Fast Auroral SnapshoT Explorer
Home
Introduction
Pictures
Publications
Data
Software
Science Operations
Mission Operations
Education Site
Links
Sitemap
    

User guide for analysis of FAST fields data.

John W. Bonnell

The following document is designed to help you get started using IDL routines to analyze data from the fields experiment on FAST. For details on the instrument design see the paper by Ergun et al. [Space Science Reviews, 2001] (The FAST Satellite Electric Field and Magnetic Field Instrument). The fields experiment consists of electric field, density, and magnetic field sensors, and the sampling and processing hardware that supports them. This document begins with a description of the fields experiment and data products that are available from it, followed by a description of the known problems and limitations of the data. The third section is a categorization of the the IDL routines available for fields data analysis, including a discussion of the most useful programs. The fourth section is an example IDL crib sheet that one can use as a starting point for fields data analysis. The last section is a list of current fields analysis programs; each has been tested, but should not be used blindly; be sure to keep the limitations of the instruments and analysis techniques in mind. An appendix follows the main text, and presents some more detailed descriptions of probe shadowing, despun coordinate systems, etc.

Description of the Experiments and Data Products

The FAST Fields Experiment consists of many sensors with lots of flexibility in what data is gathered, at what rates, and what sorts of on-board processing is applied before telemetering the data to ground. Thus things get complicated, and users need to learn a bit before getting comfortable with the data and using it and the analysis routines proficiently.

The on-orbit configuration of the FAST fields sensors is shown in the figure below. Each of the electric and magnetic field sensors is situated on a boom in order to get the sensor well away from the spacecraft body. The electric and magnetic field sensors are described below, followed by the various data products available from the sensors, and an example of a fields mode sheet.

Figure courtesy C. C. Chaston.

Electric Field and Plasma Density (Current) Sensors

The electric field sensors on FAST consist of four radial wire booms in the spin plane, each carrying a double probe at its end, along with two rigid axial booms, each carrying a single probe. The outboard probes of all the spin plane booms were designed to operate as potential (electric field) probes only, while the inboard probes can be operated as density probes as well. Both (?) of the axial probes can be operated in either potential or current mode. The biasing scheme (current bias for potential probes, and voltage bias for current probes) is set by the mode of the fields instrument, and the parameters of the given biasing scheme are described in detail on the individual Fields Mode Sheets. As noted in the Pitfalls section, one of the radial booms failed to deploy, leaving the spin plane measurements with only three well-separated antennas. Only one of the axial booms has been deployed to date (probe 9).

Magnetic Field Sensors

The magnetic field sensors consist of a three-axis fluxgate magnetometer (the DC magnetometer) and a three-axis searchcoil magnetometer (the AC magnetometer). For details of the magnetometer sensors see Elphic et al., [Space Science Reviews, 2001] (Magnetic Field Instruments for the FAST Auroral Snapshot Explorer).

Data Products

A variety of different data products are available from the fields experiment. All of the different data products are described in detail in the Fields Experiment Article; the real workhorse data products are the time series data, along with spectral estimates, and these will be described in some detail below.

Time Series

Time series data comes in two flavors: survey and burst. Survey data comes in two speeds: slow and fast. The exact meaning of the two depends upon the Fields Mode, but typically Fast Survey data is sampled at sixteen times the rate of Slow Survey (up to 2048 samples/s for Fast Survey). DC magnetometer data also comes in in Back Orbit rate as well (8 samples/s), up until the P12S7V anomaly; see the Pitfalls section below. The FAST satellite enters the auroral zone in Slow Survey, and then depending upon trigger events arising due to cues from various fields and particles measurements, it will shift back and forth between Fast and Slow Survey mode throughout a given pass. Note that both the fields and particle (ESA and TEAMS) experiments switch between fast and slow survey together. In addition to Survey rate data, the fields experiment will also enter Burst mode during a given pass in response to fields and particle cues. Depending upon the Mode, data from selected channels will be acquired at either 8192, 32768, or both samples/sec to produce the 4k and 16k Burst data streams. Note that both the fields and particle experiments enter into burst mode together, and that Burst mode data is gathered concurrently with Survey mode data; ie. both rates of data are available during the Burst intervals. Data will also be gathered from a very select set of channels at rates of up to 2 Msamples/s to produce the high-speed burst memory (HSBM) data stream, subject to a similar, but distinct set of fields and particle cues.

