ALBERT

Description: The orbit of the Polar spacecraft has been ideally suited for studying the turbulent region of the cusp that is located near or just outside the magnetopause current sheet at 7-9 RE. The wave data obtained in this region show that electromagnetic turbulence is dominant in the frequency range 1-10 Hz. The waves responsible for this turbulence usually propagate perpendicular to the local magnetic field and have an index of refraction that generally falls between the estimated cold plasma theoretical values of the electromagnetic lower hybrid and whistler modes and may be composed of both modes in concert with kinetic Alfvén waves and/or fast magnetosonic waves. Fourier spectra of the higher frequency wave data also show the electromagnetic turbulence at frequencies up to and near the electron cyclotron frequency. This higher frequency electromagnetic turbulence is most likely associated with whistler mode waves. The lower hybrid drift and current gradient instabilities are suggested as possible mechanisms for producing the turbulence. The plasma and field environment of this turbulent region is examined and found to be extremely complex. Some of the wave activity is associated with processes occurring locally, such as changes in the DC magnetic field, while others are associated with solar wind and interplanetary magnetic field changes.

Description: We present observations from the Polar satellite that confirm the existence of two types of depletion layers predicted under southward interplanetary magnetic field (IMF) conditions in magnetohydrodynamic simulations. The first depletion type occurs along the stagnation line when IMF BX and/or dipole tilt are/is present. Magnetic merging occurred away from the equator (Maynard et al., 2003) and flux pile-ups developed while the field lines drape to the high-latitude merging sites. This high-shear type of depletion is consistent with the depletion layer model suggested by Zwan and Wolf (1976) for low-shear northward IMF conditions. Expected sites for depletion layers are associated with places where IMF tubes of force first impinge upon the magnetopause. The second depletion type develops poleward of the cusp. Under strongly driven conditions, magnetic fields from Region 1 current closure over the lobes (Siscoe et al., 2002c) cause the high-latitude magnetopause to bulge outward, creating a shoulder above the cusp. These shoulders present the initial obstacle with which the IMF interacts. Flow is impeded, causing local flux pile-ups and low-shear depletion layers to form poleward of the cusps. Merging at the high-shear dayside magnetopause is consequently delayed. In both low- and high-shear cases, we show that the depletion layer structure is part of a slow mode wave standing in front of the magnetopause. As suggested by Southwood and Kivelson (1995), the depletions are rarefactions on the magnetopause side of slow-mode density compressions. While highly sheared magnetic fields are often used as proxies for ongoing local magnetic merging, depletion layers are prohibited at merging locations. Therefore, the existence of a depletion layer is evidence that the location of merging must be remote relative to the observation.

Description: 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)

Description: We demonstrate that high-resolution 557.7-nm all-sky images are useful tools for investigating the spatial and temporal evolution of merging on the dayside magnetopause. Analysis of ground and satellite measurements leads us to conclude that high-latitude merging events can occur at multiple sites simultaneously and vary asynchronously on time scales of 30s to 3min. Variations of 557.7nm emissions were observed at a 10s cadence at Ny-Ålesund on 19 December 2001, while significant changes in the IMF clock angle were reaching the magnetopause. The optical patterns are consistent with a scenario in which merging occurs around the rim of the high-latitude cusp at positions dictated by the IMF clock angle. Electrons energized at merging sites represent plausible sources for 557.7nm emissions in the cusp. Polar observations at the magnetopause have directly linked enhanced fluxes of ≥0.5keV electrons with merging. Spectra of electrons responsible for some of the emissions, measured during a DMSP F15 overflight, exhibit "inverted-V" features, indicating further acceleration above the ionosphere. SuperDARN spectral width boundaries, characteristic of open-closed field line transitions, are located at the equatorward edge of the 557.7nm emissions. Optical data suggest that with IMF BY〉0, the Northern Hemisphere cusp divides into three source regions. When the IMF clock angle was ~150° structured 557.7-nm emissions came from east of the 13:00 MLT meridian. At larger clock angles the emissions appeared between 12:00 and 13:00 MLT. No significant 557.7-nm emissions were detected in the prenoon MLT sector. MHD simulations corroborate our scenario, showing that with the observed large dipole-tilt and IMF clock angles, merging sites develop near the front and eastern portions of the high-altitude cusp rim in the Northern Hemisphere and near the western part of the cusp rim in the Southern Hemisphere.