Solar wind-magnetosphere coupling, substorms, and ramifications for ionospheric convection Steve Milan Adrian Grocott (Leics, NIPR) Suzie Imber (GSFC) Peter Boakes (Leicester) Benoit Hubert (Liège) SuperDARN Workshop Dartmouth, 2011

What is the magnetic flux throughput of the magnetosphere? Milan (2009)

Faraday (1831) Siscoe and Huang (1985) Cowley and Lockwood (1992) The expanding/contracting polar cap substorms

0.0 GWb 0.3 GWb 0.6 GWb 0.9 GWb 5 June 1998 Milan et al. (2003) Substorm Polar UVI Wind

0.0 GWb 0.3 GWb 0.6 GWb 0.9 GWb 5 June 1998 Milan et al. (2003) Substorm Polar UVI Wind

0.0 GWb 0.3 GWb 0.6 GWb 0.9 GWb Substorm Milan et al. (2003) 5 June 1998 Polar UVI Wind

Cross polar cap potential Expansion/contraction of polar cap Cross polar cap potential for symmetric, circular polar cap, measured along dawn/dusk meridian in absence of viscous interaction, lobe reconnection, frictional drag Cross polar cap potential is not a good measure of dayside coupling, nor is it constrained to be instantaneously equal in both hemispheres

20 August – 6 September, 2005 Questions Solar wind-magnetosphere coupling leads to the occurrence 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)

Magnetotail signatures Good comparison with ground signatures of substorms in AU and AL Cluster shows magneto- tail inflation during growth phase, and deflation and dipolar- ization after expansion phase onset Milan et al. (2008)

Substorm occurrence and size Substorm occurrence increases with solar wind coupling And the change in size of the polar cap increases, i.e. the amount of flux released in each substorm Occurrence x size gives a linear dependence: flux out = flux in =  0.6 Milan et al. (2008)  0.4

Open flux control of substorm intensity Superposed epoch analyses of auroral intensity, open flux, AU and AL, Sym-H, and SW-coupling during 40 substorms Substorms binned by open flux at onset auroral intensity open flux AU, AL Sym-H  D Milan et al. (2009a)

Milan et al. (2009a) auroral intensity open flux AU, AL Sym-H  D Open flux control of substorm intensity Superposed epoch analyses of auroral intensity, open flux, AU and AL, Sym-H, and SW-coupling during 40 substorms Substorms binned by open flux at onset

Milan et al. (2009a) Proton auroraElectron aurora

Proton aurora

Grocott et al. (2009) -20 min 69 kV -10 min 69 kV onset 71 kV +10 min 51 kV +20 min 62 kV +50 min 55 kV -20 min 41 kV -10 min 43 kV onset 40 kV +10 min 42 kV +20 min 46 kV +50 min 48 kV onset latitude Convection velocity in onset region Superposed epoch analysis of convection - High latitude substorms have prompt convection response - Low latitude substorms have convection decrease at onset; convection delayed! Substorm electrodynamics influenced by auroral bulge conductivity

Low latitude onset substorms are more 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 relationship between oval radius and ring current intensity Milan et al. (2009b)

Changes in oval radius associated with substorms Milan et al. (2009b)

Conclusions The 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 ionospheric convection and is central to SuperDARN science The polar cap expands more prior to substorm onset when the ring current is enhanced Lower latitude substorms have a greater auroral intensity and stronger electrojets This in turn changes the ionospheric convection response to tail reconnection, delaying convection until dissipation of auroral signatures Northward IMF: lobe reconnection (Imber et al., 2006, 2007)

IMAGE FUV IMAGE data courtesy of Stephen Mende, Harald Frey and the IMAGE FUV team

IMAGE FUV IMAGE FUV/WIC observations allow identification of substorms and quantification of changes in polar cap flux F PC F PC increases during substorm growth phase and decreases after expansion phase onset Milan et al. (2008)

Superposed epoch analysis of ~2000 substorms keyed to Frey et al. (2004) substorm list, binned by onset latitude binned by onset latitude -1 to +2 hours from onset 10-min bins After Frey et al. (2004)

Solar wind parameters and other substorm indicators are also well-organized by substorm onset latitude IMF B Z V SW N SW P SW AU, AL Sym-H Milan et al. (2009a)

The polar cap flux should grow largest when a lot of flux is opened between substorm onsets Flux accumulation Integrated dayside reconnection rate Milan et al. (2008)

Substorm occurrence: greatest when solar wind coupling is enhanced The level of fluctuation in F PC, a measure of the flux closure during substorms: larger when the solar wind coupling is enhanced How big? How often? Milan et al. (2008)

Boakes et al. (in preparation) Superposed epoch analysis of open flux, sub-divided by geosynchronous particle injection signatures Classic, isolated substorm injectionContinuous, disturbed injectionNo injection F PC BZBZ BTBT |B Y | Substorms driven by B Z < 0 nT; SMC driven by large B Y ?