The Mission


Electric Field Instrument

EFI Modes and Data Products

A detailed description of the EFI instrument modes and available data products can be found in the THEMIS EFI Instrument paper (Bonnell et al., 2008), as well as in the THEMIS Digital Fields Board (DFB) paper (Cully et al., 2008x). The following tables and discussion are abstracted from the EFI Instrument paper, and should serve to introduce the data products that are available from the EFI instrument.


Table 2: Data products available from EFI.

Data Product Range (not adjusted for frequency-dependent gain) Bits Resolution Available Sampling Rates
V1..V6 +/- 105 V
(+/- 100 V supply limited)
16 3.2 mV/ADC 2-8192 samp/s
EDC12 (49.6-m) +/- 300 mV/m 16 (9.2 μV/m)/ADC 2-8192 samp/s (16384 samp/s for AC)
EAC12 (49.6-m) +/- 51 mV/m 16 (1.6 μV/m)/ADC 2-8192 samp/s (16384 samp/s for AC)
EDC34 (40.4-m) +/- 370 mV/m 16 (11 μV/m)/ADC 2-8192 samp/s (16384 samp/s for AC)
EAC34 (40.4-m) +/- 63 mV/m 16 (1.9 μV/m)/ADC 2-8192 samp/s (16384 samp/s for AC)
EDC56 (6.2-m) +/- 2.7 V/m 16 (81 μV/m)/ADC 2-8192 samp/s (16384 samp/s for AC)
EAC56 (6.2-m) +/- 450 mV/m 16 (14 μV/m)/ADC 2-8192 samp/s (16384 samp/s for AC)
HF 4 μV/m to 12 mV/m 8 0.01 decade of amplitude/ADC 2-8192 samp/s
Spin Fit Exy Same as EDC12 or EDC34, depending upon source setting. 16-bit floating point Same as EDC12 or EDC34, depending upon source setting. ≈1 deg. In angle. 1 vector/spin (typ. 3-s period)
Spin Avg Ez Same as EDC56 16-bit floating point Same as EDC56. 1/spin (typ. 3-s period)
Spacecraft Potential   16   1/spin (typ. 3-s period)
Filter Bank and FFT Spectral Products See Digital Fields Board description [Cully et al., this issue] for details.      


Table 3 presents a typical on-orbit configuration for EFI data collection in terms of waveform sampling rates, spectral cadences and resolutions, and gain states in the four modes of operation. Also shown (in bold italics) are the data types that represent each data quantity in the Level-1 and Level-2 CDF data files, as well as the processed versions available through either the THEMIS Data Analysis Software (TDAS) IDL libraries, or the Science Data Tool (SDT) stand-alone browser. These tags (vaf, efp, etc.) indicate both the data type (va = sensor voltages; ef = E-field estimates) and acquisition mode (f, p, w = fast survey, particle burst, wave burst) for each data type. For more detailed information, the interested reader should reference the THEMIS Fields Variables Names documentation [pdf file: THEMIS FIELDS L1 Data files Variable Name Definition].


Table 3: EFI On-Orbit Data Quantities.

  Slow Survey Fast Survey Particle Burst Wave Burst
Duration Up to full orbit (24 hours maximum). Up to 12 hours. Variable, few to tens of minutes, up to 8/orbit. Variable, few to tens of seconds, up to 8/PBurst.
Data Quantity        
Vn (n=1..6) 0, 2, or 4 samp/s vaf 2 samp/s vaf 16 samp/s vap 8192 samp/s vaw
Enm (nm=12,34,56) 0, 2, or 4 samp/s DC-coupled, eff 4 samp/s DC-coupled, eff 128 samp/s DC-coupled, efp 8192 samp/s DC-coupled, efw
Spin Fit Exy, Spin Avg Ez 1/spin (1/3 s typ.) efs      
Filter Banks, HFF EDC12, SCM1 1 spec/4 s fb_∗, fbh      
FFT Spectra -- -- 16 or 64 bins. 4 or ¼ spec/s. ffp_16_∗, ffp_64_∗. 16 or 64 bins. 4 or ¼ spec/s. ffw_16_ ∗, ffw_64_ ∗.


The EFI waveform data are available in a variety of operationally and geophysically relevant coordinate systems. These are described in detail in the, "THEMIS Coordinate Systems," document ( [pdf file: THEMIS Science Coordinate Systems Definition]). At Level-2, as well as through special processing of Level-1 data, the so-called "_0" and "_dot0" versions of the 3D E-field waveform data are available. These two data types address the radical difference in accuracy between the spin plane and spin axis estimates of the E-field (see discussion in Axial Performance section below).

The "_0" data quantities replace the axial E-field estimate with zero prior to transforming the E-field into despun and/or geophysically relevant coordinate systems. They are useful for estimating the impact of the axial field component upon the full 3D E-field estimate.

The "_dot0" data quantities replace the axial E-field estimate with a value computed from the spin plane E-field estimates and the ambient B-field under the assumption that E||=0, or equivalently E•B = 0; in other words, one replaces Eaxial by Eaxial = -( (Bx/Bz)∗Ex + (By/Bz)∗Ey). It is often the case, especially at the macroscopic scales of interest for THEMIS, that the dominant contribution to the electrodynamics arises from the perpendicular component of the electric field, and this technique allows one to estimate that component, and then transform it into other coordinate systems without having to remove the large systematic errors in the axial E-field estimate. If the B-field is too close to the spin plane (i. e. Bz is too small relative to Bx and By), then the error associated with this method grows, leading to a ten-to-one increase in error for angles smaller than ~6 degrees.

In addition to the science quantities, a complete set of EFI housekeeping quantities are collected by the IDPU and telemetered as part of the IDPU State-Of-Health packets (APID 0x404 and 0x406). These housekeeping quantities are summarized in Table 4 below. The current monitors are sampled once per second, while the other housekeeping channels that monitor signals on the BEB are sampled at either variable rates between once per 256 s to once per 216 s (≈ 18 hours), or on demand, when a bias update command is sent to the BEB. The relatively slow sampling rate for the BEB-derived housekeeping channels is driven by both the relatively slow variations of the quantities involved (either fixed for many orbits (bias settings, for example), or varying on minutes to hours time scales), as well as a modest, but noticeable level of noise injection associated with the housekeeping sampling seen on the EDC and EAC channels (unipolar signals of amplitudes ≈ few tenths of mV/m).


Table 4: EFI housekeeping quantities.

Quantity Description Resolution [bits] Measurement Range
IMON_EFI_BOARD Aggregate primary-side current drawn by DFB and BEB fixed-voltage supplies (analog and digital). 8 0 to 150 mA. Typ. 100-130 mA
IMON_EFI_X, Y, Z Primary-side current drawn by EFI floating supplies (axis-by-axis). 8 0 to 130 mA. Typ. 55-70 mA in sunlight, up to 80 mA in eclipse (cold, saturated).
IEFI_IBIAS1..6 Readback of BIAS offset voltage DAC output. 16 -528 to 528 nA
IEFI_USHER1..6, GUARD1..6, BRAID Readback of USHER1..6, GUARD1..6, and BRAID offset voltage DAC output. 8 -40 to 40 V
ISPB_TEMP, IAXB_TEMP Preamp board temperature (connector) 8 -258 to 357 C. Typ. 20 to 30 C (SPB), 30 to 40 C (AXB). -135 C (3-hr eclipse).
IBEB_TEMP BEB board temperature (BEB FPGA) 8 -76 to 176 C. Typ. 20 to 30 C.