Reconstruction of Fine-Scale Auroral Dynamics

Earth, Planets and Space, 2015

Auroral microphysics still remains partly unexplored. Cutting-edge ground-based optical observations using scientific complementary metal-oxide semiconductor (sCMOS) cameras recently enabled us to observe the fine-scale morphology of bright aurora at magnetic zenith for a variety of rapidly varying features for long uninterrupted periods. We report two interesting examples of combinations of fine-scale rapidly varying auroral features as observed by the sCMOS cameras installed at Poker Flat Research Range (PFRR), Alaska, in February 2014. The first example shows that flickering rays and pulsating modulation simultaneously appeared at the middle of a surge in the pre-midnight sector. The second example shows localized flickering aurora associated with growing eddies at the poleward edge of an arc in the midnight sector.

Journal of Geophysical Research, 2001

For the first time we produce high-resolution maps of the characteristic energy of precipitating electrons from ground-based instrumentation in the auroral zone over northern Scandinavia. This is done by combining intensity-calibrated optical data at 557.7 nm from the Digital All-Sky Imager (DASI) with auroral absorption images from the Imaging Riometer for Ionospheric Studies (IRIS). Energy maps are produced with high temporal (10 s) and spatial (10 km) resolution within a common geographic area of 240 ϫ 240 km. Both IRIS and DASI have the European Incoherent Scatter (EISCAT) radar within their common field of view. EISCAT is capable of making accurate measurements of the electron density height profile which, with the assistance of an atmospheric model, are inverted into equivalent energy spectra of the flux of precipitating electrons. However, incoherent scatter radars generally have a very small field of view (Ͻ1Њ), making studies of the energy spectrum of the precipitating particles over a wide field of view impractical. Since IRIS and DASI are sensitive to high-and medium-energy electrons, respectively, EISCAT data are used to calibrate the characteristic energy of the precipitating particles for an assumed energy spectrum against a combination of IRIS and DASI data. This empirical calibration is then used throughout the common field of view of IRIS and DASI. An initial study illustrates the spatial relationship between the different energy ranges during a substorm onset and illustrates a new way to interpret auroral phenomena. 1984; Penman et al., 1979]. However, satellite data do not have the spatial resolution in the ionosphere required for detailed studies of the spatial morphology of the precipitating particle energies, which is possible from the ground.

Space Weather, 2013

1] The strong horizontal ionospheric currents in the auroral oval constitute an important space weather parameter. Here we present a method to estimate the latitude location and intensity of these currents from measurements of variations in the magnetic field magnitude made by low Earth polar orbiting satellites. The method is simple enough to be implemented for real-time monitoring, especially since it does not require the full vector field measurement. We demonstrate the method on 5 years of Challenging Minisatellite Payload (CHAMP) data and show how the monitoring depends on the local time of the satellite orbit and how it varies with local time and season in both hemispheres. Statistically, the strongest currents are observed in the predawn and predusk local time quadrants at latitudes that depend on the general magnetic activity level. We also show how the satellite-derived parameters relate to and complement existing ground-based indices. The CHAMP magnetometer in 350-450km altitude easily measures an electrojet which on the ground would produce an Auroral Electrojet (AE)-type signal as small as 20 nT. Thus, while the signal decreases roughly proportionally to the square of the distance to the current, this does not significantly affect the utility of the method for space weather applications even for satellites at substantially higher altitudes. The results for several individual magnetic storm periods demonstrate that large variability can exist in both the latitude and intensity of the currents during the progression of a storm. In the storms analyzed, the latitude of the strongest observed currents are seen to vary between 52 ı and 84 ı magnetic latitude.

Geophysical Research Letters, 2000

We analyze simultaneous visible, ultraviolet (UV), and X-ray auroral images of an isolated substorm on January 25, 1998. The total precipitating electron energy flux and mean energy extracted near substorm maximum from each of the imagers are in acceptable agreement. The expansion phase visible, UV, and Xray emissions are morphologically similar, with the brightest emissions coming from the auroral surge. However, during the recovery phase, there are regions of bright X-ray emissions near local midnight and dawn that do not correspond to obvious features in the visible and UV images. X-ray energy analysis in these regions reveals a high energy (∼30-100 keV) component of the precipitating electron population that contributes ∼30% of the total electron energy flux. The temporal development of the X rays indicates that the dawn emissions originate in a drifting electron population, while the newly revealed midnight emissions originate in a continuous injection of energetic electrons in a narrow local time region.