Spectral Estimates

Spectral estimates in several frequency bands are made on-board, either from the time series data using a digital signal processor (DSP), or using an analog swept-frequency analyzer (SFA). The DSP is also used to analyze the data from a 16-kHz-wide band around a given frequency using the plasma wave tracker (PWT). The estimated local electron cyclotron frequency is typically used as the center frequency, although other selections are possible. The DSP can also produce estimates of cross-spectral coherence and phase shift. The details of the operation of each of these data products (averaging intervals, sweep rates, tracking frequency, etc.) can be found on the Mode Sheets as well in the Fields Experiment Article.

Other Data Products

Several other data products are created on-board, but do not currently enjoy wide use or a well-developed user interface in IDL. These data products include the Low-Frequency Filter (LFF), Broadband Filter (BBF and HFQ ), and Wave-Particle Correlators (WPC). See the Fields Experiment Article for the details on these data products, and contact one of the Fields experiment team for aid and discussion before using these data products.

Fields Mode Sheets

All that one needs to know about what particular data products are available at a given time from the fields experiment can be determined using the Fields Mode Sheets. One can determine the Fields Mode for a given timespan or orbit in several ways:
  • Use the Search or Listing utilities available on the FAST Website.
  • Examine the value of a Fields Mode Data Quantity with a SDT session.
  • Use the FA_FIELDS_MODE call from within IDL (see the IDL Utility Routines section below).

A typical Fields Mode sheet is shown in the PDF-format document below:

Fields Mode Sheet for Mode 19.

Now for some unfortunate, but necessary jargon and acronyms. The data from FAST is organized into bundles of channels known as APIDs. Each APID consists of several different fields quantities, or a particular type of particle data, or spacecraft ephemeris/housekeeping data. Each quantity is identifed by its Data Quantity Descriptor or Identifier (DQD or DQI, both acronyms are used interchangably throughout the FAST SDT/IDL documentation and software package).

A Fields Mode sheet consists of several sections that describe the channels (DQDs) available in a given instrument mode, the APID that those channels appear in, and the rates at which the data from those channels are acquired. The most important sections to look at are Sections A, B, C, and D, covering the Sphere Configuration, Slow and Fast Survey data channels and Burst mode data channels respectively.

Section A can be found near the top of the first page of the mode sheet, and is a simple listing of which probes are in current mode. Here, one can see that probes XXX and YYY are in current mode for Fields Mode 19.

Sections B and C are also found on the first page of the mode sheet, and detail what DQDs are available in Slow and Fast Survey modes (first column of each section), as well as the sampling rates of those channels (second column of each section). Here, for example, one can see that the estimated electric field from probes 5 and 8 (V5-V8_S) is available at 256 and 2048 samples/s in Fields Mode 19 for Slow and Fast Survey.

Section D can be found on the second page of the mode sheet, and details what DQDs are available in Burst mode, as well as the sampling rates of those channels (the first and second columns of the section, respectively). The DQDs and sampling rates available via the HSBM processor are also detialed in Section D. Here, for example, one can see that XXX is available at 8192 samples/s and YYY is available at 32768 samples/s in Burst mode, while AAA, BBB, and CCC are available at 2 Msamples/s via the HSBM in Fields Mode 19.

Back to top

Known Problems and Limitations of FAST fields data

Boom Deployments:
The radial spin plane boom carrying probes 3 and 4 failed to deploy, leading to probe 4 being situated right outside of the spacecraft skin, and probe 3 being situated within the spacecraft. Due to this problem, any DQD derived from probe 3 should not be used (V3-V4, for example); in fact, nearly all the Fields Modes do not include any probe 3 dependent DQDs because of this failure. As noted in the Shadowing Effects section above, probe 4's proximity to the spacecraft skin makes it more prone to solar and magnetic shadowing effects, as well as making it susceptible to spacecraft potential variations. Both can be deal with via Notching, as is discussed in the section on Despin routines below. While the effects of shadowing are quite strong at quasi-DC and low-frequencies (10 Hz and below), the higher-frequency response of probe 4 (and potential differences derived therefrom) seems to match probes further from the spacecraft, and can thus be used at those higher frequencies without notching if due care is taken to understand the contamination at lower frequencies.

