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Seminar at KTH, September 2004. Pseudobreakups and substorms in comparison. Anita Kullen 1 and Tomas Karlsson 2 1 IRF Uppsala 2 Alfvenlaboratory, KTH, Stockholm. Content. Introduction The classical substorm Other types of substorm activity
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Seminar at KTH, September 2004 Pseudobreakups and substorms in comparison Anita Kullen1 and Tomas Karlsson2 1IRF Uppsala 2Alfvenlaboratory, KTH, Stockholm
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
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)
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)
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)
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)
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.
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.
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
The dependence of pseudo- breakups and substorms on different solar wind parameters
The dependence of pseudobreakups and substorms on epsilon and AE index
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).
Summary • Substorms need less solar wind energy than polar arcs due to the better energy transfer during southward IMF.
Results • Pseudobreakups appear during quiet times, during substorm growth phase or during substorm recovery. • Pseudobreakups do not appear during large substorm cycles
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
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. .
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: ?
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.