1. Auroral Arcs in the Late Growth Phase
Eric Donovan
Thanks to: Knudsen, Spanswick, Liu, Uritsky, Liang, Jackel, (G) Baker, Nishimursa, Mende
Theories about and observations of arcs abound; however, we do not agree on what arcs are, how they “start”, how they are generated, what modulates their brightness, whether or not there are different types of arcs (meaning different underlying physics), and what they correspond to in the magnetosphere.
Arcs are (almost) ubiquitous. Resolving key questions we have about arcs is an important objective in space physics.

2. 01
Mesoscale Arcs are observed at essentially all local times and at latitudes throughout the oval (there is a population of “arcs” in the polar cap but they are likely not the same phenomenon that we see in the CPS – further we have not for example determined whether or not they are on closed field lines).
noon
dusk

3. 02
The black line is total downward e- energy flux, and the grey shaded region is total downward ion energy flux. The coloured curves are ion Jperp/Jpar for four energy channels. The ion IB is where the four coloured curves deviate from 1.
The black curve shows four mesoscale arcs in the PS (eg., multiple arcs). The arcs are in the PS, and poleward of the ion IB. I think is generally true (Lyons stated this once in a paper in the 1970s without any proof – but I think he “knew” this too): All mesoscale arcs are poleward of the ion IB. Thus ALL mesoscale electron arcs are on field lines where there is a full downward ion loss cone…. What does this mean? Are the ions required for the electrodynamics?

4. 03 Region of the tail “populated” by mesoscale arcs.
Ion IB maps to transition between dipolar and tail-like
Note again that the arcs are poleward of the IB and we think the IB marks the transition from dipolar to tail-like – thus multiple arcs spanning the PS poleward of the IB suggest mesoscale arcs are a phenomenon that occur on field lines threading the (thin) tail current sheet.

5. 04
Arcs exist as unbroken features that span many hours of MLT (in this case >6 hours of MLT). This is a mosaic created from THEMIS ASI images.

6. 05 [Lassen & Danielsen, JGR, 1989]
The arcs that are in the oval are typically aligned with the oval... In other words arc alignment is generally parallel to the oval pole/equatorward boundaries.
Arcs must correspond to magnetotail gradients (the oval is the projection into the ionosphere of the CPS). Is that gradient too small to be measured?

7. 06 Late growth phase (“onset”) arc plotted in geomagnetic coordinates.
The arc is essentially exactly parallel to the constant geomagnetic latitude. This is something we are seeing as often true for a class of onsets studied this year by Liang et al. and Donovan et al. [both in GRL in 2008].
Onset arcs seem to be aligned along a constant MLat.

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9. 07 θ

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11. 09

12. 10
Mapping Factor

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14. Dramatically Reduced Bz12
Sharp Transition
Dramatically Reduced Bz

15. 13
We need a more systematic observations of auroral parameters: orientation, widths, temporal variation, etc. (reconcile with models).
(2) Arcs are always a magnetotail phenomena (poleward of IB).
(3) Mesoscale arcs align with the oval… arcs correspond to the gradient of some magnetospheric parameter, but we do not know what that is.
(4) In the late growth phase the onset arc is more consistently aligned along an MLat constant.
(3) The alignment of arcs in the late growth phase is a consequence of how extreme thinning affects the mapping of that magnetotail gradient into the ionosphere.
(5) Arc theories need to take account of the relation between arcs and the current sheet.
(6) We need to identify the magnetotail gradient that corresponds to arcs.

16. The Knudsen et al. [GRL, 2001] study determined the mesoscale auroral arc width distribution (figure at left). We asserted that the mesoscale population was possibly distinct from the small-scale arcs (eg., not just an extension of the same distribution) and /or that there may be scale sizes that are suppressed by the MI coupling electrodynamics.

18. FLRs and Arcs: Certainly some auroral arcs oscillate in a way that supports the FLR model which is widely viewed as the most successful model of the generation of arcs.

19. ... but here is the problem... I do not want to sound flippant but the reality is that some arcs oscillate and some do not (in fact I assert most do not). The plot at left shows an oscillating arc reminiscent of an FLR and the figure at right shows an essentially stationary arc.

20. Arcs
FLRs
More than that, multiple arcs are hard to reconcile with the FLR model. Further, Pc5 pulsations peak in occurrence at dawn and dusk while arcs peak in occurrence around 2300 MLT (that result is to be published soon). All in all, it is safe to say “the jury is out on what causes arcs”.

21. Waves on arcs (as opposed to arcs as waves): azimuthally propagating waves are virtually always present – undoubtedly some are a projection of magnetotail waves and some have their origin in lower altitude processes...
Although these waves seem to be always present, they are small perturbations of the arc brightness (~1% level).
Uritsky and colleagues are exploring using arc waves to remote sense magnetotail dynamics.

22. Often, multiple macroscale auroral arcs span the latitude extent of the auroral oval. In these cases there are typically 4 or 5 arcs more or less evenly spaced in latitude. The plot at left is the brightness along the white line superposed on the image at right.

23. Liu and Burrows, GRL, 1978
Onset arc location: In many cases, the onset arc is “glued” to the equatorward boundary of the redline aurora and on the poleward slope of the H+ aurora. This, the late growth phase arc alignment and 2D observations of DI lead me to assert that the onset arc is a marker of a sharp transition between dipolar and tail-like field lines.

24. Onset arc evolution: There is a sudden transition to “brightening”
Onset arc evolution: There is a sudden transition to “brightening”. The brightening is exponential (τ~ 45 seconds ) and lasts ~3τ. Saturation is sudden and usually corresponds to larger scale vortex formation.