Pseudobreakups and substorms in comparison

1. Seminar at KTH, September 2004 Pseudobreakups and substorms in comparison Anita Kullen1 and Tomas Karlsson2 1IRF Uppsala 2Alfvenlaboratory, KTH, Stockholm

2. Content Introduction • The classical substorm • Other types of substorm activity • Ionospheric and magnetospheric substorm signatures • Substorm models Results • Method • Solar wind dependence of pseudobreakups and substorms • The place of pseudobreakups within a substorm cycle • Solar wind dependence of different pseudobreakup types Conclusions

3. Introduction

4. SUBSTORM PHASES Growth phase Onset Expansion Recovery What comes first at onset ? Formation of a near-Earth neutral line at 15-25 Re Tail current disruption at 5-12 Re (closure via the ionosphere, causing the breakup) The classical substorm(see slides)

5. What causes a substorm ? • non-triggered substorms (40%) • during prolonged southward IMF • triggered substorms (60 %) • IMF Bz northturn • IMF By sign change • pressure pulse (Results from Hsu and McPherron, JGR 2003)

6. Other types of substorm activity(see slides) • Steady magnetospheric convection (SMC) event (endless recovery) • Auroral expansions during a magnetic storm (no clear equatorward onset) • Poleward boundary intensification (PBI) (no expansion/during all levels of auroral activity) • Pseudobreakup (no expansion/appear outside substorm expansion and recovery)

7. Models Most substorm models describe only the classical substorm. Conceptual models, trying to cover all types of substorm-like auroral activity are: • Coupled-mode model (Sergeev et al., 1996) • Basic energy dissipation events (pb, pbi, su breakup) are overlaid on global slow mode substorm activity • Global concequences only for the most equatorward breakups having their source region near the inner plasma sheet boundary • Sand-pile model (reference)

8. Signatures of substorm breakups, pseudobreakups and PBI’s

9. What prevents pseudobreakups from expanding globally ? • Observations: No difference between ionospheric and magnetospheric signatures of pseudo-breakups and substorm breakup. • Assumption: The rate of the solar wind energy transfer controls substorm activity. • Goal of this study: Find out the characteristic solar wind conditions for pseudobreakups and substorms.

10. Results

11. Method • Polar UV images and ACE data are taken from three winter months in 1998/99. • All pseudobreakups and substorms are selected that appear on Polar UV images. • Solar wind parameters for each type of auroral phenomenon are analyzed statistically.

12. Solar wind dependence of pseudobreakups and substorms

13. Classification • Pseudobreakups: auroral intensification without expansion The substorm size is estimated from the location of the equatorward oval boundary at 0 MLT. • Small-oval substorms: > 63 CGlat • Medium-oval substorms: 60-63 CGlat • Large-oval substorms: < 60 CGlat

14. The dependence of pseudo- breakups and substorms on different solar wind parameters

15. The dependence of pseudo- breakups and substorms on IMF Bz

16. The dependence of pseudobreakups and substorms on epsilon and AE index

17. Results There is a systematic shift of all solar wind parameters from low values for pseudobreakups to increasingly higher values for substorms of increasing strength. Conclusion: Pseudobreakups are the weakest type of substorms, appearing when there is a very low energy in the solar wind (IMF, v, n) and the transfer rate into the magnetosphere is low (northward IMF).

18. Summary • Substorms need less solar wind energy than polar arcs due to the better energy transfer during southward IMF.

19. The place of pseudobreakups within a substorm cycle

20. Pseudobreakups overlaid on AE index data

21. Results • Pseudobreakups appear during quiet times, during substorm growth phase or during substorm recovery. • Pseudobreakups do not appear during large substorm cycles

22. The solar wind dependence of different pseudobreakup types

23. Classification of different pseudobreakup types(see slides) • Classification with respect to oval location • Poleward pseudobreakups • Middle pseudobreakups • Equatorward pseudobreakups 2. Classification with respect to nearest substorm • Single pseudobreakups • Growth phase pseudobreakups • Recovery phase pseudobreakups

24. 1. Poleward, middle and equatorward pseudobreakups

25. 1. Results for poleward, middle and equatoward pseudobreakups There is no clear difference between solar wind parameters for poleward, middle and equatorward pseudobreakups. Conclusions: a) the bad resolution of Polar UVI prohibits clear results or, b) pseudobreakups may occur on arbitrary latitudes, independent on the solar wind conditions. .

26. 2. Single, growth phase, and recovery pseudobreakups

27. 2. Single, growth phase and recovery pseudobreakups

28. 2. Results for single, growth phase and recovery pseudobrekups • Single pseudobreakups appear during quiet times with constant IMF. They do not differ much from very weak substorms. Mechansim: No external trigger • Growth phase pseudobreakups appear at the end of a 1-2 hour long IMF southturn, just before a weak substorms. Mechanism: Reduced energy transfer quenches further expansion • Recovery phase pseudobreakups appear after IMF northturn triggered substorms, much poleward of the main oval. They are a special PBI type. Mechanism: ?

29. Summary

30. Conclusions • There is no difference between poleward and non-poleward pseudobreakups (magnetospheric signatures, characteristic solar wind parameters) Thus, they are probably caused by the same mechanism. • An extreme tailward extension of the closed field line region may be unstable towards local instabilities (causing bursty bulk flows). • A small (or decreasing) amount of energy transfer into the tail prevents a global expansion.