I. W. McCrea

Relationship between large horizontal electric fields and auroral arc elements

High time resolution optical measurements in the magnetic zenith are compared with European Incoherent Scatter (EISCAT) field-aligned measurements of electron density at 0.2-s resolution and with horizontal electric field measurements made at 278 km with resolution of 9 s. In one event, 20 min after a spectacular auroral breakup, a system of narrow and active arc elements moved southward into the magnetic zenith, where it remained for several minutes. During a 30-s interval of activity in a narrow arc element very close to the radar beam, the electric field vectors at 3-s resolution were found to be extremely large (up to 400 mV m−1) and to point toward the bright optical features in the arc, which moved along its length. It is proposed that the large electric fields are short-lived and are directly associated with the particle precipitation that causes the bright features in auroral arc elements.

Novel artificial optical annular structures in the high latitude ionosphere over EISCAT

The EISCAT low-gain HF facility has been used repeatedly to produce artificially stimulated optical emissions in the F-layer ionosphere over northern Scandinavia. On 12 November 2001, the high-gain HF facility was used for the first time. The pump beam zenith angle was moved in 3° steps along the north-south meridian from 3°N to 15°S, with one pump cycle per position. Only when pumping in the 9°S position were annular optical structures produced quite unexpectedly. The annuli were approximately centred on the pump beam but outside the −3 dB locus. The optical signature appears to form a cylinder, which was magnetic field-aligned, rising above the pump wave reflection altitude. The annulus always collapsed into the well-known optical blobs after ∼60 s, whilst descending many km in altitude. All other pump beam directions produced optical blobs only. The EISCAT UHF radar, which was scanning from 3° to 15°S zenith angle, shows that enhanced ion-line backscatter persisted throughout the pump on period and followed the morphology of the optical signature. These observations provide the first experimental evidence that Langmuir turbulence can accelerate electrons sufficiently to produce the optical emissions at high latitudes. Why the optical annulus forms, and for only one zenith angle, remains unexplained.

Extreme solar‐terrestrial events of October 2003: High‐latitude and Cluster observations of the large geomagnetic disturbances on 30 October

Extreme solar-terrestrial events of October 2003: high latitude and Cluster observations of the large geomagnetic disturbance on October 30

Coordinated EISCAT/DMSP measurements of electron density and energetic electron precipitation

A campaign was recently undertaken to obtain simultaneous radar and satellite measurements that would provide insight into the temporal and spatial morphology of the energetic particle precipitation. It included coordinated field-aligned measurements of the altitudinal profile of the E region electron density from the European Incoherent Scatter (EISCAT) radar, and measurements of the energetic particle precipitation from the SAMPEX and DMSP F10 and F12 satellites. The Strickland auroral electron transport code, with the recent addition of auroral proton transport, was employed to produce density profiles from the energetic particle precipitation measurements by the SSJ/4 sensors on the DMSP F10 and F12 spacecraft for comparison with the EISCAT measurements. We found good agreement between the EISCAT measurements and the model results for three of the four conjunctions including a very close conjunction (< 12 km) when the integral ion energy flux was greater than the integral electron energy flux by a factor of 3 to 4. All of the conjunctions for which the results of the comparison were good occurred within a stable diffuse aurora. Conductivity calculations using the model results varied less than 15% from calculations using the radar measurements. The comparison was good even when the separation between the satellite and the radar was greater than 200 km. The fourth conjunction, for which the agreement was poor, occurred within the discrete aurora during a very active period when the ionospheric conditions were dynamic both spatially and temporally.

Mesoscale observations of Joule heating near an auroral arc and ion‐neutral collision frequency in the polar cap E region

We report on the first meso-scale combined ionospheric and thermospheric observations, partly in the vicinity of an auroral arc, from Svalbard in the polar cap on 2 February 2010. The EISCAT Svalbard radar employed a novel scanning mode in order to obtain F- and E-region ion flows over an annular region centred on the radar. Simultaneously, a co-located Scanning Doppler Imager observed the E-region neutral winds and temperatures around 110 km altitude using the 557.7 nm auroral optical emission. Combining the ion and neutral data permits the E-region Joule heating to be estimated with an azimuthal spatial resolution of ∼64 km at a radius of ∼163 km from the radar. The spatial distribution of Joule heating shows significant meso-scale variation. The ion-neutral collision frequency is measured in the E-region by combining all the data over the entire field of view with only weak aurora present. The estimated ion-neutral collision frequency at ∼113 km altitude is in good agreement with the MSIS atmospheric model.

Flow dependence of COSCAT spectral characteristics

Abstract During the last three years, a series of EISCAT experiments has been carried out involving the COSCAT UHF transmitter, which enables coherent scatter to be studied at an irregularity wavelength of 16 cm. The most recent experiments have utilised a geometry in which the Kiruna radar is pointed at low elevation to detect E-region coherent scatter, while the Tromso and Sodankyla radars receive incoherent scatter from the F-region on a field line conjugate with the scattering irregularities. The velocity data from Tromso and Sodankyla are then combined to construct a bistatic determination of flow speed and direction, assuming that there is no plasma flow parallel to the magnetic field. This flow velocity is approximately equal to the E-region electron drift. Observations of coherent scatter have been correlated with the flow speed and direction of the derived F-region flow, and some general conditions for the excitation of the coherent scatter have been determined. It appears that there is some evidence of marginal excitation of the scattering irregularities for line-of-sight flow components of order 400 m s−1, although there is appreciable variability between the different days of the study. The phase speed of the E-region irregularities has the same sense as the line-of-sight component of the F-region ion drift. There is a clear tendency toward higher phase speeds as the flow speed increases.

Global scale ionospheric monitoring — Future development

Incoherent scatter radars have developed considerably in recent years with the deployment of multiple new systems (Poker Flat, Alaska, Resolute Bay, Canada, and in development in China, Argentina, Antarctica, and Scandinavia, as well as a second system at Resolute Bay) and operational changes to support continuous and remote measurements. We will discuss plans to add further observational sites, built around phased array incoherent scatter radars, to cover a complete geomagnetic meridian; plans to further integrate the routine operation of many radars around the globe; and the potential for hardware collaboration for future incoherent scatter radar systems.