Joshua Semeter

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.

Simultaneous Multispectral Imaging of the Discrete Aurora

Abstract A unique multispectral imager and an associated multispectral analysis framework are described which together constitute a new diagnostic tool for auroral research. By acquiring spatial and spectral data simultaneously, multispectral imaging allows one to exploit physical connections between auroral morphology and the auroral optical spectrum in a way that sequential spectral imaging cannot. The initial research focus is on imaging the transition in the incident energy spectrum during the formation of discrete arcs—that is, when the precipitating population is characterized by keV electrons. A technique is presented which uses two spectral bands (centered at 4278 and 7325 A) to extend the effective dynamic range of passive imaging to much lower energies.

Emission of OI(630 nm) in proton aurora

A red aurora occurred over southern Canada and central Maine on April 11, 1997, producing a brightness of OI(630 nm) of several Kilorayleighs, which lasted for several hours. Two passes of the Defense Meteorological Satellite Program (DMSP) F12 satellite occurred during this time, and optical data were obtained from four CEDAR Optical Tomographic Imaging Facility (COTIF) sites. The DMSP F12 particle spectrometers observed proton precipitation south of the electron aurora with energy fluxes of several mW m -2 . Tomographic inversion of the COTIF optical observations gives the altitude profile of emissions along a magnetic meridian. We combine all available data using an ionospheric auroral model. Our analysis shows that the model produces the observed auroral brightness from the proton precipitation alone.

Ionospheric response to wave-accelerated electrons at the poleward auroral boundary

[1] The local ionospheric response to an auroral intensification at the poleward auroral boundary has been investigated using the incoherent scatter radar (ISR) and optical instrumentation at Sondrestrom, Greenland, in conjunction with space-bome measurements by the IMAGE and FAST satellites. ISR elevation scans through the illuminated region revealed filamentary columns of enhanced plasma density, ∼5 km in latitude by ∼200 km in altitude. Column densities were typically 5 × 10 11 /m 3 above background and often constant over a broad range of altitudes. The brightness of the O + 732-733 nm multiplet, monitored simultaneously by a near-infrared spectrometer, exceeded 1.2 kR during one 4-min period (a factor of ∼4 brighter than previously reported auroral measurements). A time-dependent model was developed to relate O + emission intensities to O + column densities for a given illumination time. The results suggested that the electron source was composed of kilometer-scale flux tubes locked in the E x B flow for several minutes whose average energy varied temporally between 1 keV over their illumination lifetime. Conjugate electrons measured by the FAST satellite at 1700 km showed evidence for energization by inertial Alfven waves. Ionization rates computed from these spectra were sufficient to account for the observed filamentary ionospheric structure. The implications of such ionization patterns for electrodynamic coupling with the magnetosphere are discussed.

Multispectral tomographic imaging of the midlatitude aurora

The CEDAR Optical Tomographic Imaging Facility (COTIF) consists of a midlatitude chain of meridional imaging spectrographs whose slit apertures subtend a common volume in the ionosphere. The facility provides a set of simultaneous multispectral brightness measurements that are used in a two-dimensional tomographic reconstruction (horizontal versus vertical) of auroral and airglow features in the midlatitude, subauroral ionosphere. A tomographic inversion method is summarized which regularizes the vertical (field-aligned) dimension using a nonlinear low-dimensional basis expansion. Three experimental results are presented for atomic oxygen 557.7- and 630.0-nm emissions at midlatitude. Time sequences of the reconstructed emission fields illustrate the variability in O I excitation patterns in both the diffuse aurora and SAR arcs. The results are interpreted with respect to outstanding issues associated with midlatitude aurora. Extracted vertical profiles are compared with published experimental and theoretical results.

Shear velocity profiles associated with auroral curls

Optical observations using high-resolution television cameras frequently show that auroral curls are associated with shear velocities in the apparent optical flow. The present study examines in detail one particular curl system event which happened to yield sufficient resolution to determine the fine structure of velocity and vorticity profiles by means of a new analysis technique. Those observations of curl system evolution are contrasted with large velocity shear events where small-scale quasiperiodic distortions were subject to sudden decay rather than development into vortices. The results are discussed in light of an electrostatic picture of auroral acceleration and the Kelvin-Helmholtz instability model. We suggest that the latter cannot fully explain the nonlinear phase of the observed curl system event and that curl models should take auroral acceleration processes into account.

