Characterising Substorm Auroral Dynamics; A Statistical Analysis of the Morphology and Motion of the Aurora through the Substorm Growth and Early Expansion Phase.

Tuesday, 16 December 2014
Kyle R Murphy1, Sarah Bender2, Larry Kepko3, Clare Watt4, Jonathan Rae5, David G Sibeck6, Ian Robert Mann7, Eric Donovan8, Emma Spanswick8, Brian J Jackel8 and Harald U Frey9, (1)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (2)Pepperdine University, Malibu, CA, United States, (3)NASA GSFC, Greenbelt, MD, United States, (4)University of Reading, Reading, United Kingdom, (5)University College London, Mullard Space Science Laboratory, London, United Kingdom, (6)NASA Goddard Space Flight Center, NASA, Greenbelt, MD, United States, (7)University of Alberta, Edmonton, AB, Canada, (8)University of Calgary, Calgary, AB, Canada, (9)University of California Berkeley, Berkeley, CA, United States
Careful analysis of auroral morphology can be used to indirectly probe the magnetospheric counterparts to energetic particle precipitation and magnetic perturbations in the ionosphere, providing insight into the processes generating these perturbations and the location in the magnetotail where these processes occur. During the substorm growth phase and early expansion phase, observations of the electron and proton aurora provide the necessary measurements to map magnetospheric boundaries to the ionosphere, in particular the open-closed field line boundary and the nightside transition region, which separates the nearly dipolar inner field line topology from the highly stretched field line region that corresponds to the thin current sheet. In this study we present a statistical analysis of the auroral substorm through the growth and early expansion phase using the open-closed field line and transition region boundaries to provide spatial context to ionospheric observations. Using the THEMIS all-sky imagers we provide a detailed analysis of the location and brightness of the onset and poleward arcs through the substorm growth phase. We also characterize the motion of the aurora, detailing its velocity and direction of motion through the substorm growth phase and early expansion phase. The purpose of this study is to develop a clear and consistent picture of auroral dynamics and morphology during the growth phase, through substorm onset, and during the early expansion phase. Preliminary results demonstrate that substorm auroral onset occurs on closed field lines, that the growth phase is characterized by the equatorward motion of the onset arc and that the motion of aurora features in the onset and poleward arcs during the growth phase is predominantly azimuthal.