Optical tomography of the aurora and EISCAT

Annales Geophysicae, 1996

Several authors have published models of auroral conductances in the past. Some of them are based on theoretical approaches; others are the result of observations. The data bases for the latter models range from 8 h to 3 years of experiments. In this paper, we show the results of our own modeling, based on a coupled kinetic/fluid approach. We compare these results to the statistical model based on 3 years of experiment. We then model the auroral ionospheric conductances above EIS-CAT and the EISCAT-Svalbard Radar for different solaractivity indices. These modeled conductances are fitted with a simple law depending on the f index and on the solar zenith angle.

Radio Science, 1976

The Chatanika, Alaska, incoherent scatter radar has been used to measure the spatial variation of auroral ionization. A two-dimensional (altitude, latitude) cross-sectional map of electron densities in the ionosphere is produced by scanning in the geomagnetic meridian plane. The altitude variation of ionization is used to infer the differential energy distribution of the incident auroral electrons. The latitudinal variation of this energy distribution and the total energy input are obtained by use of the meridian-scanning technique. Examples are shown of observations made during an active aurora.

Journal of Geophysical Research, 1999

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.

Journal of Geophysical Research, 1994

The purpose of this paper is to examine the viability of using LBH emission ratios to infer auroral conductances and to quantify the strengths and weaknesses of this technique. We show that column-integrated Hall and Pealersen conductances may be determined from a single remote measurement of a pair of auroral LBH emissions, one in the region of strong 02 absorption (1464 ]i) and one lying outside of this region (1838 ]i). The dependence of the determined conductivities on incident average energy, total energy flux, and changes in solar and magnetic activity levels is examined. We show that, for energies above 5 keV, auroral conductances may be scaled by the square root of the incident energy flux with errors less than about 20%. For lower energies, however, the scaling may deviate significantly from a power of 0.5. We provide appropriate scaling factors as a function of average energy. We also note that the choice of either a Gaussian or Maxwellian distribution can significantly affect low-energy (<1 keV) conductance calculations. Finally, we quantify the conditions under which the mean energy of a Gaussian energy distribution may differ significantly from the characteristic energy.

Journal of Geophysical Research, 1989

Auroral luminosities at vacuum ultraviolet (VUV) wavelengths are combined with simultaneous and coincident ionospheric electron density measurements made by the Chatanika radar to relate ionospheric conductances to optical emissions. The auroral luminosities are obtained along the magnetic meridian through Chatanika with the auroral imaging photometers on the Dynamics Explorer 1 (DE 1) satellite as the radar scans in the magnetic meridian to measure electron density and conductivity as a function of altitude and latitude. The observations are used to determine an empirical relationship between the luminosities measured at VUV wavelengths and the Hall and Pedersen conductances. Of particular interest is the response of the photometer when using the VUV filter designated 123W. This filter admits the 130.4-and 135.6-nm emissions of atomic oxygen and the Lyman-Birge-Hopfield (LBH) bands of N 2. Model calculations of the LBH and O I (135.6 nm) contributions to the total measured luminosity indicate that the relation between 123W luminosity and Pedersen conductance is less sensitive to the average energy of the precipitating electrons than the corresponding relation between the Hall conductance and 123W luminosity. This is because both the luminosity and Pedersen conductance decrease with increasing electron energy. The luminosity decreases with increasing energy because the emissions are more strongly absorbed by 0 2 above the region of production. The Pedersen conductance decreases with increasing energy because the Pedersen mobility maximizes at an altitude of about 140 km. In contrast, the Hall conductance increases with increasing electron energy, so that the relation between Hall conductance and luminosity depends on the hardness of the precipitation. These qualitative results are confirmed using the simultaneously and coincidentally obtained measurements of conductance and 123W luminosity. The absolute difference between the model calculations and the experimental results indicates that the 130.4-nm emissions account for 50-65% of the total photometer response with the 123W filter. on ISIS 2 and from the Chatanika radar to show that the ionospheric electron density profile can be determined from the emissions of N 2 + and neutral atomic oxygen at 391.4 and 630.0 nm, respectively. However, remote sensing at visible wavelengths in the dark hemisphere presents several difficulties, particularly contamination from Earth albedo. In addition, intense dayglow emissions prevent auroral imaging in the sunlit hemisphere. For these reasons, auroral imaging at ultraviolet and X ray wavelengths is highly desirable. Recent observations with the Dynamics Explorer, Hilat, S3-4, P78-1, and Copyright 1989 by the American Geophysical Union. Paper number 88JA03796. 0148-0227/89/88JA-03796505.00 Viking satellites have demonstrated some of the advantages of remote sensing at ultraviolet wavelengths [

