The EMMA Instrument on the Astrid-2 Micro-Satellite

The EMMA instrument on Astrid-2 is designed to provide simultaneous sampling of two electric and three magnetic field components up to about 1 kHz. The spin plane components of the electric field are measured by two pairs of opposing probes extended by wire booms with a separation distance of 6.7 m. The probes have titanium nitride (TiN) surfaces, which has proved to be a material with excellent properties for providing good electrical contact between probe and plasma. The wire booms are of a new design where the booms in the stowed position are wound around the exterior of the spacecraft body. The boom system was flown for the first time on this mission and worked flawlessly. The magnetic field is measured by a tri-axial fluxgate sensor located at the tip of a rigid, hinged boom extended along the spacecraft spin axis and facing away from the sun. The new advanced-design fluxgate magnetometer uses digital signal processors for detection and feedback, thereby reducing the analogue circuitry to a minimum. In addition to measuring the electric field by current biasing the electric probes, they may also be used to estimate plasma density and temperature by means of sweeping the bias to obtain a current-voltage characteristic. From this, information about the coupling between the probe and the plasma may be derived as well. Sampling is done at 16, 256, or 2048 samples per second. EMMA is equipped with 12 MB memory for storage of data from times without ground station contact and also for use with the highest sampling rate where the data throughput exceeds the capacity of the telemetry link. The EMMA instrument worked very well during its seven months of operation. A wealth of scientific data was collected and significant progress on its interpretation has already been made, as evidenced by several already published papers. The instrument charateristics as well as a brief review of the science accomplished and planned are presented.