Substorm time scales from polar cap convection measurements

A recent study by Jayachandran et al. (Geophys. Res. Lett., 30, 2064, 2003) reported repeated and detectable changes in the polar cap convection associated with substorms. We report here our use of ionospheric convection measurements to determine the time scales of growth, expansion, and recovery/reconfiguration associated with the substorms. The average time scales, determined from polar cap convection, associated with growth, expansion, and recovery are 31.6, 22.4, and 38.8 min, respectively. A comparison of the growth and expansion time scales determined from the convection measurements with those determined from the geosynchronous satellite measurements revealed interesting differences. The growth time scale determined from the ionospheric convection measurement is smaller than that of the time scale determined from geosynchronous measurements, whereas the expansion time scale determined using the ionospheric convection is much larger than that of the time scale determined using the geosynchronous measurements. The implications of the results in the Magnetosphere-Ionosphere (M-I) coupling are discussed.