Journal of Geophysical Research, 1974

Data taken by the incoherent scatter radar facility at Chatanika, Alaska, have been used to investigate ionospheric conductivities and electrical currents. During quiet days the conductivities appear to vary in a way consistent with ionization arising from solar EUV radiation. At these times the ratio between the height-integrated Hall and Pedersen conductivities is close to 2. In the evening hours, enhancements in the northward electric field are found to precede small increases in the conductivities that, from analysis of electron density profiles, arise from precipitation of 3-10 X 106 el cm-•' s-• sr-• in the energy range 3-20 keV. Str9ng enhancements of the Hall conductivity relative to the Pedersen conductivity occur during negative bays when the electric field is in a southwestward direction. These enhancements are caused by a relatively strong flux of 1-3 X 10 ? el cm-•' s-• sr-• in the energy range 15-20 keV. The ionospheric currents calculated in the geomagnetic east-west direction are in good agreement with the H component measured by a nearby magnetometer; this result indicates that the current causing the ground level magnetic fluctuations is a broad horizontal sheet current. The north-south ionospheric current, however, consistently disagrees with the observed D component in a manner that cannot easily be explained unless currents parallel to the earth's magnetic field are present. The parallel currents needed in daytime, however, are directed opposite to the Sq p current system, often inferred to explain the quiet day variations in auroral zone magnetograms. Finally, it is found that the neutral wind is of considerable importance in driving currents during quiet days in the auroral zone. Often these wind-driven currents oppose the current driven by the static e, lectric field. The incoherent scatter radar at Chatanika, Alaska (L-5.6, 3_ = 65ø), has been used previously by Doupnik et al. [1972], Banks et al. [1973, 1974], and others to investigate the behavior of electric fields in the auroral zone ionosphere. More recently, a number of new experiments have been made, and improved methods have been developed for analyzing the radar data to determine the E region Hall and Pedersen currents. In this paper the earlier studies are continued in an attempt to obtain a more comprehensive view of the relationship between electric fields, particle precipitation, ionospheric currents, and magnetic perturbations in the auroral zone. The relationship between ionospheric currents and ground level magnetic perturbations has been studied several times by using chemical clouds and rockets. Rees [1971], for example, has obtained short-term data indicating orthogonality between the drift direction of ion clouds in the auroral zone F region and the magnetic perturbation vector observed at ground level. Such results agree well with the earlier barium cloud experiments by Haerendel et al. [1969], Haerendel and Liist [1970], Fbppl et al. [1968], and Wescott et al. [1969]. In contrast, in the polar cap, Wescott et al. [ 1970] found that the ionospheric currents deduced from drifting barium clouds were inconsistent with the direction of the magnetic perturbation on the ground. Haerendel et al. [1971] arrived at the same conclusion from investigations of the electric field inferred from motions of a barium cloud released at 12.4 R x on field lines originating on the morning side polar cap. In both cases the failure to obtain agreement between magnetic perturbation on the ground and the direction of the ionospheric currents inferred from E. was explained as a consequence of field-aligned currents. As will be shown in this paper, studies of the correspondence between magnetic perturbations AB measured on •Now at the Auroral Observatory, Troms½, Norway. Copyright ¸ 1974 by the American Geophysical Union. the ground and electric fields inferred from ion drifts in the auroral F region must take into account the presence of E region neutral winds. During periods when the ionospheric electric field is small it is found that these winds significantly influence the ionospheric current system. In fact, such an effect may be present in the results of Rees [1971] and Haerendel and Li•st [1970], in which the deviation from orthogonality between the F region ion cloud velocity vector and the surface magnetic perturbation vector increases as the ion velocity decreases. In such circumstances the influence of the neutral

Advances in Space Research, 1988

Multipoint measurements are necessary to address many pressing problems in space physics. The combination of satellite, airborne, and ground-based platforms for in-situ and remote sensors is a powerful approach to meeting these needs. The AFGL Airborne Ionospheric Observatory (AlO) has solved problems in this manner by operating in concert with VIKING, DE-B, AE-E, POLAR BEAR, HiLat, DMSP, WIDEBAND, and other satellites, and with Sondrestromfjord, EISCAT, Millstone Hill, Arecibo, and Jicamarca incoherent scatter radars. The aircraft platform has "parked" in satellite orbital planes for up to five consecutive passes, "parked" in the dark moon auroral oval for hours tracing plasma flow through the cusp into the polar cap, and traced detailed plasma boundaries by flying "racetrack orbits" under persistent boundaries. We illustrate here some of the potential of such a combination of sensor platforms by presenting findings associated with auroral arcs, polar cap F-region stable sun-aligned arcs, theta auroras, and other auroral features.

