POLAR-CAP MAGNETIC ACTIVITY AS A SIGNATURE OF SUBSTORM DEVELOPMENT

On the basis of the 5.46 min IMF data and the 3-min data on magnetic field at polar cap station Alert, various characteristics of the interplanetary magnetic field and polar cap magnetic activity are examined for the purpose of separating the substorm precursors. It is shown that the most suitable characteristics toward this aim are the following: 1.σ(BZ)-index, defined as the 15-min sum of values of the southward (BZS) components of the IMF with an account of the negative gradient of the IMF vertical (BZ) component; and 2.PC(BZ)-index, defined as the 15-min sum of values of the polar cap magnetic disturbances, concerned with southward component BZS, with an account of variability of these disturbances. Every intense peak in the substorm activity is preceded by a corresponding increase in σ(BZ) and PC(BZ) indices. Thus, the conclusion is made that moderate and large substorms have a growth phase and as a result such substorms may be forecasted using the above indices. © 1979.