A study of oxygen 6300 Å airglow production through chemical modification of the nighttime ionosphere

The Release Experiments to Derive Airglow Inducing Reactions (RED AIR) conducted on April 3, 1989, and December 6, 1991, offer a unique set of observations for studying the specific processes associated with the production of the O(3P–1D) emission at 6300 A. In these experiments, sounding rockets were used to place equal quantities of CO2 above and below hmax of the nocturnal F region. CO2 leads to 6300 A emission by a three-step process: (1) CO2 + O+ → O2+ + CO, (2) O2+ + e− → O* + O, (3) O* → O + hv6300. Direct measurements of plasma parameters and indirect measurements of the neutral atmosphere densities were used in conjunction with the Fluid Element Simulation (FES) computer code to model the temporal and spatial evolution of the observed 6300 A airglow enhancement and accompanying plasma depletion. Using the currently accepted set of reaction rates relevant to F region chemistry, the quantum yield of O(1D) from reaction (2) was found to have a mild altitude dependence, decreasing by 16% from 275 to 350 km. Since the initial vibrational distribution of the nascent O2+ was the same for the two releases, this result implies an altitude dependence in the quenching of O2+ vibrational states. Building on previous evidence that O2+ is vibrationally excited in the nighttime thermosphere, we further conclude that this vibrational distribution is altitude dependent. In terms of 6300 A airglow production, the effect is manifested in an altitude dependence of f(1D). Additionally, quenching by O(3P) was found to contribute very little to the depopulation of the nascent O(1D), with QO = 0 giving the best fit to the RED AIR observations.

Mobile crowd sensing in space weather monitoring: the mahali project

Space weather refers to the conditions and evolution of Earths near space environment including electron density variations in the ionosphere. This environment is influenced by both the Sun and terrestrial processes, and has an impact on communications, navigation, and terrestrial power systems. The recent discovery of clear signatures in the ionosphere related to tsunamis and earthquakes suggests that the ionosphere itself may serve as a valuable and versatile sensor, registering many types of Earth- and space-based phenomena. To realize this potential, ionospheric electron density must be monitored through a dense wide-area sensor mesh that is expensive to realize with traditional deployments and observation techniques. Crowdsourcing can help pursue this novel direction by providing new capabilities, including an increase in the number of sensors as well as expanding data transport capabilities through participating devices that act as relays. This article describes the Mahali project, which is currently at the beginning of exploring these promising techniques. Mahali uses GPS signals that penetrate the ionosphere for science rather than positioning. A large number of ground-based sensors will be able to feed data through mobile devices into a cloud-based processing environment, enabling a tomographic analysis of the global ionosphere at unprecedented resolution and coverage. This novel approach brings the exploitation of the ionosphere as a global earth system sensor technologically and economically within reach.

Persistent quasiperiodic precipitation of suprathermal ambient electrons in decaying auroral arcs

An analysis of ground-based images and rocket-borne electron data from the Physics of Auroral Zone Electrons II rocket experiment suggests that suprathermal ambient electrons, precipitating in field-aligned bursts, may play an active role in regulating the decay of auroral potential structures. A set of discrete arcs were observed to form in quasiperiodic succession behind a passing westward traveling surge. The nascent arcs faded rapidly, losing 90% of their luminosity within 15 s. The fading then abruptly stopped at a brightness consistent with a peak electron energy of ∼1.5 keV. Rocket-borne measurements of energetic electrons over the residual arcs 2 min later revealed a superposition of hot isotropic and cold ionospheric electrons, the latter precipitated in dispersive periodic (3-5 Hz) bursts from a source altitude of 3000-6000 km. A combined analysis of the optical and particle data showed that the FAB component (1) was present for at least 1 min, (2) precipitated in spatial scales of ∼10 km and (3) carried a number flux comparable to that of the overlying isotropic population. Bulk plasma parameters calculated in the vicinity of the fading arcs revealed an inverse relationship between parallel potential drop and number flux carried by the field-aligned population.

Tomographic reconstruction of 630.0 nm emission structure for a polar cap arc

Monochromatic imagers located at two sites in the Canadian Arctic were operated concurrently during the poleward transit of a duskside sun-aligned polar cap arc on 19 February 1996. The arc was observed by both stations in 630.0 nm images over a 5-min period. Profiles of 630.0 nm brightness versus elevation angle were extracted from pairs of images along a great circle defined by the two ground stations. These data were then supplied as inputs to a tomographic reconstruction for the arc-related 630.0 nm volume emission rate in a vertical (latitude vs. altitude) plane: η630 (lat,z). The time evolution of η630 (lat,z) structure for this polar cap arc indicates that a modification to the electron source energy distribution and not a thermospheric process (such as neutral heating and plasma diffusion/decay) controlled the arc-related 630.0 nm emission.