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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: regular paper 29 Aug 2019

Submitted as: regular paper | 29 Aug 2019

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Annales Geophysicae (ANGEO).

The increase of curvature radius of geomagnetic field lines preceding a classical dipolarization

Osuke Saka Osuke Saka
  • Office Geophysik, Ogoori, 838-0141, Japan

Abstract. Downstream observations at geosynchronous altitudes of field line dipolarization exhibit fundamental component of substorms associated with high velocity magnetotail flow bursts referred to as Bursty Bulk Flows. In growth phase of substorms, we found that the magnetosphere at geosynchronous orbit are in unstable conditions for Ballooning instability due to the appreciable tailward stretching of the flux tubes, and for slow magnetoacoustic wave due to the continuing field-aligned inflows of plasma sheet plasmas towards the equatorial plane. We propose following scenario of field line dipolarization in downstream locations; (1) The slow wave was excited through Ballooning instability by the arrival of Dipolarization Front at the leading edge of Bursty Bulk Flows. (2) In the equatorial plane, slow wave stretched the flux tube in dawn-dusk directions, which resulted in the spreading plasmas in dawn-dusk directions and reducing the radial pressure gradient in the flux tube. (3) As a result, the flux tube becomes a new equilibrium geometry in which curvature radius of new field lines increased in meridian plane, suggesting an onset of field line dipolarization. (4) Increasing curvature radius induced inductive electric fields of the order of few mV/m pointing westward in the equatorial plane, as well as radial electric fields associated with stretching flux tubes in dawn-dusk directions. Westward electric fields transmitted to the ionosphere produce a dynamic ionosphere where the E layer contains both dynamo (E · J < 0) and dissipation (E · J > 0) processes in it for generating field-aligned current system of Bostrom type. The dipolarization processes associated with changing the curvature radius occurred in the transitional intervals lasting for about 10 minutes preceding classical dipolarization composed of reduction of cross-tail currents and pileup of the magnetic fields transported from the tail.

Osuke Saka
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Osuke Saka
Osuke Saka
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Publications Copernicus
Short summary
Field line dipolarization at geosynchronous orbit can be regarded as increase of curvature radius of field lines in association with the combined effects of BBFs, Ballooning instability, and slow magnetoacoustic wave. Change of curvature radius created Bostrom type current system in ionosphere. These are new scenarios describing field line dipolarization and associated magnetosphere-ionosphere coupling and may resolve controversies to some extent in substorm onset scenario.
Field line dipolarization at geosynchronous orbit can be regarded as increase of curvature...