another example shows the Appleton anomaly much farther north than normal. Instability processes are indicated in the former case, while physical mechanisms associated with a magnetic storm are explored in the latter case.

The middle ultraviolet dayglow spectrum

Spectroscopic measurements of the thermospheric dayglow in the wavelength range 1900 to 3400 A are presented. These measurements were made during two rocket experiments conducted on March 30, 1990, and March 19, 1992, from White Sands Missile Range, New Mexico. The data are presented to provide reference spectra in the lower, middle, and upper thermosphere. The 1990 observations, which were made during high geomagnetic activity, showed considerably enhanced nitric oxide (NO) intensities. Self-absorption theory is applied to the υ″ = 0 bands of the NO γ system. It is found that a recently published self-absorption algorithm correctly accounts for the attenuation of the γ(1,0) bands. There is a small discrepancy between the theory and observation for the (2,0) band and the (0,0) band intensity. The fact that there is reasonable agreement for all three bands suggests that both the NO slant column density and oscillator strengths for these bands are correct.

Quenching rate coefficients for O+(2P) derived from middle ultraviolet airglow

[i] O + ( 2 P) is produced in the sunlit thermosphere primarily by photoionization of atomic oxygen. Thermospheric atomic oxygen concentrations can be inferred from measurements of airglow produced near 732.0 nm by the transition of this excited state to the 2D state and at 247.0 nm by the transition to the 4 S ground state. The accuracy of these concentrations depends on the accuracy of the important chemical reaction rates used in the airglow model, including quenching of O + ( 2 P). We obtain coefficients for the quenching of O + ( 2 P) by O and N 2 by modeling rocket and satellite limb measurements of thermospheric middle ultraviolet (MUV) airglow at 247.0 nm. We derive a reaction rate for N 2 of 1.8 x 10- 10 cm 3 s -1 , which is lower than the value obtained by other airglow studies but in agreement with laboratory measurements. We obtain a best fit value for the O reaction rate of 5.0 x 10 -11 cm 3 s -1 , with an upper limit of 8.4 x 10 -11 cm 3 s -1 . The value of the O reaction rate determined by fits to 172 altitude profiles of the 247.0 nm emission shows a strong correlation with the magnitude of the excitation g factor. However, the airglow profile above 260 km favors the upper limit we have identified.

Electron densities determined by the HIRAAS Experiment and comparisons with ionosonde measurements

We present electron density profiles derived by inversion of ultraviolet limb scans made by the High Resolution Airglow and Aurora Spectroscopy (HIRAAS) experiment on the Advanced Research and Global Observing Satellite (ARGOS). The ultraviolet limb scans were inverted using an iterative algorithm based on discrete inverse theory. We present two comparisons with nearly coincident ionosonde measurements of the F-region peak density and peak height. Our observations took place on 24 November 1999 when the 10.7 cm radio flux was 181 × 10−22 Watt m−2 Hz−1 and the daily ap was 21, indicating moderate geomagnetic activity. The retrieved peak electron density and peak height were in good agreeent with the ionosonde measurements, demonstrating the accuracy of the ultraviolet technique for sensing the ionospheric state.

Far ultraviolet equatorial aurora during geomagnetic storms as observed by the Low‐Resolution Airglow and Aurora Spectrograph

We report the detection of storm time enhancements in the low-latitude far ultraviolet airglow as observed by the Low-Resolution Airglow and Aurora Spectrograph on the Advanced Research and Global Observation Satellite. The enhancements are present in several of the dayside and nightside emission lines, including the prominent 1304- and 1356-A lines of atomic oxygen as well as the N2 Lyman-Birge-Hopfield bands near 1465 and 1495 A. Time histories of the average low-latitude intensities of all emissions show a correlation with geomagnetic activity, as measured by the Dst index. Comparisons between the prestorm and storm time latitude profiles indicate that the emission increases are confined to magnetic latitudes < 20°. We have used the ratio of 1356 A/1495 A as a measure of O/N2 composition changes at these low latitudes. Although this ratio shows composition changes during the storm, no change in the ratio is observed during the peak in the emission. On the basis of the emission morphology, we conclude that these emission enhancements are most likely the result of energetic neutral atoms, which are created in the ring current and collisionally excite ambient atomic oxygen and molecular nitrogen in the low-altitude, low-latitude ionosphere and thermosphere.

High-resolution Ionospheric and Thermospheric Spectrograph (HITS) on the Advanced Research and Global Observing Satellite (ARGOS): quick look results

The High-resolution Ionospheric and Thermospheric Spectrograph (HITS) is a very high resolution (> 0.5 angstroms resolution over the 500 - 1500 angstroms passband) Rowland circle spectrograph that is currently flying on the USAF Advanced Research and Global Observing Satellite (ARGOS, launched 23 February 1999). The ARGOS is in a sun- synchronous, near-polar orbit at 833 km altitude with an ascending node crossing time of 2:30 PM. The instrument is designed to spectrally resolve the 834 angstroms triplet to demonstrate a new technique for remotely sensing the electron density in the F-region ionosphere. In addition, the HITS can spectrally resolve the rotational structure of the N2 Lyman-Birge-Hopfield bands, which can be used to infer the thermospheric temperature. The HITS can resolve the radiative recombination continuum produced by recombining O+ ions and electrons, which can be used to infer the electron temperature. The HITS will also produce a high spectral resolution array of the 500 - 1000 angstroms passband to produce a more accurate identification of some of the previously unresolved features of the dayglow spectrum. The instrument operates as a limb imager with a limb scan occurring every 100 seconds throughout the expected three year mission life. Its field-of-view is 0.06 degree(s) X 4.6 degree(s), which corresponds to 3 km (altitude) X 230 km (along the horizon) at the limb. The instruments field-of-regard is 17 degree(s) X 4.6 degree(s), which covers the 100 - 750 km altitude range. We will present an overview of the instrument and discuss its calibration and in-flight performance.

On-orbit characterization and performance of the HIRAAS instruments aboard ARGOS: LORAAS sensor performance

The Advanced Research and Global Observation Satellite (ARGOS) has been operating since February 1999 and includes three spectrographs comprising the High Resolution Airglow and Auroral Spectroscopy (HIRAAS) experiment. The HIRAAS instruments remotely sense the Earths mid-, far- and extreme-ultraviolet airglow to study the density, composition, and temperature of the thermosphere and ionosphere. The Low Resolution Airglow and Aurora Spectrograph (LORAAS) is a limb scanner covering the 80-170 passband nm with 1.8 nm spectral resolution. Repeated serendipitous observations of hot O- and B-type stars have been used to improve the aspect solution, characterize the instrument field-of-view, and monitor relative sensitivity degradation of the instrument during the mission. We present the methodology of performance characterization and report the observed performance degradation of the LORAAS wedge-and-strip microchannel plate detector. The methods and results herein can be utilized directly in on-orbit characterization of the SSULI operational sensors to fly aboard the DMSP Block 5D3 satellites.