Omega Bands Research Articles

Improved electrodynamic modeling of an omega band and analysis of its current system

We improve a previous electrodynamic model of an omega band by applying a nonuniform spatial distribution of the Hall to Pedersen conductance ratio instead of using a constant value. The analysis of the current system due to our model reveals that after separating the background westward electrojet off, the omega band currents mainly consist of Hall currents circulating around concentrated upward field‐aligned currents in the auroral tongue. However, the upward field‐aligned currents exhibit a nearly circular configuration around the center of the tongue and almost vanish directly in the center. Equally strong downward field‐aligned currents are missing. Additionally, at the western flank of the tongue we notice an about northwest–southeast aligned Cowling channel. Its currents are mainly responsible for the spike‐like D peak in magnetograms and for the general asymmetry in D and Z longitudinal profiles on the ground. An additional enhancement of the Hall conductance in this channel further improves the agreement between calculated and measured ground magnetic disturbance profiles.

Field‐aligned currents associated with spatially periodic X ray structures in the morningside auroral oval

On February 9, 1992, the Atmospheric X ray Imaging Spectrometer (AXIS) on the Upper Atmosphere Research Satellite (UARS) observed spatially periodic structures in X ray luminosity from the top of the atmosphere during a pass over the dawnside auroral oval. The patches of luminosity were several hundred kilometers in size and separated by a mean distance of about 700 km. As many as a dozen distinct patches could be identified extending from local times just after midnight to the noon meridian. The characteristics of the precipitating electrons responsible for the patches were determined from the measured X ray fluxes. Peak electron energy fluxes were between 4 and 10 ergs cm−2 s−1 with e‐folding energies of 10–25 keV. In situ magnetic field data from the onboard magnetometer were used to model the field‐aligned currents associated with two of the patches. The model that best fit the data had upward currents in the western portions of the patches and more intense downward currents confined to the eastern edges. The modeling also indicated that these small‐scale currents were embedded within the large‐scale region 2 field‐aligned current sheet. Data from a ground‐based magnetometer near the satellite ground track indicated the presence of Ps 6 pulsations. We have interpreted the observations in terms of eastward‐drifting current patch systems. Under this assumption, the patches moved at 1.7 km s−1, the drift speed of ∼30‐keV electrons. This motion, and the correlation with Ps 6 pulsations suggests that the X ray patches were associated with omega bands that are seen in the morning sector during geomagnetically active times.

Direct determination of the local ionospheric Hall conductance distribution from Two‐dimensional electric and magnetic field data: Application of the method using models of typical ionospheric electrodynamic situations

The applicability of a method to directly deduce the ionospheric Hall conductance distribution ΣH from ground magnetic and ionospheric electric field observations, here called “method of characteristics”, is tested by using input data from models of some typical ionospheric electrodynamic situations. We shall show that the method in the form of a fully automatical computer algorithm is able to reproduce well the ΣH distributions of all modeled situations, i.e. a two‐dimensional eastward electrojet, a Harang discontinuity, an omega band, and a westward traveling surge. Furthermore, we will show quantitatively that the ambiguity implied by the necessary assumption of the distribution of the Hall to Pedersen conductance ratio has only a small effect on the results obtained. We also prove quantitatively that the assumption of vertical, straight geomagnetic field lines made in the derivation of the method leads only to very small errors if the case of oblique, but straight geomagnetic field lines is taken to be realistic. Moreover, we review the general theory of the method and discuss some additional theoretical aspects. In particular, we will show that isolated points with , but nonvanishing are associated with extrema or saddle points of the ΣH distribution, and calculate the magnetic field disturbance of an ionospheric current system with oblique, but straight field‐aligned currents directly below the ionospheric plane. The method does not require an electrostatic situation; i.e., is allowed.

A Particle Simulation of Auroral Omega Bands and Torch-Like Structures.

The mechanism for omega bands and torch structures as observed on the poleward boundary of the postmidnight diffuse aurora is studied by a two-dimensional electrostatic particle simulation for the motion of magnetospheric plasma perpendicular to the geomagnetic field. This type of diffuse aurora is caused by precipitation of energetic electrons with energies in excess of a few keV. In the model, the diffuse aurora (which may be identified with the central plasmasheet) is embedded within a less energetic but hot plasma which extends to higher latitudes. The temporal evolution of the diffuse auroral pattern can be visualized by following the cluster of the energetic electrons. Electric dipoles can grow on the poleward boundary of the hot plasma, due to charge separation induced by magnetic drifts. As a result of these dipoles, there is a spatial modulation of the energetic electrons, which leads to the formation of omega bands and the subsequent evolution into torch structures. The magnetospheric (or ionospheric) potential distribution obtained in the simulation is consistent with observed distribution of ionospheric electric fields and field-aligned currents in events of torches/omega bands.

