EFI data

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This page contains the information about the Polar Electric Field Instrument (EFI) archived data.

Contents 1 Polar Electric Field Instrument (EFI) Data Archive 1.1 Overview 1.2 Web Access: 1.3 Date Coverage: 1.4 Coordinate Systems: 1.5 Time Resolutions: 2 Description of Data: 2.1 Electric Field (vector) Data: 2.1.1 Computation of the Spin Plane Components 2.1.2 Computation of the Spin Axis Component 2.1.3 Summary of the available Electric Field quantities: 2.2 Spacecraft Potential and Plasma Density 2.2.1 Summary of the available Spacecraft Potential / Plasma Density quantities: 2.3 Orbit Data 2.3.1 Summary of the available POLAR Orbit quantities: 2.4 Attitude Data 2.4.1 Summary of the available POLAR Attitude quantities: 2.5 Data Gaps and Filler Values Polar Electric Field Instrument (EFI) Data Archive

Overview

The Polar satellite included an electric field instrument (referred to as the "EFI" instrument) which was able to measure electric field in three dimensions and which operated throughout the duration of the mission (March 1996 though April 2008). The instrument was designed and built at the Space Sciences Laboratory (SSL) at the University of California, Berkeley. The Principal Investigator was Professor Forrest Mozer. Peter Harvey designed the EFI instrument and wrote the instrument's flight software. The electric field booms (one rigid spin-axis pair and two wire spin-plane pairs) were designed and built by a team lead by Dr. David Pankow. Henry Heetderks helped design the electric field sensors. Dr. John Wygant was an important scientific advisor in the design of the instrument. Winston Teitler and Jack Vernetti wrote the data analysis software (part of the "SDT" software package at SSL).

The POLAR EFI archive provides plots or ASCII files of electric field data at the following data rates:

Burst:	Data at various rates > 1000 samples/second depending on the telemetry mode.
Real time:	20, 40, or 80 samples/second depending on the telemetry mode.
Spin period:	One point every six second spin period.

The electric field data is available in GSE and GSM coordinates. Also presented are the spacecraft potential and
plasma density deduced from it at data rates from 0.17 to 8000 Hz; and the spacecraft orbit and attitude parameters.

Two types of electric field data, E_0 and E_Dot0, are available. E_0 is computed from the spin plane measurements
and the assumption that the electric field component along the spacecraft spin axis was zero. The E_Dot0 data are
computed from the two spin plane components and the assumption that the parallel electric field was zero. If the
Z-direction is the spin axis, the assumption that the parallel electric field is zero gives Ez = -ExBx/Bz - EyBy/Bz.
This computation is limited to the cases that |Bz| is greater than or equal to 2 nT and both |Bx/Bz| and |By/Bz| are
less than or equal to 5.

The measured electric field is reduced by the conducting wire booms such that the measurement underestimates the

The measured electric field is reduced by the conducting wire booms such that the measurement underestimates the
true field by a factor between 1 and 1.3, depending on plasma conditions. The constant factor 1.2 has been used in
the production of this data set. There is a sunward electric field that is produced by asymmetries between the
sunward and anti-sunward sensors and that has a magnitude of 0-2 mV/m, depending on plasma conditions. This
offset has been set to 1.2 mV/m in the production of this data.

NO DATA HAS BEEN REMOVED DUE TO TELEMETRY NOISE, CROSS TALK, INSTRUMENT OPERATION IN AN
INCORRECT MODE, ETC.   THUS, THE USER MUST BE CAUTIOUS WHEN INTERPRETING THIS DATA.

The magnetic field data is also available, at Real time and Spin period time resolutions, in GSE and GSM coordinates.
The magnetic field and the electric field have been updated according to the latest calibrations for the magnetic field,
provided by the MFE experimenters. For further information about the magnetic field, please go to the MFE Website.

The Polar EFI PI, Forrest Mozer, is available at fmozer@ssl.berkeley.edu to assist with data interpretation.
This archive was prepared by Jack Vernetti and Winston Teitler.

Web Access:

The Polar EFI archive can be accessed with a browser at:

http://polarefi.ssl.berkeley.edu/go.html

This site allows a user to view the EFI data as plots, or download the data as ASCII files.

IMPORTANT WARNING:

This data is for archival purposes and should not be considered as highly accurate. For example, accurate data requires a correction in the form of an offset to the Sunward component of the electric field.

A constant offset of 1.2 mV/m has been used for all the data, this being an approximate average value. In fact, the offset varies with time, and must be determined by analysis of the particular time of interest.

