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Ground-based observations of the auroral zone and polar cap ionospheric responses to dayside transient reconnection

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journal contribution
posted on 2017-01-09, 16:15 authored by J. A. Davies, T. K. Yeoman, I. J. Rae, S. E. Milan, M. Lester, M. Lockwood, A. McWilliams
Observations from the EISCAT VHF incoherent scatter radar system in northern Norway, during a run of the common programme CP-4, reveal a series of poleward-propagating F-region electron density enhancements in the pre-noon sector on 23 November 1999. These plasma density features, which are observed under conditions of a strongly southward interplanetary magnetic field, exhibit a recurrence rate of under 10 min and appear to emanate from the vicinity of the open/closed field-line boundary from where they travel into the polar cap; this is suggestive of their being an ionospheric response to transient reconnection at the day-side magnetopause (flux transfer events). Simultaneous with the density structures detected by the VHF radar, poleward-moving radar auroral forms (PMRAFs) are observed by the Finland HF coherent scatter radar. It is thought that PM-RAFs, which are commonly observed near local noon by HF radars, are also related to flux transfer events, although the specific mechanism for the generation of the field-aligned irregularities within such features is not well understood. The HF observations suggest, that for much of their existence, the PMRAFs trace fossil signatures of transient reconnection rather than revealing the footprint of active reconnection itself; this is evidenced not least by the fact that the PMRAFs become narrower in spectral width as they evolve away from the region of more classical, broad cusp scatter in which they originate. Interpretation of the HF observations with reference to the plasma parameters diagnosed by the incoherent scatter radar suggests that as the PMRAFs migrate away from the reconnection site and across the polar cap, entrained in the ambient antisunward flow, the irregularities therein are generated by the presence of gradients in the electron density, with these gradients having been formed through structuring of the ionosphere in the cusp region in response to transient reconnection.


The authors are indebted to the director and staff of EISCAT for operating the facility and supplying the data and to the EISCAT group at the Rutherford Appleton Laboratory for maintenance of the analysis software and assistance therewith. EISCAT is an international facility funded collaboratively by the research councils of Finland (SA), France (CNRS), the Federal Republic of Germany (MPG), Japan (NIPR), Norway (NAVF), Sweden (NFR) and the United Kingdom (PPARC). CUTLASS is funded jointly by PPARC (Grant number PPA/R/R/1997/00256), the Swedish Institute for Space Physics, Uppsala, and the Finnish Meteorological Institute, Helsinki. The authors acknowledge the principal investigators R. Lepping and N. Ness for enabling use of the Wind and ACE MFI key parameter data, respectively. Thanks are also due to the Canadian Space Agency for providing the CANOPUS magnetometer data. J. A. Davies and S. E. Milan are supported on PPARC grant number PPA/G/O/1999/00181. I. J. Rae is supported by the Natural Sciences and Engineering Council of Canada (NSERC). K. A. McWilliams is funded by the Association of Commonwealth Universities of the British Council.



Annales Geophysicae, 20, 781-794, 2002

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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Annales Geophysicae


European Geosciences Union (EGU), Copernicus Publications, Springer Verlag (Germany)



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