1. Substorm Onset – Belief vs. Fact
Eric Donovan

2. Remind everyone where this started with Akasofu 1960s work/publications.
Remind everyone that brightening of a new or pre-existing arc is fundamental to the substorm.
State that the talk finishes with a list of unanswered questions including “What causes the arc?” and “Where do the field lines threading the arc map to?”

3. Remind everyone where this started with Akasofu 1960s work/publications.
Remind everyone that brightening of a new or pre-existing arc is fundamental to the substorm.
State that the talk finishes with a list of unanswered questions including “What causes the arc?” and “Where do the field lines threading the arc map to?”

4. Remind everyone where this started with Akasofu 1960s work/publications.
Remind everyone that brightening of a new or pre-existing arc is fundamental to the substorm.
State that the talk finishes with a list of unanswered questions including “What causes the arc?” and “Where do the field lines threading the arc map to?”

5. Remind everyone where this started with Akasofu 1960s work/publications.
Remind everyone that brightening of a new or pre-existing arc is fundamental to the substorm.
State that the talk finishes with a list of unanswered questions including “What causes the arc?” and “Where do the field lines threading the arc map to?”

6. Irrefutable: [There is a class of substorm for which] auroral onset is on field-lines threading the “night-side cusp” or region of transition between tail-like & dipolar field lines.

7. Onset Arc Maps Here
From this Tony Lui (Lui & Burrows, GRL, 1978) and many that follow have used the location of
the arc relative to the Hbeta as evidence that the onset arc maps, topologically, here (above)….
The others are (in order) Roux (91), Samson, Voronkov, Lyons, Donovan, Liang, Nishimura, Haerendal, Rae, etc.
For now I’ll take this as fact… the onset arc maps to the inner edge of the TCS.

8. Inner Edge of TCS e- CPS Onset at OCB - PBIs Inner Edge TCS
Poleward Border 630 nm
Remind everyone where this started with Akasofu 1960s work/publications.
Remind everyone that brightening of a new or pre-existing arc is fundamental to the substorm.
State that the talk finishes with a list of unanswered questions including “What causes the arc?” and “Where do the field lines threading the arc map to?”
NER
Inner edge e- CPS
5-7 Re
Onset Arc maps
to here
Hβ>5 R

9. 1 What is wrong with these arguments?
Our understanding of the proton auroral mapping is a bit overstated.

10. Inner Edge of TCS e- CPS Onset at OCB - PBIs Inner Edge TCS
Poleward Border 630 nm
Remind everyone where this started with Akasofu 1960s work/publications.
Remind everyone that brightening of a new or pre-existing arc is fundamental to the substorm.
State that the talk finishes with a list of unanswered questions including “What causes the arc?” and “Where do the field lines threading the arc map to?”
NER
Inner edge e- CPS
5-7 Re
Onset Arc maps
within this region
Hβ>5 R

12. Remind everyone where this started with Akasofu 1960s work/publications.
Remind everyone that brightening of a new or pre-existing arc is fundamental to the substorm.
State that the talk finishes with a list of unanswered questions including “What causes the arc?” and “Where do the field lines threading the arc map to?”

13. Arc Stops Moving
Equatorward
Pi1B Onset
Structure Onset
Optical Onset

14. 0508
0530
THEMIS D Bz
THEMIS B Bx
THEMIS A
1e7,8
ESA Electrons
1e3,4
ESA Ions
THEMIS E

15. 1e7,8
THEMIS A
1e3,4
Note eletron range is 4-8

18. 3

19. In the TCS away from the dipolar region… NENL
What (magnetotail instability) causes substorm (auroral) expansion phase onset?
The brightening aurora signifies something dramatic happening in the magnetotail.
There are two scenarios/paradigms for how this unfolds in the tail.
The two scenarios differ in terms of where the *instability* occurs (and what the free energy source is for the initial instability).
People speak of NENL vs CD, or alternately Out-In vs In-Out… I personally like a third way of saying this ….
“starts in a region surrounded by Thin Current Sheet” or “starts at the inner edge of TCS”.
The key thing is that in the two different scenarios, the release of free energy that corresponds to the
expansion phase is fundamentally different (Lobe Flux Reconnection vs Interchange for example)
“Out-In”
“In-Out”
In the TCS away from the dipolar region… NENL
In the transition between the TCS and the dipolar region… CD (CLI) or Ballooning

