THEMIS Electric Field Instrument Operational Concept

Deploy and Commissioning Draft plan developed Nov 2003. Draft plan refined Mar 2004 in response to revised launch date (21 Aug -> 21 Oct 2006). EFI SOH to be determined on all probes using stowed DC and AC functional test capability during initial on-orbit check-out. Instrument SOH used to determine probe assignment. EFI deployed on all probes after placement in initial science orbits. EFI deployed in 6 to 7 steps: 5 to 6 intermediate deploy lengths with spin up for SPB. 1 step to deploy both AXB. Primary constraints on deploy and commissioning: 1 30-minute TM contact per 3-1/2 hours (thermal). Desire to gather science data at intermediate deploy lengths and in different plasma regimes.

Deploy and Commissioning Deploy cycle on single probe (each step >= 3-1/2 hours): Deploy SPB-X, wait for transmitter to cool down, run Slow Sweep if desired, take Diagnostic Mode data if desired. Deploy SPB-Y, wait for transmitter to cool down, run Slow Sweep if desired, take Diagnostic Mode data if desired. Spin Up to desired spin rate for beginning of next deploy cycle. Slow Sweep: 4 BRAID, 3 USHER, 3 GUARD and 32 BIAS settings; 1152 steps, 1 step/spin, approx. 1 hour duration, data rate TBD (baseline is Diagnostic Mode, which may be overkill). Diagnostic Mode data: 2 or 3 DCE channels at 32 samp/s (3.75 deg/samp). 4 to 6 V channels at 8 samp/s (few mm spatial resolution). 1536 to 2304 bps; supported out to P1 Apogee for Real-Time contacts.

Deploy and Commissioning P3 and P4 deploy first: 2-day, 2-orbit cycle: P3 deploying one orbit, P4 quiescent, then swap. Full deploy and commissioning takes 14 days. P1 and P2 deploy next: P2 on 2-day, 1-orbit cycle: deploy on outbound, quiescent on inbound; 14 days total. P1 on 4-day, 1-orbit cycle: deploy on outbound, quiescent on inbound; 28 days total. P1 deploy cycle maybe accelerated depending upon experience gained and data gathered during P3 and P4 deploy and commissioning. P5 deploys last: 1 or 2-day cycle, with full deploy and commissioning taking 7-14 days.

Deploy and Commissioning Nominal Deploy and Commissioning Schedule

Operations and Data Validation Slow Sweeps and diagnostic data taken several times per year, upon first entry into a new plasma regime, and after long-duration eclipses to maintain optimal bias settings and monitor EFI state-of-health. EFI instrument mode set by ~30 registers on BEB; a typical mode will be specified with ~200 commands, valid over a typical one month period, once deploy and commissioning are completed. Data Validation Both sphere potentials and differential E-field data gathered in all waveform modes, allowing for initial verification of E-field data. Intercomparison of Eperp and –VxB from ESA and FGM to establish offsets and boom shorting effects.

Operation and Data Validation Data Rates (can change on-orbit; DFB mode commands) Slow Survey Spin-fit radial and spin-avg’d axial E-fields; spin-avg’d SC potential (via ptcls). Filter Bank of one axis. Fast Survey 3 E-field at 32 S/spin; 2-3 sphere potentials at 8 S/spin. Filter Banks of usual and ExB/E.B projected fields. Particle Burst 3 E-field at 128 S/s; 2-3 sphere potentials at 32 S/s. Filter Banks as in Fast Survey. Wave Burst 3 E-field at 1024 or 4096 S/s; 2-3 sphere potentials at 256 or 1024 S/s. Diagnostic Mode 3 E-field at 32 S/s; 6 sensor potentials at 8 S/s.

Operations and Data Validation Nominal Sensor Biasing (CPS and PSBL; Prediction): SC Potential <20 V and stable for CPS-PSBL plasma regimes When sensors run at 20 nA/sensor; Rsheath approx. 50 Mohm. CPS PSBL