Presentation on theme: "The influence of solar wind parameters on pseudobreakups, substorms and polar auroral arcs Anita Kullen."— Presentation transcript:
The influence of solar wind parameters on pseudobreakups, substorms and polar auroral arcs Anita Kullen
Content Polar auroral arcs Dependence on solar wind parameters Different polar arc types Polar arc models Substorms and pseudobreakups Dependence on solar wind parameters The place of pseudobreakups in a substorm cycle Different pseudobreakup types Summary
Method Polar UV images and ACE data are taken from three winter months in 1998/99. All polar arcs, pseudobreakups and substorms are selected that appear on Polar UV images. Solar wind parameters for each type of auroral phenomenon are analyzed statistically.
All substorms of one month overlaid on ACE solar wind data
All pseudobreakups of one month overlaid on ACE solar wind data
All polar arcs of one month overlaid on ACE solar wind data
Occurrence frequency Type of auroral activity Number of events Occurrence frequency Large-scale polar arcs Small splits Substorms Pseudobreakups > 10% > 6% > 50% > 9%
Polar auroral arcs
The dependence of polar arcs on different solar wind parameters
The dependence of polar arcs on the anti-epsilon parameter
Results Large-scale polar arcs occur preferably during northward IMF Bz and high solar wind energy flux. This relation can be expressed with the anti-epsilon parameter:
Classification into different polar arc types
The IMF By dependent motion of polar arcs Static polar arcs (oval-aligned, midnight arcs) appear on the dawn side for dawnward IMF and on the duskside for duskward IMF. Polewardly moving arcs (moving, bending arcs) move from dusk to dawn for dawnward IMF and from dawn to dusk for duskward IMF. (Valladares et al., 1994)
The dependence of different polar arc types on the IMF clock angle
The dependence of different polar arc types on IMF changes Polar arc typeBx sign change By sign change Bz sign change Bending arc32%23%82% Oval-aligned arc29% 39% Moving arc36%82%45% Midnight arc17%67% Multiple arcs43% 57%
Results Oval-aligned arcs occur during constant IMF. Moving arcs occur after an IMF By sign change. Bending arcs occur after an IMF Bz sign change. Midnight and multiple arcs occur during varying IMF.
Polar arc models The tail is twisted when an oval-aligned arc appears. The tail changes its twist when a moving arc appears. A tailward moving plasma sheet tounge is connected to a midnight arc. (Makita et al., 1991) (Kullen, 1999, Kullen and Janhunen, 2004) (Rezhenov, 1995)
Substorms and pseudobreakups
Classification into different substorm types Pseudobreakups: onset 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: CGlat Large-oval substorms: < 60 CGlat
The dependence of pseudo- breakups and substorms on different solar wind parameters
The dependence of pseudo- breakups and substorms on IMF Bz
The dependence of pseudobreakups and substorms on epsilon and AE index
Results Results from other studies: (Sergeev et a., 1986; Koskinen et al., 1993; Nakamura et al., 1994 etc.) Pseudobreakups have the same ionospheric and magnetospheric signatures as substorm breakup. Results from this study: 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 transfer into the magnetosphere.
All pseudobreakups overlaid on AE index data
Results Pseudobreakups appear during quiet times, during substorm growth phase or during substorm recovery. Pseudobreakups do not appear in large substorm cycles
Classification into different pseudobreakup types Single pseudobreakups Growth phase pseudobreakups: onset within 30 minutes before substorm onset Recovery phase pseudobreakups: onset when substorm recovery signatures are still visible
The dependence of different pseudobreakup types on IMF signs
The dependence of different pseudobreakup types on epsilon and AE index
Results Single pseudobreakups appear during quiet times. They do not differ much from very weak substorms. Growth phase pseudobreakups appear after an IMF southturn, just before weak substorms. Recovery phase pseudobreakups appear after IMF northturn triggered substorms, much poleward of the main oval. They may in fact be some kind of poleward auroral intensification.
Summary Auroral phenomenon IMF Bz IMF magnitude SW velocity SW density AE/ Epsilon Polar arcsnorthhigh average No activityzerolow Pseudobr.zerolow Substormssouthaverage high Substorms need less solar wind energy than polar arcs due to the better energy transfer during southward IMF.