Characteristic size and shape of the mirror mode structures in the solar wind at 0.72 AU

We investigate the structure of mirror modes in the solar wind at 0.72 AU using Venus Express magnetic field measurements. The mirror mode structure is identified as the presence of magnetic depression or magnetic "holes" in the solar wind with little or no directional change across them. We determine the characteristic size and shape of these structures by examining their durations as a function of the orientation of the magnetic field to the solar wind flow. The mirror mode structure is best fitted with an ellipsoid of revolution, and the resultant shape of the mirror mode structure is a prolate spheroid, or in other words, a rotational ellipsoid. We introduce two parameters, namely the width across the field and the eccentricity to give a full description of the size and shape of the structures. We find that the mirror mode structures in the solar wind are two-dimensional and are more elongated along the magnetic field direction.