Auroral Radar Echoes Research Articles

A model of secondary Farley‐Buneman waves in the auroral electrojet

We present a model of type II radar echoes associated with Farley‐Buneman instability in the auroral E region. The plasma waves responsible for broad‐type auroral radar echoes are described as secondary ion acoustic structures driven by the electric fields of the primary Farley‐Buneman waves. Numerical solutions of a one‐dimensional model of the secondary structures exhibit density irregularities advecting in the background electron flow direction while propagating and evolving in the transverse direction. The velocity of transverse propagation depends on amplitude, a circumstance that leads to nonlinear proper evolution in the form of compression or steepening. On the basis of symmetry, radar Doppler spectra should exhibit spectral shifts and widths proportional to the cosine and sine of the electron flow angle, respectively. This prediction is in line with a number of theoretical and experimental observations.

Aspect angle dependence of the radar aurora Doppler velocity

The aspect angle dependence of the Doppler velocity of radar auroral echoes was studied by means of a comparison between the signals received simultaneously on four 50‐MHz CW links. In most of the events, the observed Doppler velocity decreased with increasing aspect angle but the rate of decrease was lower than that predicted by linear theory. The velocity decrease was about a factor of 0.4 between geometrical aspect angles of 1.5° and 4.0° and 0.6 to 0.7 between 3.2° and 5°. A model for the Doppler velocity behavior of the radar auroral signals is proposed, combining both propagation and scattering effects. Allowance is made for “ray spreading” arising from processes like refraction and diffraction during propagation. In addition, height integration over the scattering volume is taken into account.

Lifetime measurements of auroral scatterers

A method for directly measuring the lifetimes of auroral radar scatterers from continuous wave time series is introduced. This method depends on the sharp phase and/or amplitude changes which occur between individual scattering bursts. Six different periods, with examples of type I, type III and type IV radar auroral echoes, were examined to find scatterer lifetimes. Two main observations were noted. Frequently, there were scatter signal bursts with no phase or amplitude discontinuities, indicating the presence of only a single coherent scattering region or a small number of such regions. Second, the lifetimes of the scatterers were strongly anticorrelated with their Doppler speeds.

Ion cyclotron harmonics in auroral radar echoes: Real effect or analysis artifact?

Certain analyses of Canadian radar studies of auroral electrojet echoes have apparently shown spectra with multiple ion cyclotron harmonic peaks. On the basis of these, it has been argued that ion cyclotron effects must be important for all echo types seen by VHF radars, not just type 3. We reject this conclusion for two reasons: (1) at the altitude of auroral backscatter (∼ 100–120 km) the ion‐neutral collision frequency is much higher than the gyro frequency—ions are essentially unmagnetized; (2) we show that the “high resolution” signal processing algorithm employed does not distinguish between echoes with and without harmonic structure. With a simulation, we show that the algorithm itself can impose apparent quasi harmonic structure onto the spectrum of random noise.

Radar observations of ion cyclotron waves associated with two barium shaped‐charge releases

A 50‐MHz Doppler radar interferometer and a 138‐MHz Doppler radar were operated from Kennedy Space Center to study 3‐m and 1‐m plasma waves associated with two shaped‐charged barium releases from Wallops Island, Virginia, on May 13, 1986. During the first release, interferometer and Doppler power spectral studies showed the existence of short‐lived (<2 s) coherent 3‐m and 1‐m waves centered near 30 Hz. The 3‐m waves were located about 100 km above the release on a magnetic field line in contact with the first release. No radar backscatter was observed by the radars during the second release. The coherent 30‐Hz radar echoes associated with the first barium release are very similar to some natural occurring auroral radar echoes that are believed to be generated by resistive current‐driven electrostatic ion cyclotron waves. A detailed linear dispersion analysis of electrostatic ion cyclotron (EIC) waves showed that only 3‐m O+ EIC waves were unstable for field‐aligned electron drifts greater than 0.7υthe at the altitude of 510 km in a multispecies (O+, NO+, or similarly O2+) ionospheric plasma. We interpret the 30‐Hz waves seen by the two radars far above the release as strong electrostatic ion cyclotron waves generated by intense field‐aligned currents associated with the barium stream acting like an MHD generator coupled to the ionosphere.