20. Classic Tony: Instability at inner edge of TCS arises spontaneously
Classic Tony: Instability at inner edge of TCS arises spontaneously. Rapidly changing topology draws flux tubes in from TCS leading subsequently to NER.
“In-Out”
Toshi: Burst of reconnection at the DNL forms a low-entropy flux tube which meanders through CPS all the way to inner edge then moves azimuthally along inner edge… makes a marginally unstable inner edge unstable (necessary for onset in this scenario).
Shiokawa: Instability in TCS drives NER… BBFs to inner edge lead subsequently to flux pileup (dipolarization).
So what is my point… well I have two….
1) the new variations of In-Out & Out-In that are being brought forward as a consequence of the Toshi et al results are at odds with one another;
2) I do not see how a causal connection between the streamers and onset have been made on the one hand nor how the fast mode to PSBL to
ionosphere explanation of the PBIs/steamers can be correct on the other. More than that --- I think the new Toshi scenario may be
shown to be necessary and causal, but I simply do not think the NER to PSBL to ionosphere explanation of the streamers can be correct.
So I guess in terms of the age-old argument, I believe that the NER camp should not be buoyed by Toshi’s results….
Toshis’s work, if it holds up, is a triggered in-out scenario. *Period* ----
Alternate: The NER is magnetically conjugate to, and directly causes the brightening arc.
“Out-In”
Modified Toshi: Instability in TCS drives NER… fast mode waves propagate to PSBL then ionosphere… here the PBI, streamer, auroral brightening occurs via the path NER ►► PSBL ►► Ionosphere.

21. Auroral/Ionospheric data points (mostly) towards Inside-Out.
We want to understand the magnetotail instability that corresponds to “onset”
Don’t get lost in semantics: do not base connections between phenomena on labels.
We do not know what causes the onset arc [the onset arc is likely not an FLR].
The brightening might signify CD, ballooning, flux pile up, or runaway stretching.
There is uncertainty in where the onset arc maps to at the end of the growth phase.
There is uncertainty in the mapping of the poleward shoulder of the H+ aurora.
We do not know what the equatorward boundary of 630 nm aurora corresponds to.
Auroral/Ionospheric data points (mostly) towards Inside-Out.
Magnetotail data points many towards Outside-In.
Tony: some (most “first) onsets are Inside-Out.
Larry: PBI/DNL-MR►Streamer/BBF►Instability at TCS-IE accounts for >80%
Vassilis: >80% of events are Outside-In
Kepko: pre-onset faint auroral signatures prior to optical onset supports outside-in.
Donovan: events where optical data supports untriggered inside-out.
Others: Different interpretation of Toshi scenario.
Toshi/Larry scenario: compelling images but correlation does not prove causality.
I bet there will be more definitive statements from the NENL/NER folks, however I think that the jury really is still out!

22. Challenge: There are things we have a very poor understanding of, but that may be of central importance to our ultimate understanding of the expansion phase onset. What, for example, is the role of ions of ionospheric origin in the expansion phase onset?
Chappel, C., T. Moore, and J. Waite, The ionosphere as a fully adequate source of plasma for the earth's magnetosphere, J. Geophys. Res., 92, 5896–5910, 1987.

23. Convection moves mass to, through, and ultimately out of the system.
Inner CPS
Mid-Tail
High-latitude CPS

24. The substorm loading/unloading (growth, expansion, recovery) cycle is superposed on the large-scale convection cycle.
BBF Braking
Dipolarization
Injection
Current Disruption
Current Diversion
Fast Flow
Reconnection
Rarefaction Wave
Inner CPS
Sausage & Kink
Modes
BBFs
Reconnection
Challenge: We have inadequate observations of the convection electric fields during the expansion phase. We do not include background convection in most (all?) simulations of the onset instability.
Plasmoids
Mid-Tail
Flux Ropes
High-latitude CPS

25. Remind everyone where this started with Akasofu 1960s work/publications.
Remind everyone that brightening of a new or pre-existing arc is fundamental to the substorm.
State that the talk finishes with a list of unanswered questions including “What causes the arc?” and “Where do the field lines threading the arc map to?”

26. Remind everyone where this started with Akasofu 1960s work/publications.
Remind everyone that brightening of a new or pre-existing arc is fundamental to the substorm.
State that the talk finishes with a list of unanswered questions including “What causes the arc?” and “Where do the field lines threading the arc map to?”

27. 1 Why do I believe it is Inside-Out?
I have many reasons for being an adherent of the in-out scenario… I show five of them (I have more reasons!) on this and the following four slides
This slide… 1) Location of auroral onset on field lines threading the transition region (see my concerns below though)
We know where the arc is relative to the H+ aurora, and we have a good idea what the H+ aurora corresponds to (topologically) in the magnetotail.

