Magnetic Storm-substorm Relationship and Some Associated Issues
  The impinging solar wind and its magnetic field perturbed the Earth's magnetosphere and create magnetic storms and substorms. The Earth's magnetosphere expands (contracts) during periods of southward (northward) IMF. It is shown that these magnetospheric expansions and contractions account for poorly understood aspects of magnetic storm-substorm relationship, bifurcation of the magnetotail and the appearance of theta aurora. Quantitative theory and calculations in agreement with the suggested model of solar wind/IMF-magnetosphere coupling are presented. Pre-noon and post-noon dents on the magnetopause are expected to appear during a long period of strong northward IMF. The contracting and expanding magnetosphere accumulates solar wind plasma and its magnetic field. The accumulated energy is delivered into the polar regions and creates auroral activity intensification. 
E. P. Savov