GEOSYNCHRONOUS SIGNATURES OF AURORAL SUBSTORMS PRECEDED BY PSEUDOBREAKUPS A. Kullen (1), S. Ohtani (2), and H. Singer (3) A. Kullen (1), S. Ohtani (2), and H. Singer (3) (1) Swedish Institute of Space Physics Uppsala (1) Swedish Institute of Space Physics Uppsala (2) The John Hopkins University Applied Physics Laboratory (2) The John Hopkins University Applied Physics Laboratory (3) NOAA Space Environment Center (3) NOAA Space Environment Center IAGA conference, Toulouse, France 2005 IAGA conference, Toulouse, France 2005

Introduction Motivation Motivation Whether/how is the near-Earth condition different between pseudobreakups and main onset ? Method Method Comparison between global auroral images (Polar UVI) and magnetic signatures at geosynchronous orbit (GOES 8,10). Event selection Event selection Of 57 growth phase pseudobreakups detected during a 3 month period ( Kullen and Karlsson, 2004 ), all clear events are selected, where a GOES satellite is located between 21 MLT and 3 MLT (10 events).

GOES position mapped on Polar UVI images (using T96 model with 1h averaged sw data from ACE)

GOES magnetic field data Subtraction of the T89 B-field for quiet times from GOES data Pseudobreakup Substorm

IMF Bz, AE index and GOES data

Reason for delayed tail dipolarization Previous results: Previous results: Tail current disruption and dipolarization are initiallized locally and expand from there -azimuthally ( e.g. Nagai, 1982 ) -tailward ( e.g. Ohtani et al ) -earthward ( Ohtani, 1998 ). This study: This study: Pseudobreakup and main onset appear during relative quiescence on high latitudes. Hence, they map probably tailward of GOES position ( Frank and Sigwarth, 2000 ) i.e., an earthward delay can be expected. Date Pseudo- breakup (CGlat) Main onset (CGlat) Large dipolarizat ion (Cglat) Jan 7a, Jan 7b, Dec 28, Dec 6, Dec 14, Dec 23, Dec 3, Feb 24, 99 ?5857 Feb 25, 99 ?ovalwidens Jan 15, Equatorward oval boundary at 0 MLT

No delay of dipolarization: GOES maps to onset position

No dipolarization seen: III. GOES always equatorward of oval

Reason for delayed dipolarization: I. Oval expansion after onset

Reason for delayed dipolarization: II. Dawn- or duskward substorm expansion

Results: 1. Dipolarization A clear dipolarization is observed at GOES when the region of enhanced auroral activity reaches the mapped position of GOES. This suggests a connection between expansion of bright aurora and expansion of tail current disruption region. A clear dipolarization is observed at GOES when the region of enhanced auroral activity reaches the mapped position of GOES. This suggests a connection between expansion of bright aurora and expansion of tail current disruption region. Date GOES - main onset Distance Dipolariz ation Delay Reason for delayed dipolarization Dec 6, MLT 29 mn Equatorward expansion Dec 14, MLT 22 mn Equatorward expansion Dec 3, MLT 25 mn Equatorward expansion Feb 24, MLT 26 mn Unclear (bad UVI) Jan 15, MLT 31 mn Equatorward expansion Feb 25, MLT 2 mn GOES at onset Jan 7b, MLT - GOES always equatorward Dec 28, MLT 8 mn Dawnward motion Dec 23, MLT 17 mn Dawnward motion Jan 7a, MLT 11 mn Duskward motion

H(GOES)-H(T98) at geosynchrounous orbit GOES at same longitude as pseudobreakup

Summary tail observations Pseudobreakups are connected to a small hickup of Bh (in 7 of 10 cases). Pseudobreakups are connected to a small hickup of Bh (in 7 of 10 cases). Tail B-field stretching continues between pseudobreakups and substorms (in 8 of 10 cases). Tail B-field stretching continues between pseudobreakups and substorms (in 8 of 10 cases). A clear dipolarization appears minutes after onset of the auroral substorm (in 9 of 10 cases). A clear dipolarization appears minutes after onset of the auroral substorm (in 9 of 10 cases). Tail B-field stretching continues between auroral substorm onset and large dipolarization. Tail B-field stretching continues between auroral substorm onset and large dipolarization.

Results: 2. Pseudobreakups Previous results ( Kullen and Karlsson, 2004 ): Pseudobreakups appear during quiet times with low solar wind velocity, density and IMF. Tyically IMF Bz was weakly southward hours before a growth phase pseudobreakup. An IMF northturn appears during the following substorm. This study: Continued tail B-field stretching between pseudobreakup and main onset. Conclusions: These results indicate that growth phase pseudobreakups occur while there is not yet enough free energy in the tail available for a substorm to appear. growth phase pseudobreakup