The axial boom carrying probe 9 was not deployed until orbit XXXX, and the axial boom carrying probe 10 has not been deployed as of this date (May 2001). Prior to deployment, each of these two probes was in close proximity to the spacecraft skin, and thus suffered similar shadowing and spacecraft potential problems to those of probe 4. The DC calibration of these probes is not well known on-orbit, due to the lack of availability of spin-periodic signals with which to estimate gains and offsets. Their AC calibrations seems to agree with pre-flight estimates (properly adjusted for the probe separation), and after probe 9 was deployed, allow for 3D AC electric field estimates in modes that support it.

Shadowing Effects:
As described in detail in the Shadowing Effects section below, the potential of a given probe (density or potential) relative to the surrounding plasma depends upon the current carrying populations available at its surface. That availability depends strongly upon the solar illumination of the probe, and the relative orientation of the probe, the ambient magnetic field, and the rest of the spacecraft. These aspect-sensitive effects will occur to one extent or another on all of the spin plane probes of the electric field experiment, with magnetic aspect effects showing up on almost every orbit due to the close alignment of the FAST spin plane and ambient magnetic field, and the most pronounced effects observable on any potential or potential difference derived from the probe 4 measurement (probe 4 is quite near the skin of the spacecraft, and so is easily blocked during large portions of the spin period. Solar aspect effects show up most regularly in the spin plane probes in noon-midnight orbit configurations; most readily in the spin axis probes in the dawn-dusk orbit configurations.

The current-collection capabilities of the density probes also seems to show solar and magnetic aspect sensitivity.

AC versus DC effects: contamination of spectral density at low frequencies, but little evidence for gain changes between illuminated and shadowed situations.

Sweeps:
Early in the mission (prior to orbit 5700; ), the bias current applied to the potential probes was swept several times (one to three) over the course of a given pass in order to better characterize the current-voltage characteristic of the coupling between the potential and density probes and the surrounding plasma (see the discussion on Biasing Schemes for the motivation behind this). A master list of swept orbits and times is being compiled, but an individual sweep is fairly obvious in the data when one compares the potential differences from swept and unswept probe pairs in SDT or IDL. A sweep will affect the estimation of the despun DC electric field, as well as estimates of the AC electric field up to a few Hz (at most).

Data Gaps:
The different channels can have data gaps at different times, and so care must be taken in comparing or combining data from different channels, such as in the various DESPIN routines. The FA_FIELDS_COMBINE routine is built to detect gaps in different data streams, and either interpolate across or leave the gap alone, doing this to match up two different data channels prior to applying any of the higher-level data reduction programs (any of the DESPIN routines, for example).

Sequential versus Simultaneous Sampling of Data:
survey vs burst vs hsbm sampling; does this have any real effect on the data that appears in IDL, or already accounted for via linear interpolation during calibration?

Failure to detect HG channel gain state setting:
A factor of four gain change has not been properly taken into account in calibration procedures for HG time series and DSP data, as well as SFA, BBF, and HSBM data taken from the two spin plane probe pairs V1-V4 and V5-V8. The source of this failure to detect the gain state setting is under investigation. Until this ambiguity is resolved, the affected channels (V5-V8HG and V1-V4_HG in Survey or Burst mode; V1-V4 and V5-V8 in DSP, SFA, BBF, or HSBM data) should not be used for any quantitative estimates of wave amplitude (including the estimation of wave polarization using multiple antennas), but may be used for qualitative description of wave spectra.

disparity between DSP and time series spectral density estimates:
Comparison of the spectral density estimates derived from the on-board DSP data dn 16k Burst data shows that there is a discrepancy between those estimates. The DSP estimate of the electric field spectral density (V5-V8, etc.) is roughly a factor of 500 higher than that resulting from the 16k Burst data. The DSP estimate of the magnetic field spectral density (Mag3ac only) is roughly a factor of 3160 lower than that resulting from the 16k Burst data. The source of this discrepancy is not known, but arises from the DSP calibration, rather than the time series (Burst) calibration. The above quoted figures may be used to correct the DSP data for qualitative work, and a package of analysis routines (DSP_AUTOCAL) has been developed to allow for a more detailed analysis of the discrepancy for a given interval of analysis; Contact the FAST Fields Team for further information.