28. 1 Why do I believe it is Inside-Out? Onset Arc in ~10 R H+ aurora
Onset Arc Maps Here
We know where the arc is relative to the H+ aurora, and we have a good idea what the H+ aurora corresponds to (topologically) in the magnetotail.

29. 2 Why do I believe it is Inside-Out? 10 nT
The arc that brightens brightens in a way that looks like a parameter describing an instability…. More or less always it starts on an azimuthally extended stretch of the arc essentially simultaneously (unstable azimuthally extended region in the tail), more or less always brightens like e^(t/Tau) for 3Tau or so until saturation (Igor V. showed this was what one expects for growth of certain types of ballooning at the inner edge), the saturates (again as one expects from an instability at the inner edge). [by the way – a challenge here for the BBF leading to flux pileup would be to show through simulation or other that flux pileup would lead to this e^t/tau growth of brightness of the arc

30. 2 Why do I believe it is Inside-Out?
As the arc brightens, azimuthal structure develops (high-m number, azimuthally propagating “beads”).
It is compelling to relate that structure and how it evolves in time to azimuthal wavelengths of the magnetotail instability (as projected into the ionosphere)
**A lot of work has been done on this starting with Roux, Samson, Voronkov, Donovan, Liang, Liu, Saito…
this work (probably culminating in the Liang 2008 and Saito 2008 papers) concludes that if these beads are manifestations of distortions of the TCS then
characteristics of the growing mode in the magnetotail are consistent with ballooning at the inner-edge** THIS WOULD HAVE BEEN MY ENTIRE TALK IF NOT TUTORIAL
This structure has not been proven to be a projection of magnetotail time-evolving structure; however, Uritsky’s “arc wave” work does prove that
sometimes waves on arcs like these do result from tail-flapping.
Note… this is not an FLR if anyone asks… it just is not. If anyone presses you – tell them “Eric said he’d shoot me if I gave an inch on that one” 
Partial images (and difference frames) from THEMIS ASI at Athabasca.

31. 3
Why do I believe it is Inside-Out?

32. 3
Why do I believe it is Inside-Out?

33. 3
Why do I believe it is Inside-Out?

34. 3
Why do I believe it is Inside-Out?

35. 4 Why do I believe it is Inside-Out? THEMIS A 1e7,8 1e3,4
My events (this and others I’ve looked at) show evolution in the tail from in to out. I think the work in my 2008 GRL is pretty careful in terms of mapping argument and probably needs to be addressed at some point,

36. 1 What is wrong with these arguments?
Our understanding of the proton auroral mapping is a bit overstated.

37. 1
What is wrong with these arguments?

38. 1
What is wrong with these arguments?
12%
23%
30%
50%

39. 2 We do not know what causes the onset arc!
Auroral onset occurs on a new (in this case) or pre-existing quiet arc at the equatorward edge of the redline and poleward slope of the proton aurora. It is widely agreed that this statement is true.
In fact, since this is embedded in Akasofu’s original definition of the phenomena, it is irrefutable.
However, we do not know what causes the onset arc (late growth phase). Therefore, we cannot say why, for example, the arc got brighter, or why the arc beads (we can infer… but we can’t say).
The single most agreed-upon substorm observation is of limited use in terms of assessing theories.
I have to say I’m heartened by data requests from Xiaoyan Xing (Larry’s student) that indicate this point is now getting some more serious attention.
We do not know what causes the onset arc!

40. 3
What is wrong with these arguments?

41. 4 What is wrong with these arguments?
There is an argument against the “in-out” scenario that is very difficult to refute (but equally difficult to substantiate)… fast flows from the NENL cause the breakup (precede the auroral brightening) but are narrow and missed by the satellites and have no ionospheric signature…. What are the ionospheric signatures of NENL formation? What are the ionospheric signatures of the fast flow? Is it enough that the arcs poleward of the onset are often undisturbed until after the breakup?
My events (this and others I’ve looked at) show evolution in the tail from in to out. I think the work in my 2008 GRL is pretty careful in terms of mapping argument and probably needs to be addressed at some point,

42. Summary
The central substorm question, “what is onset?”, is not resolved.
I believe some events are “inside out” (onset at inner edge of TCS).
There are significant difficulties with much of the evidence used as evidence of onset at the inner edge of the TCS. Progress forward proving or disproving the “inside out” scenario requires…
1) We need to determine quantitatively where the onset arc maps to (which means determining the magnetospheric projection of the late growth phase proton aurora;
2) We need to determine what causes the growth phase arc.
3&4) We need to resolve the “you did not see it but it was there” issues.
I bet there will be more definitive statements from the NENL/NER folks, however I think that the jury really is still out!