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MEDIAN_README
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2007-02-26
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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.