Stefan E. Thonnard

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.

Caribbean Ionosphere Campaign, year one: Airglow and plasma observations during two intense mid‐latitude spread‐F events

A series of campaigns has been carried out in the Caribbean over a one-year period to study intense mid-latitude spread-F events using a cluster of diversified instrumentation. These events are relatively rare but a number of them have now been captured and will be discussed in this and several companion papers. This paper focuses on 630 nm airglow images obtained by the Cornell All-Sky Imager for two of the more spectacular cases that began on February 17, 1998 and February 17, 1999. In the latter case, and for the first time, a poleward surge of depletion/enhancement airglow zones was captured by radar as well as an airglow imager. In the former case structures grew in place overhead and produced strong VHF F-region backscatter as observed by the CUPRI and University of Illinois radars; the other event, exactly one year later, did not result in detectable 3-m backscatter. The two data sets show quantitatively that the low airglow region is elevated in height and depleted in plasma density and Pedersen conductivity. We suggest an enhanced eastward electric field inside the low conductivity zone may be responsible for the surge. The data also suggest small scale turbulence can only be observed in developing structures.

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 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.

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.

Electron densities determined by inversion of ultraviolet limb profiles

We present electron density profiles derived by inversion of ultraviolet limb radiances observed on November 24, 1999, by the Low-Resolution Airglow and Aurora Spectrograph instrument on the Advanced Research and Global Observing Satellite. The solar 10.7-cm radio flux was 181 solar flux units, and the daily ap was 21, indicating moderate geomagnetic activity. The O+ density profile, which is approximately equal to the electron density profile in the F region ionosphere, was determined by inverting the limb radiance profile of O I 911-A emission of atomic oxygen. The 911-A emission is produced by radiative recombination of O+ ions and electrons. Sounding rocket and satellite measurements of the Earths extreme ultraviolet dayglow indicated significant contamination in the 900- to 920-A passband by emissions from atomic, molecular, and ionized nitrogen [Gentieu et al., 1979, 1981; Chakrabarti et al., 1983]. As a result of these observations, the radiative recombination emission was thought to be of little use for ionospheric sensing during the daytime. Feldman et al. [2001] have recently measured spectra in the 905- to 1184-A passband and show that the contamination at F region altitudes is less than that present in the earlier observations [Gentieu et al., 1979, 1981; Chakrabarti et al., 1983]. We found the contamination of the O I 911-A emission to be negligible at F region altitudes and have been able to use the 911-A emission to accurately characterize the ionospheric state. We have compared the peak electron density and peak height determined by inversion of the 911-A altitude profiles with nearly coincident ionosonde measurements, and we find the measurements from the two techniques to be in good agreement, demonstrating the accuracy of this technique for sensing the ionospheric state.

Enhanced empirical models of the thermosphere

Abstract The Naval Research Laboratory (NRL) has embarked on a development program to upgrade empirical models of the neutral upper atmosphere (thermosphere and upper mesosphere) and to apply these models to scientific and engineering problems. The program focus has been the Mass Spectrometer — Incoherent Scatter Radar (MSIS) model of composition and temperature. The new NRLMSIS model, due for release in 2000, has ingested additional data sets, including, for the first time, the drag and accelerometer data of Jacchia and others. The formulation in the lower thermosphere now has improved flexibility, and a new species, “anomalous oxygen,” allows for appreciable O + and hot atomic oxygen contributions to the total mass density at high altitudes. A new full disk proxy for the solar chromospheric EUV driver of thermospheric variability is available for studies in combination with the NRLMSIS model and database. This will determine the value of augmenting F 10.7 in the model formulation — a longstanding issue. Whereas F 10.7 correlates more closely with coronal EUV flux, chromospheric fluxes provide the primary thermospheric heating.

O+, O, and O2 densities derived from measurements made by the High Resolution Airglow/Aurora Spectrograph (HIRAAS) sounding rocket experiment

We present the results of an analysis of the O II 834 A and O I 1356 A altitude profiles measured during a sounding rocket flight on March 19, 1992. The profiles were analyzed using a new set of models that used discrete inverse theory to seek a maximum likelihood fit to the data. Both profiles were fit simultaneously to ensure consistency of the retrieved ionosphere and thermospheric neutral density. During the analysis the thermospheric neutral density and temperature were modeled using the Mass Spectrometer Incoherent Scatter (MSIS-86) model [Hedin, 1987]. Two parameters were used to scale the absolute MSIS O and O2 densities; the exospheric temperature was altered by varying the 10.7 cm solar flux (an MSIS-86 input). The ionospheric O+ density was modeled by a three-parameter Chapman layer. The retrieved MSIS scalars for the O and O2 densities were 0.47 ± 0.09 and 0.58 ± 0.14, respectively. These scalars indicate that the MSIS-86 model predicted significantly higher O and O2 densities. The inferred exospheric temperature was 1125 K in good agreement with the MSIS-86 prediction. The derived O density is in good agreement with the O density inferred from midultraviolet spectra observed during the same rocket flight [Bucsela et al., 1998]. The retrieved F region peak density, 1.98 ± 0.63 × 106 cm−3, peak height, 291 ± 22 km, and plasma scale height, 138 ± 24 km, all agreed with coincident digisonde measurements. Thus we have demonstrated that the ionospheric state can be accurately determined by inversion of observed O II 834 A limb radiance profiles.

Update on the calibration and performance of the special sensor ultraviolet limb imagers (SSULI)

The Naval Research Laboratory has built give Special Sensor Ultraviolet Limb Imagers (SSULIs) for the Defense Meteorological Satellite Program. These sensors are designed to measure vertical intensity profiles of the Earths airglow in the extreme and far ultraviolet (800 to 1700 angstroms). The data from these sensors will be used to infer altitude profiles of ion, electron and neutral density. The first SSULI is scheduled to launch in 2000. An identical copy of the SSULI sensor called LORAAS was launched aboard the ARGOS spacecraft on February 23, 1999. Data from LORAAS will be used to verify the performance of the SSULI sensors, ground analysis software and validate the UV remote sensing technique. Together with the LORAAS instrument the SSULI program will collect data on the composition of the upper atmosphere for a complete solar cycle.