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Multi-instrument observational data from an experiment on 13 October 2006 at the EISCAT/HEATING facility at Tromsø, Norway are analysed. The experiment was carried out in the evening hours when the electron density in the F-region dropped, and the HF pump frequency fH was near and then above the critical frequency of the F2 layer. The distinctive feature of this experiment is that the pump frequency was just below the third electron gyro harmonic frequency, while both the HF pump beam and UHF radar beam were directed towards the magnetic zenith (MZ). The HF pump-induced phenomena were diagnosed with several instruments: the bi-static HF radio scatter on the London-Tromsø-St. Petersburg path, the CUTLASS radar in Hankasalmi (Finland), the European Incoherent Scatter (EISCAT) UHF radar at Tromsø and the Tromsø ionosonde (dynasonde). The results show thermal electron excitation of the HF-induced striations seen simultaneously from HF bi-static scatter and CUTLASS radar observations, accompanied by increases of electron temperature when the heater frequency was near and then above the critical frequency of the F2 layer by up to 0.4 MHz. An increase of the electron density up to 25% accompanied by strong HF-induced electron heating was observed, only when the heater frequency was near the critical frequency and just below the third electron gyro harmonic frequency. It is concluded that the combined effect of upper hybrid resonance and gyro resonance at the same altitude gives rise to strong electron heating, the excitation of striations, HF ray trapping and extension of HF waves to altitudes where they can excite Langmuir turbulence and fluxes of electrons accelerated to energies that produce ionization.

Two plasma instability mechanisms were thought until 2007 to dominate the formation of plasma irregularities in the F region high latitude and polar ionosphere; the gradient-drift driven instability, and the velocity-shear driven instability. The former mechanism was accepted as accounting for plasma structuring in polar cap patches, the latter for plasma structuring in polar cap sun aligned arcs. Recent work has established the need to replace this view of the past two decades with a new patch plasma structuring process (not a new mechanism), whereby shear-driven instabilities first rapidly structure the entering plasma, after which gradient drift instabilities build on these large "seed" irregularities. Correct modeling of cusp and early polar cap patch structuring will not be accomplished without allowing for this compound process. This compound process explains several previously unexplained characteristics of cusp and early polar cap patch irregularities. Here we introduce additional data, coincident in time and space, to extend that work to smaller irregularity scale sizes and relate it to the structured cusp current system.

Two long runs of EISCAT Svalbard Radar (ESR), in February 2001 and October 2002, have been analysed with respect to variability in the F2 region peak density and altitude. The diurnal variation in the F2 peak density exhibits one maximum around 12:00 MLT and another around 23:00 MLT, consistent with solar wind controlled transport of EUV ionized plasma across the polar cap from day to night. High density plasma patch material is drawn in through the cusp inflow region independent of IMF BY. There is no apparent IMF BY asymmetry on the intake of high density plasma, but the trajectory of its motion is strongly BY dependent. Comparison with the international reference ionosphere model (IRI2001) clearly demonstrates that the model does not take account of the cross-polar transport of F2-region plasma, and hence has limited applicability in polar cap regions.

A sequence of 3 patches of high-density (1012 m−3) cold plasma on a horizontal scale-size of 300–700 km was observed near magnetic noon by the EISCAT VHF radar above Svalbard on 17 December 2001. The patches followed a trajectory towards the cusp inflow region. The combination of radar and all-sky observations demonstrates that the patches must have been segmented equatorward of the cusp/cleft auroral display, and hence their properties had not yet been influenced by cusp particle showers and electrodynamics on open flux tubes. The last patch in the sequence was intersected by radio tomography observations, and was found to be located adjacent to a broader region of the same high electron density further south. The patches occurred under moderately active conditions (Kp=3) and the total electron content (TEC) of the high-density plasma was 45 TEC units. The train of patches appeared as a segmentation of the tongue of ionization. The sequence of patches occurred in association with a sequence of flow bursts in the dusk cell return flow. It is proposed that reconnection driven pulsed convection is able to create sub-auroral patches in the region where high density mid-latitude plasma is diverted poleward toward the cusp. It is the downward Birkeland current sheet located at the equatorward boundary of the flow disturbance that represents the actual cutting mechanism.

