Kenneth F. Dymond

Two-dimensional mapping of the plasma density in the upper atmosphere with computerized ionospheric tomography (CIT)

Tomographic imaging of the ionosphere is a recently developed technique that uses integrated measurements and computer reconstructions to determine electron densities. The integral of electron density along vertical or oblique paths is obtained with radio transmissions from low-earth-orbiting (LEO) satellite transmitters to a chain of receivers on the earth’s surface. Similar measurements along horizontal paths can be made using transmissions from Global Position System (GPS) navigation satellites to GPS receivers on LEO spacecraft. Also, the intensities of extreme ultraviolet (EUV) emissions can be measured with orbiting spectrometers. These intensities are directly related to the integral of the oxygen ion and electron densities along the instrument line of sight. Two-dimensional maps of the ionospheric plasma are produced by analyzing the combined radio and EUV data using computerized ionospheric tomography (CIT). Difficulties associated with CIT arise from the nonuniqueness of the reconstructions, owi...

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.

Special Sensor Ultraviolet Limb Imager: an ionospheric and neutral density profiler for the Defense Meteorological Satellite Program satellites

The Naval Research Laboratory is developing a series of far- and extreme-ultraviolet spectrographs (800 to 1700 A) to measure altitude profiles of the ionospheric and thermospheric airglow from the U.S. Air Force Defense Meteorological Satellite Programs Block 5D3 satellites. These spectrographs, which comprise the Special Sensor Ultraviolet Limb Imager (SSULI), use a near-Wadsworth optical configuration with a mechanical grid collimator, concave grating, and linear array detector. To image the limb, SSULI employs a rotating planar SiC mirror that sweeps the field of view perpendicular to the limb of the Earth. In the primary operating mode, the mirror sweeps the instrument field of view through 17 deg to view tangent heights from about 50 to 750 km. The SSULI detectors use microchannel plate intensification and wedge-and-strip decoding anodes to resolve 256 pixels in wavelength dispersion. The detector is windowless and uses an o-ring sealed door to protect the Csl photocathode from exposure prior to insertion in orbit. The altitude distributions of the airglow measured by the SSULI sensors will be used to infer the altitude distributions of electrons and neutral species. At night, electron densities will be determined by measurement of ion recombination nightglow. Daytime electron densities will be obtained from measurements of multiple resonant scattering of O+ 834-A radiation produced primarily by photoionization excitation of atomic oxygen. Dayside neutral densities and temperatures will be inferred from the measurement of dayglow emissions from N2 and O produced by photoelectron impact excitation.

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.

A new technique for spectral analysis of ionospheric TEC fluctuations observed with the Very Large Array VHF system: From QP echoes to MSTIDs

We have used a relatively long, contiguous VHF observation of a bright cosmic radio source (Cygnus A) with the Very Large Array (VLA) through the nighttime, midlatitude ionosphere to demonstrate the phenomena observable with this instrument. In a companion paper, we showed that the VLA can detect fluctuations in total electron content (TEC) with amplitudes of <0.001 TECU and can measure TEC gradients with a precision of about 0.0002 TECU/km. We detail two complementary techniques for producing spectral analysis of these TEC gradient measurements. The first is able to track individual waves with wavelengths of about half the size of the array (~20 km) or more. This technique was successful in detecting and characterizing many medium-scale traveling ionospheric disturbances (MSTIDs) seen intermittently throughout the night and has been partially validated using concurrent GPS measurements. Smaller waves are also seen with this technique at nearly all times, many of which move in similar directions as the detected MSTIDs. The second technique allows for the detection and statistical description of the properties of groups of waves moving in similar directions with wavelengths as small as 5 km. Combining the results of both spectral techniques, we found a class of intermediate and small scale waves which are likely the quasi-periodic (QP) echoes that have been observed to occur within sporadic-E (Es) layers. We find two distinct populations of these waves. The members of one population are coincident in time with MSTIDs and are consistent with being generated within Es layers by the E-F coupling instability. The other population seems more influenced by the neutral wind, similar to the predominant types of QP echoes found by the Sporadic-E Experiments over Kyushu (Fukao et al. 1998; Yamamoto et al. 2005).

Simulations of the effects of vertical transport on the thermosphere and ionosphere using two coupled models

We have explored the sensitivity of the thermosphere and ionosphere to dynamical forcing from altitudes near the mesopause (~95 km). We performed five simulations, all for the year 2009, with the National Center for Atmospheric Research (NCAR)/Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). Two simulations were driven with the NCAR Global Scale Wind Model, and three used output from the Advanced Level Physics High Altitude (ALPHA) version of the Navys Operational Global Atmospheric Prediction System (NOGAPS). Use of NOGAPS-ALPHA allows for realistic meteorological variability from the lower atmosphere to propagate up into the TIEGCM, including a rich spectrum of nonmigrating tides. We find that the additional vertical transport from these tides causes a significant reduction in the calculated peak electron density of the ionospheric F2 layer (NmF2). The mechanism for this effect is the enhanced downward transport of atomic oxygen to the base of the thermosphere. In turn, this yields a greater relative abundance of N2 and hence enhanced recombination of ions and electrons. To get improved agreement with observed electron densities, we must reduce (Kzz) by a factor of 5. However, even with lower Kzz, our calculation still underestimates the NmF2 compared with radio occultation observations by the Constellation Observing System for Meteorology, Ionosphere and Climate satellite system. This underestimate of NmF2 may be linked to an overestimate of the nonmigrating tides in the coupled TIEGCM-NOGAPS calculations or to uncertainties in the bottom boundary for atomic oxygen in the TIEGCM.

A far and extreme ultraviolet limb imaging spectrograph for DMSP satellites

The Naval Research Laboratory is developing a limb imaging far- and extreme-ultraviolet (FUV/EUV) spectrograph (800-1700 A) to measure vertical profiles of the ionospheric and thermospheric airglow from DMSP Block 5D3 satellites. The spectrograph, called the Special Sensor Ultraviolet Limb Imager (SSULI), uses a near-Wadsworth optical configuration with a mechanical grid collimator, concave grating and linear array detector. Measured airglow profiles from the SSULI sensors will be used to infer vertical profiles of electron density and neutral density. At night, electron densities will be determined by measurement of ion recombination nightglow. Daytime electron densities will be obtained from measurements of multiple resonant scattering of O(+) 834 A radiation produced primarily by photoionization excitation. Dayside neutral densities and temperatures will be inferred from measurement of dayglow emissions from N2 and O produced by photoelectron impact excitation.

An algorithm for inferring the two‐dimensional structure of the nighttime ionosphere from radiative recombination measurements

We present a technique for retrieving the two-dimensional structure of the ionosphere using the measured variations of ultraviolet emissions produced by radiative recombination at 911 and 1356 A as measured by satellite-based limb scanning instruments. The algorithm uses an iterative scheme based on discrete inverse theory to determine the best fit to the data. We present the results of a simulation that demonstrates the algorithms performance for an inversion covering the nighttime half of the orbit. The algorithm was tested using “data” generated with the international reference ionosphere (IRI-90; Bilitza [1990]). The algorithm accurately retrieved the nighttime F region electron density.

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.