On the relationship between the magnetic and VLF signatures of the substorm expansion phase

We firmly establish a certain class of post-midnight ELF/VLF wave events as a signature of the substorm expansion phase. These substorm chorus events (SCEs) have been observed near L = 4 at Halley, Antarctica (76°S, 27°W), by the VELOX instrument. By examining 4 years (1992–1995) of nearly continuous Halley VELOX data, we identified 973 SCEs. From this database a subset of 258 events was selected, occurring within ±1 hour of local magnetic midnight at Halley, that is, between 0200 and 0400 UT. The epochs of these events were used to perform a superposed epoch analysis on the three components of the Halley fluxgate magnetometer data. A significant negative bay in the magnetic H component was observed to start at the SCE epoch, as expected for a ground station near the westward electrojet of the substorm current wedge. Positive and negative bays, respectively, were seen in the D and Z components. We interpret the detailed features of the composite magnetograms and conclude that the SCE is an unambiguous signature of substorm expansion phase onset. Given this association of the SCE with the substorm expansion phase based on the qualitative features of the superposed epoch ground magnetic signature, this paper also provides the most complete quantitative description to date of both the VLF and magnetic substorm signatures observed at a near-auroral zone ground station.