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Solar wind-magnetosphere coupling, substorms, and ramifications for ionospheric convection. SuperDARN Workshop Dartmouth, 2011. Steve Milan Adrian Grocott (Leics, NIPR) Suzie Imber (GSFC) Peter Boakes (Leicester) Benoit Hubert (Liège). [email protected]. - PowerPoint PPT Presentation
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Solar wind-magnetospherecoupling, substorms,and ramifications for ionospheric convection
Steve MilanAdrian Grocott (Leics, NIPR)Suzie Imber (GSFC)Peter Boakes (Leicester)Benoit Hubert (Liège)
SuperDARNWorkshopDartmouth, 2011
Faraday (1831)Siscoe and Huang (1985)
Cowley and Lockwood (1992)
NDPC
dt
dF
The expanding/contracting polar cap
substorms
0.0 GWb
0.3 GWb
0.6 GWb
0.9 GWb
PCF5 June1998
Milan et al.(2003)
Substorm Substorm
Polar UVI
Wind
0.0 GWb
0.3 GWb
0.6 GWb
0.9 GWb
PCF5 June1998
Milan et al.(2003)
Substorm Substorm
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)
5 June1998
Polar UVI
Wind
NDPC
dt
dF
Cross polar cap potential
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?
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)
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)
<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
Milan et al. (2009a)
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
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)
Close relationshipbetween ovalradius andring currentintensity
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)
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)
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
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 ?
