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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.

We report a new type of spatially coherent plasma structure that is associated with quasistatic, magnetic-field-aligned electric fields in space plasmas. The solitary structures form in a magnetized plasma, are multidimensional, and are highly supersonic. The size along B 0 is a few l D and increases with increasing amplitude, unlike a classical soliton. The perpendicular size appears to be influenced by ion motion. We show that the structures facilitate ion-electron momentum exchange and suggest that an aggregate of structures may play a role supporting large-scale, parallel electric fields.[S0031-9007(98)06705-2] PACS numbers: 94.30. Kq, 52.35.Mw, 52.35.Sb, 94.30.Tz

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Abstract. The Fast Auroral Snapshot (FAST) explorer satellite was designed to investigate the microscale structure of the auroral acceleration region that was unresolved by previous satellites. This paper will present highlights from the first 2 years of the FAST mission and compare them with previous observations and auroral models. In particular, we find good agreement with the overall field-aligned current systems previously discovered; however, we present evidence that the downward currents are often carried by energetic upgoing electrons. These upgoing electrons are correlated with diverging electrostatic shocks, indicating that quasi-static parallel electric fields are responsible for their energization. Some of these field-aligned fluxes contain large-amplitude fast solitary waves which produce strong modulations of the electrons. Observations in inverted-V arcs show that the parallel acceleration region contains narrow (-10 kin) fingers of potential that extend along the magnetic field. On these narrow kilometer scales, ion beam energies are found to agree with the inferred potential determined from the perpendicular electric field or from the widening of the electron loss cone, implying acceleration is typically quasi-static on ion transit times. We also find evidence that both the ion and electron upgoing beams produced by the parallel electric fields have plateaued parallel distribution functions. Fast solitary waves are a prime candidate to stabilize the electron beams and may provide the resistance that allows the downward directed parallel electric fields to form in the highly conducting ionosphere. Intense ion cyclotron waves and ion solitary waves are often observed during ion beams, but the stabilizing waves have not been identified. In addition, intense electromagnetic ion cyclotron waves are also observed in inverted-V arcs, along with strongly modulated electron fluxes, indicating that turbulent acceleration is occurring in addition to simple acceleration by a static potential drop.

We study a transverse instability of the nonlinear equilibria known as electron phase-space holes in the presence of a magnetic field. The instability is intrinsically two dimensional and is determined by the dynamics of the trapped electrons. It depends on hole amplitudes, ambient magnetic fields, and the perpendicular velocity spread. The long-standing hole stability problem in multiple dimensions can be characterized by the gyro-to-bounce frequency ratio. A low ratio associated with a small perpendicular velocity spread results in a disintegration of the positive potential spikes.

The Digital Fields Board (DFB) performs the data acquisition and signal processing for the Electric Fields Instrument and Search Coil Magnetometer on each of the THEMIS (Time History of Events and Macroscale Interactions during Substorms) satellites. The processing is highly flexible and low-power (∼1.1 watt orbit-averaged). The primary data products are time series waveforms and wave power spectra of the electric and magnetic fields. The power spectra can be computed either on the raw signals (i.e. in a system co-rotating with the spacecraft) or in a coordinate system aligned with the local DC magnetic field. Other data products include spectral power from multiple passbands (filter banks) and electric power in the 30-500 kHz band. The DFBs on all five spacecraft have been turned on and checked out in-flight, and are functioning as designed.Keywords THEMIS · Signal processing · Electric field instrument · Search-coil magnetometer PACS 07.50.Qx · 07.87.+v · 94.80.+g · 95.55.-n · 84.40.Ua

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