ICS-13 Abstract View


Recent progress in the understanding of the substorm onset instability
Kalmoni,N.M.E., nadine.kalmoni@ucl.ac.uk (1)
Rae, I.J., jonathan.rae@ucl.ac.uk (1)
Murphy, K.R., kyle.r.murphy@nasa.gov (2)
Forsyth, C., colin.forsyth@ucl.ac.uk (1)
Watt, C.E.J., c.e.watt@reading.ac.uk (3)
Samara, M., marilia.samara@nasa.gov (2)
Michell, R.G., robert.g.michell@nasa.gov (2)
Grubbs, G., guy.grubbs@nasa.gov (2)
Yeoman, T.K., yxo@leicester.ac.uk (4)
Owen, C.J., c.owen@ucl.ac.uk (1)
Fazakerley, A., a.fazakerley@ucl.ac.uk (1)
Recent work has highlighted that exponentially growing periodic features, nicknamed ‘auroral beads', form longitudinally along the substorm onset arc, indicating the action of a plasma instability. Since the substorm onset arc lies on closed magnetic field lines, this demonstrates that the instability exists in the near-Earth magnetotail, Earthward of the tail reconnection site.

We conduct a statistical study of independently identified substorm onset arcs to identify any characteristic spatial scales present along the substorm onset arc. In contrast to the prevailing assumption that auroral beads are a special case phenomenon, we show that they are present along the vast majority of substorm onset arcs. These beads have significantly smaller amplitudes relative to the background auroral arc, making them invisible to the eye without such quantitative analyses. This reveals that auroral beads are highly likely to be ubiquitous to all onset arcs.

Our results statistically show that these auroral beads grow exponentially through onset, with growth commencing prior to the large-scale exponential growth of auroral intensity typically associated with auroral substorm onset. We thus conclude that a magnetospheric plasma instability on closed field-lines is fundamental to the release of stored energy in the magnetotail during substorms.

Finally, we utilise multispectral observations of a substorm onset arc to further understand the auroral acceleration processes driving auroral beads. Together with in-situ measurements, we discuss the potential magnetospheric drivers of periodic perturbations of the substorm onset arc.
(1) Mullard Space Science Laboratory, University College London, Dorking, UK
(2) NASA GSFC, Greenbelt, MD, USA
(3) Department of Meteorology, University of Reading, Reading, UK
(4) Department of Physics and Astronomy, University of Leicester, Leicester, UK