In this paper we focus on flux transfer events (FTEs) and poleward moving auroral forms (PMAFs) in the cusp region, combining data from the EISCAT Svalbard radar, SuperDARN HF radars, ground-based optics, and three low-altitude polar-orbiting spacecraft. During an interval of southward interplanetary magnetic field the EISCAT Svalbard radar tracked a train of narrow flow channels drifting into the polar cap. One 30-60 km wide flow channel surrounded by flow running in the opposite direction is studied in great detail from when it formed equatorward of the cusp aurora, near magnetic noon, until it left the field-of-view and disappeared into the polar cap. Satellite data shows that the flow channel was on open field lines. The flow pattern is consistent with field-aligned currents on the sides of the flow channel; with a downward current on the equatorward side, and an upward current on the poleward side. The poleward edge of the flow channel was coincident with a PMAF that separated from the background cusp aurora and drifted into the polar cap. A passage of the DMSP F13 spacecraft confirms that the FTE flow channel was still discernable over 15 minutes after it formed, as the spacecraft revealed a 30–40 km wide region of sunward flow within the anti-sunward background convection. From the dimensions of the flow channel we estimate that the magnetic flux contained in the event was at least 1 MWb. This data set also shows that Birkeland current filaments often seen by low-altitude spacecraft in the cusp/mantle are really associated with individual FTE events or a train of FTEs in progress. As the region 0 or cusp/mantle current represents the statistical average consistent with the large-scale flow pattern, we therefore introduce a new term – FTE currents – to denote the unique pair of Birkeland current sheets that are associated with individual meso-scale FTE flow disturbances. The poleward moving auroral forms (PMAFs), often referred to in the literature, are the optical signature of the upward FTE current. Keywords. Magnetospheric physics (Current systems; Magnetopause, cusp and boundary layers) – Ionosphere (plasma convection)

A continuous band of high ion temperature, which persisted for about 8h and zigzagged north-south across more than five degrees in latitude in the dayside (07:00-15:00MLT) auroral ionosphere, was observed by the EISCAT VHF radar on 23 November 1999. Latitudinal gradients in the temperature of the F-region electron and ion gases (Te and Ti, respectively) have been compared with concurrent observations of particle precipitation and field-perpendicular convection by DMSP satellites, in order to reveal a physical explanation for the persistent band of high Ti, and to test the potential role of Ti and Te gradients as possible markers for the open-closed field line boundary. The north/south movement of the equatorward Ti boundary was found to be consistent with the contraction/expansion of the polar cap due to an unbalanced dayside and nightside reconnection. Sporadic intensifications in Ti, recurring on ~10-min time scales, indicate that frictional heating was modulated by time-varying reconnection, and the band of high Ti was located on open flux. However, the equatorward Ti boundary was not found to be a close proxy of the open-closed boundary. The closest definable proxy of the open-closed boundary is the magnetosheath electron edge observed by DMSP. Although Te appears to be sensitive to magnetosheath electron fluxes, it is not found to be a suitable parameter for routine tracking of the open-closed boundary, as it involves case dependent analysis of the thermal balance. Finally, we have documented a region of newly-opened sunward convecting flux. This region is situated between the convection reversal boundary and the magnetosheath electron edge defining the open-closed boundary. This is consistent with a delay of several minutes between the arrival of the first (super-Alfvénic) magnetosheath electrons and the response in the ionospheric convection, conveyed to the ionosphere by the interior Alfvén wave. It represents a candidate footprint of the low-latitude boundary mixing layer on sunward convecting open flux.

The 2D morphology of coherent HF radar and optical cusp aurora has been studied for conditions of predominantly southward IMF conditions, which favours low-latitude boundary layer reconnection. Despite the variability in shape of radar cusp Doppler spectra, the spectral width criterion of > 220 m s–1 proves to be a robust cusp discriminator. For extended periods of well-developed radar backscatter echoes, the equatorward boundary of the > 220 m s–1 spectral width enhancement lines up remarkably well with the equatorward boundary of the optical cusp aurora. The spectral width boundary is however poorly determined during development and fading of radar cusp backscatter. Closer inspection of radar Doppler profile characteristics suggests that a combination of spectral width and shape may advance boundary layer identification by HF radar. For the two December days studied the onset of radar cusp backscatter occurred within pre-existing 630.0 nm cusp auroral activity and appear to be initiated by sunrise, i.e. favourable radio wave propagation conditions had to develop. Better methods are put forward for analysing optical data, and for physical interpretation of HF radar data, and for combining these data, as applied to detection, tracking, and better understanding of dayside aurora. The broader motivation of this work is to develop wider use by the scientific community, of results of these techniques, to accelerate understanding of dynamic high-latitude boundary-processes. The contributions in this work are: (1) improved techniques of analysis of observational data, yielding meaningfully enhanced accuracy for deduced cusp locations; (2) a correspondingly more pronounced validation of correlation of boundary locations derived from the observational data set; and (3) a firmer physical rationale as to why the good correlation observed should theoretically be expected.Key words: Ionosphere (ionospheric irregularities; polar ionosphere)