The HSBM ``one-second'' problem:
The HSBM data often appears to be roughly one second early relative to other data streams; ie. if HSBM and 16k Burst data is available from the same antenna at a given time, individual features of the waveforms will appear about one second earlier in the HSBM data than in the Burst data. The cause of this problem is under investigation, and is not known at this time, but is an aspect of the HSBM data, not the Burst data. This problem is intermitent. A rough work-around exists if precise timing of HSBM bursts relative to other events is required. Contact the FAST Fields Team for further information.

degradation in performance of sphere 5:
An intermittent DC offset appeared in all DQDs derived from the sphere 5 potential (V5-V8, for example), starting around January 2000, and continuing through the present. The offset is believed to arise from increased leakage current in the sphere 5 preamp, and may depend upon the ambient plasma density.

Prior to January 2000, a typical DC offset in V5-V8 would be a few mV/m, corresponding to a difference in the floating potentials of spheres 5 and 8 of approximately 0.25 V. The extra offset, when it occurs, it quite obvious in a plot of V5-V8, and is currently around 760 mV/m (corresponding to a difference in floating potential of 30-40 V).

In addition to the increased offset voltage, the leakage current appears to make the sphere potential unstable in some plasma environments, leading to large, non-geophysical fluctuations in the estimated electric fields derived from sphere 5.

Power supply problems in on-board processing of DC magnetometer data (the `P12S7V' problem):
An anomaly occured in the power supply for the on-board data processor supporting Survey DC magnetometer data on orbit 8431 (09 October 1998). The power supply voltage fell far enough to degrade the quality of the DC magnetometer data. Something else happened between orbits 9200 and 9937 (19 Dec 1998 to 24 Feb 1999). Something else entirely happened after orbit 9937. Details to come from RJS.

The collection of BackOrbit DC magnetic field data was curtailed at orbit XXXX, reducing the accuracy of the in-flight calibration performed by UCLA_MAG_DESPIN (see perturbation magnetic field analysis below). Small-scale perturbations in the magnetic fields can be trusted, but larger-scale offsets (possibly indicative of global modifications to the magnetospheric field) can not; contact the FAST Fields Team for further details.

effects of torquer operations on magnetometer data:
Roughly once a day, a magnetic torquing system is used to adjust the orientation of FAST's spin axis so as to insure the best possible alignment between the spin plane and the ambient magnetic field in the northern auroral regions. Torquer operations have two effects on magnetic field measurements. First, the torquers produce a DC offset in the magnetic field measurements while on (several hundred nT), and also slightly change the offsets and gains of the magnetometers themselves after turning off. The DC offset can be seen directly in the magnetometer data, and while compensated for to some extent in UCLA_MAG_DESPIN, means that DC B fields should not be trusted during torquer operations. The changes in instrumental gains and offsets are accounted for in the on-orbit calibration procedures that UCLA_MAG_DESPIN implements. Second, the torque upon the spacecraft leads to nutation with a period of roughly 20 seconds that persists for up to two orbits after the torquer has been turned off. This effect is also obvious in the magnetometer data, as a few tens of nT oscillation in all three components of B. Care must be taken in interpreting the DC and low-frequency magnetic field data while nutation is occuring (ULF pulsation fans, take note!).

AC (searchcoil) magnetometer background levels:
All three axes of the AC (searchcoil) magnetometer have significant spin-dependent background noise levels. The source of this noise floor is currently under investigation, and can be described by the model accessible here: TBD.

searchcoil magnetometer not properly calibrated in Survey mode:
All three axes of the AC (searchcoil) magnetometer are not sampled at high enough rates in Survey mode to allow for proper use of the standard searchcoil calibration procedures. The only exception to this is mag3ac in the fastest FastSurvey sampling rate (2048 samp/s)). The under-sampling of the data distorts the time series and spectral shape of the `calibrated' data through aliasing effects. Magac Survey data should thus be used for only the crudest qualitative estimates of the ELF magnetic spectrum, if at all.

