solar wind-magnetosphere coupling, substorms, and ramifications for ionospheric convection

Solar wind-magnetospherecoupling, substorms,and ramifications for ionospheric convection

steve.milan@ion.le.ac.uk

Steve MilanAdrian Grocott (Leics, NIPR)Suzie Imber (GSFC)Peter Boakes (Leicester)Benoit Hubert (Liège)

SuperDARNWorkshopDartmouth, 2011

What is the magnetic flux throughputof the magnetosphere?

steve.milan@ion.le.ac.uk

Milan (2009)

Faraday (1831)Siscoe and Huang (1985)

Cowley and Lockwood (1992)

NDPC

dt

dF

The expanding/contracting polar cap

steve.milan@ion.le.ac.uk

substorms

0.0 GWb

0.3 GWb

0.6 GWb

0.9 GWb

PCF5 June1998

Milan et al.(2003)

Substorm Substorm

steve.milan@ion.le.ac.uk

Polar UVI

Wind

0.0 GWb

0.3 GWb

0.6 GWb

0.9 GWb

PCF5 June1998

Milan et al.(2003)

Substorm Substorm

steve.milan@ion.le.ac.uk

Polar UVI

Wind21

222 sin3 ZYSWED BBVR

0.0 GWb

0.3 GWb

0.6 GWb

0.9 GWb

PCFSubstorm Substorm

Milan et al.(2003)

steve.milan@ion.le.ac.uk

5 June1998

Polar UVI

Wind

NDPC

dt

dF

Cross polar cap potential

steve.milan@ion.le.ac.uk

Expansion/contraction of polar cap

Cross polar cap potential forsymmetric, circular polar cap,

measured along dawn/dusk meridianin absence of viscous interaction,lobe reconnection, frictional drag

Cross polar cap potential is nota good measure of dayside

coupling, nor is it constrainedto be instantaneously

equal in both hemispheres

NDPC 21

20 August – 6 September, 2005

QuestionsSolar wind-magnetosphere

coupling leads to theoccurrence of substorms,

but...

- What “triggers” onset?

- What controls the rate and size of substorms?

- Why does the auroral oval move to very low latitudes during disturbed

conditions?

steve.milan@ion.le.ac.uk

Milan et al. (2008)

Magnetotailsignatures

Good comparison withground signatures of

substorms in AU and AL

Cluster shows magneto-tail inflation duringgrowth phase, and

deflation and dipolar-ization after expansion

phase onset

Milan et al. (2008)

steve.milan@ion.le.ac.uk

Substorm occurrenceand size

Substorm occurrenceincreases with solar wind coupling

And the change in size ofthe polar cap increases,i.e. the amount of fluxreleased in each substorm

Occurrence x size gives alinear dependence:

flux out = flux in <ΦN> = <ΦD>

<ΦD>0.6

Milan et al. (2008)

steve.milan@ion.le.ac.uk

<ΦD>0.4

Open fluxcontrol ofsubstormintensity

Superposed epochanalyses of auroralintensity, open flux,AU and AL, Sym-H,and SW-couplingduring 40 substorms

Substorms binnedby open flux at onset

auroralintensity

openflux

AU, AL

Sym-H

D

steve.milan@ion.le.ac.uk

Milan et al. (2009a)

steve.milan@ion.le.ac.uk

Milan et al. (2009a)

auroralintensity

openflux

AU, AL

Sym-H

D

Open fluxcontrol ofsubstormintensity

Superposed epochanalyses of auroralintensity, open flux,AU and AL, Sym-H,and SW-couplingduring 40 substorms

Substorms binnedby open flux at onset

steve.milan@ion.le.ac.ukMilan et al. (2009a)

Proton aurora Electron aurora

Proton aurora

steve.milan@ion.le.ac.uk

Grocott et al. (2009)

-20 min

69 kV

-10 min

69 kV

onset71 kV

+10 min

51 kV

+20 min

62 kV

+50 min

55 kV

-20 min

41 kV

-10 min

43 kV

onset40 kV

+10 min

42 kV

+20 min

46 kV

+50 min

48 kV

onse

t la

titu

de

Convection velocity in onset region

Superposed epochanalysis of convection

- High latitude substorms have prompt convection response

- Low latitude substorms have convection decrease at onset;

convection delayed!

Substorm electrodynamicsinfluenced by auroral bulge

conductivity

Low latitude onset substorms aremore intense than high latitude

onset substorms, but...

What controls the onset latitude?Why does the magnetosphere allow itself to accumulate more

open flux prior to some substorms than others?

Milan et al. (2008)

steve.milan@ion.le.ac.uk

Close relationshipbetween ovalradius andring currentintensity

steve.milan@ion.le.ac.uk

Milan et al. (2009b)

Changes inoval radiusassociated

withsubstorms

Milan et al. (2009b)

ConclusionsThe expanding/contracting polar cap paradigm provides a

theoretical framework for understanding solar wind-magnetosphere coupling and substorms

The ECPC is fundamental to the excitation of ionosphericconvection and is central to SuperDARN science

The polar cap expands more prior to substorm onsetwhen the ring current is enhanced

Lower latitude substorms have a greater auroralintensity and stronger electrojets

This in turn changes the ionospheric convectionresponse to tail reconnection, delaying convection

until dissipation of auroral signatures

Northward IMF: lobe reconnection(Imber et al., 2006, 2007)

steve.milan@ion.le.ac.uk

IMA

GE F

UV

IMA

GE d

ata

cou

rtesy

of

Ste

ph

en

Men

de,

Hara

ld F

rey a

nd t

he IM

AG

E F

UV

team

IMAGE FUV

IMAGE FUV/WICobservations allow

identification ofsubstorms and

quantification ofchanges in

polar cap flux FPC

FPC increases duringsubstorm growth

phase and decreasesafter expansion

phase onset

Milan et al. (2008)

steve.milan@ion.le.ac.uk

Superposed epochanalysis of ~2000substorms keyedto Frey et al. (2004)substorm list, binnedby onset latitude

• binned by onset latitude • -1 to +2 hours from onset • 10-min bins

AfterFrey et al. (2004)

Solar windparametersand othersubstormindicatorsare alsowell-organizedby substormonset latitude

IMF BZ

VSW

NSW

PSW

AU, AL

Sym-Hsteve.milan@ion.le.ac.uk

Milan et al. (2009a)

The polar cap fluxshould grow largestwhen a lot of flux isopened betweensubstorm onsets

Fluxaccumulation

Integrated daysidereconnection rate

Milan et al. (2008)

Substorm occurrence:greatest when solarwind coupling isenhanced

The level of fluctuation in FPC,a measure of the flux closureduring substorms:larger when the solarwind coupling is enhanced

How big?

How often?

Milan et al. (2008)

Boakes et al. (in preparation)

Superposed epoch analysis of open flux, sub-divided bygeosynchronous particle injection signaturesClassic, isolated substorm injection Continuous, disturbed injection No injection

FPC

BZ BT

|BY |

Substorms driven by BZ < 0 nT; SMC driven by large BY ?