Doppler spectrum observations of long discrete echoes

By Long Discrete Echoes (LDE) we refer to an uncommon class of radar auroral echoes observed in regions of low electric fields equatorward of the electrojet, mostly in the early morning time sector. This phenomenon has been reported first by Nielsen et al. (1982, Nature 299, 238). In this paper we present further experimental evidence on LDE based on Doppler spectrum measurements. We found the spectral properties of echoes to differ significantly as compared to the commonly observed backscatter associated with typical auroral electrojet activity. The main observations are summarized as follows: (1) the spectra are confined at sub-ion acoustic velocities and are very narrow, in sharp contrast with the broad spectra of typical auroral echoes; (2) the signal strength and spectrum width are isotropic in the observing plane; and (3) the spectrum width tends to increase linearly with drift velocity magnitude. These results suggest the presence of only one type of irregularity which is isotropic in the propagation plane perpendicular to the magnetic field.

High Latitude E-Region Irregularities: New Results

Radar observations have shown that the phase velocity of small scale (a few meters and smaller) auroral two stream waves remains close to the ion acoustic speed. However, during periods of very large plasma drifts the electron temperature, and consequently, the ion acoustic speed in the auroral electrojet region is enhanced possibly by strongly driven two stream waves. Recent nonlinear theories can explain the large anomalous electron heating as well as the variation of the two stream phase velocity. However, the observations of plasma waves at angles larger than a few degrees from the perpendicular to the magnetic field remain unexplained. Radar interferometer measurements show that during very active periods strong echoes are often observed from highly localized and dynamic scattering regions. On these occasions, the two stream waves have phase velocities which can exceed 900 m/s, and the gradient drift waves have very broad spectra with mean phase velocities sometimes larger than 600 m/s. Furthermore, resonant (i.e., sharply peaked) auroral radar echoes, with Doppler shifts independent of the ambient cross field plasma drift are often present near the edges of auroral arcs. These waves have been associated with ion cyclotron waves driven by very large field aligned drifts in the upper E-region. This paper will review recent experimental results on the study of meter scale plasma waves in the auroral E-region, particularly during highly disturbed periods, and will also discuss the physical processes responsible for their generation and saturation.

Radio aurora observations at medium latitude over two solar cycles

Quasi-continuous radio aurora observations have been made at Kuehlungsborn. D.D.R. ( φ = 54°N) by a 33 MHz radar during the period 1958–1981. From these observations characteristic diurnal, seasonal and solar cycle variations of the probability of occurrence of auroral radar echoes at medium latitude have become evident. Whereas the solar cycle variation of auroral activity is very similar to that of the geomagnetic activity, this is only partly true for the seasonal variation, and not at all for the diurnal variation. The seasonal variation is slightly modified in addition by a tendency to higher aurora probability in winter than at the same level of geomagnetic activity in summer, and the diurnal variation is almost totally determined by the diurnal variation of the horizontal electric field in the ionosphere. Also, an influence of the sector structure of the interplanetary magnetic field upon the diurnal and seasonal variations of radio aurora activity has been identified.

Ion cyclotron waves as a possible source of resonant auroral radar echoes

Auroral backscatter radar observations were made from Ithaca, New York, at 50 MHz during the early morning of April 1, 1976, a period of high magnetic disturbance (Kp ∼ 8). The backscattered power showed large rapid (time scale of a few minutes or less) variations, characteristic of discrete radar aurora, from L = 3.5–4. Doppler spectra of waves propagating in nearly the north‐south direction from up to 28 different ranges were obtained simultaneously with good spatial (7.5 km) and temporal (2 s) resolution. Some unusual spectra with very narrow peaks at Doppler shifts between about 70 and 90 Hz were observed and were apparently associated with sheared flows of the auroral plasma. These echoes were observed at the southern edge of the scattering region, which moved poleward with a velocity which sometimes exceeded 800 m/s. There is evidence that the echoes were generated above about 140 km. Present electrojet instability theories cannot explain these narrow spectra, but the Doppler shift suggests that they might be caused by ion cyclotron waves generated by field‐aligned currents (electron drifts). However, the excitation of ion cyclotron waves requires currents considerably stronger than typically quoted mean values (∼10−5 µA/m²). Furthermore, the most easily excited ion cyclotron waves have wavelengths longer than 3 m and do not propagate in quite the right direction for detection by our radar. The threshold for the waves that we observe is considerably higher than the minimum. Intense localized currents, such as those known to be associated with auroral curls and vortices (J ≳ 10−4 µA/m²), appear to be sufficient to excite the longer (∼10–20 m) ion cyclotron waves in the upper E region; whether or not the field‐aligned electron drifts are ever large enough to excite directly the 3‐m waves we observe is an open question, but there is at least some evidence which suggests that such drifts do exist in very localized regions during highly disturbed conditions.

