The error bar represents an estimate of the accuracy of the prediction. The estimated error is based on the mathematical average of two sources of errors: the geophysical variability (standard deviation) and the standard error of the mean. The geophysical variability is a measure of the scatter of the data used to produce the model. This represents the standard deviation, or scatter, about the mean, which is the error on the model prediction when applied to a given location for a particular storm. The standard error of the mean is the error of the fit to the data, and represents how well the prediction matches the average ionospheric storm response for a given location. The tables of estimated errors show all three: the geophysical variability, the standard error of the mean, and their mathematical average. The latter is the value used on the ionospheric prediction display.
Standard Error of the Mean Geophysical Variability (standard deviation)