The occurrence of substorms was examined over a complete 11-year solar cycle, identifying over 5000 substorms. It was found that high-speed streams strongly modulate the substorm occurrence rate, peak amplitude and ionospheric dissipation in the form of Joule heating and auroral electron precipitation. Substorms occurring during the years of frequent interplanetary high-speed streams (1994 and 2003) are 32% more intense, on average, and transfer twice as much magnetic energy to the auroral ionosphere as the substorms occurring during the years of few or no high-speed streams (1993, 1995--2002). To characterize and to predict the substorm activity we form a new measure, the substorm activity parameter Rsu, which we expect to become a powerful tool in analyzing the near-Earth space climate.