Splitting and divergence of STARE auroral radar velocities

A study of the power spectra of auroral radar echoes shows that they are sometimes asymmetrical. This is probably due to two waves (at different heights) having different line‐of‐sight velocities. The velocity splitting occurs over a limited region of space (∼300 km in geographic extent) that moves toward the equator. The velocity splitting seems to be associated with divergence of the STARE drift velocities. This has been calculated and found to have significant time and space variations—much too large to be explained by magnetic field changes. The divergence is greatest at times when the power spectrum is asymmetrical and more than one velocity may be present. Several possible corrections (including a few unlikely ones) have been tried, but none reduce the magnitude of the divergences appreciably. We must conclude that STARE does not measure the electric field exactly and that a correction velocity of the order of 25 m s−1 and divergence of the order of 10−4 s−1 is required. We have not been able to find what it depends on, except its association with velocity splitting.

On reducing the inherent noise in the double‐pulse Doppler technique

The double‐pulse technique for measuring the Doppler shift of radar auroral echoes contains inherent noise arising from overlapping echoes. By modifying the method of reducing the data, it is possible to reduce this noise so that either greater precision is achieved or less time is required.

STARE observations of long discrete echoes

Early studies of auroral radar echoes by Unwin1,2 showed the existence of a particular class of ‘long discrete’ echoes, distinguished as being weak and relatively long-lasting (1–60 min). The backscatter usually consists of several narrow (∼20 km) bands which drift slowly in latitude, and the height of the backscattering region appears not to exceed 100 km. Here, we report on observations of this type of radar aurora made during the past few years with the Scandinavian Twin Auroral Radar Experiment (STARE). Whereas Unwin's observations covered the range of geomagnetic latitudes corresponding to l-values ∼3−∼5, the present study is concerned with higher latitudes, corresponding to the range l ∼ 5.7−∼8.

Observations of auroral fading before breakup

We have obtained detailed observations of the onset of auroral breakup using a variety of instruments with time resolution of some tens of seconds. Rapid sequences of all‐sky photographs and fast meridian scans by photometers show that breakup is usually preceded by moderate brightening, followed by fading of the auroral brightness lasting 1 or 2 min, before the actual breakup itself. At the time of the fading there is a brief darkening of the poleward sky. Often the breakup is preceded by one or more rapid intensifications, each one preceded by local fading. Pseudobreakups may also occur without the development of a major event. A bona fide breakup may begin on the fading arc, on an adjacent arc, or in an entirely new region nearby. This optical activity is closely correlated with the development of auroral radar echoes, a feature suggesting that variations in the ionospheric and magnetospheric electric and magnetic fields are responsible for the observed auroral variations. Data from the International Magnetospheric Study magnetometer network provide some indication of a correlated response by the local auroral and ionospheric currents, although this could be partly due to changes in conductivity. Riometer recordings show a slow decrease in ionospheric radio wave absorption over a period of about 10 min prior to breakup, with the largest decrease essentially to quiet time values in the region of auroral fading and subsequent breakup. The implications of these observations regarding the trigger mechanism for the expansion phase of a magnetospheric substorm are discussed.

Measurement of the drift of auroral radar echoes with the radar auroragraph

The auroragraph is a device for the analysis of any movements of patterns on movie films. Its application to the PPI record from an auroral radar is described, and examples from Syowa Station, Antarctica are given. The radar auroragraph has advantages over other more conventional methods of deriving the speed and direction of movement of auroral radar echoes in space.

On a crossed field two-stream plasma instability in the auroral plasma

This paper presents a detailed analysis of the Buneman-Farley two-stream plasma instability. We use a fluid plasma model appropriate for the E region ionosphere, including the effects of the geomagnetic field and electron and ion temperatures and their collisions with neutrals. We derive a linearized dispersion equation for the coupled electron thermal wave and ion thermal wave. The results are compared with those derived from a kinetic theory. It is shown that in the long-wavelength limit the agreement between the above described fluid theory and kinetic theory is exact. The physical arguments for the agreement are clearly stated. From the dispersion equation we obtain closed form expressions for the phase velocities, frequencies, and growth rates describing the threshold condition and the unstable region. With these closed form results we are able to determine the physical behavior of the instability under a variety of ionospheric conditions. The closed form results show that unstable plasma waves having the maximum growth rate propagate at finite off-perpendicular angles with respect to the geomagnetic field. This property, together with the fact that the two-stream instability is a convective instability, is used to infer that radar auroral echoes scattered from these waves should occur at finite off-perpendicular angles. Spatial maps of radar auroral echoes obtained with a 1210-MHz backscatter radar at Homer, Alaska, are presented as supporting evidence. By taking data during different magnetic conditions, we were able to exclude magnetic field line distortion as a possible contributing factor.

