D. André

Comparison of plasma flow velocities determined by the ionosonde Doppler drift technique, SuperDARN radars, and patch motion

We compare measurements of polar cap ionospheric plasma flow over Resolute Bay, Canada, made by a digital ionosonde using the Doppler drift technique with simultaneous measurements at the same location made by the first operational pair of SuperDARN HF radars. During the 3-hour comparison interval the flow varied widely in direction and from 100 to 600 m/s in speed. The two measurement techniques show very good agreement for both the speed and direction of flow for nearly all of the samples in the interval. The difference between the velocities determined by the two techniques has a scatter of about ±35° in direction and ±30% in speed, with no systematic difference above the level of the scatter. The few samples which strongly disagreed were usually associated with strong spatial structure in the flow pattern measured by SuperDARN in the vicinity of the comparison point. The drift speed measured by the ionosonde was independently verified by observing the time taken for polar cap F layer ionization patches to drift between ionosondes sited at Eureka and Resolute Bay. These results confirm that the speed and direction of the polar cap ionospheric convection can be reliably monitored by the ionosonde Doppler drift technique.

Evolution of ionospheric multicell convection during northward interplanetary magnetic field with |Bz /By | > 1

During northward interplanetary magnetic field (IMF), it is generally believed that ionospheric convection appears as a four-cell structure for |Bz/By| > 1 and as a distorted two-cell structure for |Bz/By| 1 on November 11, 1998. We show in detail the evolution of the convection patterns as |Bz/By| changes. Nearly symmetric four-cell convection, with two reverse cells in the polar cap and two normal cells at lower latitudes, occurs for |Bz/By| ≈ 7. The ionospheric flow associated with the reverse cells is closed almost completely on the dayside. A shifted four-cell convection pattern, with the reverse cells shifted toward earlier magnetic local time (MLT) for negative By and toward later MLT for positive By, is observed for |Bz/By| ≈ 2.3. When |Bz/By| decreases to ∼1.7, the convection appears as a three-cell pattern, with a single reverse cell focused near noon and two normal cells. The normal morning and afternoon cells are focused at quite high magnetic latitudes (between 76° and 80°); the spatial extent of the normal cells is 10°–15° or 1000–1500 km in the latitudinal direction. We also present Defense Meteorological Satellite Program (DMSP) satellite data which show sunward convection over the polar cap in the Southern Hemisphere at the same time as the Northern Hemisphere radar observations. We propose a new model of convection patterns during northward IMF for |Bz/By| > 1 and By < 0 on the basis of the combined observations. In the model the convection appears as a symmetric four-cell structure for |Bz/By| ≥ 3, a shifted four-cell structure for |Bz/By| = 2–3, and a three-cell structure for |Bz/By| = 1–2.

SuperDARN interferometry: Meteor echoes and electron densities from groundscatter

The SuperDARN radars are now able to measure the angles of arrival of the backscattered radiation. We describe the analysis procedure and present results for meteor scatter from slant ranges below 500 km and ground scatter from either the E region or the F region at greater slant ranges which can be used to determine peak electron densities and their heights. The angle of arrival measurements can also be used to identify “unwanted” backscatter from the backward lobes of the antenna radiation pattern. Electron densities can also be measured in the back lobes.

Traveling convection vortices as seen by the SuperDARN HF radars

Two impulsive traveling convection vortex (TCV) events observed simultaneously by ground based magnetometers and the SuperDARN HF radars in the prenoon sector were studied. In both cases, disturbances traveled westward at speeds of 4–6 km/s. Convection patterns derived from magnetometer measurements and radar observations were overall in reasonable agreement; observed differences at some points might be caused by both the nonuniform ionospheric conductivity distribution and difference in the integration time of the radar and magnetometer data. For one event, the convection patterns obtained from magnetometer data and SuperDARN radar measurements were relatively simple; they can be interpreted as a result of the westward motion of a convection vortex system associated with a pair of field-aligned currents separated in azimuthal direction. This TCV event was associated with relatively low Pc5 pulsation activity, contrary to the second TCV event that was accompannied by a train of Pc5 magnetic pulsations of large amplitude. Convection patterns for the second event were complicated. A simple scenario for the interpretation of the generation of TCVs and Pc5 pulsations is suggested. A sudden impulse in the solar wind dynamic pressure produces disturbances on several boundaries of magnetospheric plasma: on the magnetopause, the LLBL inner edge, and the plasma sheet inner edge. These boundaries are elastic so that surface waves can propagate along them. The high-latitude wave is responsible mainly for TCVs, whereas the low-latitude waves may be responsible for excitation of Pc5 field line resonance pulsations. The scenario explains important features of both TCV events and Pc5 pulsations: both phenomena appear simultaneously and show westward (eastward) propagation, but the TCVs are observed at latitudes close to the LLBL inner edge, whereas the Pc5 pulsations occur at lower latitudes, close to the inner boundary of the plasma sheet.

