USE OF MEDIAN AS BASELINE SENSOR VALUE
                        1/10/2001

The color coded relative intensity of POES energetic particles displayed on our 
plots is a ratio of a sensor measurement taken at the satellite on a given day 
at a particular geographic location to the "typical" level of such particles at 
that location.  In the past, the typical value has been found by calculating the 
mean (average) value for that sensor/location over a whole year.  These typical, 
so-called baseline values have been updated periodically to account for sensor 
degradation.  

We have decided to change the definition of "typical" to be the median* value 
for the sensor/location over the previous year.  This is a better value to use 
for our purpose, because it makes the displayed information much less sensitive 
to extremely high values introduced into the typical value due to a relatively 
rare particle event.  One of these abnormally high values would raise the mean 
by a significant amount, thus lowering the perceived level of energetic 
particles from the recent measurement, as well as lowering the 'belt index' used 
as a numerical indicator of the level of energetic particles at certain L-
values. The median will not be affected by these rare events, and will also be 
much less sensitive to noisy data and telemetry bit errors that could have a 
significant effect on the mean value.

The baseline plots, showing the typical values of the energetic particles 
detected by the various sensors at all geographic locations, obtained from a 
whole year's data, have also been revised to use the median values.

Other than rendering the recent observations less subject to depression due to 
rare, very high levels of particles, this change should have little effect on 
the meaning of the 'belt indices' or the display showing the departure of sensor 
responses at a given location from their nominal values because, in general, the 
yearly mean and median sensor values differ from one another by no more than a 
factor of two.

The labeling of the color bar and the baseline plots that are accessed through 
the 'Select Baseline Plot' link have been changed to reflect the switch to 
median sensor values.

* The median is found by sorting all the values, then selecting the centermost 
value.  For example, the median of [0, 1, 4, 6, 80] is 4, but the mean is 18.2.