Imaging riometer observations on energetic electron precipitation at SANAE IV, Antarctica

Abstract

The expansion phase of a substorm on the evening of 1 May 1997 was studied from recordings of all‐sky optical aurora and a 64‐beam imaging riometer at the Antarctic base SANAE IV (L = 4.1). Digitized all‐sky, low‐level white light images of auroral optical emissions were mapped onto the angular directivity functions of the 64‐beam imaging riometer. During the period of investigation, several optical arcs appeared and disappeared, while a persistent absorption structure appeared in the south (poleward). This and new structures changed only slowly in time. The differences in morphologies of optical emissions and absorption regions, together with temporal differences, imply that there are two categories of energetic auroral electrons: the softer electrons (<10 keV) causing optical emissions in the E and F regions of the ionosphere and the harder (>10 keV) electrons ionizing down into the D region for cosmic radio noise absorption (CRNA). Optical emissions tend to lead the appearance of cosmic radio noise absorptions by 0–60 s in a specific region of ionospheric space. Furthermore, in the all‐sky optical data, westward traveling surges (WTS) were observed coincident with a CRNA WTS and leading the CRNA WTS by ∼30–50 s. In the CRNA data the WTS was ribbon‐like, traveling westward at a speed of 2.3 km s−1. These observations suggest that the softer electrons, causing E and F region optical emissions, may be injected onto drift paths closer to Earth than the more energetic electrons, ionizing down into the D region for cosmic radio noise absorption. The harder electrons could have been accelerated by, for instance, an increasing dawn‐to‐dusk electric field for later precipitation.