Properties of Auroral Beads Observed During Non-Substorm Times

Abstract: The formation of auroral beads often occurs before auroral substorms and is closely related to substorm onset instabilities. Auroral beads are uniquely discussed in terms of their relationship with substorm onset, and almost all substorms are preceded by auroral beads. Substorms are easily identifiable in auroral data and by using pre-existing catalogs or automatic routines. Conversely, auroral beads are relatively difficult to identify automatically, and if they appear at other times are often overlooked. Using the Time History of Events and Macroscale Interactions During Substorms All Sky Imagers (THEMIS ASI), we identified and categorized over 300 auroral bead events from 2007 to 2016. We observed auroral beads leading to substorms around 90% of the time, but 10% of the auroral beads formed preceding non-substorm events (e.g., pseudo-breakups). To differentiate between substorm and non-substorm times, we analyzed auroral signatures in THEMIS ASI focusing on properties such as the location and poleward movement of the initial brightening arc. In this presentation, we show results of a statistical study comparing properties (e.g., growth rates, propagation directions) of substorm and non-substorm beads as well as their dependence on geomagnetic indices and solar wind parameters. The presence and characteristics of auroral beads preceding non-substorm events is significant for the identification of their source instability and relationship to the substorm onset process.