Daniel M. Cotton

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...

A statistical framework for space‐based EUV ionospheric tomography

We present a statistical reconstruction framework for space-based extreme ultraviolet (EUV) ionospheric tomography. The EUV technique offers a means to invert the nighttime F region electron density on global scales from a single spaceborne spectrograph, using prominent optically thin emissions produced by radiative recombination of O+. Since the EUV technique does not rely on ground receivers to make the measurements, the observations do not suffer from limitations on the viewing angles. The EUV tomography is an ill-conditioned inverse problem in the sense that its solution is sensitive to perturbations of the measured data. With large condition numbers of a typical projection matrix, simple least squares inversion techniques yield unacceptable results in the presence of noise. This reflects the fact that more degrees of freedom are being sought than are supported by the noisy data. To overcome this limitation, we cast the tomographic inverse problem in a stochastic framework and incorporate a statistical prior model. In doing so we also obtain measures of estimation uncertainty for the solutions. Through simulations, we demonstrate the applicability of these techniques in the context of a space mission designed for EUV ionospheric tomography, namely, the Tomographic Experiment Using Radiative Recombinative Ionospheric EUV and Radio Sources (TERRIERS). The simulations show promising results for EUV tomography as a viable ionospheric remote sensing technique.

Sounding rocket observation of a hot atomic oxygen geocorona

A sounding rocket measurement of the ultraviolet, atomic oxygen dayglow reveals an excess of emission compared to standard thermospheric model calculations at exospheric altitudes. We explore two explanations for this discrepancy: a breakdown of the radiative transfer model due to nonlocal thermal equilibrium (non-LTE) conditions above the exobase and a hot atomic oxygen geocorona. In particular, the effects of non-LTE on the ³P2,1,0 sublevel populations are modeled, and a hot O component in the upper thermosphere and lower exosphere is added to investigate the effects on the modeled emissions. For both cases, the data are reanalyzed and compared with the results using a standard LTE model. A hot O geocorona having a peak density of 106 cm−3 at 550 km and a temperature of 4000 K is consistent with the data and appears to be the most reasonable explanation of the high-altitude enhanced emissions observed in the data.

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.

Self-compensating, all-reflection interferometer

A grating interferometer that uses all-reflecting optical components has been developed for operation in the extreme and far UV. The instrument uses a V-groove, ruled grating as its beam splitter and has no moving parts. A self-compensating optical design is employed that makes it tolerant to small misadjustments of optical alignments and convenient for space-flight applications. The instrument described here uses a 600-groove/mm plane diffraction grating that operates in the second order and obtains a resolving power of ~ 100,000 at 1216 Å.

Interplanetary H Lyα Observations from a Sounding Rocket

We describe a Self-Compensating All-Reflecting Interferometer used to measure diffuse interplanetary H Ly? emission (? ~ 1216 ?) of interstellar origin at high spectral resolution (?/?? ~ 100,000). The instrument is capable of resolving the separation between the Doppler-shifted, interplanetary neutral H emission from the Earths geocoronal emission line. We present the optical layout and mechanical design that enable the instrument to maintain alignment through a sounding rocket flight. We find an H Ly? spectrum, including both geocoronal emission and interplanetary scattered features, which we have fitted using Gaussian line profiles. The interplanetary H line is fitted with a 7900 ? 2400 K line width. The geocoronal line width is fitted with a 1900 ? 500 K Gaussian. The Doppler shift between the geocoronal and interplanetary line is 0.145 ?, corresponding to an apparent interplanetary wind velocity of 16 ? 7 km s-1. We show how full line-profile measurements of interplanetary H emission can advance current studies of the heliospheric interface region and very local interstellar medium.

Single-element imaging spectrograph

A spectrograph concept designed for both high wavelength and high spatial resolution (in one dimension) is briefly described. This design is referred to as a single-element imaging spectrograph (SEIS). It is a one-bounce diffractive system that combines the spectral properties of a Rowland mount spectrograph with the imaging (spatial resolution) properties of a Wadsworth mount spectrograph through the use of a toroidal diffraction grating. No primary optics are necessary, making the system especially attractive for use in the extreme and far ultraviolet, where low reflectivity of common optical coatings can severely limit instrument sensitivity.

A position sensitive detector for EUV remote sensing

The authors describe a photon-counting extreme ultraviolet (EUV) detector system used in a rocket-borne spectroscopic instrument for remote sensing of upper atmospheric composition and temperature. The detector uses a KBr coated microchannel plate (MCP) Z stack in combination with a wedge-and-strip image readout system. Three separate detector fields of view are used to sense the Earth dayglow spectrum (980 AA to 1040 AA, and 1300 AA to 1360 AA) and the solar EUV spectrum (250 AA to 1400 AA). The authors demonstrate high gain (2*10/sup 7/), tight pulse-height distribution (35% FWHM), and a spatial resolution of approximately 35 mu m FWHM (full width at half maximum), which is the highest resolution for a wedge-and-strip anode MCP detector flown to date. The background, image linearity, and flat-field performance are discussed. Raw spectra from the rocket flight are also presented. >

SPINR: two-dimensional spectral imaging through tomographic reconstruction

We describe a novel technique that enables us to conduct 2-D imaging spectroscopy with a 1-D imaging spectrometer. A typical imaging spectrometer obtains a series of 1-D monochromatic images that contain the entire field of view, but the spatial information in the dispersion direction is lost. By rotating the instrument (hence, rotating the field of view), we compile a series of time-dependent profiles. We use a numerical tomographic reconstruction method to recover the second spatial dimension and thereby obtain 2-D monochromatic images of the field of view. This technique is more sensitive and, hence, collects a full 2-D image more quickly than a conventional push-broom scan. We present the results of numerical simulations and discuss the prospects of this method for magnetospheric imaging applications.

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.