D. W. Danskin

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.

Electron precipitation from EMIC waves: a case study from 31 May 2013

On 31 May 2013 several rising-tone electromagnetic ion-cyclotron (EMIC) waves with intervals of pulsations of diminishing periods (IPDP) were observed in the magnetic local time afternoon and evening sectors during the onset of a moderate/large geomagnetic storm. The waves were sequentially observed in Finland, Antarctica, and western Canada. Co-incident electron precipitation by a network of ground-based Antarctic Arctic Radiation-belt Dynamic Deposition VLF Atmospheric Research Konsortia (AARDDVARK) and riometer instruments, as well as the Polar-orbiting Operational Environmental Satellite (POES) electron telescopes, was also observed. At the same time POES detected 30-80 keV proton precipitation drifting westwards at locations that were consistent with the ground-based observations, indicating substorm injection. Through detailed modelling of the combination of ground and satellite observations the characteristics of the EMIC-induced electron precipitation were identified as: latitudinal width of 2-3° or ΔL=1 Re, longitudinal width ~50° or 3 hours MLT, lower cut off energy 280 keV, typical flux 1×104 el. cm-2 sr-1 s-1 >300 keV. The lower cutoff energy of the most clearly defined EMIC rising tone in this study confirms the identification of a class of EMIC-induced precipitation events with unexpectedly low energy cutoffs of <400 keV.

A statistical approach to determining energetic outer radiation belt electron precipitation fluxes

Subionospheric radio wave data from an Antarctic-Arctic Radiation-Belt (Dynamic) Deposition VLF Atmospheric Research Konsortia (AARDDVARK) receiver located in Churchill, Canada, is analyzed to determine the characteristics of electron precipitation into the atmosphere over the range 3  30 keV precipitation flux determined by the AARDDVARK technique was found to be ±10%. Peak >30 keV precipitation fluxes of AARDDVARK-derived precipitation flux during the main and recovery phase of the largest geomagnetic storm, which started on 4 August 2010, were >105 el cm−2 s−1 sr−1. The largest fluxes observed by AARDDVARK occurred on the dayside and were delayed by several days from the start of the geomagnetic disturbance. During the main phase of the disturbances, nightside fluxes were dominant. Significant differences in flux estimates between POES, AARDDVARK, and the riometer were found after the main phase of the largest disturbance, with evidence provided to suggest that >700 keV electron precipitation was occurring. Currently the presence of such relativistic electron precipitation introduces some uncertainty in the analysis of AARDDVARK data, given the assumption of a power law electron precipitation spectrum.

Energetic electron precipitation characteristics observed from Antarctica during a flux dropout event

Data from two autonomous VLF radio receiver systems installed in a remote region of the Antarctic in 2012 is used to take advantage of the juxtaposition of the L=4.6 contour, and the Hawaii-Halley, Antarctica, great circle path as it passes over thick Antarctic ice shelf. The ice sheet conductivity leads to high sensitivity to changing D-region conditions, and the quasi-constant L-shell highlights outer radiation belt processes. The ground-based instruments observed several energetic electron precipitation events over a moderately active 24-hour period, during which the outer radiation belt electron flux declined at most energies and subsequently recovered. Combining the ground-based data with low- and geosynchronous-orbiting satellite observations on 27 February 2012, different driving mechanisms were observed for three precipitation events with clear signatures in phase space density and electron anisotropy. Comparison between flux measurements made by Polar-orbiting Operational Environmental Satellites (POES) in low Earth orbit and by the Antarctic instrumentation provides evidence of different cases of weak and strong diffusion into the bounce-loss-cone, helping to understand the physical mechanisms controlling the precipitation of energetic electrons into the atmosphere. Strong diffusion events occurred as the 30 keV flux than was reported by POES, more consistent with strong diffusion conditions.

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.

Analysis of geomagnetic hourly ranges

In an attempt to develop better forecasts of geomagnetic activity, hourly ranges of geomagnetic data are analyzed with a focus on how the data are distributed. A lognormal distribution is found to be able to characterize the magnetic data for all observatories up to moderate disturbances with each distribution controlled by the mean of the logarithm of the hourly range. In the subauroral zone, the distribution deviates from the lognormal, which is interpreted as motion of the auroral electrojet toward the equator. For most observatories, a substantial deviation from the lognormal distribution was noted at the higher values and is best modeled with a power law extrapolation, which gives estimates of the extreme values that may occur at observatories which contribute to the disturbance storm time (Dst) index and in Canada.

