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This study investigates the contribution of energetic (E > 30 keV) particle precipitation during a pulsating aurora event over Kilpisjarvi (L = 6.2) on 26 February 2014. It is based on the comparison of auroral blue-line emission (427.8 nm) data from an all-sky camera and cosmic noise absorption (CNA) data obtained from a multi-beam experiment of the Kilpisjarvi Atmospheric Imaging Receiver Array (KAIRA) riometer. The data sets are compared for three KAIRA beams close to magnetic zenith. Results show a clear correlation between the measured CNA and the auroral blue-line emission during the event, for each beam. In addition, individual pulsations are observed for the first time in the cosmic noise absorption data measured by KAIRA, and are found to be close-to-identical to the optical pulsations. This suggests that the modulation of electron precipitation during pulsating aurora takes place in a consistent way over a broad range of energies.

[1] The exceptionally high solar wind stream activity in 2003 caused a record intensity in the auroral electrojet currents, leading to a major reduction of the horizontal field at auroral latitudes and to a notable strengthening of the vertical geomagnetic field in the polar cap. This strengthening is clearly visible in the observatory annual values as a significant deflection in the corresponding secular variation. A similar but weaker deflection also occurs during the strongest high speed stream years of the earlier solar cycles, e.g., in 1983 and 1994. We also found that, in addition to the disturbed times, the westward electrojet was often enhanced even during the most quiet times of the strongest high speed stream years. The quiet time level was more disturbed in 2003 than in other high speed stream years, when an exceptionally clear signal was seen in the polar cap Z intensity even in the annual mean curve in this year. We exclude other current systems like the ring current or the DPY current as possible explanations.

[1] The EISCAT Svalbard radar (at 75° MLAT) was used to investigate the ionospheric plasma flows along auroral arcs in the 7–11 MLT sector with the radar beam directed along the L-shell. In nine cases auroral arcs appeared in the immediate vicinity of the radar beam allowing for observations of plasma flow simultaneously at different distances across the arc. In eight cases the arcs were associated with a plasma flow shear indicating sheets of upward field-aligned current (FAC) over the arcs. However, in one case the data indicate a downward net current in the region of the arc, with a flow shear and, especially, a downward FAC sheet adjoining the arc on the equatorial side. We suggest that in the downward FAC sheet, the Kelvin-Helmholtz instability can generate a series of auroral spots forming an arc, if the current sheet is co-located with a plasma boundary inside the magnetosphere.

[1] Using data from the Super Dual Auroral Radar Network (SuperDARN) and the ultraviolet imager on the Polar satellite, we have earlier shown that the IMF By-related electric field can penetrate into the closed magnetosphere and produce convection changes in the region of the postnoon auroral oval [Kozlovsky et al., JGR, N12, 2002]. In such a case, contra-directed azimuthal (East–West) plasma flows should be expected in the conjugated hemispheres. In this paper we present a theoretical model, which can well explain these observations. Due to the solar wind-magnetosphere dynamo, IMF By generates a voltage between the two polar caps, so that field-aligned currents flow between the two hemispheres within some layer just equatorward of the open polar cap boundary. In this region a field-aligned potential difference (i.e., a magnetic field line discontinuity) can occur when the field-aligned current exceeds a threshold value. The IMF By-related contra-directed ionospheric plasma flows can co-exist in the northern and southern hemispheres within the region of the inter-hemispheric field-aligned currents. The latitudinal width of this region depends on the IMF By and the ionospheric conductance, and can be of the order of 1–2 degrees.