Journal of Geophysical Research, 1991

A specialized incoherent scatter radar scanning mode has been developed for use in conjunction with simultaneous real-time all-sky images. These complementary diagnostics are used to examine the aeronomy and electrodynamics of stable auroral arcs that delineate the boundary between the polar cap and the auroral oval. The first arc discussed, observed at 2000 MLT, represents the boundary between antisunward plasma flow in the polar cap and sunward return flow equatorward of the arc. The arc defined an equipotential in the high-latitude convection pattern in that no plasma flowed across the arc. The radar line-of-sight velocity measurements also indicate that this arc is consistent with a convergent electric field and an associated weak upward field-aligned current. The second arc was observed at 2330 MLT and was associated with a nightside gap or magnetic reconnection region. Strong antisunward flow was observed directly across the arc, although a velocity shear was superposed on this steady flow along the poleward edge of the arc. Detailed plasma density, temperature, and line-of-sight velocity measurements from the radar are presented for both arcs to define the electric field, horizontal and field-aligned currents, and thermal plasma parameters associated with these arcs. 1. INTRODUCTION Several previous experiments have exploited simultaneous radar and optical measurements from Sondrestrom, Greenland. These studies encompass aeronomical processes within polar cap and oval arcs [Niciejewski et al., 1989; Vallance Jones et al., 1987; Hecht et al., 1991], plasma flow near polar cap arcs [Mende et al., 1988; Robinson et al., 1987; Carlson et al., 1984], and drift of plasma patches from the polar cap toward the auroral oval [Doolittle et al., 1989; Weber et al., 1986]. Coordinated all-sky camera and Chatanika radar measurements were used by de la Beaujardiere et al. [1977, 1981] and de la Beaujardiere and Vondrak [1982] to investigate electric fields and currents associated with auroral arcs. In this paper we report results from two campaigns in which the radar scan mode was optimized to provide high-resolution plasma density, temperature, and line of sight (LOS) velocity measurements within and in the vicinity of auroral arcs. A new and unique aspect of these measurements is the use of real-time optical images to determine and maintain an optimal (perpendicular) orientation between the radar scans and the observed arcs. Because important ionospheric and magnetospheric boundaries are believed to coincide with the arc boundaries, the advantage of this mode is to ensure that the measurements are made in a coordinate system aligned with the arc. Moreover, the

IEEE Transactions on Geoscience and Remote Sensing, 2016

We present a feasibility study for a high frame rate, short baseline auroral tomographic imaging system useful for estimating parametric variations in the precipitating electron number flux spectrum of dynamic auroral events. Of particular interest are auroral substorms, characterized by spatial variations of order 100 m and temporal variations of order 10 ms. These scales are thought to be produced by dispersive Alfvén waves in the near-Earth magnetosphere. The auroral tomography system characterized in this paper reconstructs the auroral volume emission rate to estimate the characteristic energy and location in the direction perpendicular to the geomagnetic field of peak electron precipitation flux using a distributed network of precisely synchronized ground-based cameras. As the observing baseline decreases, the tomographic inverse problem becomes highly ill-conditioned; as the sampling rate increases, the signal-to-noise ratio degrades and synchronization requirements become increasingly critical. Our approach to these challenges uses a physics-based auroral model to regularize the poorlyobserved vertical dimension. Specifically, the vertical dimension is expanded in a low-dimensional basis consisting of eigenprofiles computed over the range of expected energies in the precipitating electron flux, while the horizontal dimension retains a standard orthogonal pixel basis. Simulation results show typical characteristic energy estimation error less than 30% for a 3 km baseline achievable within the confines of the Poker Flat Research Range, using GPS-synchronized Electron Multiplying CCD cameras with broad-band BG3 optical filters that pass prompt auroral emissions.

Annales Geophysicae, 2011

Two discrete auroral arc filaments, with widths of less than 1 km, have been analysed using multi-station, multimonochromatic optical observations from small and medium field-of-view imagers and the EISCAT radar. The energy and flux of the precipitating electrons, volume emission rates and local electric fields in the ionosphere have been determined at high temporal (up to 30 Hz) and spatial (down to tens of metres) resolution. A new time-dependent inversion model is used to derive energy spectra from EISCAT electron density profiles. The energy and flux are also derived independently from optical emissions combined with ion-chemistry modelling, and a good agreement is found. A robust method to obtain detailed 2-D maps of the average energy and number flux of small scale aurora is presented. The arcs are stretched in the north-south direction, and the lowest energies are found on the western, leading edges of the arcs. The large ionospheric electric fields (250 mV m −1) found from tristatic radar measurements are evidence of strong currents associated with the region close to the optical arcs. The different data sets indicate that the arcs appear on the boundaries between regions with different average energy of diffuse precipitation, caused by pitch-angle scattering. The two thin arcs on these boundaries are found to be related to an increase in number flux (and thus increased energy flux) without an increase in energy.