Back to top

Categorization of IDL routines for fields data analysis

FAST fields routines can be grouped into five categories:

The FAST fields data analysis routines were written primarily by Bill Peria and Robert Ergun, with contributions from Greg Delory, Chris Chaston and John Bonnell (ANY OTHERS?). UCLA_MAG_DESPIN was written by Robert Strangeway, with contributions by Richard Elphic and ???.

Questions regarding the fields analysis programs as well as bug reports and fixes can be directed to FAST Fields Team; a good faith effort on the part of any experimenter to understand the nature of their fields analysis problem prior to contacting the fields team is expected and appreciated.

Routines that return fields data structures

One needs to know what data is available, and then transfer it from shared memory to data structures accessible from within IDL. Currently the FAST fields routines return two sorts of data. The first sort is an anonymous IDL structure known as a `FAST fields data' structure. Such a structure contains sample times, data values, units, calibration information, and data gap information. The second sort is known as a `TPLOT quantity', which is a set of anonymous data structures stored on the IDL memory heap and accessible to the TPLOT plotting package in IDL.

Most of the data reduction routines (FA_FIELDS_COMBINE, FF_FILTER, FA-FIELDS_SPEC, etc) operate on FAST fields data structures to produce either other FAST fields data structures or TPLOT quantities.

Data Acquisition Routines

GET_FA_FIELDS: basic routine for acquiring data from shared memory. Data can be loaded in calibrated or uncalibrated form (enforced calibration depending upon setting of the FAST_CALIBRATE environment variable), and is returned as either a FAST fields structure or as a TPLOT quantity.

FF_POTENTIAL:

GET_DENSITY:

FA_FIELDS_CYCLOTRON: computes electron, H+, He+, and O+ cyclotron frequencies from measured magnetic field data (the DQI MagDC is required), as well as |B| and angle between spin plane and B.

FA_FIELDS_DSP: acquire DSP spectral data and compute OMNI spectral density, then store results as TPLOT quantities.

FA_FIELDS_SFA: acquire SFA spectral data and compute OMNI spectral density, then store results as TPLOT quantities.

FA_FIELDS_PWT: acquire PWT data, compute spectral density, offset frequencies, and store as a TPLOT quantity.

FF_DSP_POWER: compute integrated spectral density of OMNI electric field and Mag3ac magnetic field DSP data. Frequency interval for integration can be specified.

DC Perturbation Magnetic Field Estimation:

UCLA_MAG_DESPIN: The extraction of an estimate of the vector DC perturbation magnetic field from low- or mid-altitude satellite magnetometer data is a daunting task. It requires on-orbit estimation of fluxgate magnetometer calibration parameters, precision attitude estimation, despin and projection of estimated field into various coordinate systems, and finally subtraction of model (IGRF) magnetic field, all to a precision of at least 1 part in 1000. This task is accomplished on FAST using the UCLA_MAG_DEPSPIN routine. This routine is conservative, and will warn the user about both the assumptions that it is using in order to accomplish its task, and when those assumptions appear to have broken down. When operating correctly, it estimates of the vector perturbation magnetic field suitable for studies of current systems, low-frequency electromagnetic perturbations, and stress transmission. Details of the algorithm can be found in the IDL code itself (ucla_mag_despin.pro). Be certain to read the caveats on DC Magnetic Field Data in the Pitfalls section above prior to interpreting any reduced magnetometer data.

Ignore all the other DC B-field reduction routines (FA_FIELDS_MAGDC and supporting code).

Despin:

The electric field estimates from a spinning spacecraft such as FAST are useful in and of themselves, but are much easier to interpret if transformed (or despun) into a geophysically-relevant coordinate system, such as one organized around the local magnetic field. This process of despinning the data is described in greater detail in the Despinning section below, and involves estimating and correcting for any gain or offset differences between the two (or three in the case of 3d despin of HSBM data) antennas used as input to the despinning procedure. An accurate estimate of the relative orientation of the antennas and the ambient magnetic field is also required (although not with the precision needed in the DC magnetic field estimation procedure).

FA_FIELDS_PHASE: estimates the angle of the vector pointing towards the Sun and along the ambient magnetic field in the spacecraft spin plane using on-board sun sensor and magnetometer data.

FA_FIELDS_DESPIN: estimates despun spin plane electric field (original version, optimized for DC E-field estimation (few Hz and below)).