Journal of Geophysical Research, 2003

1] The FUV imager on board the IMAGE satellite provides simultaneous images of the north polar aurora every 2 min in three spectral channels. The Wideband Imaging Camera (WIC) responds primarily to the N 2 LBH bands while one of the Spectral Imagers (SI13) includes the OI 135.6 nm emission and nearby LBH bands. The third channel (SI12) is sensitive to Doppler-shifted Lyman-a emission at 121.8 nm generated by proton precipitation. The relative magnitude of the WIC and SI13 signals depends on the altitude distribution of the energy deposition, in response to the differential O 2 absorption and the altitude dependence of the neutral composition. The ratio of simultaneous images from WIC and SI13 is used to derive the spatial distribution of the characteristic energy of the precipitating auroral electrons and the energy flux they carry. The method is described and the uncertainties introduced by possible perturbations of the neutral composition known to occur in the auroral thermosphere are discussed. The first part of this study describes a validation of this method performed by comparing precipitation characteristics derived from FUV with in situ measurements from two coincident passes of the NOAA-16 satellite. They are shown to agree within about 45%. The second part applies this ratio method to analyze the time evolution of auroral activity which occured during two substorms on 28 October 2000. The time evolution is displayed in the form of magnetic local time and magnetic latitude keograms. It is shown that the pattern of the electron average energy distribution exhibits both spatial and temporal changes. Comparison with FAST in situ electron spectrograms confirms the ability of IMAGE to detect precipitation events with a $200 km spatial scale. However the characteristics of the physical process leading to electron acceleration cannot be identified with FUV. The highest values of the average energy are colocated in time and space with the largest electron precipitation fluxes. A dawnward motion of bright features is observed in the postmidnight at speeds on the order of 5 magnetic local time hours/UT hour.

Journal of Atmospheric and Solar-terrestrial Physics, 2009

The Poker Flat Incoherent Scatter Radar (PFISR) is the first dedicated ISR built with an electronically steerable array. This paper demonstrates the capabilities of PFISR for producing three-dimensional volumetric images of E-region ionization patterns produced by the aurora. The phase table was configured to cycle through 121 beam positions arranged in an 11 Â 11 grid. A 13-baud Barker coded pulse was used, which provided $1:5-km range resolution out to a maximum range of 250 km. Backscattered power was converted to electron density by correcting for path loss and applying the Buneman approximation assuming equal electron and ion temperatures. The results were then interpolated onto a three-dimensional cartesian grid. Volumetric images are presented at 5-min, 1-min, and 14.6-s integration times (corresponding to 960, 192, and 48 pulses-per-position, respectively) to illustrate the tradeoff between spatio-temporal resolution and data quality. At 14.6 s cadence, variability in plasma density within the volume appears to be fully resolved in space and time, a result that is supported by both observational evidence and theoretical predictions of ionospheric response times. Some potential applications of this mode for studying magnetosphere-ionosphere interactions in the auroral zone are discussed.

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.

Geophysical Research Letters, 1998

Electric field and energetic particle observations by the Fast Auroral Snapshot (FAST) satellite provide convincing evidence of particle acceleration by quasi-static, magnetic-fieldaligned (parallel) electric fields in both the upward and downward current regions of the auroral zone. We demonstrate this by comparing the implied parallel potentials of electrostatic shocks with particle energies. We also report nonlinear electric field structures which may play a role in supporting parallel electric fields. These observations include large-amplitude ion cyclotron waves in the upward current region, and intense, spiky structures in the downward current region.

Journal of Geophysical Research, 2010

Measurements obtained with the electronically steerable Poker Flat Incoherent Scatter Radar (PFISR) and collocated all-sky camera were used to construct composite images of ionospheric convective flows and auroral forms during a substorm cycle (onset 26 March 2008, 1146 UT). PFISR was configured to sample an array of 5 × 5 regularly spaced beams on a pulse-by-pulse basis, from which velocity vectors were computed via statistical inversion of groups of beams. Flow fields were resolved at 30 km spatial resolution at 2 min temporal resolution over a 100 × 100 km field and then geographically registered with all-sky imagery recorded at 20 s cadence. An analysis of the composite images has revealed interesting contrasts between growth-, expansion-, and recoveryphase auroras, for example, (1) anticorrelation of ion velocity (electric field) and luminosity (plasma density, hence, conductance) in both space and time during growth phase and expansion phase; (2) identical flow (magnitude and direction) inside and outside the aurora during recovery phase; (3) a large tangential flow component along auroral boundaries during both growth and recovery phase (consistent with electric field directed into the aurora), irrespective of the orientation of the arc boundary; and (4) large relative drift (∼2 km/s) between auroral forms and convective flow during recovery phase. These features are interpreted in the context of previous ground-based and space-borne observations. Future PFISR experiments are expected to enable flow field construction at 30 s cadence, which will resolve Alfvén transit time dynamics to putative substorm initiation regions and significantly clarify the observations presented herein.

Applied Optics, 1998

A large throughput transmission spectrometer, with a grating on a prism as the diffraction element, has been developed to study altitude distributions of auroral emissions. The imaging spectrometer disperses spectrally in one dimension while spatial information is preserved in the orthogonal direction. The image is projected onto a CCD array detector. Image processing methods have been developed to calibrate for wavelength, uniform field, spectral sensitivity, curvature of field, and spatial mapping. Single images are processed to represent a measured signal brightness in a unit of Rayleighs͞pixel, from which area integrations can be made for desired spatial-spectral resolution. System performance is ϳ1.5-nm resolution over a 450-nm bandwidth ͑420-870 nm͒. Two spectrometer systems of this design were operated simultaneously, one with additional optical instruments and an incoherent scatter radar at Sondrestrom, Greenland, and the other at Godhavn, Greenland, which lies 290 km to the northwest and nearly in the magnetic meridian of Sondrestrom. The developed system, calibration method, and examples of performance results are presented.