Two substorms on March 6, 1970, were obserlved by the Vela 5A satellite in the magnetotail plasma sheet and by all-sky cameras along the Alaska meridian. The first substorm started early in a 3-hour interval of southward polarity of the interplanetary magnetic field (IMF). The second substorm, which was much more intense than the first and whose beginning was particularly well marked by the classical auroral and magnetic signatures of an expansive phase onset, started near the end of the 3-hour interval as the IMF was becoming less southward. Only sporadic and partial thinning of the plasma sheet was observed during the first substorm, whereas very rapid and extreme thinning of the plasma sheet occurred within minutes after the onset time of the second, more intense substorm. The observations during these two substorms thus do not fit in a straightforward way a theoretical framework which holds that the southward turning of the IMF leads to plasma sheet thinning and then to the substorm onset unless such thinning is localized in azimuth or in radial distance. An observed correlation of the sporadic partial thinnings during the first substorm with passage of westward traveling surges through the evening sector of the aurral oval suggests that surges may be magnetic projections, to the earth, of westward propagating 'waves' in the radial distance to a magnetic neutral line. The peak-to-peak amplitude of the waves may be as great as 5--10 earth radii.