Comparison of observed and predicted MUF(3000)F2 in the polar cap region

The maximum usable frequency for a 3000 km range circuit (MUF(3000)F2), computed from ionosonde measurements at Resolute (74.75∘N, 265.00∘E) and Pond Inlet (72.69∘N, 282.04∘E), has been compared with values obtained from the Ionospheric Communications Enhanced Profile Analysis and Circuit Prediction Program (ICEPAC), Voice of America Coverage Analysis Program (VOACAP), and International Telecommunication Union Recommendation 533 (REC533) models over a 4 year period. Predictions and observations show diurnal and seasonal variations; however, the VOACAP and ICEPAC models fail to reproduce the diurnal variation trend observed in the measurements during the summer period. The performance of these models has been statistically analysed: REC533 gives a better performance in winter and equinox months, while VOACAP has a better performance for both equinox and summer months. ICEPAC shows poor performance during low solar activity.

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.

A new look at type 4 echoes of radar aurora

Type 4 echoes represent a rare but distinct spectral signature of auroral electrojet backscatter during times of large electric fields and strong two-stream-generated ion acoustic turbulence. On the basis of many bistatic CW Doppler radar observations made over the past 15 years the great majority of type 4 echoes have a composite spectrum showing two main features, a broadened type 1 component peaking at elevated ion acoustic velocities and the type 4 narrow component at roughly double the ion acoustic speed. The most important property is the large phase velocities, mostly seen in the 900 to 1100 m/s range, associated with irregularities propagating in a narrow azimuthal cone along the destabilizing Hall current. In the present paper it is argued that the existing interpretation, in which it is hypothesized that the large Doppler velocities are simply from two stream waves originating in regions of highly elevated electron temperatures, cannot explain why the observed phase velocities of type 4 waves are mostly seen at preferential values near 1000 m/s and why these waves are highly directional. On the basis of the existing evidence we conclude that there is not yet satisfactory explanation for the type 4 echoes. The fact that type 4 waves have frequencies of about two times the frequency of the simultaneously observed ion acoustic waves led us to suggest the possibility of a resonant three-wave coupling interaction process which might be in operation under some appropriate conditions in the plasma. At present this is only a preliminary proposition which would be investigated further in a future study.

Large‐amplitude GPS TEC variations associated with Pc5–6 magnetic field variations observed on the ground and at geosynchronous orbit

Large-amplitude variations in GPS total electron content (TEC) at Pc5–6 (<6.67 mHz) frequencies have been observed, using a high data rate Global Positioning System (GPS) receiver of the Canadian High Arctic Ionospheric Network. TEC variations with peak-to-peak amplitudes of 2–7 TEC units (1 TECU = 1016 el m−2) were observed over a 2.5 h period in the postnoon sector on 9 September 2011, during a period of high auroral activity within a moderate geomagnetic storm. TEC observations were from the Sanikiluaq, Nunavut (56.54°N, 280.77°E) GPS receiver located in the auroral region. Over this same time period, compressional Pc5–6 magnetic field variations were observed by the geosynchronous GOES 13 magnetometer and the ground-based Sanikiluaq magnetometer. GOES 13 has a northern magnetic footprint in close proximity to Sanikiluaq. Cross-correlation analysis indicates that magnetic field and TEC variations were possibly linked. No natural hazards or nuclear explosions capable of exciting TEC perturbations were reported on this day. Using a triangulation technique involving TEC measurements of multiple GPS satellites, the propagation velocity of TEC variations in the ionosphere was also calculated. This calculation revealed two distinct events: lower frequency (~0.9 mHz) TEC variations that propagated westward, consistent with the westward propagation of compressional Pc5 waves observed by GOES 13 and 15 satellites, and higher-frequency (~3.3 mHz) TEC variations that propagated southward. This is the first report of variations in ionospheric TEC linked to satellite observations of Pc5–6 ULF waves.