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Abstract

Ionospheric irregularities at low-latitudes, known as equatorial plasma bubbles, tend to occur preferably at post-sunset hours when the vertical plasma velocity during the pre-reversal enhancement is particularly strong. Recently, significant occurrence rates of plasma irregularities have also been reported in the high-latitude regions. Their generation mechanisms are quite different and at present still not fully understood. As these irregularities tend to cluster in three prominent regions of the polar area - cusp, polar cap, and pre-midnight substorm onset sector - we investigate their characteristics for these regions separately. One suggestion for high-latitude irregularity generation is ionisation by soft electron precipitations. These beams of soft electrons are commonly accompanied by bursts of small-scale field-aligned currents (FAC). In a superposed epoch analysis we investigate the relation between the occurrences of plasma irregularities and collocated small-scale FACs. In the cusp, a clear coincidence of the two phenomena is observed. Conversely, in the polar cap small-scale FACs are not so frequent. Therefore we suggest a transport of the irregularities from the cusp region into the polar cap by the general anti-sunward plasma convection pattern. In order to discuss possible relations between plasma irregularities detected in the pre-midnight sector and substorm phases we also compare the event times with the substorm catalogue of Frey and Mende (2006).