Quasi‐periodic ionospheric disturbances with a 40‐min period during prolonged northward interplanetary magnetic field

It has been observed that quasi-periodic oscillations in the solar wind and interplanetary magnetic field (IMF) can result in the generation of auroral electrojet bursts and gravity waves with a similar period. In this paper, we present radar and magnetometer observations of quasi-periodic ionospheric disturbances during stable, northward IMF. Magnetic deviations of ∼10 nT with a 40-min periodicity were registered with ground magnetometers between magnetic latitudes 75° and 80° over an extended region along the longitudinal direction in the morning sector. A sequence of gravity waves were observed by HF radars at lower latitudes and appeared to be closely related to the high-latitude 40-min current disturbances. There were no obvious source perturbations in the upstream solar wind for the ionospheric disturbances. We suggest that field line resonances could be excited in a closed magnetosphere during prolonged northward IMF and result in the generation of the ionospheric current disturbances and gravity waves.

Localized and strongly unstable plasma regions in the auroral E region ionosphere and implications for radar experiments

The results in this paper were obtained with SAPPHIRE, a new auroral Doppler radar experiment designed to study meter-scale E region irregularities. SAPPHIRE is a dual 50-MHz continuous wave, phased array, multibeam, bistatic system which is capable of performing cross-beam measurements from two widely different directions. There are two transmitters, each of which probes the auroral electrojet plasma over a large spatial target grid area of multiple intersections that determine 16 scattering regions or cells. Initial observations using untapered antenna arrays showed a class of scatter characterized by a narrow power spectrum peaking at the same Doppler shift in all, or several, observing cells simultaneously. These are strong echoes ranging in lifetime from a few tens of seconds to a few minutes and occurring preferentially in the midnight and morning magnetic time sectors. The analysis showed that this scatter is strongly anisotropic in azimuth and comes from localized regions of spatially coherent large-amplitude plasma waves that produce mostly type III, but also type I and the rare type IV, radar auroras. By using many events and analyzing a large number of Doppler spectra, we found that type III echoes are the strongest observed, having on the average relative intensities at least 15 dB higher than the type I echoes. The observations relate to the “short discrete radar auroras” which are known to originate in spatially confined, dynamic plasma regions. The possibility exists that the large free energy for instability in these active regions is provided from intense electric fields and/or very sharp electron density gradients, both expected to occur at times near the edges of discrete auroral arcs. Finally, the present results confirm that, because of the large dynamic range of radio auroral echoes, strong scattering regions lead to the complete domination, at times, by backscatter through antenna sidelobes. For the localized regions of strong type III and type I echoes, this means that the conventional 3-dB antenna beam width scale size of the scattering region is unrealistic. Obviously, this has important implications for the radar auroral experiments and the interpretation of observations.

Ionospheric convection and equivalent ionospheric currents in the dayside high‐latitude winter ionosphere

Ionospheric convection inferred from Super Dual Auroral Radar Network (SuperDARN) HF radar measurements is compared with an equivalent ionospheric convection derived from ground magnetometer data in the dayside winter high-latitude ionosphere. Although there was general agreement between observed convection patterns produced by radars and magnetometers, there were significant differences in details. The orientation of equivalent convection vectors inferred from magnetic data was often opposite to the convection vectors determined by the SuperDARN radars in the poleward part of the convection vortex structure, though the agreement was reasonable in its equatorward part. The magnitudes of convection vectors determined from radar data and those inferred from magnetometer data were often different. The observed differences are attributed to strong horizontal inhomogeneity in the ionospheric conductivity distribution for winter conditions. It is possible that magnetic disturbances in the dark high-latitude ionosphere are strongly affected by field-aligned currents at the terminator that separates regions of the sunlit highly conducting ionosphere and dark poorly conducting ionosphere.

