Recent THEMIS and coordinated GBO measurements of substorm expansion onset: Do we finally have an answer? Larry Kepko NASA/Goddard Space Flight Center 2011 ILWS Science Workshop Beijing, China
Jan 15, 2016
Recent THEMIS and coordinated GBO measurements of substorm
expansion onset: Do we finally have an answer?Larry Kepko
NASA/Goddard Space Flight Center
2011 ILWS Science WorkshopBeijing, China
ICS-9, Seggau, Austria - 2008
- 45 minute discussion: “What is a substorm?”“[...] the community is more interested in traveling than in working, especially since the same ideas about substorm processes are being debated today as were being debated twenty years ago. One cynic has even suggested a conspiracy theory of substorms, that the principals have agreed not to solve the problem in order to keep the field alive.” - Chris Russell, 1994.
A substorm is a global process by which the magnetosphere unloads stored energy,
arising from imbalanced convection.
IMF turns South
Magnetopause rate
Tail rate
~20 min
Substorm
Eastward electrojets
Westward electrojets& substorm current
wedge
But the solar wind doesn’t always play nice.
Distilling the substorm problem to an inside-out/outside-in controversy ignores the question of what is happening all the time
‣ How do CD models account for non-substorm activations, that appear to have similar temporal sequence as substorm onsets?
Does the magnetosphere generate events with same characteristics (aurora, Pi2, flows) etc., in 2 different manners?
What is a substorm?
In the question of what is or is not a substorm, there is (often) an underlying, tacit
disagreement:
Or:
Isolated flow bursts, pseudo-breakups and substorms are a continuum of the same
underlying physical process(es), and differ only in scale (region) or magnitude/intensity
Isolated flow bursts, pseudo-breakups and substorms are fundamentally different
phenomena.
Distant X-line
Transition region
Earth
Equatorward
boundary
Open/closedboundary
Dipolarization
Pi2 & SCW
white light
Expansion
Traditional NENL
NENL
Flow
Nothing?
Time
Au
rora
l Z
on
eM
ag
neto
sphere
Ionosphere
Magnetosphere
White light ASI
8
Rx
Rx
Auroral onset region
The reconnection site and auroral onset are
not magnetically connected
SDO AIA 094SDO AIA 304
SS
TESA
24 hours
SDO AIA 4500
0523:15 - 6300 activity0526:47 - 5577 arc forms0530:07 - WL Beads0530:31 - Poleward Exp
Quick Review
We observe a pre-onset,
equatorward moving diffuse auroral patchKepko et al., 2009 GRL
Distant X-line
Transition region
Earth
Equatorward
boundary
Open/closedboundary
Dipolarization
Pi2 & SCW
6300 diffuse aurora (this
talk)
white light
Expansion
NENL
Flow
Traditional NENL
Time
Au
rora
l Z
on
eM
ag
neto
sphere
Distant X-line
Transition region
Earth
Equatorward
boundary
Open/closedboundary
Pi2 & SCW
Instability growth
???
Poleward expansion
Raref
actio n
Flow
Expansion
NENL
Flow impact effects ?
NENL
Flow
Current Disruption (CD)
Time
Au
rora
l Z
on
eM
ag
neto
sphere (further)
dipolarization
Two impulses
NENLCD
6300
white light
Two drivers for the ballooning believers: 1) Theory & modeling (always appealing!) 2) Auroral beads at onset
Rays Beads ExpansionGrowth
Beads occur just after flow impact & after geo
SCW perturbations
Distant X-line
Transition region
Earth
Equatorward
boundary
Open/closedboundary
(further) dipolarization
Pi2 & SCW
???
Poleward expansion
Raref
actio n
Flow
Expansion
NENL
Flow impact effects ?
white light
NENL
Flow
Time
Au
rora
l Z
on
eM
ag
neto
sphere
Instability growth
Current Disruption (CD)
6300
Azimuthal motion
Distant X-line
Transition region
Earth
Equatorward
boundary
Open/closedboundary
(further) dipolarizatio
n
Pi2 & SCW
Flow
???
Instability growth
???
Poleward expansion
Raref
actio n
Flow
Expansion
NENL
Flow impact effects ?
white light
Nishimura/Lyons (triggered CD)
Three impulses
Time
Au
rora
l Z
on
eM
ag
neto
sphere
Azimuthal motion
Distant X-line
Transition region
Earth
Equatorward
boundary
Open/closedboundary
(further) dipolarizatio
n
Pi2 & SCW
Flow
???
Instability growth
???
Poleward expansion
Raref
actio n
Flow
Expansion
NENL
Flow impact effects ?
white light
Nishimura/Lyons (triggered CD)
Three impulses
Time
Au
rora
l Z
on
eM
ag
neto
sphere
1. What are the ground effects of the first flow impact?2. What are the ground effects of the second flow impact?3. When do particle injections occur?4. Two-stage dipolarization?
Conclusions•Majority of THEMIS substorm papers support NENL picture
•Dozens of papers showing flows prior to auroral onset, consistent temporal sequence of events
•Nishimura/Lyons has support (but lacking in situ observations)
•Almost no magnetospheric evidence for CD (theory & ASI)
•Still lack a theory of auroral arcs
•NENL (flow burst) model explains the continuum of activity
•Other models may require different modes
•Coordinated measurements has revolutionized substorm studies
Backup
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White light auroral onset
Gillam THEMIS ASI
South
North
EastWest
reflection
J||
EJ||
J
Generation of 5577 emissions
‣Current system of the flow burst
‣Flow diversion, magnetic shear ahead
e.g., Chen & Wolf (1993); Nakamura et al. (2001); Kepko et
al. (2004)Similar cartoon for Nishimura
Distant X-line
Transition region
Earth
Equatorward
boundary
Open/closedboundary
Dipolarization
Pi2 & SCW
6300 diffuse aurora (this
talk)
white light
Expansion
NENL
Flow
Traditional NENL
Time
Au
rora
l Z
on
eM
ag
neto
sphere
QuickTime™ and aPNG decompressor
are needed to see this picture.
White LightSimilar
response as 5577
6300 Å Responds to low energy
electrons (eV - 1 keV)
5577 Å Responds to both protons and electrons
(>1 keV)
4278 Å Responds to
medium energy
electrons (keV and up)
Gillam multi-spectral all sky imager
“Energetic”