The design, performance, and on-orbit operation of the three-axis electric field instrument (EFI) for the NASA THEMIS mission is described. The 20 radial wire boom and 10 axial stacer boom antenna systems making up the EFI sensors on the five THEMIS spacecraft, along with their supporting electronics have been deployed and are operating successfully on-orbit without any mechanical or electrical failures since early 2007. The EFI provides for waveform and spectral three-axis measurements of the ambient electric field from DC up to 8 kHz, with a single, integral broadband channel extending up to 400 kHz. Individual sensor potentials are also measured, providing for on-board and ground-based estimation of spacecraft floating potential and high-resolution plasma density measurements. Individual antenna baselines are 50-and 40-m in the spin plane, and 6.9-m along the spin axis.The EFI has provided for critical observations supporting a clear and definitive understanding of the electrodynamics of both the boundaries of the terrestrial magnetosphere, as well as internal processes, such as relativistic particle acceleration and substorm dynamics. Such multi-point electric field observations are key for pushing forward the understanding of electrodynamics in space, in that without high-quality estimates of the electric field, the underlying electromagnetic processes involved in current sheets, reconnection, and waveparticle interactions may only be inferred, rather than measured, quantified, and used to discriminate between competing hypotheses regarding those processes.

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Abstract. Electric fields, magnetic fields, and plasmas measured on the Polar satellite are studied to determine the altitude extent of the upward field-aligned current portion of the auroral acceleration region and the physical processes that populate it with parallel electric fields. This region extends upward to a geocentric altitude of 3 or 3.5 Earth radii at premidnight local times during the spring and fall. In a model for this region a necessary but not sufficient condition for the appearance of parallel electric fields is an upward field-aligned current. Parallel electric fields will exist where the plasma density is insufficient to carry this imposed field-aligned current through the converging magnetic field. These parallel fields are associated with (1) high-altitude electron acceleration required to carry the imposed current through the low-density region; (2) a low-altitude sheath required to inject compensating positive charge into the region and to restrict entry of secondary electrons from below; and (3) the midaltitude maintenance of local charge neutrality. The parallel electric field in the low-altitude sheath is directly measured to be as large as several hundred mV m -l, while the higher-altitude fields appear to l•ave magnitudes of a few mV m -1 . The low-altitude sheath is corrugated in a manner that does not depend on striations of the field-aligned current but that may depend on density variations.

[1] We report in situ observations from the Cluster and FAST spacecraft showing the deposition of energy into the auroral ionosphere from broadband ULF waves in the cusp and low-latitude boundary layer. A comparison of the wave Poynting flux with particle energy and flux at both satellites indicates that energy transfer from the broadband waves to the plasma occurs through field-aligned electron acceleration, transverse ion acceleration, and Joule heating. These processes are shown to result in precipitating electron fluxes sufficient to drive bright aurora and cause outflows of energized electrons and O + ions from the ionosphere into the low-latitude boundary layer. By solving an eigenmode equation for Alfvén waves in the observed plasma environment, it is shown that the broadband waves observed at Cluster and FAST are dispersive Alfvén waves. It is demonstrated that these waves have wavelengths perpendicular to the geomagnetic field extending from significant fractions of an L shell down to ion gyroradii and electron inertial lengths and wave frequencies in the plasma frame from 1 mHz up to 50 mHz. These waves are shown to have wavelengths along the geomagnetic field of the order of the field line length between the ionosphere and the equatorial plane and become field line resonances (FLRs) when on closed field lines. It is shown that the inclusion of nonlinear and/or nonlocal kinetic effects is required in the description of these waves to account for accelerated particles observed. On the basis of the wave polarization and spectral properties observed from Cluster and FAST it is speculated that these waves are generated through the mode conversion of surface Alfvén waves driven by tailward flows in the low-latitude boundary layer.Citation: Chaston, C. C., et al. (2005), Energy deposition by Alfvén waves into the dayside auroral oval: Cluster and FAST observations,

[1] Satellite observations have established that parallel electric fields of both upward and downward current regions of the aurora are supported, at least in part, by strong double layers. The purpose of this article is to examine the role of double layers in auroral electron acceleration using direct measurements of parallel electric fields and the accompanying particle distributions, electrostatic waves, and nonlinear structures; the concentration is on the upward current region. Direct observations of the ionospheric boundary of the auroral cavity suggest that a stationary, oblique double layer carries a substantial, albeit a minority fraction ($10% to $50%) of the auroral potential. An order of magnitude density gradient results in an asymmetric electric field signature. Oblique double layers with amplitudes greater than 100 mV/m have been verified in $3% and may occur in up to 11% of auroral cavity crossings, so it is feasible that strong double layers are a principal acceleration mechanism. In this article we also present a second type of double layer that has a symmetric electric field signature and is seen inside of the auroral cavity. These structures are a possible signature of a midcavity or high-altitude acceleration mechanism. Numerical solutions of the Vlasov-Poisson equations support the possibility of midcavity double layers and indicate that trapped electrons can play an important role in the double-layer structure.