The relationship between arc characteristics and magnetotail topology in the late growth phase E. Spanswick, E. Donovan, J. Liang, J. McFadden, T.-S. Hsu, C.-P. Wang, S. Zaharia, L. Kepko, T. Lui, and V. Angelopoulos

Where does the arc map to? Mapping using the proton aurora Orientation of growth phase arcs Implications for topology of the late growth phase * Note: I’m ignoring the fact that we don’t know what causes the arc **

Where does the arc map to? We know… In the ionosphere, substorms initiate on arcs [Akasofu 1964, Lui & Murphree 1998, Lyons 2002, ….] Leading up to onset, the arc is embedded in the poleward shoulder of the proton aurora [Fukunishi 1975, Vallance Jones 1982, Samson 1992, Donovan 2008, Sergeev 2012, ….]

The proton aurora – A mapping tool The proton aurora is relatively stable The generation mechanism is thought to be well understood (scattering due to field line curvature). If we can map the proton aurora, we are mapping the arc.

The proton aurora – A mapping tool

The proton aurora – A mapping tool If the luminosity @THEMIS is greater than the observed proton auroral brightness on the ground, we KNOW that the spacecraft maps inside the b2i. If the luminosity @THEMIS is much lower than the observed proton auroral brightness on the ground, we KNOW that the spacecraft maps outside of the region of bright proton aurora.

The proton aurora – A mapping tool

The proton aurora – A mapping tool Tailward of 10Re the predicted proton auroral luminosity is very small. The predicted proton auroral luminosities are extremely large inside of 6Re  the IB is almost always outside of 6Re.

The proton aurora – A mapping tool 12% 23% 30% 50%

The proton aurora – A mapping tool 12% The onset arc is often embedded in 15-20R proton aurora, which is not likely to be coming from the region tailward of 10Re 23% 30% 50%

Where does the arc map to? From the proton aurora… The peak in proton auroral brightness almost always maps to inside 9 Re. The ion IB almost always maps to outside 6 Re. The onset arc almost certainly maps to inside 10 Re. This location is consistent with many other studies and observations. Sergeev et al. 2012, “the breakup arc is situated in a very peculiar place, at the outer edge of a ‘magnetic wall’ region” Jiang et al. 2012, arc is ~1-2o poleward of the equatorward edge of the electron plasma sheet, at or very near the boundary between R1 & R2 currents Lui and Burrows 1978

Where does the arc map to?

Growth Phase Arcs – Orientation Yellowknife, NWT 2012-04-08 UT05

Growth Phase Arcs – orientation How does the orientation of arcs change in the late growth phase?

θ

Alignment of Arcs in the Late Growth Phase: In the late growth phase there is a region of dramatically reduced Bz and radially localized transition between highly stretched and less stretched field lines. 6Re 10Re

Sergeev et al. 2012 6Re 10Re

Take away points The onset arc is located between 6Re and 10Re down tail The change in orientation of arcs prior to breakup is consistent with a sharpening of the transition from less-stretched to highly-stretched field lines during the late growth phase.