Furthermore, the data has not been culled to exclude times at which data is usually less reliable, such as eclipses, times closely following instrument turn-on or turn-off, etc.

Date Coverage:

The first date contained in this archive is:

1996/03/16

The last date contained in this archive is:

1998/04/12

Coordinate Systems:

The following coordinate systems are used for the vector data in this archive:

1. Geocentric Equatorial Inertial (GEI).

   This coordinate system is also known as Earth-Centered Intertial (ECI),

   or Geocentric Celestial Inertial (GCI).

   
   
2. Geocentric Solar Ecliptic (GSE)
   
   
3. Geocentric Solar Magnetic (GSM)

The electric field vector data is provided in GSE and in GSM.
The orbit and attitude vector data is provided in GEI, GSE, and GSM.

Time Resolutions:

The data from the EFI instrument was produced at three time resolutions:

1. Real time
2. Spin period
3. Burst

Most often, data from the instrument was received at Real time resolution. Until late January 2004, the POLAR spacecraft was operated almost exclusively in "Normal" (Mode 1) telemetry mode, which allocated 40 realtime samples per second to the EFI instrument. After late January 2004, POLAR was operated in "Contingency" (Mode 2) telemetry mode for significant periods of time. In this mode. EFI was allocated twice as much realtime telemetry (80 samples per second). After January 2004, both telemetry modes were used, sometimes switching modes more than once during the same day.

It should be noted that Burst data, when taken, was recorded into onboard EFI memory, and then later played back using some of the realtime telemetry allocation. When this happened, the "realtime" sampling rate was effectively cut in half (to 20 data points per second in telemetry mode 1 or 40 data points per second in telemetry mode 2).

For the POLAR Spacecraft Potential and Plasma Density data, the time resolution for realtime was one point every 0.4 seconds (telemetry mode 1), or one point every 0.2 seconds (telemetry mode 2).

For Spin Period, there was one data point about every 6 seconds (the spin period of the spacecraft). For the electric field vector data, the Spin Period data is obtained from a least-squares spin fit of Realtime data. For the Spacecraft Potential, Spin Period data is obtained from a spin period average of the Realtime data.

On occasion, EFI data was received in the form of bursts. A burst contains data sampled at high time resolution, in most cases 1600 data points per second although some were at 8000 points per second. It should be remembered, however, that bursts were sporadic and had short duration (usually less than 1 minute). Usually there were several hours between one burst and the next.

The time resolution for "orbit" (i.e. "ephemeris") data is one point per 60 seconds.

The time resolution for "attitude" data is one point per 600 seconds.

Description of Data:

Electric Field (vector) Data:

The electric field data is provided, for both GSE and GSM coordinates, at the time resolutions:

1. Real time
2. Spin period
3. Burst

as described in the section: Time Resolutions:

For Real time, there are 40 data points per second (or 80 points per second in Polar telemetry mode 2). However these rates are cut in half when data from a burst is being played back (20 points per second, or 40 points per second in telemetry mode 2).

For Spin period data, there is one data point per spacecraft spin (about every 6 seconds).

For Burst, there are usually either 1600 data points per second or 8000 points per second, throughout the duration of the burst.

The units are mV/m.

The electric field data is not available at Real time or Burst time resolutions after:

2006/02/28

This is due to instrument leakage. The electric field data is, however, available at Spin period time resolution.

The Polar EFI instrument has three pairs of electric field sensor booms and can measure electric field in three dimensions. Two of the boom pairs (denoted "12" and "34") are wire booms in the spacecraft spin plane and each provides more than 100 meters distance between opposite sensors. There is also a rigid boom pair, denoted "56", along the spin axis, with more than 13 meters between opposite sensors, which provides electric field measurement in the third dimension. However, in this archive, the "56" measured component is not used. The spin axis component in the "E_dot0" quantities are computed as described below.

Computation of the Spin Plane Components

The spin plane components of the Real time and Burst electric field are obtained from raw instrument telemetry data quantities V12L and V34L as follows:

1. A correction is applied to all of the data in the form of a 1.2 mV/m offset to the Sunward component of the electric field, this being an approximate average value. In fact, however, the offset varies over time and must be determined by analysis of the particular time of interest.

2. Each spin plane component of the electric field has its value multiplied by a constant scale factor. One reason to apply a scale factor is that the wires in the electric field instrument are at the spacecraft's potential and this has the effect of partially diminishing the potential difference between the spheres. The proper scale factor to correct for this effect varies somewhat; an average correction is a scale factor of 1.2.

3. Each spin plane component of the electric field has subtracted, from its value, a running average of the corresponding Real time component.