Preferential phase velocities for type 4 irregularities in the auroral E region plasma

This paper presents additional evidence on the nature of 50-MHz type 4 auroral backscatter. By using a large number of events, recorded during several periods of continuous wave Doppler radar operation in the past few years, and a data sequence of long lasting type 4 echoes, the mean phase velocity of type 4 waves and its relation to type 1 phase velocity have been studied statistically. On the average, type 4 phase velocities range between 800 and 1200 m/s with the great majority grouping near 1000 m/s. The mean velocities of simultaneous type 4 and type 1 spectrum components are well separated in the Doppler shift range with an approximate factor of 2 in the type 4 to type 1 velocity ratio. Also, both spectrum components have the same Doppler polarity, which is indicative of a direct or indirect role for the electric field in type 4 wave generation. On the basis of the widely accepted conviction that type 1 waves have velocities saturating near the ion acoustic speed, it is concluded that type 4 waves propagate with velocities at about twice the acoustic speed in the plasma. The evidence presented here strongly suggests that type 4 waves propagate at preferential velocities near 1000 m/s, an experimental fact which must be explained in theoretical models.

Spatial occurrence of decameter midlatitude E region backscatter

This paper provides a statistical analysis of the spatial occurrence of midlatitude E region decameter backscatter. Measurements were made using the Valensole HF (high frequency) radar located in southern France during the summer of 1995 when it operated simultaneously at four frequencies. On the basis of the premise that E region scattering is fully magnetic aspect sensitive, the spatial occurrence statistics show that the aspect sensitive region moves toward the radar (southward) with respect to line of sight propagation calculations, with the lower-frequency echoes being closer toward the radar than the higher frequency ones are, in agreement with refraction theory predictions. Ray tracing inside nighttime midlatitude electron density profiles augmented with dense sporadic E s layers was used to calculate the expected echoing region, and good agreement with the observed region was found. Another finding is the angular distribution of backscatter inside the wide azimuthal sector covered by the radar scan. The spatial distribution of echo occurrence has its maximum at small azimuths at and about the geomagnetic north, suggesting that statistically, the meridional direction is strongly preferred for backscatter. Under the postulation that these are secondary decameter waves, we concluded that the observed angular anisotropy in spatial occurrence is at odds with the concept of strong isotropic plasma turbulence [Sudan, 1983] but in general agreement with the two-step gradient drift instability theory of secondary-wave generation proposed by Sudan et al. [1973].

High‐latitude ionospheric perturbations and gravity waves: 1. Observational results

The Saskatoon HF radar has been used to observe ionospheric perturbations and gravity waves at high latitudes. The observations on December 29, 1993, taken during an extended period of northward interplanetary magnetic field, are analyzed in detail. Two distinct intervals of backscatter are received, a near-range interval of ground scatter echoes from 900 to 1500 km and a far-range interval of ionospheric scatter from 2000 to 2600 km. The ground scatter intensity shows strong quasi-periodic enhancements caused by Earth-reflected gravity waves. The ionospheric echoes, which form a circular band of radius about 600 km, occur in the auroral zone below 80° magnetic latitude, just equatorward of the convection reversal. They cover a magnetic longitude interval from −60° to 80°, corresponding to a magnetic local time period from about 1100 to about 1800 LT. From the shape of the Earth-reflected gravity wave intensity bands on the latitude-time plots, those gravity waves can be traced back to the ionospheric perturbation region, which lies in the high-latitude echo zone, namely the auroral zone. This is confirmed by a two-dimensional cross-correlation between the time series for the intensity of the gravity-wave modulated ground scatter and the auroral zone ionospheric scatter. That correlation shows evidence for a time delay corresponding to the Earth-reflected gravity wave propagation time between the regions. Also, the correlation peaks for a range separation about 1100 km, which implies that the normal one-and-a-half-hop propagation mode usually used to explain the presence of the far-range auroral echoes cannot be operative and that the Pedersen ray “trapped” ionospheric mode is responsible for the observations.