Abstract.

A magnetic cloud arrived at the WIND spacecraft in the solar wind upstream of the earth at

1900 UT on Oct. 18, 1995, during Interval # 1 of the First IACG Science Campaign. The magnetic field strength increased and its direction was observed to turn southward abruptly when WIND entered the magnetic cloud; the magnetic field then rotated gradually to a northward orientation over the course of

1230 hours as the magnetic cloud moved past the spacecraft. This magnetic signature is that of an approximately force-free magnetic field with a flux rope geometry, which is characteristic of magnetic clouds. A corotating shock and stream were overtaking the magnetic cloud and interacting with it, intensifying the northward magnetic fields in the rear of the magnetic cloud. The prolonged interval of strong southward Bz produced a major geomagnetic storm and notable aurorae. The subsequent prolonged interval of northward Bz in the latter half of the magnetic cloud was associated with a return of the magnetosphere to a quiet state. Having extensive new data sets and a powerful new means of communicating, scientists have an opportunity to provide a comprehensive account of the causes and the magnetospheric, ionospheric, and thermospheric effects of this magnetic cloud, which forms an excellent example of the kinds of phenomena that the International Solar-Terrestrial Physics Project was designed to study.