Studies of polar current systems using the IMS Scandinavian magnetometer array

As a contribution to the International Magnetospheric Study (IMS, 1976–1979) a two-dimensional array of 42 temporary magnetometer stations was run in Scandinavia, supplementary to the permanent observatories and concentrated in the northern part of the region. This effort aimed at the time-dependent (periods above about 100 s) determination of the two-dimensional structure of substorm-related magnetic fields at the Earth's surface with highest reasonable spatial resolution (about 100 km, corresponding to the height of the ionosphere) near the footpoints of field-aligned electric currents that couple the disturbed magnetosphere to the ionosphere at auroral latitudes. It has been of particular advantage for cooperative studies that not only simultaneous data were available from all-sky cameras, riometers, balloons, rockets, and satellites, but also from the STARE radar facility yielding colocated two-dimensional ionospheric electric field distributions. In many cases it therefore was possible to infer the three-dimensional regional structure of substorm-related ionospheric current systems. The first part of this review outlines the basic relationships and methods that have been used or have been developed for such studies. The second short part presents typical equivalent current patterns observed by the magnetometer array in the course of substorms. Finally we review main results of studies that have been based on the magnetometer array observations and on additional data, omitting studies on geomagnetic pulsations. These studies contributed to a clarification of the nature of auroral electrojets including the Harang discontinuity and of ionospheric current systems related to auroral features such as the break-up at midnight, the westward traveling surge, eastward drifting omega bands, and spirals.

The recovery phase of substorms

A special SPA: Magnetospheric Physics session dedicated to the recovery phase of substorms is being planned for the AGU 1992 Fall Meeting. Substorms have provided an important focus for magnetospheric research for nearly 3 decades, and continue to do so today. Of the three phases (growth, expansive, and recovery) that constitute a substorm, the growth and expansive phases have received the most attention, and a fairly good understanding of them has developed.In contrast, the recovery phase has received scant attention. Perhaps that is because it lacks the dramatic auroral and geomagnetic effects that mark the expansive phase at Earth and the similarly impressive particle and magnetic field effects that mark the growth phase to expansive phase transition at geosynchronous orbit. The recovery phase was originally defined as the period when auroras, having advanced far toward the pole during the expansive phase, become less intense and retreat back toward the auroral oval. Wavy structures—omega bands—and pulsating intensities often characterize the recovery phase auroras. It has become customary, also, to define the recovery phase as that period when the Auroral Electrojet (AE) index is subsiding from the peak value it reached during the substorm. These two definitions, probably not altogether compatible, may identify somewhat different periods of a substorm. There is no obvious feature in geosynchronous observations that has been specifically identified with the recovery phase.

Limbic system dysrhythmia: a diagnostic electroencephalogram procedure utilizing procaine activation

A diagnostic method is presented that makes it possible to distinguish patients who are most likely to show a positive response to treatment with anticonvulsant medication, thereby cutting across many DSM-III-R diagnoses. Patients are evaluated to determine whether they exhibit at least 4 of 12 groups of symptoms, and the local anesthetic procaine is used, along with an electroencephalogram (EEG) to evaluate for omega band activity (30-50 Hz) of at least 50 microvolts or approximately three times baseline values in the anterior temporal leads. This method was studied in 145 patients with varying diagnoses. Eighty-three percent of patients who were symptom- and procaine-positive responded to anticonvulsants. Specific application to patients with attention-deficit hyperactivity disorder, bipolar disorder, and panic disorder is made. This method may provide a basis for identifying subpopulations of anticonvulsant-responsive patients who often are considered treatment-resistant.

Simultaneous observations of omega band related phenomena in both hemispheres

In Antarctica, magnetic Ps6 pulsations have been observed simultaneously at a number of stations together with all-sky imagery of omega bands at Sanae. For the same nights, in the northern hemisphere, all-sky imagery from Sodankylä and Kilpisjärvi and STARE radar data are reported. These observations allow us to infer that omega band related phenomena occur on the same day in both hemispheres after local magnetic midnight. In the southern hemisphere these phenomena were observed over a range of 65 longitude between about L = 4 and L = 6. Interestingly, the activity switched on simultaneously over the range of longitudes. For the northern hemisphere observations were only available for a range of approximately 7 longitude between L = 5 and L = 6.

