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

91

146

419

330

> 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:

0

20

242 cos

lvB

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 type Bx sign change

By sign change

Bz sign change

Bending arc 32% 23% 82%

Oval-aligned arc 29% 29% 39%

Moving arc 36% 82% 45%

Midnight arc 17% 67% 67%

Multiple arcs 43% 43% 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: 60-63 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 arcs north high high average average

No activity zero low low low low

Pseudobr. zero low low low low

Substorms south average average average high

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