Fundamentals of FGM Data
The FGM instrument samples data internally at a rate of approximately 201.75 vectors/second, this decimated by different amounts, depending on the instruments mode, which itself is dictated by the spacecraft telemetry mode. The following table details the most commonly used telemetry type.
|Instrument Mode||Mode Name||Primary Vectors per second||Secondary Vectors per second|
|E||Extended Mode||0.25||0||Data recorded to the instrument internal memory.|
|All||Housekeeping||0.19||0.19||One unfiltered 8-bit values per housekeeping packet.|
At all data rates, the data can be returned over six different magnetic field ranges. At the start of the mission only ranges 2, 3 and 4 were used, but as the Cluster orbit has evolved to lower altitudes, and hence higher magnetic field, ranges 6 and 7, never designed for science operation, are now used routinely The sensor ranges differ in the span of magnetic fields which they cover, and in their resolution, the following table details the possible ranges, and their parameters.
FGM operating ranges
|Range||Maximum range (nT)||Minimum range (nT)||Resolution|
|7||+65528||-65536||8 nT||Designed for ground testing|
There are a number of different data products available, some are only used internally within the FGM operations and science teams, others are more widely available.
FGM data in the ESA Cluster Active Archive (CAA)
The aim of the CAA is to generate a repository of all Cluster data, from all instruments at their highest time resolution, of the highest scientific quality that the teams are able to generate. Therefore all FGM data are in the process of being recalibrated and reprocessed and submitted to the ESA. Three primary datasets are available: full resolution (at either 22 or 67 vectors/second), data averaged to 5 vectors/second and a spin averaged data set.
Prime Parameter Data Set
The Prime Parameter Data Set is spin averaged data for all the spacecraft. The calibration for this data set is generated soon after the data have been taken, generally with a lag of only a few months, and are widely used not just for data browsing and interval selection, but also for a broad range of scientific investigations. They are freely available through the Cluster Data Centers, and are also archived at the CAA for completeness.
Summary Parameter Data Set
The Summary Parameter Data Set is one minute averaged data for a single spacecraft (the "reference spacecraft", normally spacecraft 3). The Summary Parameter Data is available to anyone, but is of limited use for scientific analysis.
Both Prime and Summary Parameter Data Sets are available from your local data centre, click on the link for the Cluster Data Centres to find out which Data Centre you should be using.
When none of the above data types are available, other data may be available, such as Extended Mode, or Housekeeping. Generally the fidelity of this data is much lower, and it is normally only suitable for contextual use.