Auroral Signatures of Magnetospheric Substorms and Constraints which They Provide for Substorm Theories

Magnetospheric substorms reflect the most pronounced periods of energy transfer from the solar wind to the magnetosphere and ionosphere of the Earth. Modern views of the substorm process have been substantially modified in the past few years by information inferred from analysis and interpretation of auroral images acquired by the UV imager flown aboard the Viking satellite. In this paper, I shall summarize some of the findings from the ongoing study of the Viking data base dealing with the nature of auroral surges in the evening sector and omega bands in the morning sector. These auroral forms reflect the growth of instabilities in the magnetosphere-ionosphere system and provide important information on the properties of the geospace environment in which the free energy to drive the instabilities is found. Substorms were originally defined using ground based arrays of detectors and any theory of substorms must account for the temporal and spatial properties of auroral forms. Any theory that does not address these questions is incomplete. The information presented in this paper therefore provides constraints which are necessary to incorporate into any theory of magnetospheric substorms.

The neutral E region zonal winds during intense postmidnight diffuse aurora: Response to observed particle fluxes

The neutral response to the postmidnight diffuse aurora was calculated using observed fluxes of electron precipitation and a high‐resolution, time‐dependent numerical model. In an earlier work, based on an idealized representation of auroral precipitation, we simulated a dramatic supersonic E region jet stream. We suggested that hydrodynamical shear instabilities of this jet were responsible for eastward propagating features on the poleward edge of the diffuse aurora called omega bands. The results of the present study are qualitatively similar to the results of the previous study: A very strong dynamically unstable E region jet is simulated for an electric field of 50 mV m−1, and the results suggest a connection with omega bands. The winds are not quite as strong as previously simulated; zonal wind speeds of ∼600 m s−1 are achieved in ∼2 hours in the present study, whereas comparable wind speeds are achieved in ∼1 hour in the earlier study. After 1 hour, the peak winds reached are ∼400 m s−1 in the present study. The smaller acceleration for the present simulations is primarily a consequence of a smaller total energy flux in the observed fluxes than in the idealized ones. The zonal winds which we simulate are 2–3 times stronger than those previously simulated for the diffuse aurora elsewhere by a model of similar complexity. We have evaluated the effects of large‐scale cross‐arc winds, which were lacking in our earlier study, and found that strong cross‐arc winds are required to prevent the formation of a strong E region zonal jet. Our present results indicate that supersonic jets are possible, but are not apt to occur very often.

High time-resolution imaging of auroral arc deformation at substorm onset

Observations of auroral disturbances such as westward-travelling surges, omega bands and auroral folds, are by necessity usually made when the luminous features have acquired their characteristic geometry, i.e. after the actual formation process. We demonstrate an event in which the initial brightening of an arc developed into a series of auroral spirals and finally into a westward-travelling surge. Two intervals of spiral formation were observed, but only the second of these generated a surge-like feature. Since the surge formation occurred on a time-scale of a few seconds, this type of observation can only be made with high time-resolution imaging devices. The auroral developments took place in the zenith above Kiruna, where they were recorded by ground-based monochromatic imagers. The subsequent poleward expansion of the activity was observed by the EISCAT incoherent scatter radar 200 km to the North of the initial activation. These observations are discussed in the light of existing substorm theories, and it is proposed that the instability responsible for the final break-up of the arc is self-destructive.

Characteristics of auroral electron precipitation on the morningside

Quantitative information on auroral electron precipitation is needed to evaluate the effects of the precipitation on the ionosphere and upper atmosphere. In this paper we present such information for the morningside aurora. We use simultaneous observations of precipitating electrons and auroral images from the low‐altitude, polar‐orbiting DMSP‐F6 satellite, and we relate specific precipitation features to specific types of auroras. We find the equatorward region of diffuse aurora to have precipitating electron energy fluxes, εp, which typically reach 10 ergs/cm² s, and such fluxes can extend over several degrees in latitude. Energy spectra over the diffuse aurora are quite hard and show no evidence of acceleration by a field‐aligned potential drop, V∥. The poleward region of structured aurora includes a soft background of relatively weak, uniform precipitation. Arclike structured precipitation results from a variable V∥ acceleration of this background. Over the arcs, εp can exceed 1 erg/cm² s. Bursts of precipitation at energies above a few keV are identifiable in the data equatorward of the peak in the diffuse auroral precipitation. We believe these bursts are associated with pulsating auroral patches. Omega band waves are occasionally observed along the poleward boundary of the diffuse aurora. Electron spectra over the omega bands are unique for the morningside. The spectra show hard precipitation as observed within the diffuse region. In addition, the spectra have a peak at energies generally ≲1 keV indicative of acceleration by a V∥ ≲ 1 keV.