FA_FIELDS_DESPIN_SVY_LONG: newer version of FA_FIELDS_DESPIN, again optimized for DC E-field estimation.

FA_FIELDS_DESPIN_{4K,16K,HSBM}: newer versions of despin codes optimized for Burst and HSBM data; HSBM supports three-axis despin. Results are stored as TPLOT quantities.

FA_FIELDS_DESPIN_HG: refilters HG-type data from the original high-pass frequency of 3.5 kHz to 300 Hz, then despins and stores as a TPLOT quantity.

SIMPLE_DESPIN: a bare-bones despin routine that handles the transformation of contiguously sampled data from pairs of antennas. No gain or offset adjustment is performed. Suitable for careful use on AC electric fields. Produces FAST fields data structures.

All the newer versions of DESPIN support spectral density estimation of the despun electric field data within the routines themselves, rather than through a separate call to the spectral density estimation routines (see Spectral Estimates below).

Spectral Estimates:

auto and cross spectra.

FA_FIELDS_SPEC: computes the auto-spectrum (aka. power spectrum) of a given time series stored in a FAST fields data structure, and return as either a FAST fields data structure or a TPLOT quantity. The spectrum is estimated using an averaged FFT algorithm, and the parameters of that algorithm (overlap, points/FFT, FFTs/spectrum, etc.) can be specified via keywords. Specification of a valid sampling rate (allowing the selection FastSurvey over SLowSurvey data) is also allowed.

FA_FIELDS_CROSS: similar to FA_FIELDS_SPEC, but computes the cross-spectrum (coherence and phase shift) of two signals using an averaged FFT algorithm.

Utility Routines:

Informational Routines

SHOW_DQIS: lists the DQDs (or DQIs) currently loaded into shared memory by SDT, along with the timespans and number of points available for each.

interpolation, merging of channels, sensor information.

FF_INTERP, FA_FIELDS_COMBINE, FA_MODE_INFO, FF_INFO

Example IDL crib sheet for fields data analysis

Example IDL "crib" sheet (TBD)

Back to top

Catalog of FAST fields analysis programs

FAST fields IDL routines (TBD)

Back to top

Notes on shadowing, despun coordinate systems, probe biasing schemes, etc.

GENERAL DESPIN NOTES:

Each 2D despin procedure takes data from two (ideally) orthogonal, concentric antennas and projects that data on a sample-by-sample basis into a coordinate system fixed with respect to (usually) magnetic phase. Let S be the unit vector along the spacecraft spin axis, and let B be the unit vector along the local magnetic field, and let x be the vector cross product. The direction corresponding to zero degrees magnetic phase ((S x B) x S) is denoted in the results of the FAST fields despin codes by E_NEAR_B, while the direction corresponding to ninety degrees magnetic phase (the direction (S x B)) is denoted E_ALONG_V.

The S x B direction is denoted E_ALONG_V because the predominantly downward magnetic field in the Earth's northern polar region, along with the anti-parallel orientation of FAST's spin and orbital angular momenta leads to S x B being roughly parallel to the perpendicular component of the spacecraft velocity in the spin plane. Note that for FAST in the southern polar regions, (S x B) is actually anti-parallel to the perpendicular component of the spacecraft velocity in the spin plane.

Each of the despin routines has the option of masking out the data taken during the nominal (and quite conservative!) periods of Sun and magnetic shadowing of the electric field probes by the main spacecraft or other probes (see discussion of SHADOWING EFFECTS below) specified by the database drawn upon by the FF_INFO routine. This masking or Notching procedure excludes the data from a given antenna while its Sun or Mag phase lies within certain broad limits around the geometrical shadow of the spacecraft, or the magnetic alignment of the spacecraft and probe.

If the two antennas used to provide the data for the despin procedure are not orthogonal (V1-V2 and V5-V6, for example), then a spin-dependent error will be introduced into both of the despun components. If the two antennas used to provide the data are not concentric (V1-V4 and V5-V8, for example), then a spin-dependent error will be introduced into both of the despun components with a magnitude that depends upon the degree of spatial inhomogeneity present in the electric field (ie. the phase shift between the two antennas due to the finite wavelength of the electric field). In either case, the errors will lead to a spin-dependent error in the despun time series data or any spectral estimates derived therefrom.

