Polar, Cluster and SuperDARN evidence for high latitude merging during southward IMF: temporal/spatial evolution

Maynard, N. C. and Ober, D. M. and Burke, W. J. and Scudder, J. D. and Lester, M. and Dunlop, M. W. and Wild, J. A. and Grocott, A. and Farrugia, C. J. and Lund, E. and Russell, C. T. and Weimer, D. R. and Siebert, K. D. and Balogh, A. and André, M. and Rème, H. (2003) Polar, Cluster and SuperDARN evidence for high latitude merging during southward IMF: temporal/spatial evolution. Annales Geophysicae, 21 (12). pp. 2233-2258. ISSN 0992-7689

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Magnetic merging on the dayside magnetopause often occurs at high latitudes. Polar measured fluxes of accelerated ions and wave Poynting vectors while skimming the subsolar magnetopause. The measurements indicate that their source was located to the north of the spacecraft, well removed from expected component merging sites. This represents the first use of wave Poynting flux as a merging discriminator at the magnetopause. We argue that wave Poynting vectors, like accelerated particle fluxes and the Walen tests, are necessary, but not sufficient, conditions for identifying merging events. The Polar data are complemented with nearly simultaneous measurements from Cluster in the northern cusp, with correlated observations from the SuperDARN radar, to show that the locations and rates of merging vary. Magnetohydrodynamic (MHD) simulations are used to place the measurements into a global context. The MHD simulations confirm the existence of a high-latitude merging site and suggest that Polar and SuperDARN observed effects are attributable to both exhaust regions of a temporally varying X-line. A survey of 13 merging events places the location at high latitudes whenever the interplanetary magnetic field (IMF) clock angle is less than ∼150 deg. While inferred high-latitude merging sites favor the antiparallel merging hypothesis, our data alone cannot exclude the possible existence of a guide field. Merging can even move away from equatorial latitudes when the IMF has a strong southward component. MHD simulations suggest that this happens when the dipole tilt angle increases or when IMF BX increases the effective dipole tilt.

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Annales Geophysicae
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21 Apr 2008 10:55
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21 Nov 2022 20:33