Occurrence and properties of substorms associated with pseudobreakups

1. Occurrence and properties of substorms associated with pseudobreakups Anita Kullen Space & Plasma Physics, EES

2. Publications about pseudobreakups • Kullen, A., and T. Karlsson, On the relation between solar wind, pseudobreakups and substorms, J. Geophys. Res., 2004. • Kullen, A., S. Ohtani, and T. Karlsson, Geomagnetic signatures of auroral substorms preceeded by pseudobreakups, J. Geophys. Res., 2009. • Kullen, A., T. Karlsson, J. A. Cumnock, and T. Sundberg, Occurrence and properties of substorms associated with pseudobreakups, J. Geophys. Res., in press, 2010. Alfven Lab Seminar, EES, KTH

3. Pseudobreakup types Isolated pseudobreakups Growth-Phase Pseudobreakups Recovery Pseudobreakups Alfven Lab Seminar, EES, KTH

4. Key Issue • How do substorms that are preceded by growth-phase pseudobreakups differ from substorms without pseudobreakups ? response of the near-earth tail solar wind conditions energy transfer to magnetosphere response of the auroral zone response of the polar cap Alfven Lab Seminar, EES, KTH

5. Near-Earth tail signatures Alfven Lab Seminar, EES, KTH

6. Magnetotail dipolarization during substorms Bh Stretched tail B-field Bh Dipolarized tail B-field [W. Baumjohan and R.A. Treumann, Basic Space Plasma Physics, 1996] Near-Earth Neutral Line NENL versus TCD model What develops first, Near-Earth Neutral Line or tail current disruption ? Tail current disruption Alfven Lab Seminar, EES, KTH

7. Substorm signatures in the near-Earth tail GOES magnetic field data • Pseudobreakup • Substorm Subtraction of the magnetosphere model T89 B-field for quiet times from GOES data Alfven Lab Seminar, EES, KTH

8. Signatures of 10 substorms preceded by pseudobreakups IMF Bz AE index Tail Bh – Model Bh Alfven Lab Seminar, EES, KTH

9. Mapping of GOES tail position to the auroral oval Alfven Lab Seminar, EES, KTH

10. No delay of dipolarization: GOES maps to onset position Alfven Lab Seminar, EES, KTH

11. No dipolarization seen:GOES is always equatorward of oval Alfven Lab Seminar, EES, KTH

12. Reason for delayed dipolarization:Dawn- or duskward substorm expansion Alfven Lab Seminar, EES, KTH

13. Reason for delayed dipolarization: Equatorward oval expansion after onset Alfven Lab Seminar, EES, KTH

14. Dipolarization delay versus GOES mapped position in the oval Alfven Lab Seminar, EES, KTH

15. Expansion of the tail dipolarization region Previous work shows, the magnetic field dipolarisation starts locally [Ohtani et al., 1991] and spreads azimuthally [Nagai, 1982; Liou et al., 2002], and radially outward [Jacquey et al. 1991; Ohtani et al., 1992] as well as inward [Ohtani, 1998]. Propagation speed of tail dipolarization region: • Azimuthal propagation speed:0.22 MLT/min (Liou et al. [2002] found 0.37 MLT/min) • Earthward propagation speed:0.09 deg/min (Liou et al. [2002] found 0.84 deg/min) onset tail onset Alfven Lab Seminar, EES, KTH

16. Results • The dipolarization at GOES starts when bright auroral region reaches GOES mapped position.This indicates, the expansion of auroral intensification region and the expansion of the tail dipolarization region are coupled. Thus, the small azimuthal expansion of the smallest substorms indicates an only limited spread of the tail dipolarization region. • For substorms preceded by pseudobreakups, the expansion of the tail dipolarization region in azimuthal direction is as fast as expected from regular substorms.The expansion of the dipolarization region in Earthward direction is 10 times smaller than expected from regular substorms. This is probably connected to a slow Earthward motion of the inner plasma sheet boundary after onset (equatorward motion of the oval boundary). Alfven Lab Seminar, EES, KTH

17. AE index Alfven Lab Seminar, EES, KTH

18. One month AE index data with pseudobreakups overlaid Alfven Lab Seminar, EES, KTH

19. AE index Superposed epoch plots for substorms of different strengths, centered around substorm onset. (Red, yellow, green and blue correspond to strong, medium, small and very small substorms) Substorms without pseudobreakups Substorms with pseudobreakups Alfven Lab Seminar, EES, KTH

20. Solar wind conditions Alfven Lab Seminar, EES, KTH

21. IMF magnitude, solar wind velocity and density IMF magnitude sw velocity sw density Alfven Lab Seminar, EES, KTH

22. Substorms with pseudobreakups Substorms without pseudobreakups Solar wind conditions: IMF Bz Superposed epoch plots for substorms of different strengths, centered around substorm onset. Alfven Lab Seminar, EES, KTH

23. Solar wind energy transfer into the magnetosphere Alfven Lab Seminar, EES, KTH

24. Dayside reconnection Nightside reconnection IMF Bz substorm t Growing region of open magnetic flux Loading-unloading substorm model • IMF Bz turns southward 1 hour before onset. • During the southward IMF period open magnetic field-lines are added via dayside reconnection. • When ”enough” energy has been stored in the magnetosphere via addition of open magnetic flux, this energy is released via a substorm. Substorm onset Alfven Lab Seminar, EES, KTH

25. Solar wind merging E-field Substorms without pseudobreakups Oval size Em integrated over the last three hours before onset versus a) oval size, b) maximal AE during substorm c) average AE during substorm. Substorms with pseudobreakups Max AE Em integrated over the last southward IMF period before onset versus time. growth-phase pseudobreakups Average AE Em = vBtsin2(Θ/2) Alfven Lab Seminar, EES, KTH

26. Polar cap signatures Alfven Lab Seminar, EES, KTH

27. 12 PC potential drop at dayside proportional to Em [Lockwood et al., 2009] PC potential drop PC potential drop at nightisde proportional to AL [Lockwood et al., 2009] 00 Substorms without pseudobreakups Substorms with pseudobreakups Dayside high latitude nightside Alfven Lab Seminar, EES, KTH

28. Substorms without pseudobreakups Em field PC north PC south AE index Substorms with pseudobreakups AE index, PC index and Em in comparison Winter PC index (here PC north) proportional to AE [Janzhura et al., 2007] Summer PC index (here PC south) proportional to Em [Janzhura et al., 2007] Superposed epoch plots for AE index, solar wind merging field Em, northern and southern PC index, centered around substorm onset. Alfven Lab Seminar, EES, KTH

29. growth phase pseudobreakup Summary • Solar wind conditions: Typical solar wind conditions during growth-phase pseudobreakups and subsequent substorms include low IMF magnitude and solar wind velocity, and weakly southward IMF Bz. • Energy transfer into the magnetosphere: A necessary condition for growth phase pseudobreakups to occur is that the amount of energy transferred into the magnetosphere the hours before onset has not exceeded a certain limit. • Ionospheric parameters: AE index, PC indices, and PC potential drop values are low as compared to regular substorms. • Substorm characteristics:Substorms preceded by pseudobreakups are typically weak, and appear as isolated events after hours of low geomangetic activity. Alfven Lab Seminar, EES, KTH