THEMIS Electric Field Instrument Operational Concept

1. THEMIS Electric Field Instrument • Operational Concept

2. 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.

3. 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.

4. 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.

5. Deploy and Commissioning Nominal Deploy and Commissioning Schedule

6. Operations and Data Validation • Operations • 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.

7. 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. • Filter Banks as in Fast Survey. • Diagnostic Mode • 3 E-field at 32 S/s; 6 sensor potentials at 8 S/s.

8. 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