Abstract
Abstract. 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 Walén 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 Super-DARN 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°. 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 ilt angle increases or when IMF BX increases the effective dipole tilt.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; magnetospheric configuration and dynamics; solar wind-magnetosphere interactions)
Highlights
It is generally accepted that merging between the interplanetary magnetic field (IMF) and the Earth’s magnetic field (Dungey, 1961) is the principal coupling mechanism for solar wind plasma entry to the magnetosphere
From the observations presented above we may conclude for this day when the IMF clock angle was about 140◦ that: 1. Polar, while skimming the nose of the magnetosphere, observed the effects of merging at high latitudes
SuperDARN observed enhancements in the flow in the ionosphere associated with these time-dependent merging events observed at Polar
Summary
It is generally accepted that merging between the interplanetary magnetic field (IMF) and the Earth’s magnetic field (Dungey, 1961) is the principal coupling mechanism for solar wind plasma entry to the magnetosphere. Accelerated flows of magnetosheath plasma observed near the subsolar magnetopause (near the Earth-Sun line) provide in situ evidence of the merging process (Paschmann et al, 1979; Sonnerup et al, 1981). Minimum variance analyses of magnetic field measurements were employed to show that the magnetopause acted as a rotational discontinuity with a finite Bnormal, proportional to the merging rate. The lag time decreased as the directional discontinuity structures approached. The directional discontinuities have similar normals to those determined at ACE and at Wind, showing that they have maintained coherence while passing through the bow-shock. The directional discontinuities have similar normals to those determined at ACE and at Wind, showing that they have maintained coherence while passing through the bow-shock. Farrugia et al (1991) found that the planarity is maintained in the magnetosheath but the orientation may change somewhat
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