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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are needed to see this picture.

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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”