Measurements of Pc 5 ionospheric electric fields by means of balloon-borne sensors

Measurements of ionospheric pulsating electric fields by means of balloon-borne sensors are reported. The electric field pulsates with a period of ∼6 min and is apparently associated with a Pc 5 micropulsation. The relevance of the present observations to the association, reported in the literature, between pulsating auroral radar echoes and geomagnetic micropulsations in the Pc 5 range is discussed.

The interrelationship between the >130 keV electron trapping boundary, the VHF radar backscatter, and the visual aurora

A coordinated program between satellite, VHF radar, and optical auroral observations was carried out in Alaska during the spring of 1972. On 18 days in February, March, and April the positions of the >130-keV isotropic and cutoff trapping boundaries in the evening sector of the oval were determined on satellite 1971-89a. On 9 of these days, simultaneous VHF 50-MHz auroral radar backscatter echoes were observed to lie equatorward of the cutoff trapping boundary regardless of the phase of the auroral substorm. The analysis of these data with respect to substorm phase, Dst, and position of the aurora shows that the motion of the trapping boundary is a dynamic feature of the substorm that can be monitored and approximately located by the VHF radar echoes.

Auroral currents, irregularities, and luminosity

On March 16, 1972, high spatial resolution VHF backscatter measurements of evening diffuse radar aurora were obtained at the NOAA radar site near Anchorage, Alaska. These measurements indicate that the region of radar aurora has considerable time-varying and latitudinal structure. Comparison of the dynamics of this region with concurrent magnetograms indicates that the irregularities are spatially coincident with the evening eastward auroral electrojet and indeed appear to be generated by the electrojet current flow. Additional comparison of the radar data with all-sky camera photographs indicates that the diffuse radar auroral irregularities and hence the eastward auroral electrojet occur in a latitudinal zone that is equatorward and adjacent to the region of visual auroral activity. Finally, discrete radar auroral echoes, which were observed in the premidnight hours, were located on the poleward edge of the visible aurora and appeared after westward traveling spirals or surges passed through the radar beam.

VHF power spectra of the radar aurora

Power spectra of VHF radar auroral echoes were obtained near Anchorage, Alaska, during October 15–23, 1970. These spectra were obtained concurrently with standard range-time-intensity (RTI) film strips of the echoing region using the same radar system. In this paper, these spectra and the RTI film strips are examined and are compared with previously defined radar auroral classifications. The results indicate that there are at least two, and possibly three, separate types of spectra that correspond to identifiable echo configurations on the RTI film strips. Identification of the two-stream instability as an irregularity-producing mechanism for one of these spectral types is possible but not definitive. Another type of spectrum is similar to the type II spectra seen in the equatorial electrojet. The third spectral type bears no similarity to any results obtained at the equator.

Correlation between convection electric fields in the nightside magnetosphere and several wave and particle phenomena during two isolated substorms

Correlation of several magnetoionospheric wave and particle phenomena previously linked observationally to magnetospheric substorms and inferred to involve convection electric fields with whistler measurements of convection activity during two relatively isolated substorms. The events occurred at about 0600 UT on July 15, 1965, and about 0500 UT on Oct. 13, 1965. The correlated phenomena include cross-L inward plasma drifts near midnight within the plasmaphere, diffuse auroral radar echoes observed near the dusk meridian, IPDP micropulsations (intervals of pulsations of diminishing period) in the premidnight sector, apparent contractions and expansions of the plasma sheet at about 20 earth radii in the magnetotail, and Pc 1/Pi 1 micropulsation events near or before midnight. Two new vlf phenomena occurred during the October 13 event - a noise band within the plasmasphere associated with a convecting whistler path, and ?hisslers,' falling-tone auroral-hiss forms repeated at intervals of about 2 sec.