Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> The characteristics of substorm-associated Pi2 pulsations observed by the SABRE coherent radar system during three separate case studies are presented. The SABRE field of view is well positioned to observe the differences between the auroral zone pulsation signature and that observed at mid-latitudes. During the first case study the SABRE field of view is initially in the eastward electrojet, equatorward and to the west of the substorm-enhanced electrojet current. As the interval progresses, the western, upward field-aligned current of the substorm current wedge moves westward across the longitudes of the radar field of view. The westward motion of the wedge is apparent in the spatial and temporal signatures of the associated Pi2 pulsation spectra and polarisation sense. During the second case study, the complex field-aligned and ionospheric currents associated with the pulsation generation region move equatorward into the SABRE field of view and then poleward out of it again after the third pulsation in the series. The spectral content of the four pulsations during the interval indicate different auroral zone and mid-latitude signatures. The final case study is from a period of low magnetic activity when SABRE observes a Pi2 pulsation signature from regions equatorward of the enhanced substorm currents. There is an apparent mode change between the signature observed by SABRE in the ionosphere and that on the ground by magnetometers at latitudes slightly equatorward of the radar field of view. The observations are discussed in terms of published theories of the generation mechanisms for this type of pulsation. Different signatures are observed by SABRE depending on the level of magnetic activity and the position of the SABRE field of view relative to the pulsation generation region. A twin source model for Pi2 pulsation generation provides the clearest explanation of the signatures observed.
Highlights
Pi2 pulsations occur simultaneously with substorm expansion phase onset and subsequent intensifications
The differences in pulsation period may result from the complex system of field-aligned currents (FAC) and ionospheric currents present at the time of the pulsation which suggest that the Sweden and Britain Radar-aurora Experiment (SABRE) field of view is located close to the generation region of the pulsations whereas Wick is equatorward and observes the single resonant pulsation period normally seen at lower latitudes
The time series signature of the SABRE Pi2 pulsation is highly dependent on the position of the SABRE field of view relative to the substorm current wedge and/or pulsation generation region
Summary
Four Pi2 pulsations occurred in the interval 20:40 UT to 21:20 UT on day 101, 1983 (Fig. 6). The size of the bays suggest weak substorm electrojets at the time of the first two pulsations and indicate a westward electrojet at 20:47 UT centred between KAU and ALT, which changes to an eastward electrojet at :20:57 UT. There is no significant X-component bay activity at either PEL or SOD If extended westwards, this eastward electrojet would be at a similar geomagnetic latitude to the location of the poleward, continuous velocity region observed within the SABRE field of view which is located equatorward of the auroral zone generation region for the Pi2 pulsations during this interval. This region moves slowly equatorward during the interval but remains separate from the second, smaller backscatter region which is equatorward of 67.0°N and appears and region of backscatter observed by SABRE during this period has a scale size of this order or less, which may account for some of the differences in the signatures observed
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