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

1. 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) steve.milan@ion.le.ac.uk

2. What is the magnetic flux throughput of the magnetosphere? Milan (2009) steve.milan@ion.le.ac.uk

3. The expanding/contracting polar cap substorms Faraday (1831) Siscoe and Huang (1985) Cowley and Lockwood (1992) steve.milan@ion.le.ac.uk

4. Substorm Substorm 5 June 1998 0.9 GWb 0.6 GWb 0.3 GWb Polar UVI 0.0 GWb Wind Milan et al. (2003) steve.milan@ion.le.ac.uk

5. Substorm Substorm 5 June 1998 0.9 GWb 0.6 GWb 0.3 GWb Polar UVI 0.0 GWb Wind Milan et al. (2003) steve.milan@ion.le.ac.uk

6. Substorm Substorm 5 June 1998 0.9 GWb 0.6 GWb 0.3 GWb Polar UVI 0.0 GWb Wind Milan et al. (2003) steve.milan@ion.le.ac.uk

7. 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 steve.milan@ion.le.ac.uk

8. 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? 20 August – 6 September, 2005 steve.milan@ion.le.ac.uk Milan et al. (2008)

9. 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 steve.milan@ion.le.ac.uk Milan et al. (2008)

10. 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 <ΦN> = <ΦD> <ΦD>0.6 <ΦD>0.4 steve.milan@ion.le.ac.uk Milan et al. (2008)

11. 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 FD steve.milan@ion.le.ac.uk Milan et al. (2009a)

12. 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 FD steve.milan@ion.le.ac.uk Milan et al. (2009a)

13. Proton aurora Electron aurora Milan et al. (2009a) steve.milan@ion.le.ac.uk

14. Proton aurora steve.milan@ion.le.ac.uk

15. 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 -20 min 69 kV -20 min 41 kV -10 min 43 kV -10 min 69 kV onset 71 kV onset 40 kV Convection velocity in onset region +10 min 51 kV +10 min 42 kV onset latitude +20 min 62 kV +20 min 46 kV +50 min 55 kV +50 min 48 kV Grocott et al. (2009)

16. 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? steve.milan@ion.le.ac.uk Milan et al. (2008)

17. Close relationship between oval radius and ring current intensity steve.milan@ion.le.ac.uk Milan et al. (2009b)

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

19. 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) steve.milan@ion.le.ac.uk

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

22. IMAGE FUV IMAGE FUV/WIC observations allow identification of substorms and quantification of changes in polar cap flux FPC FPC increases during substorm growth phase and decreases after expansion phase onset steve.milan@ion.le.ac.uk Milan et al. (2008)

23. 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)

24. Solar wind parameters and other substorm indicators are also well-organized by substorm onset latitude IMF BZ VSW NSW PSW AU, AL Sym-H steve.milan@ion.le.ac.uk Milan et al. (2009a)

25. 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)

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

27. Superposed epoch analysis of open flux, sub-divided by geosynchronous particle injection signatures Classic, isolated substorm injection Continuous, disturbed injectionNo injection FPC BZ BT |BY | Substorms driven by BZ < 0 nT; SMC driven by large BY ? Boakes et al. (in preparation)