Magnetic Field Depression at the Earths Surface Calculated From the Relationship Between the Size of the Magnetosphere and the <i>Dst</i> Values

A linear relationship between the hourly Dst value and the inverse of the cube of the magnetosphere subsolar distance Rs -8 from Explorer 12 satellite boundary observations has been found to exist for geomagnetically quiet times and slightly disturbed periods. The proportionality constant between the Dst and Rs -8 is found to be 1.40 times the theoretical value predicted by Mead but remains unchanged for different geomagnetic conditions. The only changes are the amount of the field depression due to the ring current registered in the Dst values. The field depression at the earth's surface is found to be 19 • from the quiet time ring current and -44 • from the slightly disturbed time ring current.