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Overview

The FPI L2 data products reflect the full range of ~few eV to ~32 keV plasma populations in proximity to the spectrometers.  This includes the ambient plasma environment within that energy range and – on occasion – spurious plasma populations induced in the vicinity of the spacecraft.  For example, when the MMS EDI instrument is emitting 500 eV electrons, one or more FPI spectrometers often registers the signal in DES energy bin 16. 

It is up to the data user to review the data and carefully consider what the measured signals indicate about the natural plasma environment versus what is indicated about any spacecraft induced plasma environment.  M

The FPI team works diligently to remove as many induced effects as is possible from the FPI moment values (des-moms and dis-moms files) within our data processing pipeline.  In some cases, there remains work to be done, which is best carried out on an event-by-event basis. We provide the FPI full plasma distributions (des-dist and dis-dist files) without correction so that analysis work can be performed appropriate to the specific hypothesis under study.

Following is a checklist of known data features and recommendations for mitigation:

Quality Flags <-- heed the warnings!

FPI tags its data with a variety of quality flags to indicate intervals that need special warrant consideration -- and perhaps correction -- when using the data for event analysis.  It is important to check these and address any noted features.  The FPI quality flag definitions are identical for quicklook and L2 data products.   

Many of the features discussed below are automatically flagged in during the L2 data processing procedures. 

See child page on L2 on quality flags for detailed informationand for contributions of plotting routines to quickly plot them.  There is also a tutorial on plotting the quality flags using the SPEDAS suite of software.

Instrumentation Thresholds vs the ambient plasma distribution

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Know and use the statistical error information provided in the CDF files. The error associated with the inherent spectrometer counting statistics is provided in the CDF files for both the moments and for the distributions.  The actual counts can be recovered from the distErr variable from C = (f/distErr)**2 with distErr from the dist CDFs.

FPI moment error

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calculations

Use of the Partial Moments Calculations 

Quasi-neutrality Check 

Comparison of the FPI ion and electron density moments can give some indication as to how much of the underlying plasma distribution is represented in the separate FPI, HPCA, EIS, FEEPS data files.

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Error Flag Bit 13: significant (>=20%) penetrating radiation (DIS only)

Spin-periodicities  – Velocity Offsets

• Compression artifacts, which can be different for Survey and Burst data products
• Imperfect spectrometer response correction tables  (e.g., for signals dominating the very lowest or very highest energy channels; for data intervals very near a FPI macro change; and for data intervals for which sufficient burst calibration data was not readily available).  Some of these can be addressed with time.
• Imperfect spacecraft potential or photoelectron signal correction.  For some data intervals, data users will necessarily need to apply custom corrections.
• In some cases, even when the in-flight calibration tables have reduced the spin-period electron bulk velocity signal amplitude to below 50 km/s, there may appear more significant 20 s oscillations in the current density n*e*(Vi-Ve) (comparable to real current densities at the magnetopause).  This is a known issue and is currently beyond the scope of our current L2 processing.  If such current density oscillations are noticed, please contact the FPI team, and we will address these on a case by case basis until we are able to incorporate a correction into our routine L2 processing.

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