pseudobreakups and substorms in comparison
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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
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.
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
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.
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)
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. 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: ?
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.