Correction to “Generation of auroral omega bands by shear instability of the neutral winds”

Journal of Geophysical Research: Space PhysicsVolume 91, Issue A6 p. 7138-7138 CorrectionsFree Access Correction to “Generation of auroral omega bands by shear instability of the neutral winds” L. R. Lyons, L. R. LyonsSearch for more papers by this authorR. L. Walterscheid, R. L. WalterscheidSearch for more papers by this author L. R. Lyons, L. R. LyonsSearch for more papers by this authorR. L. Walterscheid, R. L. WalterscheidSearch for more papers by this author First published: 1 June 1986 https://doi.org/10.1029/JA091iA06p07138AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume91, IssueA61 June 1986Pages 7138-7138 RelatedInformation

Wave characteristics of Ps 6 magnetic variations and their implications for convective flow in the magnetotail

Ps 6 magnetic variations are fluctuations with periods of 10–30 min and amplitudes of tens to hundreds of nT observed in the dawn sector auroral oval during substorm activity. The causative current system appears to be associated with auroral omega bands which follow the substorm breakup, and drift eastward from the midnight sector with typical velocities of 0.5–2.0 km s −1 . Quadrature techniques are applied to Ps 6 waveforms to separate their group and phase velocities, and to obtain their wave characteristics. The Ps 6 phenomenon seems to be confined to the 03:00–09:00 L.T. sector, and over the range of longitudes involved, its wave properties are relatively constant in the ionosphere. The group velocity has an average value of 1.0 km s −1 and, on the average, marginally exceeds the phase velocity by about 15% ; both these velocities are directed eastward. When the Ps 6 wave parameters observed in the ionosphere are mapped to the equatorial magnetospheric plane, the wavelength of the disturbance shows, on an average, values ranging from ∼47 Re for the 03:00–04:00 L.T. sector to ∼12 Re for the 08:00–09:00 L.T. sector; the phase velocity changes from ∼ 300 km s −1 in the deep magnetotail to ∼ 20 km s −1 near the dawn meridian. Assuming the disturbance to originate through a Kelvin-Helmholtz instability, at the interface between the low-latitude boundary layer (LLBL) and the central plasma sheet (cps), the width of the momentum shear zone narrows from ∼ 4.5 R E at 03:00–04:00 L.T. to ∼ 1.2 R E at 08:00–09:00 L.T. The inferred large sunward convection velocity of ∼ 300 km s −1 for plasma in the 03:00 L.T. region of the LLBL/cps interface is interpreted as confirmation of magnetic field line reconnection at a neutral line antisunward of the region of formation of Ps 6 in the magnetotail.

Generation of auroral omega bands by shear instability of the neutral winds

Thermospheric neutral wind acceleration via ion drag in the conducting E‐region of the ionosphere is greatly increased by electron precipitation associated with auroras. This increased acceleration can lead to the development of significant horizontal wind shears, which we find are unstable to the Kelvin‐Helmholtz shear instability. Numerical simulation of the neutral response to an intense, postmidnight, diffuse aurora shows the formation of an E‐region “jet stream” within the aurora, with peak winds speeds >700 m/s after one hour. We propose that this jet stream produces unstable Kelvin‐Helmholtz waves, which can drive waves of discrete aurora along the poleward boundary of the preexisting diffuse aurora. We suggest that such auroral waves, driven by the neutral winds, form eastward propagating waves (omega bands) occasionally observed along the poleward boundary of postmidnight diffuse auroras. We also find neutral wind shears that develop in response to discrete auroral arcs to be unstable; however we do not expect the resulting wind waves to drive significant auroral waves along discrete arcs.

Basic science and clinical aspects of procaine HCl as a limbic system excitant

1. The literature in animals and humans which indicate that systemic procaine HCl activates limbic tissue is reviewed. Studies in cats which suggest that procaine excites limbic cells by reducing neural inhibition are then described. 2. Evidence that power spectral analysis of high frequency EEG bands (omega or 31–55 cps) in the temporal cortical EEG reflects degree of limbic (amygdala) excitation in animals and humans is reviewed. Studies in cats are described which show that procaine selectively increases omega band activity in the amygdala and temporal cortex in a dose related fashion which parallels dose related increases in amygdaloid neural activity. 3. Preliminary results of combining intravenous procaine and omega band analysis of scalp EEG in humans to predict therapeutic response to carbamazepine in borderline personality and affective disorder patients are then described. The effects of procaine on omega are compared to the effects of direct electrical stimulation of human limbic system in complex partial seizure patients undergoing assessment for temporal lobectomy. The results tentatively support the hypothesis that some psychiatric patients have hyperexcitable limbic systems, and those that do, show a positive behavioural response to carbamazepine.

