The Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE), with a small inclination of 4.8¿ and an apogee of ~8.8 RE, is capable of exploring the dynamical behavior of the near-earth magnetotial current sheet during substorms. At ~1153 UT on day 240 (August 28), 1986, the spacecraft was on the midplane of the magnetotail near midnight (~23.4 h LT) at a radial distance of ~8 RE, when the onset of a substorm took place. The magnetic field data for the ~3.5-min interval following the onset indicated a variation of the magnetic field that has not been observed by geostationary satellites or by other spacecraft flown in the near-earth tail (r≲20 RE). The variation was characterized by a large-amplitude (from less than 10 nT to greater than 40 nT) oscillation of the total field with a period of ~13 s and also by southward turning of the field during most cycles of the oscillation. At times the magnetic field became strongly southward, and in a few measurements the magnitude of the southward component exceeded 20 nT. The level of high-frequency perturbations (period shorter than ~10 s) was also enhanced during the event. The observations may be due to the formation of an X-type neutral line and its motion near the spacecraft. ¿ American Geophysical Union 1987