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Bright aurorae can be excited by the acceleration of electrons into the atmosphere in violation of ideal magnetohydrodynamics. Modeling studies predict that the accelerating electric potential consists of electric double layers at the boundaries of an acceleration region but observations suggest that particle acceleration occurs throughout this region. Using multispacecraft observations from Cluster, we have examined two upward current regions on 14 December 2009. Our observations show that the potential difference below C4 and C3 changed by up to 1.7 kV between their respective crossings, which were separated by 150 s. The field-aligned current density observed by C3 was also larger than that observed by C4. The potential drop above C3 and C4 was approximately the same in both crossings. Using a novel technique of quantitively comparing the electron spectra measured by Cluster 1 and 3, which were separated in altitude, we determine when these spacecraft made effectively magnetically conjugate observations, and we use these conjugate observations to determine the instantaneous distribution of the potential drop in the AAR. Our observations show that an average of 15% of the potential drop in the AAR was located between C1 at 6235 km and C3 at 4685 km altitude, with a maximum potential drop between the spacecraft of 500 V, and that the majority of the potential drop was below C3. Assuming a spatial invariance along the length of the upward current region, we discuss these observations in terms of temporal changes and the vertical structure of the electrostatic potential drop and in the context of existing models and previous single- and multispacecraft observations. Citation: Forsyth, C., et al. (2012), Temporal evolution and electric potential structure of the auroral acceleration region from multispacecraft measurements, J. Geophys. Res., 117, A12203, doi: 10.1029/2012JA017655.

A case study of a poloidal ULF pulsation near the dayside plasmapause is presented based on Cluster observations of magnetic and electric fields. The pulsation is detected close to the magnetic equatorial plane at L shells L=[4.4, 4.6] and oscillates with a frequency of f=23 mHz. Investigating the wave energy flux reveals the standing wave nature of the observed pulsation. An estimation of the azimuthal wave number exposes a narrow azimuthal structure of the wave field with m approximate to 160. Spatial and temporal characteristics of the pulsation are analyzed in detail by representing data in a field line related coordinate system and a range-time-intensity representation. This allows an estimation of both the spatial extension of the wave field in the radial direction and its temporal decay rate. The analysis furthermore indicates that the same field lines are excited to a standing wave oscillation twice. Furthermore an accurate identification of a phase jump of the wave field across L shells is possible. Comparing the radial localization of the detected wave with theoretically expected field line eigenfrequencies reveals that the wave field is confined in the Alfven resonator at the outer edge of the plasmapause., QC 20141021

Oscillating magnetic field lines are frequently observed by spacecraft in the terrestrial and other planetary magnetospheres. The CLUSTER mission is a very suitable tool to further study these Alfven waves as the four CLUSTER spacecraft provide for an opportunity to separate spatial and temporal structures in the terrestrial magnetosphere. Using a large scaled configuration formed by the four spacecraft we are able to detect a poloidal Ultra-Low-Frequency (ULF) pulsation of the magnetic and electric field in order to analyze its temporal and spatial structures. For this purpose the measurements are transformed into a specific field line related coordinate system to investigate their specific amplitude pattern depending on the path of the CLUSTER spacecraft across oscillating field lines. These measurements are then compared with modeled spacecraft observations across a localized poloidal wave resonator in the dayside plasmasphere. A detailed investigation of theoretically expected poloidal eigenfrequencies allows us to specify the observed 16 mHz pulsation as a third harmonic oscillation. Based on this we perform a case study providing a clear identification of wave properties such as an spatial scale structure of about 0.67 R-E. the azimuthal wave number m approximate to 30, temporal evolution, and energy transport in the detected ULF pulsations., QC 20100525

Cluster magnetic field data are studied during an outbound pass through the post-noon high-latitude magnetopause region on 14 February 2001. The onset of several minute perturbations in the magnetospheric field was observed in conjunction with a southward turn of the interplanetary magnetic field observed upstream by the ACE spacecraft and lagged to the subsolar magnetopause. These perturbations culminated in the observation of four clear magnetospheric flux transfer events (FTEs) adjacent to the magnetopause, together with a highly-structured magnetopause boundary layer containing related field features. Furthermore, clear FTEs were observed later in the magnetosheath. The magnetospheric FTEs were of essentially the same form as the original “flux erosion events” observed in HEOS-2 data at a similar location and under similar interplanetary conditions by Haerendel et al. (1978). We show that the nature of the magnetic perturbations in these events is consistent with the formation of open flux tubes connected to the northern polar ionosphere via pulsed reconnection in the dusk sector magnetopause. The magnetic footprint of the Cluster spacecraft during the boundary passage is shown to map centrally within the fields-of-view of the CUTLASS SuperDARN radars, and to pass across the field-aligned beam of the EISCAT Svalbard radar (ESR) system. It is shown that both the ionospheric flow and the backscatter power in the CUTLASS data pulse are in synchrony with the magnetospheric FTEs and boundary layer structures at the latitude of the Cluster footprint. These flow and power features are subsequently found to propagate poleward, forming classic “pulsed ionospheric flow” and “poleward-moving radar auroral form” structures at higher latitudes. The combined Cluster-CUTLASS observations thus represent a direct demonstration of the coupling of momentum and energy into the magnetosphere-ionosphere system via pulsed magnetopause reconnection. The ESR observations also reveal the

We report in-situ detection by two spacecraft of oppositely directed jets of plasma emanating from a magnetic reconnection site at the Earth’s dayside magnetopause, confirming a key element inherent in all reconnection scenarios. The dual-spacecraft (Equator-S and Geotail) observations at the flank magnetopause, together with SuperDARNHalley radar observations of the subsolar cusp region, reveal the presence of a rather stable and extended reconnection line which lies along the equatorial magnetopause. These observations were made under persistent southward interplanetary magnetic field (IMF) conditions, implying that under these conditions the reconnection sites are determined by the large-scale interactions between the solar wind magnetic field and the dayside magnetosphere, rather than by local conditions at the magnetopause. Control by local conditionswould result in patchy reconnection, distributed in a less well-organized fashion over the magnetopause surface.