Pseudobreakups and substorms in comparison

Pseudobreakups and substorms in comparison

Anita Kullen1 and Tomas Karlsson2

1IRF Uppsala2Alfvenlaboratory, KTH, Stockholm

Seminar at KTH, September 2004

ContentIntroduction

– 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

Introduction

The classical substorm(see slides)

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)

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)

Signatures of substorm breakups, pseudobreakups and PBI’s

Auroral

brightening

Auroral breakup

Tail plasma flow

Tail B-field

Substorm breakup

equator-ward

global earthward flow

global dipolarization

Pseudobreakup

(Fillingim et al., 2000)

varying bursty bulk flow

local dipolarization

PBI

(Lyons et al., 1999)

poleward bursty bulk flow

local dipolarization

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.

Results

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.

Solar wind dependence of pseudobreakups and substorms

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 pseudo- breakups and substorms on IMF Bz

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

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.

The place of pseudobreakups within a substorm cycle

Pseudobreakups overlaid on AE index data

Results

• Pseudobreakups appear during quiet times, during substorm growth phase or during substorm recovery.

• Pseudobreakups do not appear during large substorm cycles

The solar wind dependence of different pseudobreakup types

Classification of different pseudobreakup types

(see slides)

1. 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. Poleward, middle and equatorward 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. Single, growth phase, and recovery pseudobreakups

2. Single, growth phase and recovery pseudobreakups

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: ?

Summary

Pseudo-breakup

Solar wind energy flux

IMF Bz Tail length Location

Single very low weakly northward

long

(long tail)

both

Growth

phase

low (decreasing)

weakly southward

long (stretched tail)

non-poleward

Recovery phase

very low weakly northward

long (double oval)

poleward

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.