The despin routines for the three-axis HSBM data allow for a further projection of the data. Those routines use two spin plane and one spin axis electric field estimate to first determine E_ALONG_V and E_NEAR_B, and then (using E_NEAR_B and the spin axis electric field) to determine estimates of the component of E_perp orthogonal to E_ALONG_V, as well as E_para.

A related, but different procedure is the generation of an `omnidirectional' spectral density (OMNI) by adding the spectral densities derived from two orthogonal antennas in the spin plane. The spin modulation caused by any prefered polarization of the electric field in the spin plane is removed by the procedure, often allowing one to better see time variation in the spectral density at the cost of the loss of any information about the polarization (E_para versus E_perp) of the electric field. A true omnidirectional spectral density estimate would require three orthogonal antenna axes; while available on FAST in some modes and sampling rates, none of the OMNI-estimating routines currently take advantage of three-axis measurements.

SHADOWING EFFECTS:

Any conducting or insulating body immersed in a plasma will float to a potential different from that of the plasma far away in order to achieve a state of current balance. The probes that make up the electric field instrument on FAST are no different. The primary charge carrying populations in the auroral zone are the ambient electron and ion populations (cold and hot), any secondary electron emission engendered by ambient plasma impacts, photoelectrons, and electron currents sourced from or sinked to the probe as part of some biasing scheme.

Sun shadowing spikes (or Sun spikes) occur when a probe falls in the optical shadow of some other part of the spacecraft (eg. booms, other probes, the main body of the spacecraft). When the probe falls into the shadow, the photoemission of electrons ceases, and the probe will typically jump to a more negative potential relative to its illuminated value in an attempt to regain current balance. A related effect (not believed to occur on FAST) is irregularities in the surface properties of the probes causing a Sun-aspect-sensitive photoemission, leading to similar effects as Sun shadowing.

Magnetic shadowing spikes occur when a probe and some other portion of the spacecraft are aligned along the ambient magnetic field, or more generally when the two items are within an electron or ion gyroradius of the same field line. Typically this results in the interruption of the electron current carried by the field-aligned motion of the ambient electrons to the shadowed probe, and leads to the probes potential jumping to a more positive potential relative to the un-shadowed situation in an attempt to achieve current balance. Magnetic shadowing can also lead to the exchange of photoelectrons along the field, and thus can have more complex effects on the floating potential of the probe. A system of biasable conducting surfaces called guards and stubs surrounding the electric field probes is used to mitigate some of the effects of photoelectron emission and exchange.

BIASING SCHEMES:

Electric field probes and Langmuir probes are often biased in order to minimize the effect of variations in ambient plasma parameters on the measured quantities, or to optimize the DC and AC transfer characteristics of the sheath-preamp system of the probes.

Electric field probes are biased by sourcing or sinking an electron current to the probe via biasing electronics on board. The bias current is sourced from or sinked to the spacecraft ground, and will figure into the global current balance of the entire spacecraft. The bias current is chosen to optimize (in some sense) the performance of the electric field probe. In the case of FAST, the current bias is chosen so as to bring the probe close to the ambient potential of the plasma, ie. the bias current is chosen so as to equal in magnitude the total of all the other currents to the electric field probe. Note that the bias current is not continuously updated, and thus the ideal case of no floating potential is maintained only in an average sense.

Density probes are biased by placing the probe at a different potential relative to spacecraft ground. Typically this is done in order to force the probe into either the electron or ion saturation regimes of operation. Again, the choice of bias potential depends upon the ambient plasma (mostly temperature) parameters and illumination conditions, and so one should be aware that the biasing scheme may not be operating ideally on any given orbit. See the Fields Experiment Article for a description of how the bias voltage on the density probes is determined on FAST.

On FAST, different biasing schemes have been developed in an attempt to deal with the different plasma conditions at perigee and apogee altitudes, as well as the different illumination conditions during different orbital seasons. The bias schemes were developed over the course of the mission in response to on-orbit determination of their relative efficacy (ie. they were tweaked if they led to poor results or instability of the probe-sheath system, otherwise they were left alone). The parameters of the biasing schemes (including the guard and stub biasing) are recorded in Section E of each of the Fields Mode Sheets.

Back to top

Webpage maintained by Webster.