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GEOSYNCHRONOUS SIGNATURES OF AURORAL SUBSTORMS PRECEDED BY PSEUDOBREAKUPS

This study investigates the near-Earth condition during pseudobreakups and main onset of auroral substorms. The comparison between global auroral images and magnetic signatures at geosynchronous orbit reveals delayed dipolarization and expansion of bright aurora during pseudobreakups. Pseudobreakups occur during relative quiescence and are connected to tail B-field stretching. Clear dipolarization appears after the onset of the auroral substorm. These findings suggest that growth phase pseudobreakups occur when there is not yet enough free energy in the tail for a substorm to appear.

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GEOSYNCHRONOUS SIGNATURES OF AURORAL SUBSTORMS PRECEDED BY PSEUDOBREAKUPS

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

  2. Introduction • Motivation • Whether/how is the near-Earth condition different between pseudobreakups and main onset ? • Method • Comparison between global auroral images (Polar UVI) and magnetic signatures at geosynchronous orbit (GOES 8,10). • 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).

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

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

  5. IMF Bz, AE index and GOES data

  6. Reason for delayed tail dipolarization Equatorward oval boundary at 0 MLT • 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. 1992) • -earthward (Ohtani, 1998). • 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.

  7. No delay of dipolarization: GOES maps to onset position

  8. No dipolarization seen:III. GOES always equatorward of oval

  9. Reason for delayed dipolarization:I. Oval expansion after onset

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

  11. 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.

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

  13. Summary tail observations • 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). • A clear dipolarization appears 10-30 minutes after onset of the auroral substorm (in 9 of 10 cases). • Tail B-field stretching continues between auroral substorm onset and large dipolarization.

  14. 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 0.5-2 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

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