STRUCTURE OF A MAGNETIC DECREASE OBSERVED IN A COROTATING INTERACTION REGION

Magnetic decreases are often observed in various regions of interplanetary space. Many studies are devoted to reveal the physical nature and generation mechanism of the magnetic decreases, but still we do not fully understand magnetic decreases. In this study, we investigate the structure of a magnetic decrease observed in a corotating interaction region using multi-spacecraft measurements. We use three spacecraft, ACE, Cluster, and Wind, which were widely separated in the x- and y-directions in the geocentric solar ecliptic (GSE) coordinates. The boundaries of the magnetic decrease are the same at the three locations and can be identified as tangential discontinuities. A notable feature is that the magnetic decrease has very large dimension, greater than or similar to 268 R-E, along the boundary, which is much larger than the size, similar to 6 R-E, along the normal direction. This suggests that the magnetic decrease has a shape of a long, thin rod or a wide slab.