4. Despinning of the spin plane components is performed to remove the effect of having the spin plane components of the electric field measured by an instrument which rotates with the spacecraft.

5. The Spin period spin plane components of the electric field are obtained from a least-squares spin fit of the Real time V34L data.

Computation of the Spin Axis Component

Since the measurement of the spin axis component is not as accurate as the measurement of the spin plane components, two versions of the electric field are defined, depending on what is used for the value of the spin axis component. Note that the spin plane components are the same in both versions. The two versions are:

1. E_0

   For E_0, the spin axis component of the electric field is assumed to be zero.

2. E_Dot0

   For E_Dot0, the spin axis component of the electric field is assumed to have a value such that the dot product of E_Dot0 and B (the magnetic field) is zero (i.e. the component of the electric field parallel to B is zero).

   If the spin plane axes are X and Y, then the spin axis component for E_Dot0 will be determined by:

   EZ = -(EX * BX + EY * BY) / BZ

   This expression is not well-defined if |BZ| is too small; if either |BX/BZ| or |BY/BZ| is too large, then any small errors in the spin plane components of the electric field (EX or EY) will be amplified. Data points that are outside set limits for |BZ|, and for the ratios, are not included in the archive. Thus, it is possible to have fewer data points for E_Dot0 than for E_0.

Summary of the available Electric Field quantities:

QuantityName:	Time Resolution:	Coordinate System:	Spin Axis Component:	Archive Filename:
E_0_GSE_RTime	Realtime	GSE	Zero	POLAR_E_0_GSE
E_0_GSM_RTime	Realtime	GSM	Zero	POLAR_E_0_GSM
E_0_GSE_Spin	Spin	GSE	Zero	POLAR_E_0_A_GSE
E_0_GSM_Spin	Spin	GSM	Zero	POLAR_E_0_A_GSM
E_0_GSE_Burst	Burst	GSE	Zero	POLAR_E_0_B_GSE
E_0_GSM_Burst	Burst	GSM	Zero	POLAR_E_0_B_GSM
E_Dot0_GSE_RTime	Realtime	GSE	E_Dot0	POLAR_E_Dot0_GSE
E_Dot0_GSM_Rtime	Realtime	GSM	E_Dot0	POLAR_E_Dot0_GSM
E_Dot0_GSE_Spin	Spin	GSE	E_Dot0	POLAR_E_Dot0_A_GSE
E_Dot0_GSM_Spin	Spin	GSM	E_Dot0	POLAR_E_Dot0_A_GSM
E_Dot0_GSE_Burst	Burst	GSE	E_Dot0	POLAR_E_Dot0_B_GSE
E_Dot0_GSM_Burst	Burst	GSM	E_Dot0	POLAR_E_Dot0_B_GSM

Spacecraft Potential and Plasma Density

The Spacecraft Potential is computed, in Volts, as the average of the spin plane single probe voltage measurements:

(V1 + V2 + V3 + V4) / 4

The Plasma Density is obtained as a function, provided by Dr. Jack Scudder (University of Iowa) of the Spacecraft Potential. The units are cm^(-3), i.e. the number of charges per cubic centimeter. This function was determined by a fit to the POLAR Hydra particle data for 2001/04/01. The validity of this function has not been checked for dates far from 2001/04/01.

The relative accuracy of the density, estimated from the spacecraft potential on short time scales is between 10 and 30 percent, depending on plasma conditions. The absolute accuracy is better by a factor of two for densities less than about 20 particles/cm^3. Density values greater than 30 particles/cm^3 are not plotted because they are inaccurate due to the steep slope of the curve of density versus spacecraft potential.

If the value of the argument to the function is too large, then the Plasma Density is set to a filler value of 1.0e+20, or if the value of the argument to the function is too small, then the Plasma Density is set to a filler value of 1.0e-20.

In fact, the actual range of values of the Plasma Density is much smaller than the range spanned by the filler values. Therefore, when reading Plasma Density data from an EFI archive data file, a good criterion for deciding whether a value of the Plasma Density is a "true" value or a "filler" value, is as follows:

A value of the Plasma Density is a "filler" value if and only if it is either larger than 1.0e+2 (100) or smaller than 1.0e-4 (0.0001); otherwise, it is a "true" value.

Note that the Spacecraft Potential is presented, in plots on the Polar EFI archive web site (http://polarefi.ssl.berkeley.edu/go.html ), with a LINEAR Y scale, but the Plasma Density is presented with a LOG Y scale.

