Five satellite years of Vela data are examined for plasma sheet thinnings. Complete proton disappearances (plasma dropouts) are the main subject here. During such times, the Vela satellite is temporarily in the high-latitude tail lobe. The distribution of such plasma dropouts within the magnetotail suggests that the semithickness of the plasma sheet near midnight seldom reaches less than 1 RE during substorms and that the dawn and dusk portions of the plasma sheet remain thicker than the midnight portion. But it is also shown that the plasma sheet occasionally becomes very thin near the dusk magnetopause. This seems to occur when the magnetopause near the dawn and dusk meridians lies further away from the earth than is normal. Extension of the magnetopause to these greater distances does not seem to be related to magnetically quiet periods nor to the Z component of the interplanetary magnetic field (IMF). No such severe thinnings of the plasma sheet are found near the dawn mangetopause. Plasma dropouts can occur regardless of the sign of the Z component of the IMF, but their frequency of occurrence seems to be greater when the Z component is negative. The decreased frequency of occurrence with northward IMF may reflect weaker thinning of the plasma sheet at ~18 RE during the weaker substorms that occur then. Weaker thinning of the plasma sheet may also imply a greater than normal distance from the earth to the magnetic neutral line thought to form in the magnetotail at substorm onset. Three plasma dropouts which occurred in the midnight sector at unusually large distances from the estimated position of the neutral sheet were observed during geomagnetic storms. It is likely that the midnight sector of the plasma sheet can become very thick (~18 RE) at certain times during the main phase of storms. Detailed measurements in the plasma sheet were obtained near the beginning of a geomagnetic storm whose sc triggered a substorm. A compression of the plasma sheet at XSM?-15 RE occurred about 10 min after the sc onset at the earth and about 5 min after the start of plasma sheet thinning associated with the sc-related substorm. If compression-thinning of the plasma sheet initiated this substorm, the triggering action must have occurred earthward of XSM?-15 RE.