Variability of plasma sheet dynamics

From 1200 UT on October 27 to 1200 UT on October 28, 1972, Imp 7 moved across the tail plasma sheet at 35 R E and remained within 2 R E of the expected position at the neutral sheet. During this 24‐hour interval, at least five substorms occurred on the ground. We present Imp 7 observations by using high‐time resolution data that illustrate the rapid variability and multiplicity of plasma sheet phenomena both during and between substorms. Tailward plasma flows, lasting for intervals as short as 2 min and as long as 40 min, were associated with four, and possibly five, of the substorm onsets. Prolonged earthward flow was detected during the expansion phases of two substorms. Between substorms, moderate but variable plasma flows were observed, including two intervals of dawn‐directed flow and large B Y field components. Hence the tail field can be significantly distorted out of the noon‐midnight meridian, possibly by the associated plasma flow stresses. During a rapid 90‐s neutral sheet crossing the magnetic field rotated smoothly through 180° with B Y dominating the field strength at the center. The smooth field rotation suggests that the structure of the neutral sheet may sometimes resemble the hydromagnetic rotational discontinuity. Highly turbulent magnetic fields were detected during an earthward plasma flow interval in which the north‐south component changed from +8 γ to −8 γ in 1 s; the field exhibited large fluctuations in all three components on time scales of 1–10 s. The high level of magnetic turbulence may significantly contribute to plasma sheet dissipation.