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Polar observations of ion/electron bursts at the pre-dawn polar cap boundary: evidence for internal reconnection of overdraped lobe flux
oleh: P. E. Sandholt, C. J. Farrugia
Format: | Article |
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Diterbitkan: | Copernicus Publications 2008-08-01 |
Deskripsi
Observations made by Polar of ion-electron bursts on the dawn side of the polar cap are presented. They occurred when conditions external to the magnetosphere corresponded to that of the sheath region of a magnetic cloud, which was characterized by very high densities/dynamic pressure and a magnetic field which was strong in all components and which was tilted antisunward (<I>B<sub>x</sub></I><0) and northward (<I>B<sub>z</sub></I>>0) with its clock angle lying between 20 and 90° (<I>B<sub>y</sub></I>: 8–15 nT). A clear temporal development in the energy range spanned by the individual ion bursts (from 0.2–2 keV to 1–10 keV) was present. We relate this to a corresponding temporal evolution in the cloud sheath field and plasma. We analyze the solar wind-magnetosphere aspects of the observations using the concepts of (i) (i) overdraped lobe flux, (ii) <I>B<sub>x</sub></I>- and <I>B<sub>y</sub></I>-regulated sequential reconnections in opposite hemispheres (magnetopause and internal modes), and (iii) newly-closed magnetic flux. In particular, we find that the most energetic ion bursts (accompanied by bi-directionally streaming electrons at 1–10 keV and intense magnetosheath-origin fluxes) are located on newly closed field lines generated by internal reconnection occurring between overdraped lobe field lines and the closed geomagnetic field. This result corroborates a topology of lobe reconnection under conditions of dipole tilt and/or nonzero IMF <I>B<sub>x</sub></I> component advanced by Watanabe et al. (2006), which in our case is adapted to nonzero IMF <I>B<sub>y</sub></I> conditions.