The STORM model provides an estimate of the expected change in the ionosphere during periods of increased geomagnetic activity. The model estimates the departure from normal of the F-region critical frequency (foF2) every hour of the day for the current and previous day. Values are given in six separate geomagnetic latitude bands, 20° wide, from 20° geomagnetic latitude to the North and South magnetic poles. Storm-time corrections within 20° of the magnetic equator are not made. The corrections are given in terms of a scaling factor, which can be used to adjust the climatological mean. As the ionosphere departs further from normal the color of the trace changes from green, to yellow, to red, where green represents changes within 10% of normal, and red indicates departures in excess of 25%. An estimate of the error in the prediction is also shown based on an average of the geophysical variability and the standard error of the mean. A detailed description of the estimated errors can be found at Estimated Errors. The empirical model provides a useful, yet simple tool for estimating the changes to ionosphere in response to geomagnetic activity.
The STORM model was developed from ionospheric observations during many storms that were analyzed as a function of season and latitude. Within each season and latitude sector, the magnitude of the ionospheric response was determined as a function of an index parameterizing the magnitude of the storm. The storm magnitude index depends on the previous 33 hours of ap, weighted by an appropriate filter. The optimum length and shape of the filter was obtained by a singular value decomposition method. The real-time model uses the hourly values of the 3-hour running ap, provided by the USAF Hourly Magnetometer Analysis Reports. The blue line in the lower boxes shows the hourly value of the integrated ap, which is the index used to drive the model.
The storm-time correction of the F-region critical frequency is primarily of benefit for high frequency (HF; 3-30 MHz) communication users. During a geomagnetic storm the F-region ionosphere can be either depleted or enhanced. When the ionosphere is enhanced, higher communication frequencies can be used, enabling a reduction in absorption and an increase in received signal strength. If the ionosphere is depleted, the maximum usable communication frequencies must be reduced to ensure reflection of the radio signal by the ionosphere to the receiver.
The real-time web page can be used to access the results of STORM for a number of past geomagnetic storms by clicking on the Significant Storms link. The response for any day of interest, for at least the last 365 days, may also be obtained by simply inserting a given date in the appropriate box on the page. A comprehensive validation has been performed by comparing the output of the correction model with data obtained from all available ionosonde stations during all the geomagnetic storms in the year 2000. The validation shows that the model captures more than half of the increase in the storm-induced variability. Follow the links beyond Significant Storms to see graphical examples of the validation.
The references below provide more detailed information regarding the development of the model; simply click on the papers. Please contact Tim.Fuller-Rowell@noaa.gov, Eduardo Araujo-Pradere Eduardo.Araujo@noaa.gov or Mihail.Codrescu@noaa.gov for further information.
