Andrew W. Stephan

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...

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.

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.

The Remote Atmospheric and Ionospheric Detection System experiment on the ISS: Mission Overview

The Remote Atmospheric and Ionospheric Detection System (RAIDS) is a suite of three photometers, three spectrometers, and two spectrographs which span the wavelength range 50-874 nm and remotely sense the thermosphere and ionosphere by scanning and imaging the limb. RAIDS was originally designed, built, delivered, and integrated onto a NOAA TIROS satellite in 1992. After a series of unfruitful flight opportunities, RAIDS is now certified for flight on the Kibo Japanese Experiment Module-Exposed Facility (JEM-EF) aboard the International Space Station (ISS) in September 2009. The RAIDS mission objectives have been refocused since its original flight opportunity to accommodate the lower ISS orbit and to account for recent scientific progress. RAIDS underwent a fast-paced hardware modification program to prepare for the ISS mission. The scientific objectives of the new RAIDS experiment are to study the temperature of the lower thermosphere (100-200 km), to measure composition and chemistry of the lower thermosphere and ionosphere, and to measure the initial source of OII 83.4 nm emission. RAIDS will provide valuable data useful for exploring tidal effects in the thermosphere and ionosphere system, validating dayside ionospheric remote sensing methods, and studying local time variations in important chemical and thermal processes.

The Remote Atmospheric and Ionospheric Detection System on the ISS: sensor performance and space weather applications from the visible to the near infrared

The RAIDS experiment is a suite of eight instruments to be flown aboard the Japanese Experiment Module-Exposed Facility on the International Space Station (ISS) in late 2009. Originally designed, built, and integrated onto the NOAA TIROS-J satellite in 1993, the original RAIDS hardware and the mission objectives have been modified for this ISS flight opportunity. In this paper we describe the four near infrared instruments on the RAIDS experiment covering the wavelength range of 630 - 870 nm. Over the past two years these instruments have undergone modification, refurbishment, and testing in preparation for flight. We present updated sensor characteristics relevant to this new ISS mission and discuss performance stability in light of the long instrument storage period. The four instruments, operating in a limb scanning geometry, will be used to observe the spectral radiance of atomic and molecular emission from the Earths upper atmospheric airglow. The passbands of the photometers are centered on the atomic lines OI(777.4), OI[630.0], and the 0-0 band of O2 Atmospheric band at 765 nm. The spectrometer scans from 725 to 870 nm. These observations will be used in conjunction with the other RAIDS instruments to investigate the properties of the lower thermosphere and to improve understanding of the connections of the region to the space environment, solar energy flux and the lower atmosphere. These studies are fundamentally important to the understanding the effects of the atmosphere and ionosphere on space systems and their operation in areas such as satellite drag, communications and navigation.

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.

Evidence of ENA precipitation in the EUV dayglow

Observations from the STP 78-1 satellite, at 600 km altitude, of the OI 989 and 1304 A, and OII 834 A dayglow emissions between March 20-29, 1979 show brightening at low and mid-latitudes (< 30° geomagnetic latitude) during enhanced geomagnetic activity, as determined by the Dst index. We attribute this increased dayglow, which varies by up to 20% from average quiet time emissions, to increased energetic neutral atom production in the ring current. These particles precipitate to lower altitudes where they collisionally excite ambient atmospheric and ionospheric oxygen, that manifest in enhanced airglow intensities. We present our results and the evidence from which we conclude we have detected the first evidence of extreme ultraviolet dayglow excited by low-latitude precipitation of energetic neutral atoms.

Tomographic extreme-ultraviolet spectrographs: TESS

We describe the system of Tomographic Extreme Ultraviolet (EUV) SpectrographS (TESS) that are the primary instruments for the Tomographic Experiment using Radiative Recombinative Ionospheric EUV and Radio Sources (TERRIERS) satellite. The spectrographs were designed to make high-sensitivity {80 counts/s)/Rayleigh [one Rayleigh is equivalent to 10(6) photons/(4pi str cm(2)s)}, line-of-sight measurements of the oi 135.6- and 91.1-nm emissions suitable for tomographic inversion. The system consists of five spectrographs, four identical nightglow instruments (for redundancy and added sensitivity), and one instrument with a smaller aperture to reduce sensitivity and increase spectral resolution for daytime operation. Each instrument has a bandpass of 80-140 nm with approximately 2- and 1-nm resolution for the night and day instruments, respectively. They utilize microchannel-plate-based two-dimensional imaging detectors with wedge-and-strip anode readouts. The instruments were designed, fabricated, and calibrated at Boston University, and the TERRIERS satellite was launched on 18 May 1999 from Vandenberg Air Force Base, California.

Advances in remote sensing of the daytime ionosphere with EUV airglow

This paper summarizes recent progress in developing a method for characterizing the daytime ionosphere from limb profile measurements of the OII 83.4 nm emission. This extreme ultraviolet emission is created by solar photoionization of atomic oxygen in the lower thermosphere and is resonantly scattered by O+ in the ionosphere. The brightness and shape of the measured altitude profile thus depend on both the photoionization source in the lower thermosphere and the ionospheric densities that determine the resonant scattering contribution. This technique has greatly matured over the past decade due to measurements by the series of Naval Research Laboratory Special Sensor Ultraviolet Limb Imager (SSULI) instruments flown on Defense Meteorological Satellite Program (DMSP) missions and the Remote Atmospheric and Ionospheric Detection System (RAIDS) on the International Space Station. The volume of data from these missions has enabled a better approach to handling specific biases and uncertainties in both the measurement and retrieval process that affect the accuracy of the result. This paper identifies the key measurement and data quality factors that will enable the continued evolution of this technique into an advanced method for characterization of the daytime ionosphere.

Characterization of sensitivity degradation seen from the UV to NIR by RAIDS on the International Space Station

This paper presents an analysis of the sensitivity changes experienced by three of the eight sensors that comprise the Remote Atmospheric and Ionospheric Detection System (RAIDS) after more than a year operating on board the International Space Station (ISS). These sensors are the Extreme Ultraviolet Spectrograph (EUVS) that covers 550-1100 Å, the Middle Ultraviolet (MUV) spectrometer that covers 1900-3100Å, and the Near Infrared Spectrometer (NIRS) that covers 7220-8740 Å. The scientific goal for RAIDS is comprehensive remote sensing of the temperature, composition, and structure of the lower thermosphere and ionosphere from 85-200 km. RAIDS was installed on the ISS Japanese Expansion Module External Facility (JEM-EF) in September of 2009. After initial checkout the sensors began routine operations that are only interrupted for sensor safety by occasional ISS maneuvers as well as a few days per month when the orbit imparts a risk from exposure to the Sun. This history of measurements has been used to evaluate the rate of degradation of the RAIDS sensors exposed to an environment with significant sources of particulate and molecular contamination. The RAIDS EUVS, including both contamination and detector gain sag, has shown an overall signal loss rate of 0.2% per day since the start of the mission, with an upper boundary of 0.13% per day attributed solely to contamination effects. This upper boundary is driven by uncertainty in the change in the emission field due to changing solar conditions, and there is strong evidence that the true loss due to contamination is significantly smaller. The MUV and NIRS have shown stability to within 1% over the first year of operations.