Basic science and clinical aspects of procaine HCL as a limbic system excitant

1. The literature in animals and humans which indicate that systemic procaine HC1 activates limbic tissue is reviewed. Studies in cats which suggest that procaine excites limbic cells by reducing neural inhibition are then described. 2. Evidence that power spectral analysis of high frequency EEG bands (omega or 31–55 cps) in the temporal cortical EEG reflects degree of limbic (amygdala) excitation in animals and humans is reviewed. Studies in cats are described which show that procaine selectively increases omega band activity in the amygdala and temporal cortex in a dose related fashion which parallels dose related increases in amygdaloid neural activity. 3. Preliminary results of combining intravenous procaine and omega band analysis of scalp EEG in humans to predict therapeutic response to carbamazepine in borderline personality and affective disorder patients are then described. The effects of procaine on omega are compared to the effects of direct electrical stimulation of human limbic system in complex partial seizure patients undergoing assessment for temporal lobectomy. The results tentatively support the hypothesis that some psychiatric patients have hyperexcitable limbic systems, and those that do, show a positive behavioural response to carbamazepine.

Characteristics of eastward drifting omega bands in the morning sector of the auroral oval

The STARE radars and the Scandinavian networks of magnetometers, all‐sky cameras, and riometers recorded during the night of October 21/22, 1979, the occurrence of a fairly regular sequence of auroral omega bands and associated magnetic and electric field variations. The combined two‐dimensional data are used to derive a realistic model for the three‐dimensional current flow associated with the auroral forms. In the model calculations the observed structure in the particle precipitation is accounted for by an inhomogeneous ionospheric conductivity distribution. The main resulting feature of the model current system is a sequence of east–west orientated pairs of upward and downward directed field‐aligned currents, associated with the bright and dark areas of the visual aurora, respectively. The major source of magnetic disturbances on the ground is a “meandering” ionospheric Hall current, composed of a westward background electrojet and circular Hall current vortices around the locations of the localized field‐aligned currents. The total magnetic disturbance observed on the ground during different events appears, however, to be strongly dependent on the Hall to Pedersen conductivity ratio and the degree of inhomogeneity in the conductivity distribution. The three‐dimensional current system associated with the auroral omega bands drifts eastward with a velocity comparable to an E×B drift within the general southward directed electric background field. However, complete agreement was not found at all times.

Ionospheric and field-aligned current systems in the auroral zone: a concise review

During the last few years our knowledge about the real three-dimensional current flow in the auroral zone has been significantly increased due to new improved measurements, especially those made by ground-based magnetometer networks, coherent and incoherent auroral radars, sounding rockets and low-altitude satellites. Combination of two or even more of those data sets (e.g. electron densities and electric and magnetic fields) allowed for a rather accurate determination of the distribution of Hall, Pedersen and Birkeland currents in the auroral zone. In this review an attempt is made to summarize the present knowledge about the distribution of conductivity, electric field and current flow in the auroral zone as well for the large-scale electrojet systems as for the comparatively smaller current systems associated with quiet and active aurora, i.e. discrete arcs, auroral break-ups, westward travelling surges and omega bands.

On the distribution of global auroras during intervals of magnetospheric quiet

Defense Meteorological Satellite Program (DMSP) satellite data have been used to derive the instantaneous distribution of auroras during intervals of very quiet magnetic activity. The data were ordered with respect to the magnitude of the auroral electrojet index AE. Results indicate that the size of the instantaneous auroral oval is highly sensitive to relatively small changes in the magnitude of AE. Analysis of the DMSP auroral imagery data suggests that contracted oval substorms are distinct from the more violent substorms observed along the expanded oval in terms of both intensity and morphology. Certain morphological features associated with expanded oval substorms in the morning sector were not observed in this data set. These features include omega bands, torchlike structure, and, to a certain extent, patchy auroras. Sun-aligned arcs in the polar cap were observed on numerous orbits and were most frequent when the magnetic field disturbance was minimal. Finally, several near-simultaneous orbits of the DMSP and Isis 2 satellites were compared to establish a threshold sensitivity for the DMSP detector system. For the noon-midnight DMSP satellite during late 1972 the equivalent threshold sensitivity at 3914 Å was a few kilorayleighs.