The Magnetospheric Source Location of the Proton Aurora 01 The Magnetospheric Source Location of the Proton Aurora Eric Donovan, Emma Spanswick, Jim McFadden, Tony Lui, and Vassilis Angelopoulos Acknowledgements: Larry Kepko, Mike Henderson, Brian Jackel, CSA, NASA

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? Where does this aurora come from? 03a

? Where does this aurora come from? 03b Why does this matter? Arcs occur in the CPS poleward of the ion Isotropy Boundary. Data courtesy Mike Wiltberger.

? Where does this aurora come from? 03c Why does this matter? Arcs occur in the CPS poleward of the ion Isotropy Boundary. Onset arc (arc that brightens) occurs on the “poleward shoulder” of the bright H+ aurora. Use of magnetospheric in situ and ionospheric data requires better knowledge of the mapping than we currently have. We do not fully understand the magnetospheric processes that cause of the H+ aurora.

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05 We have an empirical relationship between total (FAST) ESA loss cone ion energy flux and the brightness of the proton aurora (note this excludes higher energy H+ but this is just an empirical relationship and works fairly well). The same instrument was flown on THEMIS. Using THEMIS ESA we can estimate the total loss cone ion energy flux (in the PS in the ESA energy range). If the loss cone is full, this is the same measurement as that made by FAST ESA (for that flux tube – note we can not know where the ionospheric footpoint is but we do not really need to). We identified all the times in 2008 & 2009 when a THEMIS probe was in the near midnight region (-1Re<Ygsm<1Re) and PS (β > 1).

06 Use THEMIS ESA (when in PS) to estimate total ion energy flux in the loss cone. Use ESA to get average total energy flux in the 10 degrees around the loss cone. Use THEMIS FGM |B| to estimate size of the loss cone. Estimate downward ion energy flux (in the ESA energy range) at the ionospheric footpoint (that we do not know!). That number * 65 is our estimate of the footpoint H+ auroral (Hbeta) Intensity.

07 THEMIS Plasma Sheet Observations - Equivalent Proton Auroral Brightness >10 Re 9-10 Re 8-9 Re 7-8 Re 6-7 Re 5-6 Re

08 Result 1: Peak in brightness almost always maps to <9 Re. Result 2: IB almost always maps to >6 Re. >10 Re 9-10 Re 8-9 Re 7-8 Re 6-7 Re 5-6 Re

Where does the onset arc map to in the tail? 09 First Onset

10 Result 3: Onset arc almost certainly maps to inside 10 Re. 12% 23% 30% 50%

11 1) The peak in proton auroral brightness almost always maps to inside 9 Re. 2) The ion IB almost always maps to outside 6 Re. 3) The onset arc almost certainly maps to inside 10 Re. How can we address the “almost” in result 3? Ask the question: “What is the probability that a 10-15 Rayleigh Hbeta aurora occurs on field lines threading the neutral sheet tailward of X (e.g., 8, 9, 10, etc.) Re in the late growth phase See also posters Jiang et al., The location of the onset arc relative to the inner edge of e- CPS Grant et al., The relative location of auroral boundaries inferred from FAST Thank You!

The Magnetospheric Source Location of the Proton Aurora 02 The Magnetospheric Source Location of the Proton Aurora

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THEMIS Plasma Sheet Observations – Field Strength & LC Size

The Magnetospheric Source Location of the Proton Aurora 06 The Magnetospheric Source Location of the Proton Aurora

The Magnetospheric Source Location of the Proton Aurora 05 The Magnetospheric Source Location of the Proton Aurora

Why does this matter? 09 There is a class of substorm for which many argue that it is irrefutable that auroral onset is on field-lines threading the “night-side cusp” or region of transition between tail-like & dipolar field lines.

The Magnetospheric Source Location of the Proton Aurora Eric Donovan, Emma Spanswick, Jim McFadden, Tony Lui, and Vassilis Angelopoulos Acknowledgements: Larry Kepko, Mike Henderson, Brian Jackel, CSA, NASA

The Magnetospheric Source Location of the Proton Aurora 01 The Magnetospheric Source Location of the Proton Aurora

The Magnetospheric Source Location of the Proton Aurora 04 The Magnetospheric Source Location of the Proton Aurora