The Spacecraft Potential and Plasma Density are provided at the Time Resolutions: of:

1. Real time
2. Spin period
3. Burst

For Real time, there is one data point every 0.4 seconds for telemetry mode 1 (or for every 0.2 seconds in telemetry mode 2). Note that mode 1 was the POLAR telemetry mode throughout the mission until late January 2004. After that, mode 2 was used often, but not exclusively.

For Spin period, there is one data point per spin (about 6 seconds). The Spin period Spacecraft Potential data if obtained from a spin period average of Real time data.

Burst data appears, in most cases, at either 1600 or 8000 data points per second, for the duration of the burst.

Summary of the available Spacecraft Potential / Plasma Density quantities:

QuantityName:	Time Resolution:	Archive Filename:
SC_Pot_P_Den_RTime	Realtime	POLAR_S_C_Pot_P_Den
SC_Pot_P_Den_Spin	Spin	POLAR_S_C_Pot_P_Den_A
SC_Pot_P_Den_Burst	Burst	POLAR_S_C_Pot_P_Den_B

Orbit Data

The spacecraft position vector is the position of the spacecraft with respect to the center of the Earth. The units are in "Re" (one Re is the Earth mean radius of 6371.2 km).

The spacecraft velocity vector is the velocity of the spacecraft with respect to the center of the Earth. The units are in km/sec).

The Time Resolutions: for all orbit data is one data point every 60 seconds and is provided in the following Coordinate Systems:

1. GEI
2. GSE
3. GSM

Summary of the available POLAR Orbit quantities:

QuantityName:	Units:	Archive Filename Id:
SC_Position_GEI	Re	POLAR_S_C_Position_GEI
SC_Position_GSE	Re	POLAR_S_C_Position_GSE
SC_Position_GSM	Re	POLAR_S_C_Position_GSM
SC_Velocity_GEI	km/sec	POLAR_S_C_Velocity_GEI
SC_Velocity_GSE	km/sec	POLAR_S_C_Velocity_GSE
SC_Velocity_GSM	km/sec	POLAR_S_C_Velocity_GSM
Geocentric Distance	Re	POLAR_S_C_Orbit_Other
Magnetic Local Time	Hours	POLAR_S_C_Orbit_Other
Magnetic Latitude	Degrees	POLAR_S_C_Orbit_Other
Lshell	Re	POLAR_S_C_Orbit_Other
Geographic Latitude	Degrees	POLAR_S_C_Orbit_Other
Geographic Longitude	Degrees	POLAR_S_C_Orbit_Other
Invariant Latitude	Degrees	POLAR_S_C_Orbit_Other

Attitude Data

The spacecraft attitude vector is the unit vector in the direction of the spin axis or the angular momentum. Since this is a unit vector, it has no units.

The Time Resolutions: for the attitude data is one data point every 600 seconds (10 minutes) and is provided in the following Coordinate Systems:

1. GEI
2. GSE
3. GSM

Summary of the available POLAR Attitude quantities:

QuantityName:	Coordinate System:	Archive Filename Id:
SC_Attitude_GEI	GEI	POLAR_S_C_Attitude_GEI
SC_Attitude_GSE	GSE	POLAR_S_C_Attitude_GSE
SC_Attitude_GSM	GSM	POLAR_S_C_Attitude_GSM

Data Gaps and Filler Values

Most data quantities in this archive have no "filler values" in the sense that every value in the archive file is a "true value". Whenever there is a gap in the data, it is left as a gap in the corresponding data file.

The only exception to this is the Spacecraft Potential and Plasma Density data. The Plasma Density is obtained as a function and, in the archive files, the Spacecraft Potential and the Plasma Density are linked together in the files' CDF structure and so share a common set of times, for which "true" values of the Plasma Density are not always available.

There are no "filler" values for the Spacecraft Potential; every value of this quantity is a "true" value. However, it is possible for the corresponding value of the Plasma Density at a given time to not be computable by the Plasma Density function. When this occurs, the archive file contains a "filler" value for the Plasma Density.

If the value of the Plasma Density for a data point is a "filler value", then the Plasma Density plot in the Polar EFI archive website http://polarefi.ssl.berkeley.edu/go.html will not show this point (but the corresponding data point for the Spacecraft Potential does appear).

There can be two different "filler values" for the Plasma Density:

1.0e+20

1.0e-20

In fact, the actual range of "true values" of the Plasma Density is much smaller than the range defined by the two filler values. Therefore, when reading a file of the Plasma Density data, a good criterion for deciding that whether a value of the Plasma Density is "true" or "filler" is as follows:

A value of the Plasma Density is a "filler value" if and only if it is either larger than 1.0e+2 (100) or smaller than 1.0e-4 (0.0001); otherwise it is a true value.