Rise time of geomagnetic sudden commencements —Statistical analysis of ground geomagnetic data—

The relationship between the rise time, dT, and amplitude, dH, of geomagnetic sudden commencements (SCs) observed on the ground is studied paying special attention to problems of data used in analyses by previous workers and extraction of direct effects of the solar wind interaction with the magnetopause. We measured dT and dH of nighttime SCs recorded on rapid-run magnetograms at a low latitude station, Guam, from 1957 to 1975 and made a scatter plot of dH versus dT. The rise time, dT, for a fixed value of dH scatters widely but the range of the scattering becomes narrower for larger values of d H. The upper envelope of the plot shows a clear inverse relationship between dH and dT but dT does not correlate with dH when dH is small. The amplitude, dH, also shows a clear positive relationship with the gradient dH/dT. We assumed that the rise time is essentially determined by time for an interplanetary shock to sweep geoeffective magnetopause length L for ground detection of the magnetospheric compression and calculated the relationship between dH and dT using the shock relations and an empirical relationship between d H and the dynamic pressure jump of the shock. The scatter plot of dH versus dT is reasonably interpreted if L is taken to be about 30 Re.