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4. The Geomagnetic Disturbance Index, Dst

During a typical geomagnetic storm the magnetic field is depressed (H component is negative) everywhere in the middle and lower latitudes of the Earth. The DSTDEMO program by W.H. Campbell and E.R. Schiffmacher illustrates the change of these midlatitude fields during a storm. The program also shows how the shape of the Dst index, during geomagnetic storms, can be fitted to lognormal distribution curves. For convenience in this display, all the storm values (obtained from the H component of field) are presented as positive magnitudes. The user can either select from ten sample geomagnetic storms or enter hourly magnitudes of the H component of field through the main phase and recovery phase of a storm (i.e., enter the storm-time depression of H as positive values). An example screens is shown below.

Example Screen from DST

A lognormal distribution becomes a normal (symmetric) distribution when the amplitudes are plotted using the logarithm of the x-axis values. This symmetry allows the prediction of storm decay (geomagnetic storm recovery phase) from the storm value rise to maximum (geomagnetic storm main phase). The program computes the field decay from the hourly values given from the start of the main phase of the storm until one hour past the peak magnitude of the storm-time Dst index. Screen graphics show the comparison between the predicted recovery and the observed values.

Try one of the sample Dst storms first to see how DSTDEMO works. The program needs to read the accompanying sample data files so you are asked to give the location where you have copied those files. The program asks questions about the printer so that a hard copy of the results can be an option. When the program is run with user-supplied hourly Dst index or H-component field values, negative values of Dst will be converted to positive numbers. The projection technique requires that values must be given up to an hour past the peak of the storm. If more past-peak values are entered, they will be shown as an overplot on the prediction of recovery. To run the program, insert the disk in the computer, select the drive letter, and enter DSTDEMO.

Download this program and data (self-extracting zip)

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