Magnetic structure of interplanetary counterpart of a halo CME: simultaneous observations by NOZOMI and ACE

An interplanetary counterpart of a halo CME which occured on April 13, 1999, was observed by NOZOMI and ACE on April 16-18, when the two spacecraft were within 3 degrees in heliocentric longitude and 0.2 AU in heliocentric distance. On arrival of the halo CME, both spcacecraft detected intense magnetic field whose Bz component was positive at NOZOMI while negative at ACE, although the two spacecraft had been observing almost the same magnetic fields until the arrival of the halo CME. The observed field direction cannot be explained by magnetic structure such as a single cylinder or a torus. Some filanentary structure or rotation of the whole structure would be required to account for the observation. In front of the CME, the magnetic field was highly variable, and characteristic signature of a planar magnetic structure was observed by both spacecraft. The planes of the planar structure, to which all the field lines were parallel, were only slightly (13 degrees) inclined with each other. Supposing that the planes are different parts of a curved sheath in front of the CME, the radius and position of the center are calculated from the angle between the planes and the distance between the two spacecraft.