Following the GGS key parameter philosophy Hydra produces survey quality electron density and temperatures. These parameters are not the highest quality that the instrument can produce at the expense of considerable further computer processing.

This discussion is meant to motivate the systematic issues of what is known about the key parameter data quality as judged against the computationally more costly "full up" data reduction.

The figure below illustrates the cross correlation of the key parameter density vs the moment density for 24 hours of data collection. The density is determined by a numerical integration over the average distribution function as a function of energy after correcting for the estimates of the floating potential provided by EFI (Forrest Mozer, PI). The sophistication of the corrections at low energy for unmeasured electrons is not as delicately done as in the full up moments code that is computationally more expensive. Nevertheless, the key parameter does tell you what density regime you are in at the 20% level. The key parameter temperature is from a weighted energy average of the angle anveraged observed distribution function. When the electrons are not moving and isotropic this technique gives essentially the same results as the moment code. In the presence of anisotropic pressure and or bulk speed, the angular averaged electron distribution function is surpressing information that the true temperature determined as one third the trace of the temperature tensor can only know. The POLAR orbit encompasses plasma anisotropy values that range from 0.5-2, so that occasionally there are disagreements. There is, however, little doubt as to the mean energy at the 20% level by this key parameter method.

The key parameters are not meant to be inventories for precise tests of various theories. We encourage those who need the best possible numerical values for these parameters to contact the Hydra team via its web page request form for more appropriate data quality products for such tests. Also if you think you have absolute measures for the density for example we would also be interested in supporting such tests.