The levels of geomagnetic field activity, or disturbance, currently used in the
long-term (up to 27 days) forecasts are labelled qualitatively for general
usage. For each of the three major zones (subauroral, auroral, polar cap), the
range of activity is divided into four classifications: quiet, unsettled,
active, storm. The actual parameter used for reporting and forecasting magnetic
activity is a daily index. It is known as DRX and is the average of the hourly
ranges (maximum minus minimum during each hour) in the X (northward) component
of the magnetic field intensity for a day (the UT [or GMT] day), i.e., DRX for
zone is the mean of 24 values. Because this averaging process has the effect of
smoothing (filtering) the more rapid fluctuations in the field, the qualitative
descriptors are defined rather differently than for the short-term forecasts.
Units are nanoteslas (nT).